Barry Goyette
lvl.4
Flight distance : 14928 ft
United States
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DrMrdalj Posted at 2017-3-10 08:18
Thanks for the example and comment,
If I understand your point - you say that D-Log is closer to Canon WideDR gamma then to pro-grade log curves (Cineon, C-Log2, S-Log2, V-Log)... All pro log curves strongly protect highlights but compress shadows, so cinematographers intentionally overexpose (expose-to-the-right) - if I understand your point you find D-Log to act more as WideDR so you expose for center?
A couple of things. No I wouldn't compare D-log500 to WideDR gamma, which is a linear gamma. D-log appears to be a log gamma, similar to CLog or CLog3 , but "probably" is not a cineon style log like Arri's Log-C or Canons CLog2. I say probably because DJI hasn't published any data regarding it's encoding and I haven't done the type of test that would determine the "shape" of the log (cineon style logs are typified by a long flat mid section that essentially allows you to adjust exposure within a certain range in post with little penalty in the highlights or shadows). Pseudo logs like Clog, and maybe, D-log have raised shadows (that are compressed somewhat compared to a cineon type gamma, but less than a linear gamma) and a steeper, often less-straight middle portion of the curve. These types of gammas are easier to grade and view, and generally have less noise issues in the shadows.
I've heard a number of things said on this forum that are simply not true, one is that in a linear gamma there is more information in the shadows than in the highlights. In fact the opposite is true, and is one of the reasons why Log Gammas are so valuable. I know it looks like shadows are compressed when you look at a log gamma curve, but the reality is that compared to a linear gamma, the discrimination of shadow values is greatly expanded...In reality, the entire range is being redistributed with (relatively) equal amounts of coding values for each unit of input, instead of linear gamma's extreme favoring of data in the highlights (one of the reasons ETTR became so popular).
When you look at a log gamma curve, it's easy to reference what a linear gamma curve looks like and say that the shadows are compressed, because the midpoint has been lowered. A neutral linear gamma curve is a straight line, which belies the fact that there are many, many less encoding values below 50% than above. Log Gammas are different. Each stop in a log gamma has almost the same number of encoding values. So when sony moves the midpoint lower in a log curve What's going on is not compression, but rather a translation of those tones to allow for a greater highlight range. For instance, you've sited that middle grey is often placed below 40% in log gammas. The reason for this is so that reflected white, the brightest non source, non-specular tone that can be recorded, which typically falls at 91ire in rec709. can be placed at ~60ire. to leave more room for expanded highlight range. As we increase DR and expand that highlight range, All the tones move down the scale, but in a relatively equal fashion. In a way they all compress, (relative to a camera with lower DR, anyway) This is why, as we increase DR, recording at a higher bit level becomes important, because it allows for a greater number of encoding values for each stop of range.
So for the record, Shadow tones are not compressed in LOG. They are, compared to linear gammas, expanded. Looking at a photo (not the curve) shot in log and comparing it to one shot in Linear, this should be obvious. When you apply a LUT to a log gamma image...what happens?...it compresses both the highlight and shadow tones to look more like a linear gamma. This would be hard to do if they had already been compressed compared to a linear gamma.
Now...Cinematographer's exposing LOG to the right -- There isn't technically much value to ETTR in log. For instance when you shoot RAW stills, your camera and ACR show you 7-8 stops of range relatively centered on the linear RAW range it's recorded. Because it's linear, the information below that 7-8stops has exponentially less data points than the information above that 7-8 stops displayed on your screen. So photographers choosing to ETTR are moving the exposure more towards the area where there are many more data points (encoding values or gradations).
In Log it's different. Each stop contains relatively the same number of encoding values, and so the same number of gradations within each stop. The thing is, as we get to the lower end of the log scale, because the shadows have generally been expanded, we start seeing all the crap in the signal...the noise that was recorded because light IS linear, not log..meaning even though the LOG stop has the same number of encoding values--gradations--the light itself and the sensors ability to record it fall off dramatically. So when shooting Log, generally as long as you are in the middle portion of the curve, there is no difference whether you expose a little to the right or a little to the left...each tone has the same amount of info to work with, so you just adjust in post as you desire. However, you want to steer clear of both endpoints --- clipping in the highlights and the deepest noise in the shadows, because in log...these are true endpoints, unlike how ACR and your camera show you RAW.
Take a shot like the one I posted on RC Groups yesterday. It has important deep shadows and sunlit portions and sunlit clouds, yet I still had probably 2-3 stops of adjustment where I could get a good exposure. In this situation, I could choose to simply Expose to the right and probably eliminate or minimize most noise in the shadows. However, If I had a client looking over my shoulder, I'd probably have to spend a few minutes explaining why the image looked overexposed...that I'd fix in in post. In that case, or for someone like me who really wants to WYSIWYG it...(perhaps a misnomer in LOG), it's important to be able to adjust ISO rather than simply increasing EV at a higher ISO. Thats why we need the option of choosing a lower ISO for D-Log. |
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