Crop Factors, Depth of Field, and Bokeh

ewong — Thu, 21 Jun 2007 07:27:05 +0000

It’s hard not to come across some mention about sensor-sizes and crop-factors these days when reading about DSLR’s. The commonly discussed pros and cons of different sensor-sizes usually revolve around resolution, low light sensitivity, and reach. But one thing I’ve always wondered is, would two photographs taken by two cameras with different sensor-sizes but the same effective focal-length look the same? The answer came to me the other day when my friend and I were geeking-out about camera gear, and he mentioned to me that depth-of-field is actually a function of focal-length (among many other things). It then clicked that maybe images taken with smaller sensors and shorter lenses (due to the crop-factor) might have a larger depth-of-field and thus less of that much sought-after creamy bokeh.

So to test the theory, I took a few quick photos at various focal-lengths and did a little Photoshop’ing of the resulting images to simulate various crop-factors. Unfortunately, I didn’t have a full-frame camera to test with, but regardless, I was pretty surprised by the results. The difference between the bokeh of the 1.3x and 1.6x crop-factor images was subtle, but definitely noticeable when compared side-by-side. Going down to the 4.1x crop-factor sensor (to simulate an image taken with my old Sony F707), the difference in depth-of-field and bokeh was really quite staggering.

Interestingly, the experiment answered another thing I’ve always wondered about, which is why point-and-shoot digital cameras don’t produce photos with that same creamy bokeh that DSLR’s do. It turns out that it’s not necessarily due to the fact that the point-and-shoots don’t have fast apertures (my Sony F707 was F2.0 at its widest), but more just that the small cameras have very small sensors and use lenses with very short focal-lengths.

AgentWong

Crop Factors, Depth of Field, and Bokeh