Will APS-C 50mm f/1.4 and full-frame 80–85mm at about f/2.2 give similar background blur?

Asked 9/9/2013

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If I frame the same portrait on two cameras by matching field of view:

  • APS-C / 1.6x crop camera with a 50mm lens at f/1.4
  • Full-frame camera with an 80–85mm lens at about f/2.2

Since 80mm is roughly 50 × 1.6, and f/2.2 is roughly 1.4 × 1.6, would the amount of blur / depth of field look essentially the same? Or would one setup produce noticeably more bokeh than the other?

Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0

Photography Stack Exchange contributor

12y ago

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Provided you match the field of view (by multiplying the focal length by 1.6) and keep the entrance pupil diameter the same (by multiplying the f-number by 1.6), then from a purely theoretical point of view, depth of field and the amount of background blur will be the same.

From a practical point of view there will be slight differences in the nature (including smoothness) of out-of-focus backgrounds between the two systems.

Assuming the 50mm lens on the crop sensor was designed for full frame (as almost all fifties will be), then there will be reduced occurrence of "cat eye bokeh" where out-of-focus highlights toward the edge of the frame take on an elliptical appearance due to clipping by the lens barrel.

The 80mm lens on the full-frame camera will probably be slightly smoother due to the more relaxed conditions of a longer focal length, specifically the edges to out-of-focus highlights should have a nice gentle roll off. To obtain good performance when wide open, most 50mm f/1.4s are over corrected for spherical aberration, which gives a harder edge to out-of-focus highlights which get brighter toward the edge, the opposite of what you want for smooth bokeh (for this reason the Canon 50mm f/1.2L was deliberately under-corrected for spherical aberration).

If the hypothetical 80mm lens has a wider aperture than f/2.2 then the out-of-focus highlights will take on a slightly non-circular shape (depending on the number and curvature of the aperture blades) in comparison to the 50 f/1.4, which if f/1.4 is wide open will have perfectly circular highlights as the aperture blades are fully retracted.

Originally by user1375. Source · Licensed CC BY-SA 4.0

user1375

12y ago

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AI Answer

Generated from our catalog & community — verify before relying on it.

If you match field of view and also scale the f-number by the crop factor, the amount of blur and depth of field are theoretically about the same.

So in your example:

  • APS-C: 50mm at f/1.4
  • Full frame: ~80–85mm at about f/2.2

These should give very similar depth of field for the same framing.

Why: matching focal length by 1.6 keeps the same framing, and matching f-number by 1.6 keeps the entrance pupil diameter effectively equivalent, which is what drives blur amount.

In practice, the rendering may still differ slightly. The overall blur quantity should be close, but the character of the bokeh can vary because the lenses are different designs. For example, the crop setup may show less “cat-eye” bokeh near the frame edges if the 50mm lens is a full-frame lens used only on the center of its image circle. The full-frame 80–85mm may look a bit smoother simply due to its optical design.

So: similar blur amount, but not necessarily identical blur character.

UniqueBot

AI

12y ago

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