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what aperture means to real photographs? DOF or light?

  • 28-07-2008 10:46am
    #1
    quilmore
    Registered Users, Registered Users 2 Posts: 1,037 ✭✭✭


    I've found exactly the same picture taken with a fixed lens is much brighter than the same picture (with same light, same camera and same shutter speed/aperture/iso) than even a good zoom lens at the same distance

    I did compare:
    a sigma 30mm 1.4 with a nikon 18-55 at the equivalent, both with the same aperture and camera settings (something like: 1/80 and f/5.6, ISO 200)
    and a nikon 50 1.8 and a nikon 35-70 f/2.8

    clearly the fixed lenses came out brighter even with the same aperture (and all the rest)

    is "brightness" on a lens beyond the f number?


Comments

  • #2
    ThOnda
    Registered Users, Registered Users 2 Posts: 4,699 ✭✭✭ThOnda


    Well. For the first - zoom lens equals compromise. Fixed lens equals carefully calculated pieces of glass/plastic.

    The appearance of the image depends on physical construction of the lens - number of lenses, in what amount of elements, material of lenses and also if they are clean or if you have filter. Aperture could be the same, but every piece of glass/plastic reduces amount of the light going through the whole lens. And most of the time construction of the primes is much more simple because it does not have to cope with problems related to zooms.

    Therefore the image quality from primes is more likely to be better than from zoom lenses.


  • #3
    Roen
    Closed Accounts Posts: 4,620 ✭✭✭Roen


    Yep, it pretty much is, aperture cannot effect the transmission values of the glass itself. There's variables such as the type of glass used and the coating(s) on that glass.
    Crown, Flint and Fluor-Crown glass all have different transmission levels, so even if your aperture is the same, different intensities of light will pass through them.


  • #4
    Arciphel
    Registered Users, Registered Users 2 Posts: 2,843 ✭✭✭Arciphel


    Quilmore, might also be something to do with how your D300 meters the sigma lens vs. a Nikon lens? I have the Sigma 30mm f1.4 also and have found I need to step it down about 2/3 of an ev to compensate for slight over-exposure. Although all the comments about the refractive index of lenses being different is right too.


  • #5
    quilmore
    Registered Users, Registered Users 2 Posts: 1,037 ✭✭✭quilmore


    I've learnt something new today (well, in the last few days)
    thanks


  • #6
    CabanSail
    Moderators, Category Moderators, Arts Moderators, Entertainment Moderators Posts: 9,047 CMod ✭✭✭✭CabanSail


    The F Stop number is a ratio. IIRC it's the Focal Length of the Lens divided by Diameter of the Iris. As said before, this formula does not include losses due to transmission characteristics of the glass. There can also be different transmission curves, so that a particular wavelength of light may be affected more than another. If you think about it, if you place a ND4 filter on the front of a lens it does not change the physical setting of the aperture.


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