How does FE 55mm perform in crop as a 85mm lens??

Started Jan 21, 2014 | Discussions thread
viking79
viking79 Forum Pro • Posts: 14,148
Re: How does FE 55mm perform in crop as a 85mm lens??

zither wrote:

This actually bring me another question:

How does the DOF get computed when using FE55 in crop mode?

Depth of field is based on aperture size and focal length, so depth of field of the FE 55mm f/1.8 on APS-C is the same as on full frame. What is different is your field of view (the APS-C is just a crop of the image you would get from the full frame). So the 55mm f/1.8 use on APS-C will give the same field of view and depth of field as an 85mm f/2.8 used on full frame, both have an entrance pupil/apparent aperture size of about 31mm (55 / 1.8 ~= 85 / 2.8).

Actually, I'm pretty sure FE55 will give good IQ in most case, but really not sure about the the DOF and shutter speed.

Same as any other 55mm f/1.8 lens on APS-C, or again, same as 85mm f/2.8 on full frame.

Could anybody rank the DOF and Shutter speed using the following combinations? assuming the settings are the same.

Set the full frame at 1 1/3 stop higher ISO to maintain shutter speed with aperture set 1 1/3 stop smaller (same composition), set the same ISO and longer shutter speed to improve image quality, set the same aperture and 1 1/3 stop higher ISO to get higher shutter speed but less depth of field, etc. It is a trade off with full frame, it only adds flexibility.

A7 + FE 55 full mode

55mm f/1.8. In APS-C terms this is 35mm f/1.2. 24 MP

A7 + FE 55 crop mode

Still a 55mm f/1.8, just like you cropped the above image after taking it. In FE terms this is 85mm f/2.8. 10 MP.

A7 + Nikon 85 full mode

85mm f/1.8, in APS-C terms this is 55mm f/1.2 24 MP

NEX + FE 55 crop mode

Still a 55mm f/1.8, same as A7 in crop mode. 16 or 24 MP (whatever your NEX is)

So focal length and aperture are both fixed properties of a lens that never change no matter what camera you put them on, but to get an "equivalent" image, you have to multiply the focal length and aperture by the crop factor.

Eric

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