Originally posted by Winder Pentax does it just like all APS-C manufacturers. He just doesn't mention Pentax because Pentax is such a small player.
Go to B&H Photo and look at the DA* 55mm F/1.4 lens. The last line in the overview reads:
Essentially equivalent to an 82.5mm f/1.4 lens upon 35mm SLR cameras
An 82.5mm F/1.4 lens would have an aperture area of 2727.3 mm^2
The 55mm F/1.4 lens has an aperture area of 1211.81 mm^2.
The aperture on a 55mm lens would need to be F/1.0 in order for it to equal the 82.5mm F/1.4 in terms of quantity of light projected on the sensor.s.
An 1.4 lens is equal to an 1.4 lens regardless of format. Aperture is not defined by DOF but by exposure. DOF is mentioned nowhere in the definition of a lens aperture.
Different formats are not supposed to collect the same amount of light. Insisting on "equivalence" is missing the point entirely.
Haven't seen the video but all these zealots (who should read up upon exposure theory) do the same mistakes:
1) Insisting that different formats should have the same output and/or collect the same amount of light. This is wrong according to the theory of exposure. If it was correct a Pentax 645 should be compared to a cell phone.
2) Using one format as benchmark; usually FF. This is cheating. By this method only the properties of the reference format is used as properties smaller formats has to comply with. Not the other way around.This is only a one-sided biased opinion. Obviously for two lenses to be DOF equivalent they both have to do the same. Ie an equivalent FF lens must be able to do what the smaller format lens does as well DOF wise to be "equal".
3) Pretending that DOF is only depending on aperture and equivalent focal lenghts. It isn't. It is dependent on aperture, focal lenght, focusing distances, and subject magnification. Ie lenses claimed to be DOF equivalent are patently not. Not even wide open.
Some may be surprised if they put "equivalent" lenses into a DOF calculator. For comperable lenses (eg prime vs prime, zoom vs zoom) you'll find that the smaller format lens usually has thinner minimum DOF than the larger. This is because it has larger maximum magnification.
Last edited by Pål Jensen; 10-06-2014 at 01:44 PM.