Depth of field can be a somewhat confusing topic to get to grips with if you’re new to photography. That is to say, it’s easy to see what depth of field is, but it can be tricky to understand what elements can cause it to change, whether intentionally or by accident. In this video, photographer Kellan Reck takes a look at depth of field and explains the variables that can affect your depth of field.
Aperture
This is the one that most people become familiar with first. It’s why so many people lust after those f/1.4 prime lenses to be able to get as shallow a depth of field as possible. But what is the aperture? Essentially, it’s the opening in your lens through which light passes on the way to hitting your sensor.
Wider apertures have smaller numbers and offer a shallower depth of field (less of your scene is in focus). Narrow apertures have higher numbers and give you a deeper depth of field (more of your scene is in focus).
Subject distance
The distance of your subject to the camera also plays a huge impact on your depth of field. The closer your subject is, the shallower your depth of field becomes, even if your aperture does not change. This is why many macro photographers resort to stacking images, because even at f/32, you may not get your entire subject in focus if it’s three inches from the end of the lens.
This is one area where the race to create higher megapixel cameras can actually be beneficial. It allows you to back up, and achieve more depth of field while still offering enough resolution to crop down into your subject.
This is why you might need to be stopped all the way down to f/8 or f/11 to get a good headshot of a single subject with their whole face in focus, yet shoot f/4 for larger groups of people where you’re standing much further back. Yes, f/4 is a wider aperture and should produce a shallower depth of field, but when you move away from your subject, you increase that depth of field again.
Focal length
Focal length as to how it might affect depth of field is a somewhat controversial topic. The idea goes that the wider the lens, the more depth of field you get. But do you, really? In theory, from a physics standpoint, the focal length of a lens shouldn’t affect the depth of field. It simply scales the view up or down (including all of the out of focus areas). But, when you scale down those out of focus areas, it can bring back the impression of sharpness and detail to our eyes. Exactly how much detail will often depend on the resolving power of your sensor or film stock (and how good your eyes are).
With a sensor of infinite resolution and optically perfect glass, yes, scaling up a wide shot to match a telephoto should, theoretically, reveal the same level of depth of field blur. But we don’t have infinite resolution sensors nor optically perfect glass, and there is a point at which those scaled down out of focus areas appear sharper to our eyes. So, it could be argued that focal length, does, to some degree, alter our perception of depth of field, even if it might not change it from a pure optical physics standpoint. Anyway, I’ll leave you guys to fight that one out in the comments.
