ISO 1600 vs correcting exposure in photoshop

[Daniel Browning]

In a purely digital world, you may be right. In the days of film, f/4 and 1/500th might underexpose some film and overexpose other film. Otherwise, Bryan Peterson must be wrong in calling it the Exposure Triangle...and he's certainly sold a lot of books calling it that.

There's nothing wrong with calling it the Exposure Triangle: that does not imply that changing ISO (without changing f-number or shutter speed) is changing exposure. I do not recall Bryan Peterson ever using the incorrect definition of exposure in "Understanding Exposure".

This took me a long while to digest and interpret. What I finally figured out is that you're suggesting that we shoot in manual mode,

I did use manual for illustration, but it's possible to use the "ETTR then ITTR" technique with autoexposure as well.

starting at ISO 100 "for every shot", and choose "exposure" (using your definition of exposure) values such that the desired image is (hopefully) captured while also maximizing the light upon the sensor, without so much light that important highlights are lost. If it's impossible to achieve the desired image via aperture and shutter values (i.e. slowing the shutter will result in a blurry image, or opening the aperture will thin the DoF too much),

Yep.

it's best to increase the ISO to maximize the light upon the sensor, without so much light that important highlights are lost.

Increasing ISO doesn't change the light on the sensor at all. It increases brightness and decreases read noise.

My translation of all of that is that it's best to achieve a right-justified histogram (using appropriate margins for the details we wish to keep) by prioritizing the widest artistically-reasonable aperture and the slowest artistically-reasonable shutter, and then the minimum amount of ISO increase.

That will result in the same settings (Tv/Av/ISO) that I am suggesting.

My initial interpretation was that I should set my camera to ISO 1600 and lock that in, which I don't think is the desired interpretation.

Correct. ISO 1600 always has less noise for a fixed exposure, but that doesn't mean we should always use it. High ISO has a price: clipped highlights. That price is always far too high to pay, except when we are in low light, and we no longer need the normal amount of highlight headroom.

[dmckinny]

My initial interpretation was that I should set my camera to ISO 1600 and lock that in, which I don't think is the desired interpretation.

Correct. ISO 1600 always has less noise for a fixed exposure, but that doesn't mean we should always use it. High ISO has a price: clipped highlights. That price is always far too high to pay, except when we are in low light, and we no longer need the normal amount of highlight headroom.

OK now, the lights are starting to turn on for me. Daniel, I think I was not comprehending what you meant by 'highlight headroom'. Would the following statement be correct?


Highlight headroom is roughly analogous to available dynamic range. In the case of a low light shot, we slow the shutter as much as is acceptable, and open the aperture as much as is acceptable. Then when there is still not enough light for our needs, we increase the ISO. The trade off here is that by increasing the ISO we are sacrificing dynamic range (losing highlight headroom), in order to decrease read noise and (in the case of digital) amplify the signal on the sensor. This works in low light because there is less dynamic range in the scene, so we can use the higher ISO.

[Daniel Browning]

Would the following statement be correct?


Highlight headroom is roughly analogous to available dynamic range. In the case of a low light shot, we slow the shutter as much as is acceptable, and open the aperture as much as is acceptable. Then when there is still not enough light for our needs, we increase the ISO. The trade off here is that by increasing the ISO we are sacrificing dynamic range (losing highlight headroom), in order to decrease read noise and (in the case of digital) amplify the signal on the sensor.

All correct.

This works in low light because there is less dynamic range in the scene, so we can use the higher ISO.

I would put it differently. Low light scenes tend to have very high dynamic range, and we'd love to shoot them at ISO 100 if we could. It's just that reducing the noise is *so* important that we will sacrifice the headroom to get it. That is, we'll go from 7 stops of highlight headroom down to 3 stops, but the decrease in noise will be well worth it.

[Dallasphotog]

There is the ever-so-slight chance that some of you guys are too smart.


I was just going to say that when things get on the dark side I push ISO up enough to get shutter speeds I can hand hold and that allow me to freeze the subject motion. From there my motto is "In Photshop I trust."


This is especially true shooting football or baseball at night where the camera is trying to correctly expose areas of the background and I really only care about the subject. I set up manually andlive with underexposed histograms. Usually, a little workwith the shadows and highlights slider in Photoshop curesall.


For weddings, I use RAW files for theversatility provided in Digital Photo Professional. Dark churches never allow an optimum combination of shutter speed, aperture and ISO.


I don't own a 50D or a 5D, but I can say thatI've never had very good images off the 1DMKII above ISO-800 and that the new 5DMKIIseems to produce revenue generating product atISO-12,800; with verynicematerial at ISO-6400. It certainly has the best image quality of the bodies I own or use including the 1DsMKIIIwe use at work.

[bob williams]

Wow, This is the conversation I have been waiting for and the explanations "I thought" I needed to better understandexposure and and its realtionship with ISO and noise. Unfortunately,I nowconsider myself at least 3 stops dumber than I was when I started reading this string. Let me see if I have the basics right:


1. High ISO (up to 1600) doesn't cause noise in and of itself--


2. Use of high ISO in the wrong situation can result in noise---


3. If correctly exposed, High ISO results in less noise butcan result in blown highlights--


4. I have erroneously compared high ISO digital to high ASA film -- what was "grainy" in film does not neccesarilymean noise in digital.


Please correct or confirm these assumptions.


Now---the big question----


I normally shoot wildlife, landscape and nature with a 50d and a 100-400L or a 24-105 L (70-200 f2.8L IS is comming). I do prefer shooting within 30 minutes of sunrise or sunset---but many times I find myself in broad daylight and hopefully adjust accordingly. I normally base my settings on desired effect; i.e. Faster shutter for motion control, narrow aperture for DOF etc ---and I set ISO on lowest possible for the shooting situation----What is the error with my method of set-up.


Note--As you have probably already summised, I have discovered that sometimes I get noise when it is unexpected.


Your response is greatly appreciated,





Bob

[Daniel Browning]

You've got it right, Bob!

2. Use of high ISO in the wrong situation can result in noise---

Yes, if the high ISO causes you to reduce exposure, it will result in more noise.

Faster shutter for motion control, narrow aperture for DOF etc ---and I set ISO on lowest possible for the shooting situation----What is the error with my method of set-up.

That's fine, as long as you aren't "wasting" headroom. Low ISO gives you highlight headroom. If you don't need the highlight headroom, the best thing is to increase exposure. If you can't increase exposure, increase ISO. That is the essense of "ETTR then ITTR".

[bob williams]

Thanks, that is very helpful. I appreciate the feedback Dan.





Bob