Auto iso and it's priorities....

[oldbaldgit]

Hi, sorry if this has been asked before, but.......

If I am shooting on manual, Tv for example, and I have selected auto iso, how do I know what aperture to expect? Does the auto iso seek it's lowest value, therefore giving me a smaller f number?Hi, sorry if this has been asked before, but.......

Hope this makes sense.

Thanks.

[Hereford_EOS]

Re: Auto iso and it's priorities....

That's a good but tricky question.
When choosing Tv and looking through the viewfinder you will see the aperture and ISO values.
It all depends on the light.
In low light the aperture will be as wide as possible (lowest f/number) and the ISO high.

[MX5]

Re: Auto iso and it's priorities....

I've been trying to work out the algorithm that the camera firmware uses (Safety Shift Off):

Tv + Auto-ISO works as follows as best I can tell. It first change aperture based on changing light (as it would without Auto-ISO), but as soon as the aperture reaches the widest available on the lens it will then start to change the ISO.

Av + Auto-ISO works as follows as best I can tell. It first change shutter speed based on changing light, but as soon as shutter speed would have to drop below 1/(2xfocal-length) (i.e. slower than 1/200s for 100mm), it would then adjust the ISO (It doesn't appear to take IS into consideration when doing its calculations).

With Safety Shift On - I'm still not sure if Safety shift takes precedence over Auto-ISO or vica versa.

[oldbaldgit]

Re: Auto iso and it's priorities....

Brilliant.
Thanks guys.

[Thndr_strck]

Re: Auto iso and it's priorities....

If I select auto ISO, I then shoot in M

[MX5]

Re: Auto iso and it's priorities....

If I select auto ISO, I then shoot in M

The discussion on Auto-ISO has opened up an interesting new world to many people. Especially when used in conjunction with M … this to my mind isn’t manual per se but what I’ll call Iv (ISO value, cf Av and Tv) in that shutter speed and aperture stay constant and ISO will change as light changes. With recent cameras, higher ISO settings are much more usable for everyday photography (rather than as an emergency). So using ISO up to 800 [or 1600 or higher on some bodies] isn’t going to impact quality too much or indeed produce so much noise a photograph is all but unusable.

The following is for a 7D with firmware 2.0.x (you’ll need to check your manual if you are using a different ‘Truly Auto-ISO capable’ camera [Some early implementations of Auto-ISO on Canon and other bodies, weren’t quite what you might expect]).

You can set the ISO increment (the native ISO of the 7D is probably 100-ISO and consequently speeds of 200, 400 et seq are achieved by ‘hardware amplification’ … intermediate speeds are done by a combination of ‘hardware amplification’ and push/pull a 1/3rd if a stop by software (well as far as I can tell). So I set C.Fn1-2=1.

You can limit the high water mark for Auto-ISO using the menu system, say to 800-ISO. But in M you don’t have a Safety feature.

If you have Safety-Shift set (C.Fn1-6=1) this only comes into use if you are in Av or Tv.

I was playing with Auto-ISO and one quirk I noticed. If you've set C.Fn1-2=1 (whole stop changes in ISO on the 7D) when in Auto-ISO you get third stops ... well so my brief (none scientific) experiment would seem to indicate. I wasn't expecting that but I gather other cameras behave in a similar way. So the behavior of intermediate ISO may concern some.

[Trevoreast]

Re: Auto iso and it's priorities....

Especially when used in conjunction with M … this to my mind isn’t manual per se but what I’ll call Iv (ISO value, cf Av and Tv) in that shutter speed and aperture stay constant and ISO will change as light changes.

Yes it's an enormous step towards a new semi-auto mode. IV seems like the logical name for it.
Perhaps will we see it on camera dials soon ?

Trev