My instructor lectured about perspective and depth of field yesterday, and while I had already investigated the effects of aperture on depth of field and focal length on perspective, one thing that he mentioned (and I didn’t really believe) was that focal length also affects depth of field. Ok, so to investigate this for myself, I pulled out the trusty set of zoom lenses and took some pictures. This time around, I kept the camera at a constant distance from the object, and cropped the resulting images so that the subjects take the same amount of space in each. For all of these pictures, I used an aperture of f/8 and speed of 1/60th, with an off-camera flash bounced off the ceiling for constant light.
It is reassuring to see that the apparent size of the objects (perspective) isn’t affected by this, as I ‘know’ that to be determined by the subject-to-camera distance. There is, however, a fairly noticeable change in the depth of field between the pictures. In the largest zoom (200mm), the books in the background are completely out of focus. Switching the lens focal length down to 35mm sharpens them right up. Note that there is a limit to how useful this can be for expanding depth of field, because when you crop an image, you end up loosing sensor (or film grain) resolution. This is especially noticeable in the 18mm version, which had to be blown up to cover the same subject size as the 200mm version. Of course, a higher-resolution sensor would help here as well :-).
Use the prev and next controls to jump to the next setting, or press play to see a slideshow of the entire range of settings for the control.
As a side note, this explains why the 85mm lens (drool) that I tested in a store gave me such a cool shallow depth-of-field that I couldn’t reproduce with the 50mm f/1.8.
Note: Once again, thanks to Patrick Fitzgerald for creating the handy Slideshow Wizard, which is what I used to create this example!