Equivalent Aperture

Started Oct 29, 2013 | Discussions thread
Mattoid Regular Member • Posts: 340
Re: Equivalent Aperture

Dr JLW wrote:

I have been seeing DP review evaluate camera in terms of equivalent aperture. I have figured out how the calculate it but what does it mean in terms of pictures?

It needn’t actually be expressed as equivalent aperture as it actually describes a physical maximum aperture diameter (for a particular angle of view). E.g. a 40mm f4 has a maximum aperture diameter of 10mm and a 20mm f2 has a maximum aperture diameter of 10mm. Therefore a 20mm f2 on a four thirds sensor gives an equivalent aperture of f4.

The physical aperture diameter has a direct affect on the amount of light energy. (on a larger sensor the light energy is spread over a larger area giving a darker exposure.) More light energy means less noise

It also has a direct effect on the amount of background blur, which, in turn, when combined with the factors of angle of view and viewing magnification gives the perception of depth of field (which is subjective and not an objective measurement.)

Those are the two main effects.

Physical maximum aperture diameter can be expressed as a maximum equivalent aperture just as angle of view in degrees can be expressed as an equivalent focal length.

The graph they show could easily be angle of view compared to maximum aperture diameter. And thus avoid the “equivalent” confusion.

I also noticed the graph tends to bunch up at the wide end, when in reality changes at the wide end in mm result in a much greater change in angle of view than at the long end. Drawing and labelling the graph with angle of view as the x axis (with perhaps the 135 equivalent bellow in brackets) would solve this problem.

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