gamma
Canon C-Log on the C300 compared to S-Log.
First let me say that as yet I have not used C-Log in anger, only seen it at a couple of hands on demo events and in downloaded clips.
From what I’ve seen C-Log and S-Log are two quite different things. S-Log on the F3 is a true Log curve where each stop of exposure is recorded using roughly the same amount of data and the available dynamic range is about 13.5 stops. It is inevitable that when you use a true log curve like this and play it back on an uncorrected Rec-709 (standard HD gamma) monitor that it will look very flat and very washed out. This is a result of the extreme gamma miss-match across the entire recording range. If you had a monitor that could display 13.5 stops (most only manage 7) and the monitor had a built in Log curve then the pictures would look normal.
What has too be considered is that S-Log is designed to be used with 10 bit recording where each stop gets roughly 70 data bits ( this roughly means 70 shades of grey for each stop).
Now lets consider the Canon C300. It has no 10 bit out, it’s only 8 bit. Assuming Canon’s sensor can handle 13.5 stops then using 8 bit would result in only 17 bits per stop and this really is not sufficient, especially for critical areas of the image like faces and skin tones. A standard gamma, without knee, like Rec-709 will typically have a 7 stop range, this is a deliberate design decision as this yields around 34 bits per stop. As we know already if you try to do a hard grade on 8 bit material you can run in to issues with banding, posterisation and stair stepping, so reducing the bits per stop still further (for example by cramming 13.5 stops into 8 bits) is not really desirable as while it can improve dynamic range, it will introduce a whole host of other issues.
Now for some years camera sensors have been able to exceed 7 stops of dynamic range. To get around the gamma limitation of 7 stops, most good quality cameras use something called the knee. The knee takes the top 15 to 20% of the recording range to record as much as 4 to 5 stops of highlights. So in the first 0 to 80% range you have 6 stops, plus another 4 to 5 stops in the last 20%, so the overall dynamic range of the camera will be 10 to 11 stops.
How can this work and still look natural? Well our own visual system is tuned to concentrate on the mid range, faces, foliage etc and to a large degree highlights are ignored. So recording in this way, compressing the highlights mimics they way we see the world, so doesn’t actually look terribly un-natural. OK, OK, I can hear you all screaming… yes it is un-natural, it looks like video! It looks like video because the knee is either on or off, the image is either compressed very heavily or not at all, there is no middle ground. It’s also hard to grade as mid tones and highlights have different amounts of squashing which can lead to some strange results.
So the knee is a step forward. It does work quite well for many applications as it preserves those 34 bits of data for the all important mid tones and as a result the pictures look normal, yet gives a reasonable amount of over exposure performance. Next came things like cine gammas and film style gammas.
These often share a very similar gamma curve to standard gammas for the first 60-70% of the recording range, so faces, skin, flora and fauna still have plenty of data allocated to them. Above 70% the image becomes compressed, but instead of the sudden onset of compression as with a knee, the compression starts very gently and gradually increases more and more until by the time you get close to 100% the compression is very strong indeed. This tends to look a lot more natural than gamma + knee, yet can still cope with a good over exposure range, but depending on the scene it can start to look a little flat as your overall captured range is biased towards highlights, so your captured image contains more bright range than low range so will possibly (but not always) look very slightly washed out. In my opinion, if shooting with cinegammas or similar you should really be grading your material for the best results.
Anyway, back to the Canon C300. From what I can tell, C-Log is an extension of the cinegamma type of gamma curve. It appears to have more in common with cinegammas than true S-log. It looks like the compression starts at around 60% and that there is a little more gain at the bottom of the curve to lift shadows a little. This earlier start to the compression will allow for a greater dynamic range but will mean fewer bits of data for skin tones etc. The raised lower end gain means you can afford to underexpose more if you need to. As the curve is not a full log curve it will look a lot more agreeable than S-Log on an uncorrected monitor, especially as the crucial mid tone area is largely unaffected by strong compression and thus a large gamma miss-match.
For the C300 this curve makes complete sense. It looks like a good match for the cameras 8 bit recording giving a decent dynamic range improvement, largely through highlight compression (spread over more recording range than a conventional knee or cinegamma), keeping mid tones reasonably intact and a little bit of shadow lift. Keeping the mid range fairly “normal” is a wise move that will still give good grading latitude without posterisation issues on mid range natural textures.
More Codec and Gamma Tests.
More Gemini, Samurai, AC-Log and S-Log sample frame grabs. See download box at bottom of post.
I had thought, when I first wrote this post that I had discovered a strange issue where the 444 RGB recordings from the Gemini had more dynamic range than 422 recordings. I didn’t think this was right, but it was what my NLE’s (FCP and CS5.5) were telling me. Anyway to cut a long story short, what was happening was that when I dropped the Gemini RGB files into the timeline the levels got mapped to legal levels, i.e. nothing over 100% while the YCbCr 422 clips go into the timeline at their original levels. The end result was that it appeared that the 422 clips were clipping before the 444 clips. Thanks to Waho for suggesting that it may be a conversion issue with the frame grabs, I was able to see that it was simply the way the NLE’s (both CS5.5 and FCP were behaving in the same way) were clipping off anything in the 422 clips above 100% both in the frame grabs and also on the monitor output. As the RGB files were all below 100% they were not clipped so appeared to have greater dynamic range.
Anyway….. below is a new set of frame grabs layered up in a single photoshop file showing how the various codecs and recorders and codecs perform. The levels in these have been normalised at 100% to avoid any dodgy clipping issues. I’ve included F3 Cinegamma 4, plus my AC-Log picture profile, plus Samurai ProRes, Gemini S-Log and F3 Internally recorded S-Log of a very extreme contrast situation. Use the link below to download the photoshop layers file. You’ll need to me a registered user to access the link.
Download More codec test grabs.
Photoshop Layered Frame Grabs v3
Downloads Box by SocialBlogsiteWebDesign.com
S-Log on a non S-Log PMW-F3 and Log on an EX1/EX3
Note: There is something up with the frame grabs. For some reason they are very dark. I’ll look into this in the morning and get some more accurate grabs online.
First of all let me say thanks to Ben Allan on CML list for getting me thinking about this. He has already started experimenting with creating a log style Picture Profile for the EX1. All the setting you’ll find here are my own work and based on tests done with real scenes and some dodgy home made latitude test charts 
Ben’s musings on CML made me consider what S-Log is. In essence it is nothing more than a clever gamma curve that allows you to capture a greater dynamic range than is normally possible with conventional gamma curves. The reason why the standard gamma dynamic range is normally constrained is in part simply because if you record too large a dynamic range and then show it on a conventional monitor or TV, it simply does not look right. So to make it look right it must be graded in post production. In order to do a significant grade in post, the quality of the recording has to be good enough to withstand a fair bit of pulling and pushing. As a result 10 bit recording is recommended (however it is still possible to work with lot with top quality low noise 8 bit recordings, not that I would recommend this). Anyway as both the standard PMW-F3 and EX1/EX3 have 10 bit outputs I decided to see if it was possible to come up with a picture profile that would mimic a Log curve and then see if it actually brings any real world advantage.
Genuine S-Log, mid grey @38%
First up I experimented with the F3. I already have the S-Log option, so this gave me a benchmark to work against. To mimic S-Log you need to increase the gamma gain at the lower end of the curve, you can do this with the Black Gamma function. I know that with S-Log the cameras native ISO is 800 as this is the sensitivity at which maximum dynamic range can be realised with the F3′s sensor. So I started my experiments at 800iso. I could bring up the shadow detail with the Black Gamma but I notice that I appeared to be trading off some highlight handling for shadow information, so while the images kind of looked like S-Log, they did not really gain any latitude.
AC-Log v1. Very similar to S-Log, same exposure as S-Log
During this process I realised that my mid range sensitivity was now a lot higher than with genuine S-Log, so I decreased the camera gain so I was now at 400iso and started tweaking again. Now with Black Gamma all the way up at +99 I was seeing around 1 stop further into the shadows, with no impact on highlight handling.
When I tested my new Picture Profile on a real scene, exposing as you would S-Log with mid grey at 38% I was very pleased to find some very similar images that do grade quite well. As well as the Gamma tweaks I also incorporated a few other changes into the profile to increase the overall grade-ability.
CineGamma 4, mid-grey at 38%
There is a definite improvement in shadow reproduction. It’s not as good as real S-Log, but it does give a very useful improvement for those without S-Log. One interesting point is that the exposure between the two log frame grabs posted here is not changed, so even though the camera is set at 400iso, when the picture profile is applied the camera behaves more like an 800iso camera and exposure should be set accordingly. I think my PP (which you can download at the bottom of the page) brings a little under a one stop improvement in DR, real S-Log is about 2 stops.
If you click on the image captures you can view them full frame. When you compare the AC-Log and Cinegamma 4 images you should be able to see more shadow detail in the tree on the right of frame with the AC-Log yet the sky is further from clipping as well.
So what about the EX1 and EX3, can the same be done for them? Well this is much more of a challenge as the EX cameras are much noisier. Simply bringing up the Black Gamma does help you see into the shadows a bit better but it comes at the cost of a lot of extra noise and really makes it un gradable. Normally I don’t recommend using negative gain as it can reduce the dynamic range of the camera. But I figured if I use negative gain and then increase the gamma gain that should cancel out any dynamic range loss. To then avoid the usual -3db reduction in highlight performance I adjusted the overall gamma gain to return the peak output level to 109IRE. After a bit of fiddling around with my test charts and waveform monitors I could see that it was possible to gain a small amount of dynamic range, a little under 1 stop, however there is an overall increase in the noise level of about +4db. Now that doesn’t sound too terrible, but to gain the extra stop of DR you have to under expose compared to standard gamma’s, typically with S-Log you would put mid grey at 38% (use the centre spot meter on the EX1/EX3 and a grey card). This works reasonable well with this fake log picture profile. The problem however is that when grading you may find that you have to add still further gain to bring skin tones to a normal level and this will accentuate the noise. You could use something like the Neat Video plugging to reduce the noise and in this case I think this sudo Log picture profile could be handy in tricky lighting situations. The EX1R Log picture profile, to work correctly MUST be used in conjunction with -3db gain, any other gain setting and you will loose dynamic range. Again like real S-Log, 10 bit external recording is desirable, but why not play with the picture profile and try it for yourself. It is a bit experimental, I’m not convinced that the extra stop of DR is worth the noise penalty on the EX1R, but then I’m spoilt as I have an S-Log F3.
I have uploaded both the F3 and EX1R picture profiles into a single zip file that you can download below. You will need to have an account on xdcam-user.com to download them, or register for a new account first. Un-zip the package and copy the SONY folder to the root of an SxS card, so you should have both a BPAV folder and a SONY folder in the root directory. The cameras will need the latest firmware versions to load the single profile directly. In the Picture Profile menu choose an empty PP and then in the bottom PP menu chose “load”.
Download F3 and EX1R Log Picture Profiles
F3 and EX1R Log like profiles
Downloads Box by SocialBlogsiteWebDesign.com
EX1 and EX3 Picture Profiles.
These are the picture profiles that I am currently tending to favour for the EX1, EX1R and EX3. Please remember that picture profiles are entirely subjective. These settings work for me, that doesn’t mean they are perfect or for everyone. I like the images the cameras produce when I use these profiles. Please feel free to adapt them or modify them any way you choose. They work on any of the current EX cameras.
Vivid – Designed to help match the EX to a PDW-700. Gives vivid colours with a small shift away from yellow.
Matrix – Cinema, Matrix Level +60
R-G +8, R-B +10, G-R 0, G-B +15, B-R +5, B-G +6
Detail Level -10 Frequency +20, Crispening -40 (if using gain use crispening +14)
Gamma Cinegamma 1
Black level -3, Black Gamma -35
Low Key Saturation -10
Natural C4 – Designed to give a neutral, natural looking image.
Matrix – Cinema, Matrix Level +35
Detail level -7, Frequency +30, Crispening -40 (if using gain use crispening +20)
Black Level -3, Low key Saturation -15
AC Punch – Gives a very high contrast, bold look.
Matric – Cinema, level +40
Gamma Standard 2, Knee level 80, Slope 0
R-G 0, R-B +1, G-R +12, G-B +2, B-R +11, B-G 0
Detail Level -10, Frequency +30, Crispening -45
Black Level -4, Black Gamma -20.
AC Good to Grade – a general purpose setup to give good grading possibilities.
Matrix – Cinema, Level +25
Gamma Cinegamma 1 (Do not use -3db gain)
Detail Level -7, Frequency +45, Crispening -45 (use +35 if using gain)
Black Level -3.
AC-SD Camera look. To mimic an older SD camcorder based on a DSR400, good for HD to SD conversion.
Matrix – Cinema, Level +15
Detail Level +20, Detail Frequency -35, White Limit +35, Black limit +45
Knee, Manual, Level 90, Slope 0.
Gamma Standard 2, Gamma Level +5
Black Gamma -10
Black Level -10
Enjoy! Any feedback or suggestions welcome. Let me know of any profiles that you come up with that may be of interest to others.
More on S-Log and Gamma Curves
A lot of the issues with any camera and the dynamic range it can record are not due to limitations of the cameras hardware but to retain compatibility with existing display technologies, in particular the good old fashioned TV set that has been around for half a century. The issue being that in order for all TV owners to see a picture that looks “natural” there has to be a common standard for the signal sent to the TV’s that will work with all sets from the very oldest to the most recent.
As even the most recent TV’s and monitors often struggle to display a contrast range greater than 7 stops there is no point in attempting to feed them with more, Taking 12 stops and simply squashing it into 7 stops will create a disappointing, flat and dull looking image. So for productions where extensive grading is not taking place, it is not desirable to record information beyond that which the existing broadcast system can handle. This is why the vast majority of modern camcorders with the knee off and using a standard gamma curve all exhibit very similar dynamic ranges (7 to 8 stops typically), because the limitation is generally not that of the sensor, but that of the gamma curves used in broadcast television. By adding a bit of highlight compression through a cameras knee circuit we can stretch out the dynamic range a bit as our visual system is most acute to inaccuracies in the the mid ranges of an image where faces, people and natural subjects normally appear so we don’t tend to notice strong compression occurring in highlights such as the sky or reflections. A well designed knee circuit can help gain an extra 2 or 3 stops by compressing the hell out of highlights, but as most of us are probably aware it can create it’s own issues with the near complete loss of real detail in clouds and the sky as well as color saturation issues on skin highlights, this is gamma curve compression rearing it’s ugly head. Moving on, we come to cinegammas, hypergammas and other similar extended range gammas. One of the issues with a traditional aggressive knee circuit is that it is either on or off, compressing or not compressing, there is no middle ground and this makes grading problematic as it is all but impossible to extract any meaningful data from very highly compressed highlights. Cinegammas etc address this by slowly increasing the amount of compression used as image brightness increases. In addition the gamma curve compression starts much earlier, long before you get to what would traditionally be regarded as “highlights”. This slow and gentle onset of compression grades in a more pleasing manner than a conventional knee. If you don’t grade the added mid-to-highlight compression results in a picture that looks a little flat and lacks “punch”, but is not overly objectionable to view. There is however a limit to just how much data you can cram into a compressed codec or recording system. Cinegammas and Hypergammas are tailored to give optimum performance with existing 8 bit and 10 bit high compression systems and workflows so the design engineers chose to only record a range of about 11 stops as trying to extract more than this from systems essentially designed to only record 7 to 8 stops will lead to visible compression artefacts. Technologies have continued to advance and now it’s remarkably easy (compared to just a couple of years ago) to record 10 bits of 4:2:2 or 4:4:4 data without compression or with only minimal compression. By eliminating or at least significantly reducing the compression artefacts it’s now possible to extract more meaningful data from a compressed gamma curve than was possible previously. S-Log is in effect nothing more than a heavily modified gamma curve, taking cinegammas and hypergammas to the next level. S-Log needs 10 bit recording to work as the curve compression starts much lower in the curve, so when grading those crucial skin tones and natural objects will need to be un-compressed to look natural and 8 bits of data just would not give enough range. As the image brightness increases the amount of gamma curve compression is increased logarithmically. If you look at the data being recorded this means that the majority of the 10 bit data is allocated to shadow areas then mid tones with less and less data being used to record highlights.
Most modern cameras, not just the XDCAM’s simply ignore highlight information beyond what can be recorded, this results in the image getting clipped at a given point depending on the gamma curve being used. Interestingly using negative gain on a camcorder can act as a low end clip as very small brightness changes will be reduced by the negative gain, possibly to the point where they are no longer visible. This normally results in a reduction in dynamic range (as well as noise). I suspect this is why the F3 has less noise using standard gammas because the sensor has excess dynamic range for theses curves and good sensitivity, so Sony can afford to set the arbitrary 0db point in negative space without impacting the recorded DR but giving a low noise floor benefit. For S-Log however it’s possible to record a greater dynamic range so 0db is returned to true zero and as a result the noise floor increases a little.
LUT’s are just a reverse gamma curve applied to the S-Log curve to restore the curve to one that approximates a standard gamma, normally REC-709. They are there for convenience to provide an approximation of what the finished image might look like. However applying an off the shelf LUT will impact the dynamic range as an assumption has to be made as to which parts of the image to keep and which to discard as we are back to squeezing 12 bits into 7 bits. As every project, possibly every shot will have differing requirements you would need an infinite number of LUT’s to be able to simply hit an “add LUT” button to restore your footage to something sensible. Instead it is more usual for the colorist or grader to generate their own curves to apply to the footage. Most NLE’s already have the filters to do this, it’s simply a case of using a curves filter or gamma curve correction to generate your own curves that can be applied to your clips in lieu of a LUT.
PMW F3 Picture Profile Smorgasbord.
I’ve been working some more on picture profiles for the PMW-F3, mainly matrix settings. You can download the full set by clicking here: ac-profiles. Download the zip file, unzip and place the “Sony” folder in the root of an SxS card or SD card in an adapter. Place the card in the camera and go into the “picture profiles” menu and select a picture profile and then “ppdata” and “recall” to load the data into your camera. This will overwrite any PP’s you already have.
Here’s the latest settings I have:
ALL use Detail level -17, Frequency +20, Aperture +25 unless otherwise stated.
AC Warm1: Warm look, less blue/yellow
Cinegamma 1, Black Gamma -25, Black Level -2.
Matrix: Standard, level +8, R-G +14, R-B +12, G-R +4, G-B +8, B-R +4, B-G -18
AC Cool1: Stark cool look, maybe day for night.
Cinegamma 1, Black Gamma -25, Black Level -2.
Matrix: Standard, level +22, R-G -44, R-B -24, G-R -34, G-B =28, B-R -7, B-G -69
AC Elec1: Electronic, vivid look.
Gamma STD1, Black Gamma -20, black level -3, Detail Level -10, Frequency -40
Matrix Hi-Sat,
NAT1CG-1: Neutral Look, natural colors, less yellow/green.
Cinegamma 1, Black Level -2
Matrix FL-Light, Level +3, R-G +2, R-B +2, G-R +8, G-B +8, B-R -8, B-G -6
Note that for most of these I have used a cinegamma, that is because I would assume that post work will be done on the footage. If your not planning on doing any grading or post work you should consider using a standard gamma which will give a richer looking image or cinegamma 2 which is broadcast safe.
PMW-F3 Picture Profiles. First Batch.
OK here we go. Here are some notes from testing my PMW-F3. First thing is… aliasing… a zone plate looks pretty bad with a fair amount of aliasing. I had heard rumours of this from others with pre-production units, but in the field I had not seen anything that would worry me. While the zone plate is not pretty, real world aliasing looks acceptable. I usually use brickwork and roof tiles to test for moire and these look clean on my F3. I think a fine patterned shirt could cause concern and I need to look into this further. I am surprised that there is not more about this on the web!
Excessive detail correction does increase the aliasing, however turning detail and aperture off does not reduce the aliasing significantly. Keep the detail level below -15 to avoid increasing the strength of the aliases. Above -15 the aliasing artefacts are more noticeable. Detail “Off” appears to be the same as Detail -25. Below -25 the image softens, below -45 very noticeably and there are some strange increases in aliasing below -50. For the moment I will be using detail at -17 or off.
The aperture setting can be used to add a little sharpness to the image to compensate for not using detail or a low detail setting. Aperture does not increase the appearance of the aliasing artefacts as strongly as the detail correction. I like the added crispness I can get with Aperture set to +30 combined with detail at -17. I would strongly recommend against using a raised aperture setting if you have detail higher than -15 as this will add sharpness to any detail corrected aliases and lead to twittering edges on horizontal and vertical lines.
Colours have that usual Sony look. Not bad and pretty natural looking, but for me a little on the green side. For a more natural 1:1 look I quite like these Matrix settings:
R-G +10, R-B +4, G-R 0, G-B +14, B-R +3, B-G -3, Std Matrix.
For a more Canon like look with Rec-709 Matrix I came up with these:
R-G -2, R-B +9, G-R -11, G-B +2, B-R -16, B-G -10, Std Matrix, level +14, Blk Gamma -20
For use with Cinegamma 1 I use the above with Matrix Level +25, Blk Gamma -36. Highlights are a little washy, but as with any Cinegamma the best results are obtained by grading in post production.
When should I use a Cinegamma or Hypergamma?
Cinegammas are designed to be graded. The shape of the curve with steadily increasing compression from around 65-70% upwards tends to lead to a flat looking image, but maximises the cameras latitude (although similar can be achieved with a standard gamma and careful knee setting). The beauty of the cinegammas is that the gentle onset of the highlight compression means that grading will be able to extract a more natural image from the highlights. Note than Cinegamma 2 is broadcast safe and has slightly reduced lattitude than CG 1,3 and 4.
Standard gammas will give a more natural looking picture right up to the point where the knee kicks in. From there up the signal is heavily compressed, so trying to extract subtle textures from highlights in post is difficult. The issue with standard gammas and the knee is that the image is either heavily compressed or not, there’s no middle ground.
In a perfect world you would control your lighting (turning down the sun if necessary ;-o) so that you could use standard gamma 3 (ITU 709 standard HD gamma) with no knee. Everything would be linear and nothing blown out. This would equate to a roughly 7 stop range. This nice linear signal would grade very well and give you a fantastic result. Careful use of graduated filters or studio lighting might still allow you to do this, but the real world is rarely restricted to a 7 stop brightness range. So we must use the knee or Cinegamma to prevent our highlights from looking ugly.
If you are committed to a workflow that will include grading, then Cinegammas are best. If you use them be very careful with your exposure, you don’t want to overexpose, especially where faces are involved. getting the exposure just right with cinegammas is harder than with standard gammas. If anything err on the side of caution and come down 1/2 a stop.
If your workflow might not include grading then stick to the standard gammas. They are a little more tolerant of slight over exposure because skin and foliage won’t get compressed until it gets up to the 80% mark (depending on your knee setting). Plus the image looks nicer straight out of the camera as the cameras gamma should be a close match to the monitors gamma.
Understanding Gamma, Cinegamma, Hypergamma and S-Log
Standard Gamma Curve
The graph to the left shows and idealised, normal gamma curve for a video production chain. The main thing to observe is that the curve is in fact pretty close to a straight line (actual gamma curves are very gentle, slight curves). This is important as what that means is that when the filmed scene gets twice as bright the output shown on the display also appears twice as bright, so the image we see on the display looks natural and normal. This is the type of gamma curve that would often be referred to as a standard gamma and it is very much what you see is what you get. In reality there are small variations of these standard gamma curves designed to suit different television standards, but those slight variations only make a small difference to the final viewed image. Standard gammas are typically restricted to around a 7 stop exposure range. These days this limited range is not so much to do with the lattitude of the camera but by the inability of most monitors and TV display systems to accurately reproduce more than a 7 stop range and to ensure that all viewers whether they have 20 year old TV or an ultra modern display get a sensible looking picture. This means that we have a problem. Modern cameras can capture great brightness ranges, helping the video maker or cinematographer capture high contrast scenes, but simply taking a 12 stop scene and showing it on a 7 stop display isn’t going to work. This is where modified gamma curves come in to play.
Standard Gamma Curve and Cinegamma Curve
The second graph here shows a modified type of gamma curve. This is similar to the hypergamma or cinegamma curves found on many professional camcorders. What does the graph tell us? Well first of all we can see that the range of brightness or lattitude is greater as the curve extends out towards a range of 10 T stops compared to the 7 stops the standard gamma offers. Each additional stop is a doubling of lattitude. This means that a camera set up with this type of gamma curve can capture a far greater contrast range, but it’s not quite as simple as that.
Un-natural image response area
Un-natural response
Look at the area shaded red on the graph. This is the area where the cameras capture gamma curve deviates from the standard gamma curve used not just for image capture but also for image display. What this means is that the area of the image shaded in red will not look natural because where something in that part of the filmed scene gets 100% brighter it will only be displayed as getting 50% brighter for example. In practice what this means is that while you are capturing a greater brightness range you will also need to grade or correct this range somewhat in the post production process to make the image look natural. Generally scenes shot using hypergammas or cinegammas can look a little washed out or flat. Cinegammas and Hypergammas keep the important central exposure range nice an linear, so the region from black up to around 75% is much like a standard gamma curve, so faces, skin, flora and fauna tend to have a natural contrast range, it is only really highlights such as the sky that is getting compressed and we don’t tend to notice this much in the end picture. This is because our visual system is very good at discerning fine detail in shadow and mid tones but less accurate in highlights, so we tend not to find this high light compression objectionable.
S-Log Gamma Curve
S-Log Gamma Curve
Taking things a step further this even more extreme gamma curve is similar to Sony’s S-Log gamma curve. As you can see this deviates greatly from the standard gamma curve. Now the entire linear output of the sensor is sampled using a logarithmic scale. This allows more of the data to be allocated to the shadows and midtones where the eye is most sensitive. The end result is a huge improvement in the recorded dynamic range (greater than 12 stops) combined with less data being used for highlights and more being used where it counts. However, the image when viewed on a standard monitor with no correction that looks very washed out, lacks contrast and generally looks incredibly flat and uninteresting.
S-Log Looks Flat and Washed Out
Red area indicates where image will not look natural with S-Log without LUT
In fact the uncorrected image is so flat and washed out that it can make judging the optimum exposure difficult and crews using S-Log will often use traditional light meters to set the exposure rather than a monitor or rely on zebras and known references such as grey cards. For on set monitoring with S-Log you need to apply a LUT (look Up Table) to the cameras output. A LUT is in effect a reverse gamma curve that cancels out the S-Log curve so that the image you see on the monitor is closer to a standard gamma image or your desired final pictures. The problem with this though is that the monitor is now no longer showing the full contrast range being captured and recorded so accurate exposure assessment can be tricky as you may want to bias your exposure range towards light or dark depending on how you will grade the final production. In addition because you absolutely must adjust the image in post production quite heavily to get an acceptable and pleasing image it is vital that the recording method is up to the job. Highly compressed 8 bit codecs are not good enough for S-Log. That’s why S-Log is normally recorded using 10 bit 4:4:4 with very low compression ratios. Any compression artefacts can become exaggerated when the image is manipulated and pushed and pulled in the grade to give a pleasing image. You could use 4:2:2 10 bit at a push, but the chroma sub sampling may lead to banding in highly saturated areas, really Hypergammas and Cinegammas are better suited to 4:2:2 and S-Log is best reserved for 4:4:4.
Shooting Snow and other bright scenes.
Well winter is upon us. The north of the UK is seeing some pretty heavy snow fall and it’s due to spread south through the week. I regularly make trips to Norway and Iceland in the winter to shoot the Northern Lights (email me if you want to come) so I am used to shooting in the snow. It can be very difficult. Not only do you have to deal with the cold but also difficult exposure.
First off it’s vital to protect your equipment and investment from the cold weather. A good camera cover is essential, I use Kata covers on my cameras. If you don’t have a proper cover at the very least use a bin liner or other bag to wrap up your camera. If you have a sewing machine you could always use some fleece or waterproof material to make your own cover. If snow is actually falling, it will end up on your lens and probably melt. Most regular lens cloths just smear any water around the lens, leaving you with a blurred image. I find that the best cloth to use in wet conditions is a chamois (shammy) leather. Normally available in car accessory shops these are soft, absorbent leather cloths. Buy a large one, cut it into a couple of smaller pieces, then give it a good wash and you have a couple of excellent lens cloths that will work when wet and won’t damage your lens.
Exposing for snow is tricky. You want it to look bright, but you don’t want to overexpose. If your camera has zebras set them to 95 to 100%. This way you will get a zebra pattern on the snow as it starts to over expose. You also want your snow to look white, so do a manual white balance using clean snow as your white. Don’t however do this at dawn or near sunset as this will remove the orange light normally found at the ends of the day. In these cases it is best to use preset white set to around 5,600k. Don’t use cinegammas or hypergammas with bright snow scenes. They are OK for dull or overcast days, provided you do some grading in post, but on bright days because large areas of your snow scene will be up over 70 to 80% exposure you will end up with a very flat looking image as your snow will be in the compressed part of the exposure curve. You may want to consider using a little bit of negative black gamma to put a bit more contrast into the image.
If the sun is shining, yes I know this may not happen often in the UK, but if it is then the overall brightness of your scene may be very high. Remember to try to avoid stopping down your lens with the iris too far. With 1/3? sensor cameras you should aim to stay more open than f5.6, with 1/2? more than f8 and 2/3? more than f11. You may need to use the cameras built in ND filters or external ND filters to achieve this. Perhaps even a variable ND like the Genus ND Fader. You need to do this to avoid diffraction limiting, which softens the image if the iris is stopped down too much and is particulary noticeable with HD camcorders.
Finally at the end of your day of shooting remember that your camera will be cold. If you take it in to a warm environment (car, house, office) condensation will form both on the outside and on the inside. This moisture can damage the delicate electronics in a camcorder so leave the camera turned off until it has warmed up and ensure it is completely dry before packing it away. This is particularly important if you store your camera in any kind of waterproof case as moisture may remain trapped inside the case leading to long term damage. It is a good idea to keep sachets of silica gel in your camera case to absorb any such moisture. In the arctic and very cold environments the condensation may freeze covering the camera in ice and making it un-useable. In these extreme situations sometimes it is better to leave the camera in the cold rather than repeatedly warming it up and cooling it down.
Have fun, don’t get too cold, oh… and keep some chemical hand warmers handy to help stop the lens fogging and to keep your fingers from freezing.
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