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  1. #1
    Join Date
    Dec 2006
    Posts
    26

    how fast is f/2.8 compare to f/4.0?

    as i know..... f/2.8 is about the double speed of f/4.0.....so you can use a shutter speed that is half with the f/2.8 compare to f/4.0....or you can compensate by doubling the ISO...is this doubling really makes a huge difference on speed?....because i saw many ppl saying that the sigma 18-55 f/2.8 is really fast......and nikkor VR 18-200 f/3.5-5.6 is slow....i just wonder why is that?.....nikkor VR 18-200 at 18-55mm range operates at f/4.0 or larger.....it is only half compare to f/2.8.....and giving twice as fast shutter doesnt really help with exposure much when the surrounding is dim....i've tried with my VR 18-200 in my living room....without flash...i often need to use shutter speed at less than 1/10 (aperature is at f/4.0)....which is a shutter speed almost impossible to shoot at.....i'm new to DSLR.....but do the lense make a difference even at the same aperature value?.......at the exposure value.....with the sigma at 55mm, 1/100, f/2.8 and nikkor at 55mm, 1/50, f/4.0......should i expect the same exposure result?...or i can double the ISO setting to obtain the same result?.....becuase i found that without a flash...often i'm looking at shutter speed in the 1/10 or slower range iwth ISO 400....so even with double aperature.....the shutter is still not fast enough.......my camera is D80 and my lense is nikkor VR 18-200 f/3.5-5.6

    i would understand if we are working in the film world where changing film speed is a troublesome job...but with DSLR....we can compensate the aperature with higher ISO speed or slower shutter speed or combination of both.....isnt it?....i'm new to DSLR (new in photography)...please dont get offended by my stupidity....i'm just curious

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    Nikon D80
    Nikon SB600
    Nikkor VR 18-200mm f/3.5-5.6
    KingMax 4GB 150x
    Last edited by shingfan; 01-08-2007 at 07:17 AM.

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