Are there any "rules" for using ISO?

Started Apr 3, 2013 | Discussions thread
Barrie Davis
Barrie Davis Forum Pro • Posts: 21,460
Re: Are there any "rules" for using ISO?

GreenMountainGirl wrote:

I have been experimenting with low-light images of moving subjects (mostly deer) and getting mixed results.

How low is the light? What shutter speeds and apertures are you getting presently, and at what ISOs?  Refer to you EXIF files, and then you can tell us. Better still, post some pictures complete with EXIF details.

Using aperture-priority produces slower shutter speeds,


The shutter speed will depend on the aperture YOU prioritise, so you are in charge. If the shutter is too slow to still movement, then the aperture YOU SELECTED is too small for the available light at the ISO you are using, requiring the camera to set a long shutter speed...

Remember, swapping about between different modes (A and S etc.) DOES NOT alter which shutter speed and aperture combinations are needed for correct exposure. All that changes is which one (shutter or aperture) that YOU must select. What's more, in performing the task, you are still obliged to make a sensible selection, AND to meter accurately after doing so...

... or you will still end up with duds.

The idea of the semi-automatic exposure modes is that you only have to monitor ONE exposure factor, the shutter or the aperture, instead of BOTH. It saves some of the work of Manual Mode, but not ALL of it.

and raising ISO does not seem to work.  Using shutter-priority works better, but still often have to make exposure adjustments in LR4.  Most of these photos are at/close to 300mm, so have not noticed any significant effect on DOF, just need to get a better idea of ISO (and exposure compensation) because there often isn't much time for "test photos"!

Conventional advice for best image quality is to use the lowest ISO value that yields usable shutter speed and aperture combinations in the strength of light prevailing.

However, in DIM LIGHT it may not be possible at low ISO at all, but likely needs quite HIGH ISO, especially if...

1) ... you are using a long lens, say 300mm, and need to freeze camera shake (you are)....

2) ... if your lens does not have a particularly wide aperture at long f-lengths (most do not)...

3) ... and your subject is moving... (which it is)

As may be seen when all three are combined with dim light, you have the perfect storm of worst possible shooting conditions .

The only recourse is to wind UP the ISO to a MUCH higher setting.

Q: How far up?
A: The minimum amount that yields USABLE shutter speeds and apertures...

....  but it will still be quite a lot, that's for sure.

Make a start at 1600, and go up from there.
Combine with the widest aperture your lens has got.
Check the resultant shutter speed to see that it is one you can (a) hand-hold (b) will freeze the subject.

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