One might think that photographers don't have much to argue about, but some do! In fact, I've been in the middle of a debate about whether, all other things being equal, sensor size affects depth of field. I've contended that when viewing the same portion of an image shot from the same position with the same lens at the same focal length, the depth of field (DOF), which is the area in front of and behind the focal plane (the area of maximum focus/sharpness) which will appear reasonably sharp, will be identical. The things that affect actual focus relative to the focal plane are focal length of the lens, the aperture opening of the lens, and the distance from the camera to the subject. My contention is that if you compare a cropped-sensor image with an image from a full-frame-sensor that was cropped to the same field of view, you would see no difference in the depth of field. However, this is not the same as seeing the full-frame image at the same viewing size as the cropped-sensor image, because you are shrinking the full-frame image, which skewers the perceived depth of field. It is also not accurate to use a larger focal length lens or to move the camera closer to get the same image in the full-frame sensor as you have in the cropped-sensor image: that would make the actual depth of field with the full-frame image more shallow.

So, my test: I shot a series of images with my Canon 5D (full frame) with a good fixed lens (100mm f/2.8 macro) at several aperture settings: f/2.8, 4, 5.6, 8, and 16. I repeated this with my Canon 30D, 1.6-factor cropped-sensor camera mounted on the same tripod, unmoved, with the same lens, at the same aperture settings. The nearest booklet is about 21" from the sensor plane, the farthest is about 24" away (3 inches apart).

I then cropped the full-frame picture to match the image taken with the cropped sensor, and guess what? If you look at them side-by-side, there is no noticeable difference!

I'm posting six matched pairs here for your edification. See if you can guess which is from the full-frame sensor and which is from the cropped one. After I post the three pairs, I will post the uncropped versions side-by-side so that you can see how "perceived" DOF differs from "actual" DOF as defined by actual pixel values as they represent focus.

I also repeated the captures with a 50mm f/1.4 lens at the same aperture settings to demonstrate how DOF differs when you change the focal length. In this case, when you shorten the focal lenth you widen the DOF.

The software that I use to process my RAW files was Adobe Lightroom. You can see from the pairs that Adobe has a different color interpretation for the two cameras that I didn't take the time to accurately match, Oh well, that has nothing to do with depth of field!