Bokeh the Fifth

Let me get this out of the way right off the bat: to the best of my knowledge bokeh isn’t directly affected by sensor size. I say “to the best of my knowledge” because, despite popular opinion, I’m not all crazy omniscient (crazy, yes, omniscient, no).

“But,” you say, “Derek! You told us way back when that sensor size affects bokeh!”

Why, so I did, good job paying attention. Here’s the thing: remember our lesson on Depth of Field? And I told you sensor size was a part of it? Aha, now we’re getting to the point.

So, again, there’s a lot of math involved in this (which I am definitely not qualified to talk about, but if you want an astoundingly complete and rational essay on the effects of sensor crop on various aspects of an image I will point you to my favorite reference page here), but what you basically need to know is that depth of field has what would be called an “inverse correlation” to sensor size. Basically, the smaller the sensor, the deeper the depth of field.

OK. Grab a Nikon digital SLR camera (like the new D90). It’s a DX, which has a crop factor of 1.5. Once again, we’ll compare things to the bad-old standard of 35mm. You used to shoot at 50mm f5.6 all the time on your film camera. But, now you have the D90. So, remembering what we said about field of view, you look through your Nikon lenses and you pick up that new 18-55 VR and rack it out to about 33mm (because now you know that’ll give you a field of view the same as your 50mm did) and you set your aperture to 5.6. You snap the shot and… well, and there’s less out-of-focus area in the shot then the one you shot with your 50mm. As a matter of fact, it looks pretty similar to about f8 on your old camera. That’s because of the 1.5x crop factor on the D90. If you take 5.6 x 1.5 you get a value 8.4. If you want to imitate the same DOF as the old 5.6, you have to divide it by the crop factor to get the aperture you’d need now (5.6 / 1.5 = 3.7, by the way. f3.7 on your D90 will have the same amount in focus as your film camera did at f5.6)

Kinda like with the field of view, this “conversion” is just for the depth of field. The lenes still collects as much light as an f5.6. There’s an oft-made argument that this is a crop-sensor DOF “advantage”, because it has the DOF of a slower aperture at a faster one. This is countered by it being hard to get very shallow DOF (which helps with bokeh, less we forget why I’m even on this topic at all), and by the fact that smaller sensors have more problems dealing with noise management. It is the opinion of this blogger that it can’t be called an advantage or a disadvantage, it’s just a fact that you should be aware of so you can work around it.

OK! More examples, to make sure you got it. My beloved Olympus digital cameras, the E system, have a 2x crop factor. If I shoot f5.6 I get the DOF of F 11 (5.6 x 2 = 11.2) on a 35mm system. Pick up a 1D series Canon digital SLR camera and you’ll find a 1.3x crop factor, meaning f5.6 is closer to f7 (5.6 x 1.3 = 7.28). Getting the hang of it?

Those formulas again, real quick, for reference:

Going form crop-sensor to 35mm: aperture x crop factor
Going from 35mm to crop-sensor: aperture / crop factor.

To tie it back down to bokeh. We know that wider apertures give better bokeh, simply because less is in focus. If you want even more soft bokeh, then you might consider getting a system with a larger system (the so-called “full frames” like the A900, 5D Mk II, and the D700.) If you hear someone say that full-frame cameras have film-like bokeh (which is what started these articles in the first place), this is what they mean. F2.8 on full frame sensors looks just like the f2.8 did in film SLRs. Does this mean your Canon Digital Rebel camera can’t get that same bokeh? Certainly no, but you’ll need a lens that can open up to f1.8. Just something you should be aware of.