Image Quality and ISO with DSLR

[Gail Spitler]

This is a question from a decidedly amateur photographer, not a newbie but I am trying to move up a notch in my skill and knowledge levels. I hope this is an appropriate question for this forum.

Current DSLR’s allow a wide range of ISO speeds that produce images that are not encumbered by disagreeable noise levels. I am wondering what the current wisdom is about selecting an appropriate ISO.
In the days of film, I think that the general rule of thumb was to use as low an ISO as possible to achieve good color saturation. Thus one used higher ISO film when pushed by the situation.
If one is photographing birds that are not still, do I set the ISO at 400 say and adjust up if I need a faster shutter speed? In what kind of situations would you really want to use a low ISO? What would be the reason for this choice?
I realize that ISO, aperture settings and speed are all interdependent. Most of what I read deals with how to balance lens opening with time and DOF issues. I am wondering about what are the decisions or guidelines one thinks about when choosing ISO in addition to these other parameters.

Thanks
Gail Spitler
British Columbia, Canada

[Jared Lloyd]

400 is good if if you have the available light for the shutter speeds you are trying to work with. The newer model cameras are great when it comes to dealing with noise these days. I use a Nikon D300 and have no problem shooting between 400-1600 with low noise reduction turned on - though 1600 is starting to get a bit soft. I find myself consistently shooting at 800 lately and like I said, no problem. Low ISO is great for more static subjects - those you can compensate for with a longer shutter speed - think lanscape or motionless Great Egret with reflection and twilight colors......

[Gail Spitler]

Thanks Jared. Are we saying that you even with today's more flexible DSLR's, you still get better color saturation and ... with a lower ISO of 50 or 100? So when the subject allows it, go for as low an ISO as you can? I think one basic question is all things being equal what does an ISO give you that an ISO of 400 does not?

Thanks
Gail

[Charles Glatzer]

Visualize the image beforehand. F/stop (DOF), shutter-speed or both should be paramount in selecting the appropriate ISO to render the image as desired... not the other way around. Use whatever ISO is necessary to effectively communicate your vision, be it with or without a bit of noise. Why put limits on your creativity?

Best,

Chas

[John Chardine]

I agree with Charles, but before you freely switch ISO to suit your needs I would make sure I understand the effect of ISO on the image. Gail talks about effects on saturation but I don't remember that being a concern when deciding to use Ektachrome 400 vs. 100 or 200. In DSLRs, ISO effects noise levels, not much else. As soon as I got my Canon 40D I did a controlled ISO test and noticed for this body that the effect of ISO on noise levels was almost unnoticeable up to 400 and ramped up from there with 1600 being unacceptable in my opinion for outdoor nature/birds which is what I do most. When I shoot static landscapes with a tripod I'll normally set the ISO to 100 unless I need extreme DoF and thus need to stop down. For moving objects such as birds I usually shoot at 400 or 640 with very little cost in image quality.

[Ed Okie]

Gail, you need to include in your query the specific camera you're using. The ISO choice and/or why, is highly related to the camera in use... a $200 point-&-shoot camera, or a high-grade camera. ISO 100, in the old days, used to be - the - choice. It's vastly overrated today because the recording capability of today's better cameras is so good - noise, per se, isn't an issue. And one last point: noise isn't "bad," in context to the image captured... meaningless. It's the expression/image or subject matter, plus composition that really counts. I happen to use a Canon 5D and ISO 640 is my most common setpoint.

[Robert O'Toole]

Visualize the image beforehand. F/stop (DOF), shutter-speed or both should be paramount in selecting the appropriate ISO to render the image as desired... not the other way around. Use whatever ISO is necessary to effectively communicate your vision, be it with or without a bit of noise. Why put limits on your creativity?

Excellent advice Chas, I agree 100%.

Robert

[Roger Clark]

I agree with Charles, but before you freely switch ISO to suit your needs I would make sure I understand the effect of ISO on the image. Gail talks about effects on saturation but I don't remember that being a concern when deciding to use Ektachrome 400 vs. 100 or 200. In DSLRs, ISO effects noise levels, not much else.

At the risk of being flamed again for saying something technical, this is a technical issue with multiple effects. Performance depends on the camera and mostly on the size of the pixels (larger pixels = better performance, e.g. the Nikon D3 has such great high ISO performance not because of some astounding breakthrough in electronics, its simply the large 8.4-micron pixels, the largest of any current DSLR on the market). Most noise you see in digital camera images is not due to electronics, but the noise in counting photons. a fundamental physical limit.

In general, as ISO increases, noise increases as the square root of the ISO (assuming no noise reduction is applied). Also, dynamic range decreases linearly with increasing ISO.

At the lowest ISO, performance also becomes limited by the A.D converters (and 14-bit converters used in current DSLRs show little improvement over 12-bit converters--another long technical discussion as to why. The result is that lowest ISOs, below 200, have similar or sometimes lower dynamic range than ISO 200 (but the lower ISOs still have better signal-to-noise ratios.
For example, the Canon 1D mark II at ISO 50 has lower dynamic range than at ISO 100,
but ISO 50 is less noisy

So for top image quality: of course subject and how you want to freeze action is top priority (better to have a noisy image that has the sharpness you want than a low noise blurry image). But after that paramount consideration, use the lowest ISO needed to get the image, but not necessarily too low. I use ISO 100 and 200 a lot, but push up high when needed.

More details on how digital camera sensors perform can be found at:
http://www.clarkvision.com/imagedeta...rmance.summary

Roger Clark

[John Chardine]

Roger- Many thanks for the link to your web site. It is fascinating stuff and very nicely presented.

[Gail Spitler]

Thanks to all for the comments. I am using a Canon 30D and 40D, btw. It has been a very helpful discussion.
Gail Spitler