depth of field

26 Aug 2006

So, I bought this 50mm/1.8 lens with the intention of taking lots of portraits in low-light conditions. It’s narrow, and it’s a relatively fast lens. A complication that I didn’t anticipate, however, is the super-narrow depth of field at wide aperture. This makes taking a portrait in low light tricky. Take, for example, this picture, taken at f/1.8, 0.013s exposure, 1600 ISO (50mm, obviously):

(Yes, what would an existence on the internet be without at least one picture of a cat, right?) Notice the nose and back of the cat’s head out of focus. That’s a seriously narrow depth of field. You can also see it here, although I managed to make it work there, albeit in a less-interesting picture.

The narrow depth-of-field means when you’ve got this lens wide-open and you’re at a close focal distance that you don’t really try to get someone’s face in focus so much as you try to decide what part of their face you want in focus. Difficult. I know this is photography 101 stuff, but it’s interesting to learn it by accident rather than just reading about it (and probably better).

Naturally if you have more light you can just stop down the lens and increase the exposure time, but when you’re at 1600 ISO and f/1.8 just to get enough light for a decent shot, you just have to work with the depth of field as it exists, I guess.