How sensor size affects depth of field and diffraction in macro at equal framing
Asked 1/28/2014
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For macro photography, depth of field is very shallow, so photographers often stop down heavily. If two cameras produce the same framing and final display size, how do sensor size, aperture, and diffraction interact? For example, comparing a full-frame Nikon D4 with a 90mm macro lens and a Micro Four Thirds Olympus OM-D E-M5 with a 45mm macro lens, which would provide more usable depth of field if each is shot near its diffraction-limited aperture?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
12y ago
2 Answers
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In general shooting, provided you keep angle of view, camera position and size of entrance pupil the same, depth of field and diffraction will be the same regardless of sensor size!
The entrance pupil is the focal length divided by the f-number you are shooting at, e.g. a 45mm f/2.8 at f/2.8 has an entrance pupil that is 45/2.8 = 16mm. It's the size the aperture stop appears to be when viewed through the front element.
So with the equipment you list, a 90mm lens is required to give the same field of view when using the full frame D4. So to get the same 16mm entrance pupil you need to use 90/16 = 5.6, f/5.6 provided you set the camera to f/5.6 you will get the same depth of field.
What about diffraction?
The absolute size of the airy disc (the blurred area created from a sharp point of light as it passes through a small aperture) increases linearly with a larger f-number, so f/5.6 on the D4 causes a blur that is twice the radius of the EM5 (whose lens is at f/2.8) however because the sensor on the D4 is twice the height (and width) the blur has exactly the same overall effect on the image.
It's always worthwhile to think about blur and sharpness etc. in proportion to the image size. When thinking about diffraction most people immediately start trying to work out airy discs and pixel pitch and get confused when the number of pixels is different between different formats. Instead just think about blur in terms of the percentage of image and such issues go away!
The key word in the first sentence is "in general shooting". Unfortunately when you get into the macro territory things get complicated, fast.
Firstly the focal length and aperture values stated for a lens are only valid with the lens focused at infinity. Most macro lenses focus closer by reducing focal length.
The aperture also behaves differently at high magnifications with the "effective" aperture taking over.
Finally the symmetry (difference in size between entry and exit pupils) of the lens plays a part, so without knowing what's inside the lens you can't be certain of the depth of field values.
If you're really concerned about depth of field and macro then you should be thinking about stacking shots rather than different camera bodies. A different sensor might give a small advantage but stacking gives a huge advantage.
Originally by user1375. Source · Licensed CC BY-SA 4.0
user1375
12y ago
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At equal framing and final output size, sensor size alone does not automatically give one system more usable macro depth of field. What matters is the combination of magnification, aperture, and output/viewing size.
For equivalent framing, the full-frame camera needs about twice the focal length of Micro Four Thirds. If you also keep the entrance pupil equivalent, depth of field is essentially the same. In your example, a 45mm lens at f/2.8 has roughly the same entrance pupil as a 90mm lens at f/5.6, so those settings produce similar depth of field.
Diffraction is often misunderstood here. “Diffraction-limited aperture” is tied to pixel pitch and is mainly relevant at 100% pixel viewing. In normal prints or screen viewing at the same final size, diffraction differences between formats are less dramatic than DLA charts suggest.
So the practical answer is: neither setup inherently wins just because of sensor size. With equivalent framing and comparable final presentation, depth of field and diffraction tradeoffs are broadly similar, and the result depends heavily on magnification, resolution, viewing size, and viewing distance.
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