In part one of this series we talked a little about the physical differences between a full frame sensor and a “cropped” sensor. Now let’s see what happens where the rubber meets the road.
This is where it gets a little complicated and the arguments start. For the sake of simplicity we will only deal with “full frame lenses” and not “cropped sensor lenses”. After reading the first post some of you may have wondered aloud, “If the sensor only uses a small part of the circle, then why not make a smaller lens with a smaller circle?” Well, they do. Canon, for example, makes a few lenses called EF-S that are designed specifically for the Crop Sensor cameras and are meant to be cheaper. The downside is that they do not fit on a full frame camera. In the interest of trying to compare apples with apples (if you are offended by that reference you are just looking for trouble) we will only compare full frame EF lenses.
The original listener question asked if there was a difference in the DOF when shooting with a 50mm 1.8 on a Crop Sensor camera vs a Full Frame. The answer is “NO, there is no difference.” Wait a minute. Sort of. So, “YES, there is a difference”.
Since I own a 40D (crop), a 5D mkII (full frame) and a 50mm f1.4 lens I decided to put it to the test.
Let’s deal with the “NO DIFFERENCE IN DOF” answer first. In order to demonstrate the difference I set up the shot with as many constants as possible- the cameras mounted on a tripod so the distance to subject is the same, aperture set to F4, shutter to 160, WB to Daylight, and the same 50mm lens for both shots. The only difference is the two cameras. Let’s compare the result-
Ignoring the obvious, let’s focus our attention solely on the DOF. To my eye it looks close enough to be called identical. And so it should, nothing has changed optically. Since the DOF is a function of the aperture, focal length, and distance from the camera and those values are identical in each shot we achieve the same DOF.
Remember though, that I did say it DOES affect our DOF. That part is still true but to illustrate we need to explore what is causing the obvious differences in these two shots. We’ll do that in part 3.