Circles of confusion, and circles of the confused

When some folks who have been full frame camera users move to medium format cameras, they suffer from confusion about depth of field (DOF). When this confusion spreads to a group of people they become a circle of the confused (CotC). This post is intended to reduce the size of the CotC.

Depth of field tables are computed based upon geometric optics and a threshold for acceptable sharpness called the circle of confusion (CoC). Depth of field calculators depend similarly upon specifying the CoC, Good DOF calculators allow the user to input the CoC. If yours doesn’t, get rid of it and get one that allows that.

When you change from a smaller format to a larger one, you need to examine your previous assumptions about DOF. A simple way to do that is to pick an aspect ratio and write down the height of the image on the sensor in millimeters that corresponds to the largest image you can make with that aspect ratio.

For full frame format, and a 4:3 aspect ratio, the height is 24 mm.

Then do the same for your medium format cameras. For 44x33mm format, and a 4:3 aspect ratio, the height is 33 mm.

Compute the ratio of the two, with the MF camera in the numerator. That gives you 33/24 or 1.375. You can use that number in two ways. If you want to find the focal length on your new MF camera that gives you the same angle of view as x mm on a FF camera, multiply the FF focal length by the above ratio.

Example: you want the focal length for your MF camera that gives the same angle of view as a 50 mm lens on a FF camera. That is 50*1.375 or 68.75 mm.

When you put a 69mm lens on your MF camera, what aperture do you need to have the same DOF as the FF camera with a 50mm lens, assuming same print size, same subject distance, same viewing distance, and same 4:3 aspect ratio? The FF aperture times 1.375.

Example: you want the same DOF as you’d get at f/8 on your FF camera. 8*1.375 is f/11. Whoa! That’s a one stop difference. Does that one stop difference obtain at any FF aperture? Indeed it does.

So, and I’m going to put this in bold: with the same angle of view and a 4:3 image aspect ratio, you need to stop down the lens on the MF camera by one stop to get the same DOF as the aperture you’d be using on a FF camera, assuming same print size, same subject distance, and same viewing distance.

Simple, huh? Done and dusted? Not so fast.

Presumably, you went to MF to get some things you couldn’t get with a FF camera. Quite possibly two of those things were dynamic range and resolution improvements.

If you want to take full advantage of the possibilities in those areas, you’ll have to move beyond the above calculations. Accurate as they are, they don’t tell the whole story.

Let’s deal with dynamic range first.

If you replace your 50mm, f/8 full frame setup with a 69 mm f/11 medium format one, the sensors are equally sensitive, and you use the same shutter speed on the two cameras, you won’t gain much, if any, dynamic range. To get the most dynamic range out of you MF camera, you’ll have to halve the shutter speed (double the amount of time the shutter is open).

Example: f/8 at 1/125 on a FF camera, and f/11 at 1/60 on the MF camera.

If there’s subject movement that keeps you from selecting the slower shutter speed, and you shoot the MF camera at f/11 at 1/125, you’ll get about the same DR in both cases.

Here’s another bold point: to get full dynamic range advantage of a larger format camera requires a longer shutter speed if the light is uncontrolled, and more light if you can control the light. It was always that way, even back in the film days. You needed a lot more watt-seconds in your studio strobe power pack if you were shooting with an 8×10 camera than with a 6×6 one.

Next up, resolution.

Maybe you went to medium format to get more resolution. I know that was a big reason for me. More resolution allows you to make sharper images. But the above DOF calculations assumed you wanted the same amount of blur threshold in same size prints from the two different cameras. And now, because you’ve got a higher resolution camera, you are getting greedy about sharpness. You won’t tolerate the same fuzziness at the near and far edges of what you want in focus as you did with your FF camera. So, to keep from being in the CotC, you need to make your CoC smaller, which will mean you have to stop down more.

The ins and outs of picking CoC size is beyond the scope of this post, but, if you’re hyper-critical about sharpness, you’ll want a CoC that’s not much bigger than the pixel pitch. You’ll find that that causes your DOF calculator to cough up very shallow DOF numbers. There’s a downside to resolution greed.

Addition: diffraction

I thought this would be more complete if I added a word about diffraction.

Equivalent apertures yield the same amount of diffraction blur on a same-sized print. So you can get the same DOF and diffraction blur with your FF and MF camera. But, as with the CoC criteria, that’s probably not what you want. If you were happy with the same blur for same-sized prints, you probably wouldn’t have gone to MF in the first place. That drives you to open up the lens from the equivalent aperture to minimize diffraction. But that will reduce DOF. Now you’re between a rock and a hard place. That’s why view camera movements and focus bracketing are more important as the format size increases.