Sigma 85mm or 105DC?

Started Jan 9, 2011 | Discussions thread
jtra
Contributing MemberPosts: 608
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Re: Sigma 85mm or 105DC?
In reply to PrTv, Mar 21, 2011

PrTv wrote:

Speaking about Bokeh, 105DC will give you bigger bokeh due to its being longer. I find bokeh rendered by both lenses is pleasing. One thing I like about 85 1.4 lens is its ability to separate a subject. This shot was taken with Nikkor 85 1.4D @F1.4 (exif is intact). I find that background is creamy and the subject is kinda pop-out. I know that there are a number of lenses that can do better in this regard, but compared with 105DC @F2, I find that the 85 1.4D @F1.4 can separate a subject better.

Great picture!

You can estimate relative size of bokeh for same magnification (i.e. fitting subject in frame with both lenses) by computing: FocalLength / ApertureNumber.

This will give you relative size of diameter of out of focus highlights. People will percieve the area of that so for relative area size you will have to square the numbers.

Example:
105mm f/2 gives: 52.5mm relative diameter

85mm f/1.4 gives: 60.7mm relative diameter, so it is 15% bigger in diameter, 33.7% bigger in area.
Computation is described here: http://toothwalker.org/optics/dofderivation.html

Also, bokeh is not just about size of out of focus highlights, but about distribution of light inside of them. See my article on that: http://jtra.cz/stuff/essays/bokeh/

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