[JimJohnson]

Okay, I can see why I might opt for a fairly low or a fairly high ISO value to achieve certain shutter/aperature values. Can more experienced digital shooters tell me why they take sensitivity off of auto for the broad middle ground?

[Adam]

The higher the sensitivity, the more noise. So for the best images, you should always try to use the lowest ISO possible for the current lighting conditions. And then, as you said, if you want a particular DOF or a fast shutter speed, you can also tinker with the ISO. It's a balancing act- for example, if it isn't ultra sunny outside, you'll usually get better results at iso 100 than at iso 200 because you'll be able to use a faster shutter speed.

[procyon]

Adam, I think you got that last sentence a bit mixed up.

[westmill]

Its just about control. The lower ISO range will give you not only finer and sharper detail but also more Dynamic range. In genral, most simply try and use
the lowest ISO that is needed. You could be shooting a landscape at dusk, with your camera on a tripod. Or merely using the cameras built in anti shake etc.
you may be shooting at F8 and hand held at 30th sec at 200 ISO to obtain optimum results. If your camera was on auto, it would prob decide that you should be using
F8 at 125th of a second that would bump up your ISO to 800 ISO at F8. Its a handy feature to have if getting the shot is more important than quality.
Think of it like another extension of program mode. Its the same as asking why you would use apreture priority or shutter priority instead of program.
It allows you as much or a little automation as you wish !

[GabrielFFontes]

Yeah, its the opposite. If it isn't ultra sunny outside, you'll usually get better results at iso 200 than at iso 100 because you'll be able to use a faster shutter speed. Also, the difference between iso 100 and 200 is very difficult to notice.

[bobmaxja]

Jim
Like many time for peoples you start back in phot, I suggest 1 book
"Understanding Exposure" by Bryan Patterson
This book is cheap but go trought the 3 main component of a camera: speed,aperture and sensitivity

Go and it
http://www.amazon.com/Understanding-Exposure-3rd-Photographs-Camera/dp/0817439390

[Ishpuini]

The rule is lowest ISO is best. If you want a grainy look, you can add it in postprocessing, same if you want to cut off higlights and shadows. The only parameter that determines whether moving up in the ISOs is whether there's enough light to get the shutter speeds you need or be able to stop down the aperture sufficiently for the depth-of-field you require.

I prefer to decide myself when I move up in the ISOs. This depends on the lens I have mounted and on the subject I am shooting and the position I'm shooting from. You might argue that the camera will recognize your lens so it will include both focal length and MFT in its decisions in auto-ISO, but still I feel know my lenses best. For the two other aspects the camera doesn't have a clue. I concur that you can set the auto-ISO to move up aggressively or more conservatively, and as such stay in auto-ISO, but it's far from practical to have to change this setting depending on the shooting circumstances. For instance: for action photography under pressure I would prefer the former, for landscape photography at dusk when I'm at ease the latter. And even within one shooting session you may evolve from one preference to the other.

It's really much easier to learn to control all three parameters: shutter speed, aperture and sensitivity, than to rely on the many settings hidden in the menus that have an undocumented impact on these three settings.

Wim

[JimJohnson]

Hi bobmaxja-
I think most of the folks are missing the key point of my question ... I DO understand the relationship between shutter speed, aperature and sensitivity ... perhaps better than some as EVERYTHING was manual when I started photography. I also did my own darkroom work back then. My question is why I would mess with taking sensitivity off its auto setting for general photography (my K-r seems to use ISO 200-800 for typical daylight scenes). Is there a significant change in noise level or dynamic range within this ISO range?

I can understand controlling shutter speed (blur versus freeze) and aperature (depth of field) ... but if there is no noticeable change in noise level or dynamic range, and I can achieve the desired shutter or aperature value, other than the rare need for BOTH a specifc shutter AND aperature, why would I want to manually set a specific ISO?

[bobmaxja]

Because you will control the 3 functions. If you leave it to the camera, the camera will make a choice for you. But I agree most of the time I use the auto ISO for day to day photo shots

[Anvh]

My question is why I would mess with taking sensitivity off its auto setting for general photography (my K-r seems to use ISO 200-800 for typical daylight scenes).

If it works for you then why worry?

It's your camera and your photos, i shoot as low as i can.
During day light it's most of the time at ISO80 but indoors i know i need to use higher so ISO800 is my default most of the time but that's my way of working and i like to use wide apertures.

[procyon]

If you try to brighten shadows in underexposed RAW files (very useful for high dynamic range scenes where HDR is not an option for some reason) then ISO 100 and 800 are very different.

[westmill]

Hi bobmaxja-
I think most of the folks are missing the key point of my question ... I DO understand the relationship between shutter speed, aperature and sensitivity ... perhaps better than some as EVERYTHING was manual when I started photography. I also did my own darkroom work back then. My question is why I would mess with taking sensitivity off its auto setting for general photography (my K-r seems to use ISO 200-800 for typical daylight scenes). Is there a significant change in noise level or dynamic range within this ISO range?

I can understand controlling shutter speed (blur versus freeze) and aperature (depth of field) ... but if there is no noticeable change in noise level or dynamic range, and I can achieve the desired shutter or aperature value, other than the rare need for BOTH a specifc shutter AND aperature, why would I want to manually set a specific ISO?

I beleive this was answered in my very first line ? There is a differance in noise and dynamic range. Wether or not the differance in quality
has obviously got to be down to the individual user.

[Mareket]

Because you can set a specific noise reduction setting for a bunch of photographs and not have to fine tune it for each one depending on the ISO that was chosen. That's why I do it anyway. The shutter speed is generally flexible enough to be the component I most readily change when setting exposure. And changing the ISO is only a button click away anyway.

[JimJohnson]

I'm thankful to all of you for the dialog this question has produced, but procyon's response is the kind of answer I am looking for - most of the responses seem to assume there is a noticeable difference between middle-value ISO speeds. Yours is the first to explicitly state that you have observed a difference and under what circumstances.

Does anybody else have similar experiences to share on this topic?

[Anvh]

If you want numbers and examples.
Pentax K-r Camera Imaging - Full Review

DxOMark - Pentax K-r