In parts one and two we talked a little bit about the difference between crop sensor cameras and their full frame cousins. Specifically we were asked about the effect on Depth Of Field. The basic conclusion was that sensor size does not affect DOF- when all things are equal.
That means that the same lens, same aperture and same distance from camera to subject yields the same DOF results. But obviously the pictures themselves are quite different. The initial assumption is that the crop frame sensor “zooms” in on the image but that is only partly true. In reality since it only uses the center part of the image it is in fact “cropping” the image but when printed the same size (in this case 800 pixels wide) it effectively zooms the image. Because the “zoom” is done in post it doesn’t have the same effect optically as a true in camera “zoom” would. Let’s have a look. (To recap- all images were shot with a 50mm lens at f4).
Here’s the full frame image again.
Now here is the image from the crop sensor as it would appear in relation.
Looking at the image you can see where the rest of the picture would be captured if we used a full frame. (NOTE- I actually tried to centre the image better than that but I missed. Sorry.) But to illustrate the point even further, let’s look at the two images laid on top of each other. The black and white is the extra picture captured the 5D’s full frame sensor.
So that’s where sensor size doesn’t affect DOF. Now let’s see where it DOES change the result. In order to achieve the same framing as one would get with the full frame 5d, the crop sensor user has to back up. Since one of the main factors of DOF is distance from subject to camera, and we are now changing that distance we can expect the DOF to increase (less blur).
Not only is our DOF affected, our perspective also changes slightly. Another way to achieve similar framing would be to use a wider lens on the crop sensor. For instance the 30mm 1.4 from Sigma is popular as a 50mm equivalent. However, since another main factor of DOF is focal length, using a shorter focal length with also result in a deeper DOF and a different perspective.
In the end the results in DOF don’t appear to be overwhelmingly different so my advice is to be happy with your crop sensor camera and go out there and make some great photos. Remember, it is likely 10 times the camera you were shooting with just a few years ago.
PS- I did shoot one more test, the results of which I will share in the final post in this series. Stay tuned.