What happens to bokeh when cropping?

…is a question we’re increasingly asking when comparing different cameras, because the pixel dimensions of the resulting image often allow significant cropping while maintaining image quality. After all, that was the part of inspiration for creating the Sony RX1 series. Since then, pixel counts have doubled again.

CREATOR: gd-jpeg v1.0 (using IJG JPEG v62), quality = 95

Of course, the intuitive and simple answer to the question is, background blur (sometimes, albeit not quite correctly, called “bokeh”) gets magnified by the extent of the crop. But what if we’re looking at two different cameras and comparing them? Let’s say we have (a) a 16 MP camera with a 35mm fixed lens in 135 film terms, and (b) a 24 MP 28mm (similarly equivalent) fixed lens camera. Both have the same sensor size and an f/2 lens.

Why might we be interested in comparing these two cameras and their crops? For instance, because both would be capable (if we do the maths) of making an approximately 8 megapixel image when cropped to 50mm equivalent.

But what of the bokeh? For understanding this, we need to work with the actual aperture size, not its relation to focal length. 28mm/2 = 14mm, while 35mm/2 = 17.5mm. In this case, we should expect “bokeh balls” to have a 25% larger diameter in the image derived from the 35mm camera – and this, of course, depends on cropping to the same equivalent focal length. The megapixel size of either the original or resulting image is not relevant.

But the megapixels do play a role in a different way: the camera with lower resolution gathers more light per pixel in the resulting crop. So without taking into account that a stronger crop will put more demands on the quality of the lens, we can already see that the more moderate crop would, from first principles, be at an advantage.

So as long as the actual physical aperture is the same absolute size, you should expect a roughly similar image. For a 28mm lens to match a 35mm/2.0, it would need to have the same absolute physical aperture diameter, i.e. 17.5mm, making that a 28mm/1.6 lens.

So, in conclusion, at the same f-stop, lenses with greater focal length have a bokeh advantage. Cropping a 28mm f/2 image will NOT get you the equivalent of a 50mm f/2 image. Rather, the f-stop needs to be modified to take into account the focal length difference of the two lenses. In spite of this, cropping will of course magnify the existing background blur.

A small footnote is necessary: While greater focal length could be described as “more zoomed”, many lenses available in the market today (whether fixed on a camera or interchangeable) do NOT maintain their f-stop as you zoom in and depending on the exact technical details of the lens, you may not in fact get a bokeh advantage from zooming in!

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