Question on Aperture Priority

Lawrence

Senior Member
This afternoon I put my camera in Aperture priority mode with the following settings.

f8
ISO-A (automatic ISO)
Minimum shutter speed 1/125
Maximum ISO 3200

My thoughts being that the camera would select the minimum shutter speed first (right brain logic) and adjust the ISO up to 3200 to get the best exposure.
I understand that it might select ISO first and adjust the shutter accordingly too - I am not sure which takes priority in selection.

However all my photos were shot at ISO 400 and the shutter speed varied between 1/40 and 1/80 of a second.

Can anyone explain this to me please?

I have just read the online manual for the D5100 and I think I am doing everything right but the results confuse me.
 

J-see

Senior Member
I'm not sure about yours but when I use A on mine and have auto-ISO turned on, it tends to prioritize ISO over shutter speed. I have auto-ISO off all the time and only manually increase it if I can't get away with a certain shutter speed. If auto-ISO is off, it only adjusts the shutter in function of A. On, it first tries to go to the preferred (minimal) ISO you selected and adjusts the shutter in function of that.

But that's on a D3300, not sure if yours functions identical.
 
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BackdoorArts

Senior Member
What lens were you shooting with? I suspect the shutter speed chosen was right around your focal length. Generally this doesn't happen unless you set shutter speed to AUTO, so it should have put you at 1/125 whenever possible by raising the ISO appropriately. Only once it maxes out the ISO should it drop your shutter speed below the specified minimum. So ISO 400 and 1/40 or 1/80 just doesn't make sense.

Have you verified that the settings you specified are actually the settings? Sometimes when I'm navigating that menu I exit certain selections inappropriately and they don't take (sometimes you click back, sometimes you click OK...). It really sounds like your minimum shutter speed is set to AUTO or something lower than 1/125s.

Another way around this would be to shoot in Manual mode at f8 and 1/125 with Auto ISO on at a max of 3200 as it will ignore the shutter speed portion - though it will cause dark images if ISO 3200 isn't sufficiently high for the proper exposure. But that's often fixable in post.
 
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Lawrence

Senior Member
I'm not sure about yours but when I use A on mine and have auto-ISO turned on, it tends to prioritize ISO over shutter speed. I have auto-ISO off all the time and only manually increase it if I can't get away with a certain shutter speed. If auto-ISO is off, it only adjusts the shutter in function of A. On, it first tries to go to the preferred (minimal) ISO you selected and adjusts the shutter in function of that.

But that's on a D3300, not sure if yours functions identical.

Interesting that it selects the "preferred minimum" which presumably is 100 if I have set maximum at 3200

Its messing with my head big time and the manual isn't helping me.
 

Lawrence

Senior Member
What lens were you shooting with? I suspect the shutter speed chosen was right around your focal length. Generally this doesn't happen unless you set shutter speed to AUTO, so it should have put you at 1/125 whenever possible by raising the ISO appropriately. Only once it maxes out the ISO should it drop your shutter speed below the specified minimum.

Have you verified that the settings you specified are actually the settings? Sometimes when I'm navigating that menu I exit certain selections inappropriately and they don't take (sometimes you click back, sometimes you click OK...). It really sounds like your minimum shutter speed is set to AUTO rather than 1/125.

Another way around this would be to shoot in Manual mode at f8 and 1/125 with Auto ISO on at a max of 3200.

I was shooting with my 35mm prime. I didn't have shutter speed set to Auto - just checked again.

Yes I think manual mode is the way to go but it would be good to know, and understand, why this is happening.
 

BackdoorArts

Senior Member
I was shooting with my 35mm prime. I didn't have shutter speed set to Auto - just checked again.

Yes I think manual mode is the way to go but it would be good to know, and understand, why this is happening.

It still doesn't make sense. I just tried it, albeit with a D7100 and a 28mm prime, with the exact settings you describe and it stays at 1/125s until I hit ISO 3200. I can't imagine it would work differently on your camera. Are you sure you have ISO sensitivity turned 'ON'?
 

Lawrence

Senior Member
It still doesn't make sense. I just tried it, albeit with a D7100 and a 28mm prime, with the exact settings you describe and it stays at 1/125s until I hit ISO 3200. I can't imagine it would work differently on your camera. Are you sure you have ISO sensitivity turned 'ON'?


Absolutely 100% sure

Glad it doesn't make sense to you either - that makes me feel slightly less insane
 

BackdoorArts

Senior Member
Just curious, with it in your hands now, as you look through the viewfinder is it showing you a shutter speed below 1/125s, even with the lens wide open in a bright room? Try varying the aperture and see what it does to shutter speed. It sounds like you're in Auto and not A, and I know that's not the case.
 

Lawrence

Senior Member
Just curious, with it in your hands now, as you look through the viewfinder is it showing you a shutter speed below 1/125s, even with the lens wide open in a bright room? Try varying the aperture and see what it does to shutter speed. It sounds like you're in Auto and not A, and I know that's not the case.

Jake I am sitting here laughing as I know it sounds like I am nuts. A is aperture Green is Auto. I am in A!
Doing what you suggest with aperture on f 1.8 and minimum shutter speed on 1/125 the shutter jumps around to 1/60 and 1/80th as I move the camera around - it is night time here so I am under artificial light but that shouldn't cause this.
 

BackdoorArts

Senior Member
And just curious, what is your current ISO setting? I suspect it's 400. It's possible to toggle Auto ISO on and off by pressing the ISO button on the camera back and turning the control wheel. If the camera is operating properly right now then it's possible you accidentally turned if off at some point. Has happened to me.
 

Lawrence

Senior Member
And just curious, what is your current ISO setting? I suspect it's 400. It's possible to toggle Auto ISO on and off by pressing the ISO button on the camera back and turning the control wheel. If the camera is operating properly right now then it's possible you accidentally turned if off at some point. Has happened to me.

Current ISO setting is 100
 

Horoscope Fish

Senior Member
The problem is making my brain ache as well and I'm thinking the simple solution might be using specific values in the Auto ISO menus. Give it a window of, say ISO 100 to ISO 6400, instead of using the "Auto" setting. Something tells me that's the root of the problem here. This is how I use Auto ISO when I do use it and I either set a minimum shutter speed myself, or I push the slider under "Auto" all the way right to the fastest "Faster" setting.

....
 

BackdoorArts

Senior Member
The problem is making my brain ache as well and I'm thinking the simple solution might be using specific values in the Auto ISO menus. Give it a window of, say ISO 100 to ISO 6400, instead of using the "Auto" setting. Something tells me that's the root of the problem here. This is how I use Auto ISO when I do use it and I either set a minimum shutter speed myself, or I push the slider under "Auto" all the way right to the fastest "Faster" setting.

....

That's the thing, he's not using the AUTO setting, he's got the Maximum ISO set to 3200, which when coupled with the base setting of 100 gives him the 100-3200 window. That, with a Minimum Shutter Speed of 1/125 means that he should never get a shutter speed below that unless his ISO is capped at 3200. So ISO 400 and 1/40 or 1/80 makes no sense at all.

It's a PITA, but you could try to do a green-button reset of the camera to see if it clears it. No idea if there's another setting somewhere that could screw with this, but that's my last thought. But if you're willing to take it to a shop then at least we might be able to find the answer to this mystery. Very strange, indeed.
 

Marcel

Happily retired
Staff member
Super Mod
Jake I am sitting here laughing as I know it sounds like I am nuts. A is aperture Green is Auto. I am in A!
Doing what you suggest with aperture on f 1.8 and minimum shutter speed on 1/125 the shutter jumps around to 1/60 and 1/80th as I move the camera around - it is night time here so I am under artificial light but that shouldn't cause this.

Which metering mode are you using? Using "spot" or "center weighted" will make readings change when the camera is moved around. Using the A mode, it is the shutter speed that will vary when lighting changes. If you want shutter not to change, you've got to use the Shutter priority "S" mode.
 

Horoscope Fish

Senior Member
That's the thing, he's not using the AUTO setting, he's got the Maximum ISO set to 3200, which when coupled with the base setting of 100 gives him the 100-3200 window. That, with a Minimum Shutter Speed of 1/125 means that he should never get a shutter speed below that unless his ISO is capped at 3200. So ISO 400 and 1/40 or 1/80 makes no sense at all.
Okay then I mis-read something somewhere. If both a minimum and maximum ISO have been set, as well as a minimum shutter speed, and he's falling below the minimum shutter speed while still within the specified ISO range then something's definitely wrong.


,,,,
 
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mikew_RIP

Senior Member
Got a feeling i had the same thing the first time i tried using auto iso,i cant remember what i had done but it was something i had not set correctly.
 

BackdoorArts

Senior Member
Which metering mode are you using? Using "spot" or "center weighted" will make readings change when the camera is moved around. Using the A mode, it is the shutter speed that will vary when lighting changes. If you want shutter not to change, you've got to use the Shutter priority "S" mode.

That's not the thing, Marcel. Regardless of what the meter is suggesting as the proper exposure, the exposure settings should fall within those specified in the Auto ISO settings. If he has a minimum Shutter Speed set to 1/125sec then it shouldn't drop below that without hitting the Maximum ISO setting first (in this case 3200). With those parameters set and Auto ISO turned on it should be impossible to get a shot at 1/40, f8 and ISO 400 in Aperture Priority mode.
 

J-see

Senior Member
Interesting that it selects the "preferred minimum" which presumably is 100 if I have set maximum at 3200

Its messing with my head big time and the manual isn't helping me.

I just set mine on 100 and the max on 6400 and in A and S, it directly adjusts ISO and completely ignores the shutter or aperture depending the mode. In P the same happens, the moment auto ISO is enabled, all scrolling results into ISO changes. I disabled it for exactly that reason; I couldn't make sense of the logic and disliked the cam making those decisions.
 

Vixen

Senior Member
I had the exact same happen to me today, ISO set at 400. Popped camera into shutter priority and auto ISO and all shots were at ISO 400. Turns out I had the Auto ISO settings wrong. Not sure what the D5100 settings for ISO are like in the menu so can't suggest what you have wrong unless it is like the D7100 and there is a setting for max ISO, minimum shutter speed etc......I had somehow set the max ISO to less that the 400 I had the camera set to (oops)

Try what I did and look for a youtube vid on auto ISO for your camera....was my saviour today coz I thought Auto ISO wasn't working.

It was user error :D
 
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