ISO 1600 vs correcting exposure in photoshop

[iND]

Does anyone have a feel for high ISO 800,1600,3200 on the 50D or 5D image quality vs


lower ISO and correcting the exposure in photoshop.


example.


Shoot ISO 1600 F4 60 vs ISO 400 F4 60 (and try to fix exposure in CS3)

[Chuck Lee]

What your talking about is commonly refered to as "pushing" the exposure in post. I can't speak for the 50D but I can say that I've "pushed" quite a few exposures on images recorded with my 5D.


I would say it is safe to say to use higher ISO and slightly over expose rather than under expose and push in post.


I find my 40D does extremely well at 1600 ISO if I overexpose .3-.5 ev and pull down the overexposure when developing the raw file. By comping up +.3ev I am shooting at a 1250 ISO equivalent. I am loosing 1/3 stop shutter for a given aperture. Or I could just shoot 1250 ISO at 0ev comp. Is there a difference? Not really. I'll splain..........


The 30D & 40D (I have a feeling this goes for the 50D as well) emulate interum ISO values in 1/3 stop increments. Not sure about 1/2, never used it. ( maybe someone could add to this if experienced using 1/2 stop ISO values) When shooting at 160, 320, 640 & 1250 ISO the camera shoots at 200, 400, 800, &1600 but mathematically offsetsthe raw data before writting to the CF card. This is the same as the 1/3 stop of overexposure and results in a much cleaner image than the normal ISO settings. For 125, 250, 500, & 1000 ISO the inverse is true. 100 is pushed mathematically up to 125, 200 to 250 and so forth which introduces more noise into the image. When shooting with the 40D, I use 160, 320, 640, & 1250. If I need to go higher I go to 3200 and over expose by .3-.7 which will give me an eqivalent ISO of 2400 at +.3ev or 2000 at +.7ev (this 2000 is cleaner than 1600 ISO underexposed and pushed in post) Oh, I forgot to mention that there is usually +1ev worth of headroom in a Canon Raw file at 3200 ISO. That is information un reported by the histogram. Simply compensate down during post processing and viola!! normal exposure, withless noise and optimized shutter speed.


The 5D is straight up! I hope the 5D MII is as perfect. 100, 200, 400, 800, 1600, & 3200 are as clean as it gets. Emulated interum values don't offer any benefit from a noise cancelling perspective.


I discovered all this from a post on dpreview a few years ago. I have experimented and shown this to be very true. Here is an excellent study.


http://www.pages.drexel.edu/~par24/rawhistogram/5DTest/5DTest.html


http://www.pages.drexel.edu/~par24/rawhistogram/30DTest.htmlThis is applicable to the 40D. I'm wondering if anyone can confirm this for the 50D?


Hope this helps,


Chuck

[Daniel Browning]

Does anyone have a feel for high ISO 800,1600,3200 on the 50D or 5D image quality vs lower ISO and correcting the exposure in photoshop.

The short answer is to use the highest ISO you can without blowing important highlights. High ISO reduces noise.

Here's the long answer. ETTR (Expose To The Right) means to increase exposure (f-number, shutter speed, etc.) as much as possible without blowing the highlights you care about. ETTR is the best technique for reducing noise. A single stop of exposure (e.g. f/2.8 instead of f/4) can make more of a difference than 2 stops of ISO (e.g. 400 -> 1600). That doesn't mean ISO is unimportant, just that exposure is more important.

Every camera has different performance at each ISO. The way your 50D and 5D work using "1 stop" ISO settings (not 1/3-stop) is that higher ISO always have less noise than lower ISO until you get to 3200. In other words: ISO 200 has less noise than 100. 400 has less noise than 200. And so on and so forth until you get to 3200. At 3200 and above, the rules change. 3200 has the same noise as 1600. 6400 has the same noise as 1600. 12800 has the same noise as 1600. Therefore, ISO 1600 is the highest ISO that you should use.

The only reason to ever go above 1600 is for convenience: it's necessary for JPEG (if you don't shoot raw), it gives you a brighter review image on the LCD, makes it easier to use autoexposure, flash metering, you don't have to make adjustments in post processing, etc.

But there is a cost associated with high ISO. That cost is clipped highlights. For every doubling of ISO, one stop of highlights are lost. That is why I advise folks to use ITTR: ISO To The Right. That is, increase ISO as much as you can on every shot, without blowing important highlights. But don't go over 1600, as I said above, unless you need the convenience features.

Of course, you often read that high ISO causes noise. The correct meaning of that phrase is this: "At low ISO, I use normal exposures. At high ISO, I use severe underexposure. The underexposure causes a tremendous increase in noise. Increasing the ISO helps to reduce that noise a little, but it's not nearly enough to make up for all the noise caused by underexposure." There is one exception to this: going from ISO 100 to ISO 200 decreases noise so much that it *almost* makes up for the underexposure (it only has about 1/3 stop more noise). That is why ISO 100 and ISO 200 have very similar noise levels.

Hope that helps.

[Daniel Browning]

Great post, Chuck. Thanks for covering the "tweener" (1/3-stop) ISO settings. (FWIW, there are no 1/2 stop ISO settings.)

I would say it is safe to say to use higher ISO and slightly over expose rather than under expose and push inpost.

Agreed.

I'm wondering if anyone can confirm this for the 50D?

I confirm it.

By the way, Chuck, you might be interested to see the chart of dynamic range over ISO settings, made by John Sheehy:

http://forums.dpreview.com/forums/re...ssage=31378147

The 5D is straight up! I hope the 5D MII is as perfect.

[iND, you can probably ignore the discussion between Chuck and I -- we like to get technical with eachother.]

The read noise ("high ISO noise") is improved greatly in the 5D2, but it still has pattern noise. Canon changed the way tweener ISO are handled in the 5D2. Now they are 30D-style "digital" tweeners instead of analog tweeners like the 5D Mark 1 and 1D series. That's actually a good thing, because the analog tweeners on the 1D series and 5D1 are implemented by a separate gain amplifier that adds enough of its own noise to make it just as noisy as pushing a lower ISO. So to avoid 1/3 stop of clipped highlights, it's better to avoid the tweener ISO on those cameras. (Actually, there is slightly less noise under ISO 400 using the analog tweeners, but often the loss in highlight headroom is not nearly worth it.)

[Oren]

"In other words: ISO 200 has less noise than 100. 400 has less noise than 200. And so on".


Daniel, you'll have to explain more... what the heck?


What ISO does as far as I know, is electronically amplifying the signal - and in such a process you also amplify the noise.

[Daniel Browning]

Let me take another crack at explaining it.



Here is how most people understand high ISO:

• When I use autoexposure mode.
• At low ISO, the noise is very little.
• At high ISO, the noise is very high.
• Therefore, high ISO causes noise (incorrect).

The
logical error is that correlation is not causation. In autoexposure
mode, high ISO reduces the exposure. It is the reduced exposure that
causes the noise to be very high. Not the high ISO. In fact, if you had
left ISO the same (low ISO) and just reduced exposure manually, the
noise would be even worse than high ISO. This is because high ISO
actually *reduces* noise. Of course, it doesn't reduce it enough to
make up for the reduced exposure, but every little bit helps.



Here is the correct way to understand high ISO:

• When I reduce exposure, it causes more noise.
• Sometimes when I reduce exposure, I am left with more highlight headroom than I need.
• In those cases, I can trade away the highlight headroom to get less noise by using high ISO.
• This
  is all thanks to the fact that the 50D sensor technology has an
  advanced feature where the actual read noise is reduced at high ISO.
  Some other cameras, such as MFDB, don't perform that way.

To prove this to yourself, just take two pictures with the same exposure (i.e. same f-number and
shutter). Set one to ISO 100 and the other to ISO 1600. Then examine
the shadows of both images and tell me which one has more noise. (Of
course there is a difference in blown highlights, which is why I
explained "ETTR then ITTR".)


Decreasing exposure causes an increase in noise. And people tend to
use high ISO at the same time as they decrease exposure, but that
doesn't mean the ISO caused the noise. If it did, then keeping the
exposure the same and increasing ISO would increase noise. It doesn't.
It decreases noise (and blows highlights).



This is one case where the wide use of autoexposure causes many folks to look at things incorrectly.
Saying "high ISO causes noise" is similar to saying "high ISO causes
thin DOF". Everyone knows that's not correct. High ISO *tends* to be
used with wide f-numbers, and wide f-numbers tend to result in thinner
DOF, but that doesn't mean the thin DOF was caused by ISO. In the same
way, noise is not caused by ISO, it is caused by underexposure.



Remember that the least amount of noise results from "ETTR then ITTR." That means to
expose to the right and only then increase ISO (if possible without
blowing important highlights). If someone did the opposite (increase ISO instead of increasing exposure), it would result in far more noise: that would be "ITTR then ETTR", which is the opposite of what I am describing.


Most people have their own home-grown and incorrect
explanations why low ISO has more noise than high ISO when exposure is
fixed. The real reason is that the sensor has physically lower read
noise at higher ISO.



Of course, ideally, we would like low ISO to have the same low read
noise that high ISO has. If it did, there would never be any
reason to use high ISO. A digital push of low ISO would give you the
exact same image as a high ISO shot, but it would have many more stops
of highlight headroom (more dynamic range).



In fact, some cameras are like that, including most MFDB. But the 50D is different. That is why
understanding how high ISO *reduces* noise is important.

[Dumien]

I think I'm falling in love with you Daniel...really...you make me feel brand new (as Simply Red said) hahaha!!


Ok, so, let's say i'm in a weird situation (not so weird to me because it happens all the time) i'm at an ice rink, photographing figure skating. I can stay in only one place, because moving would mean shooting from behing the plexiglass. In front of me there are windows, biiiig windows. The rink is illuminated by lightbulbs on the ceiling (which is pretty far above). I need to stop motion as well as I can.


What I do is get at f/2.8, 1/200-1/320 and ISO 1600. Now, according to you, I'm doing the worst thing possible, cause I'm not really ETTRing because of the need to stop motion, and to get a DECENT exposure I up the ISO to 1600.


In such a situation, what should one do? Maybe this is just the kind of situation you only have to deal with noise, or just get a prime...right?


ANdy

[Daniel Browning]

What I do is get at f/2.8, 1/200-1/320 and ISO 1600. Now, according to you, I'm doing the worst thing possible, cause I'm not really ETTRing because of the need to stop motion,

You are correctly ETTRing. If you shot f/22, that would not be ETTR, because you don't really need the DOF of f/22. But you are doing everything you can do ETTR, so you have done it correctly. Now that you have finished ETTR, the next step is to ITTR. And you already did that. So you are doing "ETTR then ITTR" perfectly.

Maybe this is just the kind of situation you only have to deal with noise, or just get a prime...right?

Right.

[Alan]

What ISO does as far as I know, is electronically amplifying the signal - and in such a process you also amplify the noise.
<div style="clear: both;"]</div>

That seems right to me, as well.


So, higher ISO is a trade off between lower noise in the shadows, but higher noise in the highlights? Or, is it the other way around?


Either way, isn't it trading one noise for another?

[Dumien]

thanks Daniel, really appreciated... btw, I always shoot M mode because I don't like the camera metering...but that's just me ^^" (i mess up some shots though, but oh well!)


thanks again


Andy