DF / FX and aperture question

Started Sep 28, 2012 | Discussions thread
freddyNZ
Senior MemberPosts: 2,410
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Re: DF / FX and aperture question
In reply to mainger, Sep 28, 2012

mainger wrote:
Bingo! This one makes it very clear, thanks a lot!

Cheers!

slimandy wrote:

Another way of looking at it.....

For an 85mm lens to have an f1.4 aperture it needs a physical opening of about 61mm....
85/61 = f1.4 (ish, 1.3934)

So if you work out the focal length equivalent is 85*1.5 = 127.7, the equivalent relative aperture is....

127.5/61 = f2 (ish, 2.09)

No problem with the maths there.

But worth adding (possibly at risk of being shot) that the optical construction of 50mm lenses results in fast lenses which seem to generally suck for bokeh, 85mm lenses seem to be on average much nicer. So despite the effective focal length of a 50mm lens on dx which seems perfect (focal length equivalent 75mm), you're possibly still better off with an 85mm prime on Dx, and increasing distance to the subject for shallow DOF portrait.

Please don't shoot me - I'm a dx guy. Don't shoot me again, as Ken Rockwell (sorry to mention his name) is actually one of the few who's ever bothered to do a comparison of ~ 50mm prime lens bokeh, including Zeiss ZF 1.4, and they all sucked. He didn't test the Sigma 50mm f1.4 (because he hates Sigma) - and neither have I (I don't hate Sigma), but the bokeh samples for this lens on Photozone look promising for use as a portrait lens on dx, with much less donut ringing of specular highlights and much less busy, smoother rendering of OOF areas in the foreground and background.

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