How is exposure metering done?

k.udhay

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I understand that the exposure reading should go down when the aperture opening size is reduced, keeping other parameters fixed. But when I can't observe that through my view finder, how does the exposure meter do? I understand that the aperture will take its size only just before the shutter opens. It is little confusing for me. Pl. help. Thanks.
 

cgipson1

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Because the metering software is programmed to give a correct exposure at the settings you chose, even if you are not seeing the closed down aperture when looking through the viewfinder... THE METER KNOWS! TRUST THE METER! lol!

If you are in Aperture Priority, or Manual... you should see the meter indicate over or underexposure when changing the settings. It might also help to turn off AUTO ISO. Depending on what body you have, it may have a DOF preview... but since the viewfinder can get really dark when using that, it sometimes is not that useful.
 
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k.udhay

k.udhay

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Thank you, cgipson1! :)
 

Derrel

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I understand that the exposure reading should go down when the aperture opening size is reduced, keeping other parameters fixed. But when I can't observe that through my view finder, how does the exposure meter do? I understand that the aperture will take its size only just before the shutter opens. It is little confusing for me. Pl. help. Thanks.

The process that we now have in cameras today is called "open aperture metering". It was first premiered in the 1960's I believe by Pentax. In the early days on in-camera metering, metering had to be done at the shooting f/stop, which might be wide-open at say 50mm and f/2; if the lens were closed down to f/8, then the metering would have to be adjusted by actually stopping the lens down to f/8 with a stop-down button. THat type of light metering is called "stopped-down metering".

Stopped down metering is the method commonly used for lenses or optical devices that have only ONE, single, light-admitting aperture size (like mirror telephotos, for example).

Full aperture light metering used to be accomplished by a system of mechanical connections, in a system in which the lens's f/stop maximum had to be "indexed" (in one of several ways), so the metering system could compute the right exposures when the lens was stopped down to smaller apertures. Today, that mechanical linking and indexing can be done through electronic communication methods. In some systems, like Minolta-AF, now called Sony Alpha, or Canon EF mount, there is NO mechanical f/stop control on the camera or the lenses, and all the communication and setting is done electronically.
 

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