View Full Version : which ISO
8th of September 2008 (Mon), 15:02
Im a newbie (as you can imagine from the title)
I have read "understanding exposure" per advice of the forum. The book is great at explaining the "triangle" and gives good basic information BUT I dont understand how to pick the right ISO for the situation I am in...
So far here is what I know:
use lower ISO for daylight
use higher ISO for dawn/dusk and sports or freezing action
That's far from being enough...
whats your way of determining which ISO to use. What's the basic "walk around" ISO typically used? 100? 200? what about flash photography?
8th of September 2008 (Mon), 15:15
My own rules of thumb:
1. Always use the lowest possible ISO for any shot.
2 .... hmmm there is no no.2
In practical terms this means thinking of your exposure as a window of aperture and shutter speed, adjust the ISO to get the shot in the window and always use the lowest possible option.
In film photography, no. 2 would talk about grain and creative use of grain. In digital photography there is no such thing. Digital noise is not grain, it can be used creatively but can be added at any time PP, always better to get the best quality shot in the can to begin with.
8th of September 2008 (Mon), 15:24
To paraphrase the above poster.
Thinking of exposure as a triangle is slightly misleading. It's all about priorities, it's not a perfect balance of all three.
1) Choose which is most important - shutter speed or aperture and set one for what you ideally want it to be.
2) Set your secondary setting to match with what you chose in 1
3) Set the ISO to enable the best combination of 1 and 2
This comes somewhat naturally after experience and practice.
However, when dealing with trying to keep ISO as low as you can, don't attempt to push the exposure to make the ISO as low as you can possibly get it. ISO noise becomes worse when the shot is under or over-exposed. If you're strugging to get the correct exposure, pushing ISO up higher than you expect will often get better results.
e.g. Choosing an ISO of 800 and a safe shutter speed usually give you cleaner results than an ISO of 400 and forcing a partial underexposure to trying and get the shot.
8th of September 2008 (Mon), 15:41
I boost the ISO only when needed and don't really have any formula for it, although some things are pretty much given, such as when shooting indoors I will definitely need to boost it. How much depends on how much light there is.
So for me, I'll set my aperture and shutter speed, and only if I cannot get the desired exposure (i.e. can't get fast enough shutter speed) will I change the ISO.
When shooting sports, which is not that often for me, I usually have the ISO boosted even when outdoors. I want to be using fast enough shutter speed to stop the action and that takes priority over any ISO noise.
Simple as that...
8th of September 2008 (Mon), 16:18
ISO noise becomes worse when the shot is under or over-exposed.
Underexposure certainly increases ISO noise, but overexposure is just going to blow out pixels. If you deliberately expose to the right, though, without blowing anything out, you actually end up with less noise
8th of September 2008 (Mon), 16:22
When shooting stationary objects (flowers etc.), I select the aperture that provides the depth of field wanted/needed, then select a shutter speed that will eliminate ANY motion be it the subject moving in the wind, the camera vibrations or body movements (if not using a tripod). Then I select the lowest ISO that will give correct exposure.
When shooting moving objects, determine the slowest shutter speed that will stop motion (if that's what is needed), select an aperture that will provide the depth of field that works for the subject, and then select the ISO.
In film days, we always started with the film speed because changing it was tricky in the field.;)
When you gain experience, you will look at the lighting situation, the subject, etc. and will set the ISO first, then the other two as required.
Too many of us worry about using ISO values over 100 or 200 - I often shoot at 640 and 800 with my 30D - the important thing is to expose to the right (ETTR). This minimizes noise problems.
EDIT: Jeez, I should have read Pete's post - I just repeated what he said but using more words.:o
8th of September 2008 (Mon), 18:10
Glenn's post was perfect, but I want to stress that you should not fear using higher ISO when you need it. There are two things to consider:
1. Using higher ISO will create a little digital noise, though the latest dSLR cameras are very good with this. Noise can be fixed in post processing. If you get motion blur, camera shake or too shallow of a DOF in the shot because you wanted to use low ISO you are stuck. Those flaws are fatal.
2. Some people develop a noise fear from gazing at 100% crops from 6 inches on their 30 inch monitors. In order to get a feel for noise (and sharpness) take a few high ISO shots and print them at 8x12 or larger. Costco will print big stuff for just $2.99. Look at the prints and learn just what in a 100% crop constitutes a noisy image.
8th of September 2008 (Mon), 18:24
Your lens choice has something to do with your choices also.
Say you were shooting a 70-200 f4
Minimum Shutter speed to keep from getting camera shake i around 320 for most 250 for the lucky steadier hands.
Now you want a wideish DOF so you choose an aperature of lets say 7 or 8.
Therefore you would adjust your ISO to get as close to those settings as possible.
8th of September 2008 (Mon), 18:44
Don't skimp on ISO though. A little noise is better then a lot of motion blur.
8th of September 2008 (Mon), 18:47
I don't agree about setting ISO first; for me, in dimmer situations I will sometimes set the needed shutter speed and expected wider aperture, then adjust ISO only as high as necessary to expose correctly. I will give one way or another if needed, but I'll try to never go under a certain minimum shutter speed, and the aperture will have to stay where it must be at minimum to get the subject(s) in focus.
I do it much more like the very first paragraph of Glenn's post.
8th of September 2008 (Mon), 21:51
Basically, what Pete said.
For a good starting point, first set the f-stop & shutter speed you need for the effect you want. Then the other parameter: f-stop or shutter speed. Then adjust the ISO.
Need an exposure crutch? (http://photography-on-the.net/forum/showthread.php?t=89123) Don't skimp on ISO though. A little noise is better then a lot of motion blur. Gavin shoots a lot at high ISO & provides fun captions as well. The first & many other images are at ISO Speed Ratings = 1600!
The week's sporting events in images... (http://photography-on-the.net/forum/showthread.php?t=443789)
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