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Let's talk about Dynamic Range
I've been able to find very little on the subject in the forums... And I don't mean DR theory, I mean very specific tweaking of custom presets with the intent on controlling/maximising DR.
I'm currently working on finalising my custom presets for our feature length fiction, and I've spent some time with charts and the Canon Console +waveform. I'm *not* shooting high spec perfectly lit charts, just a variety, some home made, some still-life setups. One thing I see mentioned in the forum search is that the XH-A1 has a wider dynamic range when set to 'video' gamma, rather than cine1 or cine2. This doesn't match my findings at all, am I missing something? As far as I can see, the gamma makes the curve more or less steep (obviously), effectively changing the priority/allocation of bits, but Video gamma certainly isn't expanding anything. If anything, Video gamma compresses the DR of the highs (even with low knee). Sure, this is to free up bits further down the scale (why we're applying gamma in the first place) and results in a nice bright video image, but I see less steps at the top end and basically the same at the bottom. Does anyone know what settings Adam Wilt used when he measured 8.3 stops of DR? Obviously this is >8stops which we classically say can be decently represented by 8 bits, so in theory we're on the verge of subtle banding somewhere in the scale, but I'd like to know what he used. If there's anything I'm missing please let me know. What are other people's findings? Kris |
Hi Kris,
Using one of the Cine gammas should give you more dynamic range than the standard gamma. I believe Cine2 should give you more than Cine1 because it's designed for doing a filmout (as opposed to the "look of film on video"). Also, you should set Black=Stretch and Knee=Low for maximum dynamic range. These settings will result in a somewhat low contrast look, of course, but you can always push contrast in post. |
Cheers Juan, that's exactly my findings.. I thought I was going mad. My logic at the moment is that Cine2 gives me the most range, based on visually counting steps and trying every which combination of settings (along with black stretch and knee low as you said), but at the theoretical expense of having a large portion of the image quite 'dark', and therefore needing some boost in post, and subsequent risk of increased banding in the lows. I've also got Master Ped at -4, to get it as low as possible without low-clipping the very bottom of the signal, to make sure I'm using all the bits I can.
So is this what other people have found? re: use of cinegamma specifically. |
Having no idea what the master pedestal did, I had been keeping video gamma levels at normal with blacks stretched and highlights crushed. I was happy with the amount of range I got and accepted that I would just have to lower the curve a bit in post in order to get the look I wanted.
But, after reading this thread, I decided to look at CineGamma 2 again. I liked what it did, as I always have, but I felt it made the picture look too dark, and was actually eliminating dynamic range as opposed to increasing it, because I would have to open up or gain up to get dark objects back to the same exposure as before, and lighter objects would blow out. Then...I played with the master pedestal... And amazingly, it brought the exposure on the bottom end back up while still leaving the gamma tweak in full effect. I'm not sure what the curve actually looks like, but the image is amazing. BLACK STRETCH KNEE LOW CINEGAMMA 2 MASTER PED AT +9 CINE COLOR MATRIX 2 |
Brandon,
That is what I'm doing, but raising Master Ped isn't needed! If you look at curves/console while pointing at a scene, you'll see that Master Ped moves the very bottom of the curve up and down, but doesn't EXPAND it ... so your values are scaled up from say 8-255, to say 20-255. This makes the dark things LOOK brighter, but it isn't "unclipping" anything .... Well, I should say- it may raise details out of the pitch black when watching back on your *TV*, but if you're colour correcting in post then it isn't giving your any more information that you can't get back with curves. In fact, all you're doing is wasting some bits, as you're using less of the full range. I.e. you're not making use of the bit of the scale from "REALLY BLACK" to "BLACK", all look black on your TV, but if you're grading in post you can use that ..... My in progress preset is using -4 master ped, it's as low as it can go with my current settings before the scene bottom-clips. |
Kris, that's entirely true, and most of the stuff I shoot I have to darken on the low end in post, but I find that I get better results if I crunch in post as opposed to stretch. It seems to result in less noise. I'll do some more testing.
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Brandon, I agree, I've always done it that way... shoot high and push contrast in post ... But what if a project is going to be relatively dark anyway, and cine1 or cine2 are closer to your end result than video? In the past I've assumed (and read) that by doing this we'd lose dynamic range and all our detail in the darks... but it doesn't seem to be true here.
This is what I'm toying with.. 1. How to maximize measurable dynamic range, even if it necessitates post-fixing 2. Whether this post-fixing (for example lifting things back up) is acceptable, or even beneficial in some cases (where you make use of super blacks and super whites, for example, and fix in a 10bit+ post workflow to avoid/limit rounding errors). 3. Whether one gamma mode is demontratably 'best' (in a wide range of scenarios), or whether even varying gamma mode by scene depending on desired content/result is actually the best use of bits. ie. if it's meant to be a low key scene, shoot cine2 and make the best use of bits, if it's meant to be higher key use video gamma. *please* get in touch (anyone) if you've differing results/conclusions, take none of this as gospel |
More testing, subjective finding--
1. When grading two captured videos of an identical scene, one clip Cine2 Gamma one clip Video Gamma, I vastly prefer the grading result I get with the Cine2 Gamma. I need less funky curve adjustment in order to get the right texture/contrast I'm after. By 'funky' I mean that I'm not having to use 5-6 points on a curve graph, I can get what I'm after with two points. I find that really interesting, it's a good development. If you find yourself with finicky curves trying to 'uncompress' some contrast back into your highlights, and a little bump at the very bottom to keep your lows visible before a dip in the lower/mid area to add contrast, etc., then I seriously recommend you check out cine2, it has a great aesthetic, *presuming* that you're already shooting and exposing something decent. 2. You CAN match Cine2/Video captures pretty identically, with a slightly funky Curve adjustment and saturation adjustment, at which point the dynamic range in both images looks really very close. So as I mentioned before, a lot of this might just be relating to getting as close as possible to your target aesthetic as possible, to make the most out of your 8 bits and not be re-inventing the wheel with crazy curves in the grade. .. still would love to hear the results of people's experiments Jury's still out on whether/ how much gamma affects dynamic range, beyond the (perhaps recoverable) compression of the highlights. |
Dynamic range in Picture is a difference between full white and full black points.
Video gamma is linear. Cine gamma curves are in form of "S". Cine1, Cine 2 or Video - all of them have the same begin and end points = dynamic range is always the same. Cine gammas makes even dark parts darker with less differences between dark areas (but not darker as a full black point) and brighter areas brighter (but not brighter as full white). You can do the same in postproduction. Cinegamma makes "Filmlook" but not a film dynamic range. Dynamic range depends on camera picture sensors - not on internal gamma processing. Remember - cinegamma looks good - but its only electronically distorted video response. Its not real life. |
Ivan-
A cine gamma curve isn't an "s" contrast curve, video gamma isn't "linear" .. in fact it's kind of the opposite. Cine gamma is less 'gamma corrected' in a traditional video sense, i.e. it is closer to the real life linear response. CCDs do operate in the real world. The curve is shaped as per gamma's power law relationship- http://www.dfanning.com/ip_tips/xstretch_5.jpg Lower gamma means that your image is perceptually dark at the encoding/quantisation stage, resulting in a re-biasing of the available 8bits. Your image comes into post looking dark/flat. What you describe (the 's' contrast boost) is endemic to 'video' gamma- the lows and highs are compressed/crushed, to give a perceived contrasty image. This crushing means basically what you describe- darks get closer to black, whites get closer to white... while white and black do stay where they are, the 'usable', recoverable information is often lost, or practically lose (i.e. death in quantisation). Remember also that these curves are applied before they are fully mapped/quantised/etc in camera, i.e. before the white point and black point are chosen (note- setup, master ped, etc.), so dynamic range 'can' indeed be affected. -EDIT- Here you go- http://www.usa.canon.com/app/html/HD...justment.shtml |
Brandon- cool, nice test shots, looking at them now.
The interior is easy to see- there's some bottom-clipping in the darkest bit of the curtain about 3/4 of the way across (from left), this is totally in tact right down to say -4 (i.e. you may as well shoot down here at all times), with the slightest hint of compression at -5.. getting bad at -7. To be fair, you could probably argue that the subtle extra bit-precision (for the whole image) outweighs the subtle extra DR for a tiny bit of curtain shadow detail in this case, right down to even -7 or -8. Really depends how much of your image is going to be scraping the bottom of the DR barrel..! In terms of shooting with a higher master ped to help with monitoring (on set, or tv, etc.), you probably aren't losing much. Looking at the outside image, there's almost no bottom-clipping .. pretty low DR scene interestingly (99% within the A1s lattitude). I see a hint of compression starting about 1/3 of the way from the left of image, but hard to make it out until -7. Are you seeing the same? Seems like we can safely say that (for setup=0, black=stretch, gamma=cine2, at least) MP > -4 is only really for when you need your output bright for viewing purposes, lower is better for best use of bits, ~-4 is as low as you can go without seeing even a hint of compression of near-blacks, beyond that sees some low-compression but can be entirely acceptable for a relatively bright scene, and makes best use of bits. |
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Black and White levels on Video signal are stable levels - independent from gamma settings or character of picture (in both - digital or analog video). IMHO this Cinegamma correction is aplicated to compelete videosignal in a input/output matrix. But okay - am not a Canon constructor - i can not be sure. |
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Also if you point the cam at a ramp which overexposes at the top 20%, then set zebras to 100%, then changing the gamma curve changes the point where the zebras start. I haven't yet tested whether this is because the 0.0-1.0 points for the gamma transform are different to the 0.0-1.0 for the output (i.e. internal white is higher than output white to limit DR due to 8bit/contouring), or whether it is simply that 100% zebras are broadcast legal limit, rather than output 1.0.
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CCD sensors have own maximal dynamic range - u can`t change this with gamma corrector. And after CCDs is ist always a input/ouptut matrix in picture signal chain. |
Analogue min and max IRE 'are' at points on a scale- say from 0IRE to 100IRE for luma. These are voltages as you said. But the cameras can record super blacks and/or super whites- i.e. values above and below the analogue broadcast legal values. If your TV is displaying 0 to 100 IRE when the cam records around -4 to 114 IRE (which seem to be approx the A1s min and max for composite), then the TV is displaying 0.034 to 0.881... so if you gamma correct in camera (i.e. 0-1), there will be values which scale in and out of your TV's visible range. I presume that this what you're seeing when adjusting gamma in camera seems to change where the 100% zebras are on a recorded luminance ramp?
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in the days when everyone was shooting for broadcast there was a strong imperative to have whites at 100 and blacks at 0 - it was the only way to get a full volt of signal and maximize the sn ratio.
Kris - what you are asking for is a "maximize dynamic range" setting. The default setting is about the best you will find. Dynamic range needs to be evaluated for each scene - what you will normally want is to maximize the tonal values that fall in the midrange - in many circumstances it's fine to blow out the whites and crush the blacks - and that is where the cine settings help. |
Hi Peter- I'm definitely seeing more DR with knee low and blacks stretched- cine (i.e. more linear) gamma expands the steps at the top of the step chart, so while it might not be increasing DR, I'm getting more usable/recoverable detail for grading.
You said- "in many circumstances it's fine to blow out the whites and crush the blacks - and that is where the cine settings help"- the cine gammas don't do this in my tests, they expand the highlight detail and compress the lows (without necessary affecting DR at all of course). They don't clip the blacks. "Dynamic range needs to be evaluated for each scene - what you will normally want is to maximize the tonal values that fall in the midrange"- Agreed, but in my mind this has everything to do with the gamma, recording with a result as close as possible to your end result in order to use the bits where you need em. |
Okay, I'm 'absolutely' seeing more DR with cine2, then video gamma. Sometimes you just have to trust results (and charts..). Without a doubt, the cine gamma is unclipping values in an over-exposed test scene, without clipping lower values. Obviously the lows get compressed down a bit (i.e. get less bits).
For anyone totally confused, check these links- http://www.usa.canon.com/app/html/HD...justment.shtml http://www.motionfx.gr/Files/HD%20Va...Film%20Rec.pdf (The second is regarding the varicam, but it's relevant.) To be clear- I'm not talking about getting something for free. It's clear that the camera as set up by default is optimised for legal broadcast delivery on a standard monitor/screen, for a nice bright image with minimum contouring. The CCDs *do* capture more DR than you see with typical settings, but capturing this within the 8bit file format necessitates compromises- extra bits used to expand highlights (i.e. knee low, or cine gamma) is basically taking bits from further down the scale, minutely increasing risk of contouring, etc., etc... Cine2 gives you more DR from the chips, but makes the image innapropriate for display on a monitor without tweaking-- as the dynamic range is too wide for a typical monitor/viewing environment it'll look too dark until corrected in post. I've started experimenting with cine2 on shoots and, where it's appropriate for the scene/style, I'm much prefering it in the grade. |
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As I've been further investigating this issue, I've decided to purchase a Tiffen Ultra Contrast 3 filter from Bhphotovideo.com. According to Tiffen's website, this filter will reduce contrast and bring up dark areas without flare, as shown in the image attached.
Here is the link to the filter: http://www.tiffen.com/displayproduct...&itemnum=72UC3 |
Hi Brandon! Yeah they're definitely interesting, haven't tried them yet. Charles' comment about low-levels shifting up and down as you pan around (due to the ambient lift of the ultra-con being dependent on how much light there is available in total) is a tad worrying, since I shoot a lot of hand-held cine type stuff, but I'm definitely planning on checking them out. Obviously it doesn't 'capture' the extended dynamic range, in the way that cine gamma does, but 90% of the time (when shooting video) you're mostly just worried about highlight clipping in outdoor scenes, etc., so it's definitely interesting. Our feature shoot (brevis, nikon glass, largely hand-held via shoulder mounted rod setup) will feature about 20% outdoor daylight material, so let me know how you get on.
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Kris my sentence read:
"...what you will normally want is to maximize the tonal values that fall in the midrange - .... - and that is where the cine settings help." The "- in many circumstances it's fine to blow out the whites and crush the blacks -" was parenthesized with the intent of underlining the fact that it is often OK to blow out the highlights or crush the blacks precisely because this may yield more usable tones in the midrange where you need them. |
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