Oly 45mm versus 60mm

Started Dec 15, 2012 | Discussions thread
george4908
Regular MemberPosts: 190
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Re: Oly 45mm versus 60mm
In reply to bowportes, Dec 16, 2012

bowportes wrote:

george4908 wrote:

You'll also notice that the hay strands immediately in front of the figure are a little less out of focus on the 60 than the 45. Normally you would expect the longer lens to have the shallower depth of field, but the 2.8 maximum aperature more than offsets this. Would be interesting to see the same shot with the 45 at 2.8.

You know... I've been wondering myself to what extent the additional FL of the 60 (60mm versus 45mm) compensated for its smaller aperture (f 2.8 instead of f1.8) in terms of the blurring of background.

Well, the difference in focal length from 45mm to 60mm is 33%. The difference in aperature from 1.8 to 2.8 is 55%. Not sure if there's a straight linear equivalence in this case, but it may explain why the depth of field for the 60mm at 2.8 is larger than the DOF for the 45mm at 1.8. Someone with more optical expertise may have to weigh in, but the numbers would seem to suggest that you'd have to go to a 70mm lens at 2.8 (and back up a bit more to maintain constant image size, of course) to get the same depth of field as the 45mm at 1.8.

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