I wanted to reconcile the standard advice to use a long focal length lens to get a blurry portrait background, with the fact that standing back from the subject increases the depth of field as it's a percentage of the subject to camera distance.
Everything is done on a tripod and with fairly high ISO to keep the shutter speed high. The wall is underexposed, hence the noise.
The experiment shows what's really going on with blurry backgrounds. Standing back and using a longer focal length at the same aperture does not change the depth of field, it just magnifies the blurry background with respect to the size of the subject, so the blur "appears" fuzzier.
I stepped back because the issue is how to control the background with appropriate lens choices given a certain composition one wants. That means to frame the subject the same way each time.
Guest
31-Mar-2005 14:54
Hi Howard Sandler,
I think you get this wrong. Because you have stepped back when using the longer focal lenght thereby counteracting the effect of the longer focal lenght, since shallow DOF is dependent on three factors: Aperture size, focal lenght, and focus distance.