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What's better for RAW, high ISO or underexposure?
It occured to me today that I can probably get the same result by boosting exposure compensation of an underexposed RAW low ISO image as I would by boosting ISO settings in camera. The only difference is that the ISO setting applies analog amplification in-camera while exposure compensation does the same thing in the digital domain. Can anyone comment on whether one is preferrable to another? Seems that at least in the digital domain one could apply some creative selection as to what to boost and what not to boost...

Comments (11)

Noise is usually shows up first in the dark areas of a image. Underexposure and adjusting in PP will just make the noise more noticeable. Sometimes a high ISO image will be relatively noise free if there is no dark area's in the image, on some cameras...

Comment #1

If I had such a question, I'd probably set up a tripod and a light bulb and shoot a few tests. Some people want answers, some people want reassurance. *shrug*.

[ e d @ h a l l e yc c ] http://www.halley.cc/pix/..

Comment #2

Not what is better for raw. but what makes a better image- and that is low iso and the correct exposure..

As far as boosting the exposure in raw converter or pp, no. what would happen is that you would boost the image all right, but you would boost the background noise by exactly the same amount of boost. and you ending pics would be noisier...

Comment #3

Wrong. iso1600 has lees image quality than iso200. which is not to say it cannot or shouldnot be used, but the 1600iso has less IQ to start with...

Comment #4

Under exposure has even less IQ than high ISO typically. You can get very noisy images at low ISO if they are underexposed and brought back in PP..

Some cameras have very good IQ at ISO 1600..

Use the lowest ISO for the correct exposure...

Comment #5

Mike Goltsman wrote:.

It occured to me today that I can probably get the same result byboosting exposure compensation of an underexposed RAW low ISO imageas I would by boosting ISO settings in camera. The only difference isthat the ISO setting applies analog amplification in-camera whileexposure compensation does the same thing in the digital domain. Cananyone comment on whether one is preferrable to another? Seems thatat least in the digital domain one could apply some creativeselection as to what to boost and what not to boost..

Tests show that adjusting EC is nearly, but not exactly, the same. The camera does do a better job. It's not known exactly why, but the presumption is as you stated..

If you have a good camera then underexposing by a couple of stops at a low ISO could give you an apparent increase in DR by compressing highlights as you apply EC...that seems to work well. But only if your scene is difficult...otherwise proper exposure is nearly always best...

Comment #6

I guess it makes sense. If I had thought about it some more, I would have realized it. In addition to sensor noise, you also have quantization error. That is, when the signal read from the photosite corresponds to a pixel value of 29.5 the camera has to pick 29 or 30, introducing up to half a bit of error. When you, say, use ISO 200 instead of 1600 and compensate by 3 EV in software, the value of 29.5 that should turn to 29.5*8=236 instead turns into 30*8=240. Because two neighboring 29.5 pixels might turn into 29 and 30, the 3 EV compensation may produce two adjacent pixels that should both be 236 but instead are 232 and 240.

On the other hand, a 3 EV (ISO 200 to 1600) analog amplifier will produce 29.5*8=236 precisely for both pixels. Obviously, the sensor noise might make those two pixels read 26 and 32 respectively, but that effect will be common to analog and digital EV boost. So, in-camera analog ISO boost is better...

Comment #7

Mike Goltsman wrote:.

It occured to me today that I can probably get the same result byboosting exposure compensation of an underexposed RAW low ISO imageas I would by boosting ISO settings in camera. The only difference isthat the ISO setting applies analog amplification in-camera whileexposure compensation does the same thing in the digital domain. Cananyone comment on whether one is preferrable to another? Seems thatat least in the digital domain one could apply some creativeselection as to what to boost and what not to boost..

You've pretty much answered your own question with your previous reply..

Suggestions of getting the exposure right while using lower ISOs ignore what your question is about, so I don't know why people even bothered bringing them up..

Using higher ISO will invariably give cleaner results than undexposing and boosting (mathematically) during PP. Relatively clean amplification in camera means less quantisation noise as it works on a continuum of analog values before quantising the result. Quantising a lesser exposed result and then multiplying those already quantised values in PP means considerably more quantisation error..

There are other factors that I don't fully understand, but there have been numerous threads from credible posters that have confirmed the noise difference between the two methods..

One caveat is that some cameras achieve ISO 3200 by underexposing ISO 1600 and then multiplying the already quantised result by 2 to give the ISO 3200 result. In that case, there's no noise difference between you using ISO 1600 and underexposing or getting the camera to use ISO 3200..

Cheers from John from Adelaide, South AustraliaJohn Harvey Photography http://johnharvey.com.auCanon 40D, Canon 20D & Fuji F10..

Comment #8

Ah, thanks - now I understand the meaning of ISO3200 (with BOOST)...

Comment #9

Higher iso for sure - most of the detail in a shot is in the highlights area, if you underexpose not only are you increasing noise but also throwing away detail.

Simon.

Http://www.landscapephotographyuk.com/.

North Wales photographs - Snowdonia & Anglesey..

Comment #10

Simonkit wrote:.

Higher iso for sure - most of the detail in a shot is in thehighlights area, if you underexpose not only are you increasing noisebut also throwing away detail.

Simon.

Hi Simon. What you say is half true. Higher ISO will have less noise than underexposing at a lower ISO. Part of that is from quantisation noise from the D/A conversion happening before multiplying values during PP if you underexpose at a lower ISO. Multiplying already discreet values opens up the gaps between available values. A higher ISO uses electronic amplification in camera (up to ISO 1600 in most DSLRs) to produce a properly exposed image, with D/A conversion hapening at that point from a continuous range of analog values, so you get smoother gradients..

You're not throwing away highlights as such by underexposing at a lower ISO compared to properly exposing at an equivalently higher ISO. Either way, the amount of light the sensor sees is the same. It's just the amplification and processing that's different. At the end of the amplification/A/D conversion/multiplication process, you still populate the brighter parts of the available RAW spectrum, but it's cleaner and more continous if you shoot with the higher ISO option..

Many DSLRs don't have a true ISO 3200, but use ISO 1600 and then multiply those values by 2 to create a pseudo ISO 3200. That's no different from you choosing ISO 1600 and underexposing, then correcting during post processing..

Cheers from John from Adelaide, South AustraliaJohn Harvey Photography http://johnharvey.com.auCanon 40D, Canon 20D & Fuji F10..

Comment #11

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This question was taken from a support group/message board and re-posted here so others can learn from it.

 

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