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I came to know that a 50mm prime lens on a cropped sensor behaves like an 75mm prime lens.

So, does the 35mm prime lens behave like a 50mm prime lens on a cropped sensor?
If yes, then will it show the same bokeh as the 50mm lens on a cropped sensor?

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On a Nikon DX camera there's a 1.5x crop, so 50mm is more like 75mm. –  Dan Wolfgang Feb 22 '12 at 12:17
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Bokeh is more than just depth of field, but for the parts of this question which are related to that, see Can a smaller sensor's "crop factor" be used to calculate the exact increase in depth of field? –  mattdm Feb 22 '12 at 13:12
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I think the questioner is referring to the amount of background blur not the quality of the background blur (which is what the term bokeh refers to) can we edit the question in order to get an unambiguous answer? –  Matt Grum Feb 22 '12 at 23:41
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"Bokeh" refers to the visual quality of the out-of-focus areas of a phtograph. See What is bokeh, exactly? –  mattdm Feb 23 '12 at 14:30
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The depth of field question is already answered here (as linked in a previous comment). So let's leave this about the bokeh. –  mattdm Feb 23 '12 at 14:41
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3 Answers

up vote 2 down vote accepted

The only different between full frame and crop sensors is that the crop sensor is smaller - so the smaller sensor only sees only the center of the full frame image.

Or, another way to put it is that the picture you get from a crop sensor is the same picture you get from a full frame if you crop it and only leave the middle part.

So, does cropping effect bokeh? obviously not, but...

Cropping does have the same effect as zooming in, that's why the field of view of a Nikon APS-C at 35mm (or Canon APC-C at ~30mm) is similar to the field of view of a full frame with a 50mm lens.

And that "extra zoom" means that to fill the frame with the same subject on a crop sensor you will be at a greater distance than with a full frame - and distance to subject does effect DOF.

So, the quality and shape of the bokeh doesn't change in any way but the amount of bokeh does change (simply because distance to subject changes).

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This is tricky as bokeh is hard to define. A 35mm lens has the same field of view as a 50mm lens on a cropped sensor, but the focal length is still 35mm. As a longer focal length contributes to a shallower depth of field, any bokeh will be 'reduced' on a 35mm lens compared to a 50mm.

How discernible the difference will be is debatable - certainly less than the difference between a 35mm and, say, a 200mm lens. Other factors also contribute to bokeh such as the number of and construction of the glass elements in the lens and the shape and number of aperture blades.

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I think I understand now - Focal length is same, just the FOV changes, and DOF depends on focal length not on FOV, therefore the 35mm lens will still have less bokeh as compared to 50 mm (on the cropped sensor)? Is this correct? –  TheIndependentAquarius Feb 22 '12 at 11:46
    
This is not correct and is a common myth. DOF depends on field of view. "Depth of field remains the same, regardless of lens focal length, so long as the image size (and f-stop) is the same. There is no point in changing to a shorter focal length lens and moving closer, because if the image size remains the same so will the depth of field." (David Samuelson, 'A Hands-On Manual for Cinematographers', Focal Press, London, second edition, 1998, p.218)" –  vlad259 Feb 22 '12 at 12:24
    
Oops pressed return too early. My source for the above: bluesky-web.com/dofmyth.htm but see also cambridgeincolour.com/tutorials/depth-of-field.htm. (Your sensor size will alter your DoF calculations though.) –  vlad259 Feb 22 '12 at 12:25
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Sorry, to be clearer: "DOF depends on focal length" is a myth. –  vlad259 Feb 22 '12 at 12:32
    
@vlad259 DOF doesn't depend on focal length?? –  TheIndependentAquarius Feb 22 '12 at 12:45
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The answer is almost definitely no, and it has nothing to do with being on a cropped sensor: different lenses have different bokeh characteristics. Bokeh is achieved through the optics and aperture diaphragm blades. Both the 35mm f1.8 and 50mm f1.8 have 7 blades, but their optical formula is different -- the result will be different bokeh.

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correct. "bokeh" depends on the exact optics and lens design, not the recording medium (or at least not exclusively the recording medium, it may play its part as well). –  jwenting Feb 23 '12 at 8:28
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