Depth of field
QIf I’m using a 5x4 camera with a 2 square back on it, will I get the depth of field for a 5x4 or for a 2 x 2 format? Logic tells me that I should have the same DOF as if I was using 5x4 film instead of medium format. I keep coming back to the thought that if I take a picture on a sheet of 10x8 film and keep cutting the neg down, and down and down, then by the time I’ve cropped the negative down to half an inch across, the depth of field in that half inch should be almost infinite. Fishboy (AP forum)
ADepth of field is directly dependent upon the film or sensor format, so if you use a 5x4 camera with a 2 square back, you’ll get exactly the same depth of field as if you were using a conventional medium-format body shooting 2 square exposures. The reason is all down to how the concept is defined. Depth of field, or the distance in front of and behind a focused subject that appears acceptably sharp, can be calculated using a formula based on subject distance, sensor size, focal length and aperture setting. But what is sharp and what isn’t is also a function of how the image is viewed; if you view the image as a smaller print or from further away, more of it will appear to be in acceptable focus. So a key assumption when making the depth- offield calculation is that you’ll be viewing an image at a specific size from a specific distance. Otherwise, you won’t be comparing like for like. As a result, in your comparison of taking a negative and cutting it down and down, you need to add a second stage, of making prints that are all of the same size. With this additional enlargement of the smaller crops, parts of the image that aren’t in the plane of sharp focus will appear more blurred; in other words, the depth of field will be decreased. Andy Westlake