MFT 25mm f/1.4 fails to blur background like Full Frame 50mm lens @ f/2.8

Started 9 months ago | Discussions thread
Alex Notpro
Contributing MemberPosts: 750Gear list
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Fygaren is on to something here ... FL and Crop!
In reply to Fygaren, 9 months ago

Thanks all for your responses and trying to help me figure this out. I think the effective(actual) focal length combined with cropping must be the source of the difference. (Other explanations offered don't make sense from an engineering standpoint)

The Nikkor lens has a small but noticeable amount of Focus Breathing, so it's hard to know where exactly it behaves as a 50mm as opposed to a 48mm or 52mm lens. Also, I can't find any manufacturer's specs for either lens, to see if their stated focal lengths have been rounded for marketing purposes. According to this article (http://photo.stackexchange.com/questions/13603/how-to-test-actual-focal-length) I should be able to measure the FL... perhaps I'll try that later.

Here are the crops used for posting:

  • MFT 25/1.4 original 4608x3456 cropped to 1055x1583
  • FF 50/2.8 original 4256x2832 cropped to 988x1484
  • FF 50/1.4 original 4256x2832 cropped to 958x1440

I quoted the crops above in case I'm missing something, but I don't think the crops matter by themselves. If the lens had the exact same FL, than these shots taken from the same distance and cropped down to the same framing would be exactly equivalent! It's only in combination with a difference in FL, that when cropped to match the framing, causes the equivalent aperture on the MFT lens to creep up, i.e. if the 25mm is really a 23mm, then the extra cropping would make it a 50mm f/3.0 equivalent.

 Alex Notpro's gear list:Alex Notpro's gear list
Olympus PEN E-PM2 Olympus E-M1 Sony Alpha 7R Panasonic Lumix G Vario 7-14mm F4 ASPH Sony FE 55mm F1.8 +16 more
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