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Author Topic: RAW and ISO  (Read 6557 times)

unfocused

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Re: RAW and ISO
« Reply #15 on: January 25, 2012, 11:06:10 PM »
Thanks all. Now we'll see if the student got the lesson.

My takeaways:
  • Traditional black and white film ISOs like 100, 200, 400 are what the sensors are designed for;
  • Those in-betweeny ISOs are compromises;
  • If you're a perfectionist, some of those compromises can actually yield a little less noise is you select an ISO that "pulls" the sensor (slight overexposure to capture a bit more data, but not too much);
  • Your actual mileage may vary;
  • For most normal ISOs, this isn't going to matter a heck of a lot, although shooting at ISO 100 may be a reasonable goal to keep in mind if you have that option;
  • It's at the margins (high ISO, low light) that things get critical.
  • Don't try to outsmart yourself (or Canon) by getting too creative playing with the exposure in the camera and then pushing or pulling in RAW.
  • Get the exposure as close as possible in the first place and then tweak in RAW
  • By the time it gets into print, nobody's going to be able to tell anyway

Did I pass?

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Re: RAW and ISO
« Reply #15 on: January 25, 2012, 11:06:10 PM »

briansquibb

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Re: RAW and ISO
« Reply #16 on: January 26, 2012, 01:01:09 AM »
In the general case, none of it matters, and you should just concern yourself about the things that REALLY matter...composition and framing, focus and DOF/bokeh, panning/tracking, etc.

In my world content is all, after that worry about the IQ. Great IQ is nothing without good content. If noise doesn't show on the print it isn't important.

I find sticking it on M mode and auto ISO works best for me, after all I only have to work out the DOF I want and the speed is straight forward in most circumstances. The camera works out the rest. AWB as well.

Am I the only person that uses ISO50? I get good results with that on the 5DII/1D4


Edwin Herdman

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Re: RAW and ISO
« Reply #17 on: January 26, 2012, 01:07:00 AM »
I see references to "brightness" in here.

It's true that ISO is an indirect control for brightness (especially in the case of the pushed / pulled back ISOs), but this is not ultimately useful, unless you don't care about making large prints, or you need the brightness set out of the camera (in fact you don't).  If you do any post-process work at all, however, you don't need to slave to make brightness "correct" in camera.

Instead, if you want to get the most out of every pixel, you ought to overexpose the shot (i.e. dial in 2/3 stop overexposure or whatever your camera needs) to pull as much shadow areas into the first half of the data as possible.

There will be less noise in the shot (especially in darker areas), and the RAW will be larger (more data to play with).  Pulling back brightness in post will not add noise, of course.

As always, there's a good, practical guide to this, found here (Luminous Landscape).  There are also some comments about camera makers and film camera era thinking, which is not worth beating yourself up about or trying to make sense of in strict film camera terms.

jrista

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Re: RAW and ISO
« Reply #18 on: January 26, 2012, 02:42:40 AM »
Thanks all. Now we'll see if the student got the lesson.

My takeaways:
  • Traditional black and white film ISOs like 100, 200, 400 are what the sensors are designed for;
  • Those in-betweeny ISOs are compromises;
  • If you're a perfectionist, some of those compromises can actually yield a little less noise is you select an ISO that "pulls" the sensor (slight overexposure to capture a bit more data, but not too much);
  • Your actual mileage may vary;
  • For most normal ISOs, this isn't going to matter a heck of a lot, although shooting at ISO 100 may be a reasonable goal to keep in mind if you have that option;
  • It's at the margins (high ISO, low light) that things get critical.
  • Don't try to outsmart yourself (or Canon) by getting too creative playing with the exposure in the camera and then pushing or pulling in RAW.
  • Get the exposure as close as possible in the first place and then tweak in RAW
  • By the time it gets into print, nobody's going to be able to tell anyway

Did I pass?

Pretty much. But don't forget this, as its really much more important than all of the above:

Quote from: jrista
In the general case, none of it matters, and you should just concern yourself about the things that REALLY matter...composition and framing, focus and DOF/bokeh, panning/tracking, etc.
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jrista

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Re: RAW and ISO
« Reply #19 on: January 26, 2012, 02:45:02 AM »
Am I the only person that uses ISO50? I get good results with that on the 5DII/1D4

If I had an ISO 50, I'd definitely use it. Would be invaluable for those shots where you want a really long exposure (i.e. Big Stopper type super exposure to glassify watter and blur clouds into oblivion.) I've never had the option on any of my cameras, so I've never used it, but thats certainly not for lack of wanting to.
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NotABunny

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Re: RAW and ISO
« Reply #20 on: January 26, 2012, 05:22:54 AM »
Aargh! Somewhere buried in the posts of the last week or so (I think) was a discussion of RAW files, native ISO, how digital cameras read the data, etc. etc. Now I can't find it.


I think you mean this thread (leads directly to my post there) http://www.canonrumors.com/forum/index.php/topic,2261.msg48485.html#msg48485

The problem with (a large) underexposure is that all the pixels in the image will be registered, for example, with values between 0 and 10 (compared to the maximum 255). Now you increase the exposure in post and get values between 0 and 100, but the problem is that you don't fill all the values between 0 and 100, you actually get only 0, 10, 20, 30...

If however you properly expose an image on the sensor, noise will randomly fill the gaps and thusly create a smooth image - this is a form of dithering.

One other thing: by underexposure I mean relative to what you want when you process the image, not what the camera decides with its 18% gray algorithm.

Here are examples: http://theory.uchicago.edu/~ejm/pix/20d/tests/noise/noise-p3.html
« Last Edit: January 26, 2012, 05:35:54 AM by NotABunny »

Ellen Schmidtee

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Re: RAW and ISO
« Reply #21 on: January 26, 2012, 06:05:08 AM »
If I had an ISO 50, I'd definitely use it. Would be invaluable for those shots where you want a really long exposure (i.e. Big Stopper type super exposure to glassify watter and blur clouds into oblivion.) I've never had the option on any of my cameras, so I've never used it, but thats certainly not for lack of wanting to.

Wouldn't you get the same result by using ISO 100 + 1 stop neutral density filter?

[By "same result" I mean glassify water & blur clouds, in contrast to having less noise than ISO 100.]

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Re: RAW and ISO
« Reply #21 on: January 26, 2012, 06:05:08 AM »

bycostello

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Re: RAW and ISO
« Reply #22 on: January 26, 2012, 09:42:54 AM »
there isn't really a connection between raw and iso. 

ISO sets the amount of noise so lower iso is always better, assuming you don't want noise...  normally you use the iso so you can shoot at the shutter speed you need...

raw is simply the raw file, contains all the captured data..  so a big file and that is the compromise to jpg, but allows you to pull the settings of the image more....  i use raw as only one chance to get it right and gives me a greater margin of error... 

Axilrod

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Re: RAW and ISO
« Reply #23 on: January 26, 2012, 09:51:33 AM »
Aargh! Somewhere buried in the posts of the last week or so (I think) was a discussion of RAW files, native ISO, how digital cameras read the data, etc. etc.

Now I can't find it. But, at any rate, it was way more technical than I could follow. I'm wondering if some of the more technically-minded participants might be able to give us non-techies a simplified explanation of what they were talking about and why it matters (if it does indeed matter).

When I go out to take pictures and set my 7D to ISO 400 (hey! I shot Tri-X most of my life) and shoot RAW am I really setting the ISO to 400 or am I making some compromise that I am not aware of. And, should I care?



There ARE some very specific caveats about ISO settings when it comes to Canon that do not apply to other sensors (namely, Sony sensors). Canon uses a base/push/pull approach that can really throw you for a loop. First, the base ISO settings, i.e. 100, 200, 400, 800, etc., are the only true "native" ISO settings with Canon cameras (1D X and possibly future gear excepted, they have likely moved to a different approach now). Intermediate settings, such as 125, 160, 250 320, etc. are either "pushed" or "pulled". High ISO settings can be a complex mosh of a variety of methods to achieve the final result.

Every increase in base ISO setting is going to have an impact on DR, usually about 1 stops worth, but its never quite that cut and dry in the real world. This is because you are amplifying the analog signal beyond the lowest native setting of 100 on a scale that has a hard cutoff once you surpass the maximum limit (i.e. 12 bits of luminosity), where your DR should (theoretically) be at its highest. The lower dynamic range may pose problems with clipped highlights if you are not careful. Technically speaking, this should be true for every camera, not just Canon, simply as a matter of physics.

When it comes to ISO 125, 250, 500, etc. those are all "push" settings. Its the base ISO with a +1/3rd stop of in-camera digital "overexposure". When it comes to ISO 160, 320, 640, etc. those are all "pull" settings. Its the base ISO with a -1/3rd stop of in-camera digital "underexposure". This is why some settings on Canon cameras appear to have higher noise than higher ISO settings (i.e. ISO 500 tends to be a bit noisier than ISO 800), and why some settings appear to have lower or similar noise as lower ISO settings (i.e. ISO 320 can be as clean as ISO 100). When it comes to really high ISO settings, such as ISO 3200 or 6400, the story is even more complicated. You end up with several stops of standard analog amplification to ISO 1600, then one or more additional varieties to increase ISO beyond that...you may end up with some additional but less effective analog amplification as well as some digital boost. This is usually why moving from ISO 1600 to ISO 3200 usually results in a LARGE increase in noise, where as moving from ISO 800 to ISO 1600 or ISO 400 to ISO 800 results in a more reasonable increase in noise.

The information above is based on a great post by Daniel Browning on the Canon Digital Photography Forums: http://photography-on-the.net/forum/showthread.php?t=1081982. You can also see the effects of Canon's (probably flawed) approach to ISO settings in this Vimeo video: http://vimeo.com/10473734. For comparison, the D7000 uses one of Sony's newest sensor designs that seems to do a more consistent job with gradually increasing noise: http://vimeo.com/26211959.

Hopefully, the visual examples will clarify the nitty-gritty technical stuff that you were not able to understand from whatever you were reading before. As for whether it matters, overall, mostly no, but on some level, maybe. Noise and the amounts of it tend to be way overblown most of the time. The more we push resolution, the less meaningful noise becomes. If you double the resolution of a sensor, and pixel peep an image from the previous vs. the one from the second...the second will likely "appear" noisier...however there are twice as many pixels, and the apparent noise of every 2x2 block of pixels from the higher resolution represents a single pixel from the lower resolution sensor. Scale down the larger image to the same size as the smaller, and the noise characteristics will likely be the same. The scaled-down larger image could very likely appear LESS noisy, as downscaling has the effect of absorbing small-scale undesirable artifacts. Print is similar, and in a sense, a certain amount of noise is actually USEFUL in print. Print is often as least three times to as much as ten times as dense as a computer screen. Any amount of noise that may be visible at 100% crop on a computer screen with 72-100dpi is likely to be entirely invisible in print at 300-720ppi. A perfectly smooth gradient will usually posterize (create visible banding) in print, but a bit of noise or film grain will usually eliminate any posterization.

One area where higher ISO settings could matter is dynamic range. Every full stop of ISO increase usually means you lose about a stop of DR. With Canon's approach to ISO, you may also lose an extra 1/3rd stop if you are using a push or pull setting, possibly more if you are shooting above ISO 1600. A lot of stuff we photograph doesn't need huge dynamic range, and for the things that do, such as landscapes, we can usually get away with much longer exposures and lower ISO settings (half the time, a long exposure is required for artistic effect...such as long water exposures.) If you need both maximal dynamic range and high sensitivity, then you very likely ARE making a trade-off, and you should be aware of the consequences, as it may affect your ability to get the shot you want.

I wouldn't worry about normal ISO settings...the noise we see at ISO100-800, and for newer cameras even ISO 1600, is pretty much a non-issue in real-world scenarios. The only time noise can really become a problem is when you have to use a higher setting like ISO 1600, 3200, maybe even 6400...and you simply don't have enough available light to really get a full exposure. (I have this problem a lot as I shoot wildlife like elk and deer, and birds, and they usually come out to feed right as the sun sets. The very dim light usually means I have to use ISO 3200 or even 6400, and am unable to fully expose the sensor at the shutter rates necessary to capture the action. You can have the same problem indoors with lenses with small maximum apertures, such as f/5.6.) Noise can become a real problem at that point, and it doesn't really matter what camera brand you use. There are solutions to those problems to, though. You can use flash to produce more light, find ways to increase the available lighting, find ways to reduce the necessary shutter speed (i.e. IS/VR lenses), etc. Dynamic range can really suffer at higher ISO settings, especially if you available light and shutter speed requirements are limiting your ability to fully expose and maximize the use of the sensors available DR.


I have no idea what Mr. Wizard is talking about and no way I'm reading something that long on a message board, but for shooting video use ISOs that are multiples of 160.  I've stuck with throughout all my shoots and the results are definitely better.  This is well known so someone may have mentioned it, but I think it goes against what this guy is saying.
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Re: RAW and ISO
« Reply #24 on: January 26, 2012, 10:57:49 AM »
Okay, so what you are saying is that when shooting RAW it is better to set a lower ISO and underexpose by as much as two stops, than to set an ISO above 100 and expose for that ISO. (Obviously, you don't want to set ISO 100 and then expose at 1,600)

But, I believe I have read that with digital files, it is better to expose for the shadows (overexpose) slightly, to capture more data. Do you disagree with that?

And, I guess, a more basic question. What exactly are we doing when we change ISO on a digital camera? Are we making the sensor more sensitive to light (I doubt that) or are we instead, programming in some sort of compensation through the camera's software? (Again, shooting in RAW, I couldn't care less about shooting JPEGs).

Finally, should I even give a rat's behind? Am I better off just relaxing and shooting at my preferred ISO (400) and not thinking too much about it.

The overexposing of shadows does help provided the ISO is constant. When you play with 2 variables both having competing effects then the answer is not that obvious.

Setting the higher ISO engages the DAC amplifier to boost the sensor signal. If no photons are available, the system generates digital Noise, however if enough exposure (photons) is available then it is able to drown the noise. Basically the absence of photons (shadow area) results in noise.

By always setting at ISO400, you are forcing a quicker shutter closure and/ or smaller aperture (depends on exposure mode) at the expense of Noise generated.

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Re: RAW and ISO
« Reply #25 on: January 26, 2012, 11:01:34 AM »
You are making a compromise. You are introducing more noise setting at 400ISO i.e. Electrical or S/N type sensor noise. You are forcing the Camera to come up to a certain level of exposure whether or not you have enough photons in the time your shutter closes. Kind of like in Audio, when you turn up the volume high, you get hiss.

However RAW, will let you tweak the Exposure values by +/-2 stop without much degradation, so in theory you should be able to shoot at ISO 100 and up the exposure by 2 stops in RAW thereby making up for the low light and getting better noise levels.

If the light was beyond the 2 stops needed then the ISO would also have to be bumped up in addition to the exposure in RAW PP.

Thats  how I understand it, not sure if I answered the question.

This is very interesting guys.  Based on very limitted shooting on this, the other day I noticed my ISO 400 pictures when I use my flash had some noise on it.  In fact, the minute I do anything fancy in post prod I notice that ISO 400 can show some noise which was a surprise to me.

Now again this is only based on a few test image but what I tried instead (I was using my flash) was shooting at ISO 200 or even 160.  It made a BIG difference on IQ.  Now where it gets interesting, for the aperture I was using, there was time that I actually underexposed even with the flash at ISO 160.  Pushing exposure and fill light in Lightroom afterward actually yielded me some very crisp picture - better than at ISO 400.  Not sure I was 2 stop under but I was at least 1 stop under.

Based on your theory, I am now very tempted to shoot 1 stop under for a while to use lower ISO and adjust in Lightroom and see what happen...

Does this make sense?

That is my experience as well. I try and set ISO to lowest possible without underexposing too much (i.e. that I cannot regain in LR). I have tried ISO50 as well but did not see much noise improvement compared to ISO100, but did use to to extend shutter closure. On my 5D I find ISO640 is better than 500, 320 better than 200 etc. so instead of 400, try 320.
« Last Edit: January 26, 2012, 11:19:36 AM by K-amps »
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jrista

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Re: RAW and ISO
« Reply #26 on: January 26, 2012, 11:29:01 AM »
If I had an ISO 50, I'd definitely use it. Would be invaluable for those shots where you want a really long exposure (i.e. Big Stopper type super exposure to glassify watter and blur clouds into oblivion.) I've never had the option on any of my cameras, so I've never used it, but thats certainly not for lack of wanting to.

Wouldn't you get the same result by using ISO 100 + 1 stop neutral density filter?

[By "same result" I mean glassify water & blur clouds, in contrast to having less noise than ISO 100.]

Well, with Lee's "Big Stopper", a 10-stop ND filter, at ISO 100 you might expose for 300 seconds or so. With ISO 50, you could expose for 600 seconds or more. You could use two Big Stopper filters, however they are quite expensive and very hard to get a hold of, so if you have ISO 50, its a much cheaper option.
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jrista

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Re: RAW and ISO
« Reply #27 on: January 26, 2012, 11:30:51 AM »
I have no idea what Mr. Wizard is talking about and no way I'm reading something that long on a message board, but for shooting video use ISOs that are multiples of 160.  I've stuck with throughout all my shoots and the results are definitely better.  This is well known so someone may have mentioned it, but I think it goes against what this guy is saying.

If you want to use the lowest noise ISO settings, then you are exactly correct, and that fits perfectly with what I said in my post that was too long for you to read. If you DO read it, you might actually understand WHY you would use ISO 160, 320, 640, etc. for that purpose. ;)
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Re: RAW and ISO
« Reply #27 on: January 26, 2012, 11:30:51 AM »

briansquibb

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Re: RAW and ISO
« Reply #28 on: January 26, 2012, 12:10:33 PM »
If I had an ISO 50, I'd definitely use it. Would be invaluable for those shots where you want a really long exposure (i.e. Big Stopper type super exposure to glassify watter and blur clouds into oblivion.) I've never had the option on any of my cameras, so I've never used it, but thats certainly not for lack of wanting to.

Wouldn't you get the same result by using ISO 100 + 1 stop neutral density filter?

[By "same result" I mean glassify water & blur clouds, in contrast to having less noise than ISO 100.]

With large whites there is only one filter that can be fitted - in sunny conditions I use a PL drop in. So then I use iso50 to drop it more - I haven't got a drop in ND.

I find with iso50 the pictures look better for whatever reason.


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Re: RAW and ISO
« Reply #28 on: January 26, 2012, 12:10:33 PM »