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Aperture values and numbers, what's the difference?

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  • Aperture values and numbers, what's the difference?

    Aperture number, EV, Tv, Av, aperture value, f-number, stop, f-stop - what do these all mean in relation to each other?

    This topic was discussed on the old DPNow forum but the information is very handy, so here is the information, reproduced for your convenience:

    If you examine the metadata or image properties information made available by image viewers and editing programs, you may see something like this:


    f/5.60, but Aperture Value 5

    or

    f/4.00, Aperture Value 4

    or

    f/8.00, Aperture Value 6

    So what's going on?

    An Exposure Value or EV is a linear representation of the brightness of the image projected through the lens and the shutter onto the film or image sensor, during exposure. A difference of one EV means either a doubling or a halving of the light transmitted.

    An f/number, or f-stop number, or aperture, or focal ratio, is calculated by the lens focal length divided by aperture diameter. It's a non-linear scale and not easy to work with unless you memorise key values like 2.8, 5.6, etc.

    To make things easier, an APEX (Additive system of Photographic EXposure) linear scale was invented. However, we've stubbornly stuck with the f/number scale on lenses anyway!

    Aperture values are just like EVs, in other words it's a linear scale, but relate to the actual aperture or focal ratio, so:

    Aperture value (Av) : Aperture or f/number or f-stop

    0 : f/1.0
    1 : f/1.4
    2 : f/2.0
    3 : f/2.8
    4 : f/4.0
    5 : f/5.6
    6 : f/8.0
    7 : f/11
    8 : f/16 ..etc.

    This is why Av=6 while aperture is f/8.0, for example. Each whole number increment of the Av matches one EV step. So, each increment of whole Av either doubles or halves the light transmitted through the lens.

    Av is, of course, just half the story. The other half is to do with shutter speeds and the values analogous to Av are Tv (Time value).

    Here is the corresponding shutter speed table:

    Time value (Tv) : Time (seconds)

    0 : 1
    1 : 1/2
    2 : 1/4
    3 : 1/8
    4 : 1/16 (1/15)
    5 : 1/32 (1/30)
    6 : 1/64 (1/60)
    7 : 1/128 (1/125)
    8 : 1/256 (1/250) ..etc.

    So when Tv=6, the shutter speed will be 1/60th second.

    The figures in brackets are approximations widely adopted for simplicity as it's easier to think in 5ths and 10ths than 16ths, etc. Again the whole number Tv steps are equivalent to one EV step.

    Finally, there are EVs (more correctly Ev) themselves, or Exposure values (as opposed to EV steps that represent whole number increments up and down the linear EV scale).

    The total EV of a photo is the Av added to the Tv, so a picture taken using settings of f/8 and 1/125th second would have an EV of 6 (Av) plus 7 (Tv), totalling an EV of 13.
    You can go further and play with more advanced equations based on Brightness and Incident light values, but the above is basically all you need to know for general use.

    Ian
    Founder/editor
    Digital Photography Now (DPNow.com)
    Twitter: www.twitter.com/ian_burley
    Flickr: www.flickr.com/photos/dpnow/
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