Question on camera calculation


Feb 12, 2011
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#1
I'm not sure if this is a relevant or important question but it's something that has been bugging me and i can't find the answer.

Knowing that the camera uses 3 variables to calculate, ISO,Aperature and Shutter based not he exposure you want.

By choosing any 2 of the 3, the camera will calculate the 3rd.
For example, setting ISO at 100 with Aperture at F4, the camera will calculate the shutter speed.

However, what happens when you only select 1 of the 3? For example, you set to Aperture mode at F4 but ISO AUTO, what base does the camera use to know what the optimum shutter speed to use and what ISO to set? I mean, it can choose from anything from ISO100-6400, from any shutter speed in any combination…how does it know what the "OPTIMUM" 2 settings are? I'm always curious because sometimes i do set to Auto ISO because there's really no time to fiddle with ISO settings…
 

daredevil123

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Staff member
Oct 25, 2005
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lil red dot
#2
spinworkxroy said:
I'm not sure if this is a relevant or important question but it's something that has been bugging me and i can't find the answer.

Knowing that the camera uses 3 variables to calculate, ISO,Aperature and Shutter based not he exposure you want.

By choosing any 2 of the 3, the camera will calculate the 3rd.
For example, setting ISO at 100 with Aperture at F4, the camera will calculate the shutter speed.

However, what happens when you only select 1 of the 3? For example, you set to Aperture mode at F4 but ISO AUTO, what base does the camera use to know what the optimum shutter speed to use and what ISO to set? I mean, it can choose from anything from ISO100-6400, from any shutter speed in any combination…how does it know what the "OPTIMUM" 2 settings are? I'm always curious because sometimes i do set to Auto ISO because there's really no time to fiddle with ISO settings…
It really depends on the mode you selected. If you set aperture to F4, that means you are in A mode. In Nikon Dslr at least, you need to set the minimum shutter speed, minimum iso and max iso. So if the shutter speed metered is lower than the minimum shutter speed set, the camera will raise the iso till it hits minimum shutter speed. If max iso is hit before min shutter speed is hit, the iso will stop at max iso and shutter speed is the shutter speed metered at that max iso an f4.
 

Octarine

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Jan 3, 2008
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Pasir Ris
#3
However, what happens when you only select 1 of the 3? For example, you set to Aperture mode at F4 but ISO AUTO, what base does the camera use to know what the optimum shutter speed to use and what ISO to set? I mean, it can choose from anything from ISO100-6400, from any shutter speed in any combination…how does it know what the "OPTIMUM" 2 settings are? I'm always curious because sometimes i do set to Auto ISO because there's really no time to fiddle with ISO settings…
Cameras have a lot of algorithms inside. For you example in Aperture mode: One could be "keep ISO at lowest possible level", together with "observe rule 'shutter speed <= 1/focal length". Since the camera notices the focal length (reported by the lens) it could set the shutter speed accordingly and increase ISO if necessary.
If you use these Scene modes then certain priorities are set automatically, e.g. wide aperture for Portrait, small aperture for landscape and others.
 

Feb 12, 2011
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#4
observe rule 'shutter speed <= 1/focal length"

This answers a lot :) I believe this IS how cameras decide what setting to use when almost everything is auto…
Like i said, i don't think it's an important question that will hinder photo taking, it's just something that's bugging me…thanks for answering!
Unlike Nikon, Canon doesn't have a setting for minimum shutter i think..there's a max ISO for auto but i dun recall a min shutter setting.
 

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