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Christmas Prize Draw 2017

auto iso in aperture or shutter priority


trivets12 13 1.3k
10 Aug 2017 10:55AM
Mornin'
For some reason, the use of auto iso has cropped up in a few forums on other websites that I look at and I have a couple of questions about it!
Having never used it, I would like to know how the algorithm works in the camera software i.e if used in aperture priority, the camera will select the shutter and iso to give correct exposure. Will the camera give any priority to shutter thereby using the lowest iso possible, or will it just choose an average of the two, I mean will it always choose the lowest iso possible? Similarly, with shutter priority - will it set the max aperture, say F4, first and then choose a suitable iso secondly to achieve correct exposure? I have read the camera manuals and cannot find out how this is worked out.
Many thanks
Trudy

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Chris_L Plus
3 4.2k United Kingdom
10 Aug 2017 11:18AM
Depends on the camera. Mine has a setting where I can set min shutter speed that camera can use before it bumps up the ISO.

In reality that's not all that useful. Setting 1/100th is great for a wide angle shot on an IS lens but not so good for telephoto.

Better though, is that I can set a preference, I think it's "normal, fast, faster, fastest". I believe camera takes into account focal length plus my preference, calculates a shutter speed that satisfies that goal and ups the ISO if it's unattainable at current aperture.

You should also be able to set a max ISO to avoid what you consider unusable.
Chris_L Plus
3 4.2k United Kingdom
10 Aug 2017 11:22AM
By the way, your camera should also let you use Auto ISO in Manual mode. So choose your desired aperture, then choose safe shutter for the lens and the movement in the scene and shoot to your heart's content. Keep an eye on the ISO being chosen as you don't want nasty surprises. On mine display says ISO Auto but tells me what it will use when I half press.
trivets12 13 1.3k
10 Aug 2017 11:25AM
Thanks, I'm using Canon 6D's and have never even thought about using auto iso but did consider that it might be useful in rapidly changing light conditions. Might give it a whirl when on my hols next week.
mikehit 8 8.0k 13 United Kingdom
10 Aug 2017 12:36PM
On my Canon gear, I use Auto ISO when shooting wildlife. I use Av mode and set a minimum shutter speed of 1/1,000 sec.I set a maximum ISO of 6,400 because after that the noise becomes too much.
So I set (for example) aperture of f5.6 and ISO 800. The camera uses these settings until cloud comes over, adjusting shutter speed to give the correct exposure. If cloud comes over the shutter speed drops and when it drops to 1/1,000 that is when auto ISO kicks in - the shutter speed can no longer drops so the ISO increases. If the ISO hits 6,400 then the shutter speed drops below 1/1,000.

In Av mode, you set Aperture and ISO and the shutter speed changes automatically
In Tv mode you set shutter speed and ISO and the aperture changes automatically
In Auto ISO you set Av and Tv and the ISO changes automatically
Auto ISO in Manual mode is not, in my view, Manual mode and is a variation of Av and Tv - maybe they should call it ISOv Grin and have another click on the mode dial? In true manual mode you set everything yourself with no input from the camera.

You quote 'rapidly changing light conditions' and any of these three will do the same thing but you have more options on what to control. In a lot of wildlife conditions you want to have specific DOF (control aperture) to create separation from the background and freeze motion (control shutter speed) so you are happy to let ISO drift.
In landscape using a tripod you may want to reduce noise (control ISO) and control DOF (set aperture) and happy to let the shutter speed drift.
JackAllTog Plus
8 4.9k 58 United Kingdom
10 Aug 2017 1:07PM
For out and about type shots, i'll typically choose either aperture or Shutter priority as i feel relevant and auto ISO with an exposure compensation of -1/3 or -2/3rd's to try and avoid blown highlights, then bring the image back a bit if needed in post processing.

Much more occasionally, full manual - though typically then auto ISO is rubbish if there is a change of blown highlights - as on Canon 6d and APS-c bodies you can't do exposure comp in manual mode for auto ISO.

In terms of the AV mode algorithm - it seems to follow the reciprocal rule of 1/focal length & trying to keep ISO quite low when possible - though that's not use on fast moving subjects.
In Tv mode its often Open to max aperture or one step below while keeping ISO low - fine if your zooming in on one thing, but not if you need a deeper front to back sharpness. Then in very bright conditions it'll shut the aperture right down beyond f8 and give you loads of depth of field.

I think you should try it a bit but its likely you'll swap through all 3 modes most days on your holiday. Hope you have a good break though, and don't have to carry too much kit each day.
trivets12 13 1.3k
10 Aug 2017 1:25PM
Thanks, everyone!
Hi
I shoot all my wildlife photos with my Nikon D4 with auto iso and full manual, set limit to 3200 , i choose the aperture then use the comand dial to change the speed, iso floats to give best option. ie bird moving fast up the speed with command dial to suit or if bird freezes then drop speed iso drops, i find suits me great
mikehit 8 8.0k 13 United Kingdom
10 Aug 2017 10:38PM

Quote:Hi
I shoot all my wildlife photos with my Nikon D4 with auto iso and full manual, set limit to 3200 , i choose the aperture then use the comand dial to change the speed, iso floats to give best option. ie bird moving fast up the speed with command dial to suit or if bird freezes then drop speed iso drops, i find suits me great



Being pedantic - if you are using auto ISO it is not full manual. The camera is adjusting ISO to give what it (the camera) thinks is the best exposure. If the bird (for example) flies against varying background you risk under/over exposure on the target depending on the brightness of the background.
11 Aug 2017 7:30AM
You might also want to consider your metering area, depending on what you are shooting, and experiment with that also

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