Question on using a FF lens in crop mode on A7RIV

Started 3 months ago | Discussions thread
SQLGuy Veteran Member • Posts: 9,720
Re: Question on using a FF lens in crop mode on A7RIV

A couple of other ways to think about it:

1. A bit over simplistic, but crop mode is very much like using a 1.5X teleconverter. A 1.4X TC adds 40% to the focal length, and drops one stop (1.4 X the lens' rating).

2. F stop is the ratio of the lens' effective diameter to its focal length. Cropping, or a teleconverter, increases the focal length, but doesn't make the front element any bigger, so the calculated F stop obviously increases by the crop factor as well.

For instance, a 100mm F2 lens will have an effective diameter of 50mm. E.g. the Canon 100/2 USM takes 58mm filters and probably has about a 56mm front element... there being some losses along the way in getting to 50mm effective diameter. Putting that lens on a 1.5X TC, or using it in 1.5X crop mode, will make it act like a 150mm lens. 100/50 = F2.0. 150/50 = F3.0.

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A7R2 with SEL2470Z and a number of adapted lenses (Canon FD, Minolta AF, Canon EF, Leica, Nikon...); A7R converted to IR.

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