Are there any "rules" for using ISO?

Started Apr 3, 2013 | Discussions thread
mike703 Veteran Member • Posts: 6,685
Re: avoid Tv mode

GreenMountainGirl wrote:

So slightly perversely the best way to get the fastest shutter speed that you can for given lighting is to

(i) use Av mode and open the lens up completely and

(ii) set the highest ISO you are comfortable with.

Then, the shutter speed selected by the camera will be the fastest you can get for that light.

I have to ask - are there any particular situations where you would recommend using shutter-priority?  Perhaps in bright light?

You could of course as long as you keep an eye on the aperture and make sure that it lies within the range of the lens.  But personally I have never found Tv mode of much use.  I normally want to do one of two things as regards shutter speed:

(i) most of the time, with static subjects, I just want to make sure that shutter speed is fast enough to avoid camera shake.  Beyond that I don't care whether the shutter speed comes out at 1/30 sec or 1/2000 sec - it will make no difference.  In contrast the f-stop will make a big difference, mostly to depth of field and sharpness: I don't want to end up with a very small aperture like f/22 (which will soften the picture due to diffraction effects) - optimum sharpness is usually around f/5.6 - f/11 - and I don't want the camera to try and set a wider aperture than the lens can do, which will underexpose, as mentioned earlier.  I probably wat to control depth of field to be deep (landscapes, narrow aperture like f/11), shallow (portraits, f/2.8 - f/4) or medium (group shots, usually f/5.6).  So setting the aperture in Av mode, and taking whatever shutter speed the camera chooses as long as it's hand holdable, works fine.

(ii) for fast-moving subjects of course shutter speed becomes all important and you want the fastest you can get to freeze the action.  But again I would not use Tv mode because that will not guarantee you the fastest speed possible.  Suppose you have an f/4 lens and are shooting sports.  You might choose 1/500 sec - fine.  You might find later that the lens ended up at f/5.6... which means you could actually have used 1/1000 sec, and that difference could have given you sharper pictures (somebody running fast can move 2 cm in 1/500 sec).  Similarly the difference between 1/500 and 1/1000 sec could be important if you are trying to hand-hold a 300mm lens and the magnifiied image is shaking all over the place in your viewfinder.  If you use Av mode and f/4 you'll always get the fastest shutter speed possible.

So I never use Tv mode, even for sports when shutter speed is the most important variable.  I might use auto-ISO mode (set 1/1000 sec, f.4, and let the ISO vary as it wants).  But if the fastest shutter speed is crucial I'd use Av mode, lens wide open, and a high ISO value.

Best wishes

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