Tuesday, 1 December 2015

exposure - Why would using higher ISO and faster shutter speed yield more noise than using lower ISO and slower shutter speed?


I'm trying to see how this answer correlates with practice. That answer basically says I need to let enough light into the camera and then use the highest ISO value and I'll get the lowest noise then.


So I make two shots with the same camera. Camera has aperture set to a specific value, fully manual mode, camera is on a tripod and focused onto a specific faraway object. Shooting is done around midnight, so it's rather dark. The scene is a large industrial building standing faraway across a field from the camera. The field has no light sources, all lights are on the industrial building.


I dunno if it's important but images are written as JPEG inside camera - no shooting to raw and external post-processing. Maybe that's why I see my unexpected result.


Camera highest ISO value is 3200 (not using extended ISO values).


So I first shoot with ISO 1600 and shutter speed set to 1/125 second and then I shoot with ISO 3200 and shutter speed set to 1/250 second. The amount of light should be identical and indeed both shots look properly exposed and exposed the same way.


When I open the pictures in an image viewer and zoom in to one hundred percent I see that ISO 1600 shot is much cleaner than one with ISO 3200. I checked that several times and I'm sure - it's not the other way around.


Why is this? Is this because of post-processing difference?





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