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Author Topic: RAW and ISO  (Read 6689 times)

unfocused

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RAW and ISO
« on: January 25, 2012, 04:09:24 PM »
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?
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RAW and ISO
« on: January 25, 2012, 04:09:24 PM »

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Re: RAW and ISO
« Reply #1 on: January 25, 2012, 05:26:40 PM »
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.
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Re: RAW and ISO
« Reply #2 on: January 25, 2012, 05:52:03 PM »
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.
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Meh

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Re: RAW and ISO
« Reply #3 on: January 25, 2012, 05:58:06 PM »
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?

Simple.  Non-technical.  Without any of the minor caveats.   Yes, when you set your 7D to ISO 400, you are shooting at ISO 400.  Period.  It is ISO 400 whether you shoot RAW or JPG.  RAW gives you more latitude in post to adjust many things but that is a separate issue.

Image noise goes up with ISO.  That's your compromise.   You are better off raising ISO in camera than you are underexposing and increasing the exposure in post.


Meh

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Re: RAW and ISO
« Reply #4 on: January 25, 2012, 06:07:37 PM »
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)

I don't agree with that.  Up to a point (but it's a technical issue) you are better off raising ISO in camera than raising exposure in post.

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).

1. Tricking the camera's light meter.  2. Increasing the gain in the sensor.  3. Maybe applying digital gain (for very high ISO).  4. Maybe some other techy stuff with the ADC

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.

You should care.   Shoot at the lowest ISO setting you can to get the least noisy images.   If you can't get the aperture and shutter speed you want for the shot, raise the ISO until you can.  But if you prefer to shoot at ISO 400 and that works for you because it's what you know then do it.

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Re: RAW and ISO
« Reply #5 on: January 25, 2012, 06:20:30 PM »
Shooting at iso 100 and pushing the raw-file up two stops in post is NOT AT ALL the same as shooting at 400 iso!! That's crazy...

YOu are better off using "Av" on your camera, set the aperture that gives you the depth of field you want, and check if the shutter speed is what you need. if not go only as high up in iso as you need, but make sure your exposure is bright enough.

On the 5d2 I'm very often 1 stop overexposed, and pull maybe down a little in post instead, but never underexpose and push it up, that'll give you so much more noise. Always get it right in camera, that will give the cleanest files in post.
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Re: RAW and ISO
« Reply #6 on: January 25, 2012, 07:22:28 PM »
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?
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Re: RAW and ISO
« Reply #6 on: January 25, 2012, 07:22:28 PM »

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Re: RAW and ISO
« Reply #7 on: January 25, 2012, 08:03:06 PM »
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:
Small | Large
. For comparison, the D7000 uses one of Sony's newest sensor designs that seems to do a more consistent job with gradually increasing noise:
Small | Large
.

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.
« Last Edit: January 25, 2012, 08:27:23 PM by jrista »
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Meh

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Re: RAW and ISO
« Reply #8 on: January 25, 2012, 09:35:08 PM »
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?

No.  Shoot either properly exposed or slightly overexposed and adjust in post.  Shoot at the lowest ISO you can while still using the aperture and shutter you want for your shot.   Using the full stop ISO of 100, 200, 400, 800 is generally thought to be better than the in between (see @jrista's comment for some details, although some say that may not actually be correct, only Canon knows for sure)

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Re: RAW and ISO
« Reply #9 on: January 25, 2012, 09:47:33 PM »
(see @jrista's comment for some details, although some say that may not actually be correct, only Canon knows for sure)

Check out the linked Vimeo videos for examples, and a comparison of Canon vs. Nikon/Sony ISO settings. I am not entirely certain I fully agree with Browning's analysis (he seems to indicate that Smart ISO (just storing the ISO setting in metadata and always digitally adjusting in post) is the "best" form in "all" cases, however I personally would have assumed that Bargain ISO or even Numpty ISO would be far better options...low noise analog gain), however I think the idea of base/push/pull ISO IS accurate, as it the results of that can clearly be seen in the Canon 7D test video.
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Re: RAW and ISO
« Reply #10 on: January 25, 2012, 09:56:46 PM »
@jrista - I think Meh's point wasn't that the base/push/pull idea isn't correct, but that the numbers you suggest might not be generic across sensors. While base may be ISO 160 and full stops up from there for the 7D (the video is pretty convincing), that may not be the case for a 50D or a 1DIV, where the base may be 100 or 125.
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Re: RAW and ISO
« Reply #11 on: January 25, 2012, 10:05:52 PM »
My opinion is that the ISO setting of 160 or whatever base ISO for different models is for those who are perfectionists.  Most of us will never notice the difference.

Raw is the same story, most will never know the difference.

Having said that, I always use Raw when I can.  I usually crop heavily and add noise reduction or sharpening as well as occasional other enhancements.  When you do this, the possible range of image adjustments is much wider, and if you do a lot of post processing to a jpeg image, it will look terrible, you need to stay quite close to the original.

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Re: RAW and ISO
« Reply #12 on: January 25, 2012, 10:07:09 PM »
@jrista - I think Meh's point wasn't that the base/push/pull idea isn't correct, but that the numbers you suggest might not be generic across sensors. While base may be ISO 160 and full stops up from there for the 7D (the video is pretty convincing), that may not be the case for a 50D or a 1DIV, where the base may be 100 or 125.

Well, I wasn't saying "base" was ISO 160. Lowest native (base...as I think your using the term) is still ISO 100...however the reasoning behind why ISO 160 often looks cleaner is because its a 1/3rd stop pull from ISO 200...that has the effect of reducing DR by an additional 1/3rd stop, and "pushing down" the noise floor. It would be akin to taking an ETTR shot and reducing exposure in post...same noise-reducing effect. I don't think anyone who has ever analyzed Canon's approach to ISO would claim that ISO 160 was the "base" ISO...and I believe even Canon has confirmed that their only true native ISO levels are 100, 200, 400, 800, and in some cases 1600. I can dig up some similar videos for other Canon cameras, including the 5D II, 1DsIII, etc. that show the exact same effect as the 7D video. When it comes to the Rebel line, Canon locks down selectable settings to full stops, so its difficult to verify if they have the same issue (although with my 450D, I always liked it when the camera selected 160 and 320 in Auto ISO better than 100 from a noise standpoint.)
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Re: RAW and ISO
« Reply #12 on: January 25, 2012, 10:07:09 PM »

Meh

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Re: RAW and ISO
« Reply #13 on: January 25, 2012, 10:34:02 PM »
What Canon says and what actually is may or may not be the same or similar and could be quite different or possibly close to correct and that's the point they might want us to believe or be convinced of.

How Canon or anyone else gets to specific ISO settings is not formally published I don't believe.   The sensor collects photons with a certain sensitivity/efficiency which is adjusted by some combination of methods to give us adjustable ISO.  We can't be certain what is the best or cleanest ISO settings.

I think I'm with Mt.Spokane... in most practical and reasonably well lit scenes the difference in SNR between whatever the best ISO is and its nearest neighbours isn't going to be huge so whether you keep it simple and shoot 100, 200, etc. or buy into the 160, 320 theory doesn't matter.  If you're shooting a lot of dark night scenes or club photos it might.

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Re: RAW and ISO
« Reply #14 on: January 25, 2012, 10:44:36 PM »
I think I'm with Mt.Spokane... in most practical and reasonably well lit scenes the difference in SNR between whatever the best ISO is and its nearest neighbours isn't going to be huge so whether you keep it simple and shoot 100, 200, etc. or buy into the 160, 320 theory doesn't matter.  If you're shooting a lot of dark night scenes or club photos it might.

Certainly! Couldn't agree more, and I think I said as much in my dissertation above. That aside, if you habitually shoot in more extreme situations, in answer to the OP, YES...you are making a trade-off with higher ISO settings, and in the case of Canon, its been visually demonstrated that 1/3rd stop settings require additional trade-offs. Those trade-offs, namely affecting DR far more than visible noise, can affect your shot in a meaningful way. 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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Re: RAW and ISO
« Reply #14 on: January 25, 2012, 10:44:36 PM »