Depth of field is something that is fun to play with. It can help you create stunning photos when you have control over your depth of field. Sometimes I have a hard time remembering how to narrow the depth of field, but there is a simple way that I try to remember. I had to write it down, and I think I have it now. While there are calculators out there for figuring this stuff out accurately, at this point I’ll happily live within the realm of “close enough.” In my electronics studies, I like to live in the “10% rule” as suggested by the book “The Art of Electronics” by Horowitz and Hill.
In photography, there is a lot to know, and I am not going to proclaim that I am an expert. There are those with educations in photography that can tell you much more about how things work. For me, just knowing what the results will be is good enough. If you want to know the math behind all of this, there are great and popular sites that can help with that.
Depth of field is controlled by three variables: focal length, f-stop, and subject distance. Two of these are proportional to depth of field: f-stop and subject distance. Focal length is inversely proportional to the depth of field. Finally, I’d like to note that an increasing f-stop number is equal to a decrease in the aperture.
an increase in focal length = a decrease in DOF
an increase in f-stop = an increase in DOF or a decrease in aperture size = an increase in DOF
an increase in subject distance = an increase in DOF
Now, it’s a great time for some examples…
I’d like to add that you can create some extremely shallow depths of field by having a large focal length, small f-stop (large aperture), and be as close to your subject as the lens will allow a focus.
This image was taken inside with a flash with the subject being less than 3 feet from my desk. You can see that the depth of field is so shallow that even Mr. Penguin begins to blur as his body slopes away from the camera. The opposite holds true if you decrease the focal length, increase the f-stop, and increase the subject distance. Ignoring the focal length and subject distance (assume you have a prime lens and you can’t change the distance between you and the subject), the following phrase is for you: “f/8 and be there” – Arthur “Weegee” Fellig.