This is kind of an informative piece about the photographic principle of Depth of Field that has a lot of implications in how your image turns out. This is not a definitive guide, and there are probably better ones out there… But I got to thinking about trying to describe it in this forum after shooting with my wife last weekend.
In my opinion, the biggest thing setting apart Point-and-Shoot cameras and DSLR / SLR cameras is Depth of field. To be clear, depth of field is referring to the portion of an image that is in focus. Shallow depth of field images have very narrow areas of focus. Infinite or unlimited depth of field refers to images where everything is in focus.
Here are two images, one showing a narrow depth of field and one showing an infinite Depth of Field. Click on them for much larger representations without text on them…
The main purpose of creating a Depth of Field in a photograph is to create images with very specific elements in focus. Or if you’re being all artsy about it, it’s also about what’s out of focus.
Depth of Field is most effected by many things depending on the complexity you want to read into it… But it’s easiest to concentrate on the three main variables:
- Aperture
- Distances
- Focal Length
Aperture
This is the amount of light being let onto the sensor / film that is designated by the “F Stop” of the lens. In terms of depth of field, the more light being let onto the sensor, the shallower the depth of field. Unfortunately it’s a little confusing in that if you want to let more light in with your lens, you need to set your aperture lower… An aperture setting of f/1.8 lets more light in than f/5.6… The images above were taken at the same lighting conditions, with the same lens, on the same camera – but with different aperture settings…
The image on the right of the lens with the cap off shows the aperture blades set at around f/1.8 on my 50mm lens. When cranked up to f/22, you can’t even tell there’s anything there… If you have a DSLR with a kit lens, there’s a higher probability that you have no aperture ring to adjust these things manually to see how they work. The picture below of my 20mm lens shows the aperture ring (it has the aperture values along the part of the lens that would be closest to the body).
Distances
This element is simple in that it only concerns with the distances between objects and the camera. This variable is very much tied to the other variables, but it’s important to recognize.
Here’s a simple example. Let’s say you’re taking a picture of inline objects. With a shallow depth of field you’ll see that the items get more blurry as they get further away from the focal plane.
Focal Length
This is the distance between the lens and the sensor / film. Generally speaking, the longer the focal length – the shallower the depth of field. Conversely, the smaller the focal length – the more likely that the area of focus will be very large (and infinite in many cases).
Ever notice that as you focus that the lens actually changes its length? This directly impacts the whole notion of depth of field, and some lenses illustrate this precisely. Take a look at the picture of my 20mm lens below. It shows a little window that points to values that change when you dial in your focus. This value is the distance to which you can expect the object to be in focus. Unfortunately this is only a feature of higher end lenses.
Wrapping it Up
Now to bring those three together… Any one of these three elements can produce whichever depth of field you’re going for. Combining all three in your shot composition is even more powerful.
Other things to keep in mind
Since depth of field is a result of multiple factors, you must also realize that what you see in the viewfinder is not always what you get. Changing the aperture does not affect what you see through the eyepiece in a DSLR / SLR – but it makes a world of difference what is captured on the sensor / film.
Can you produce depth of field from point-and-shoot cameras? Yes. But the limiting factor is the aperture range and focal length because most point-and-shoot cameras are made to be smaller and more cost effective than DSLR / SLR. I’ve personally had the best results of getting depth of field out of point-and-shoot cameras when using the macro mode and taking pictures up close (mostly taking advantage of a disparity of distance between what I’m focusing on and the background).
The easiest way to achieve this is to set your camera to aperture priority (this is mostly a DSLR thing, but some point-and-shoot cameras can do it). If you’re looking for a shallower focal area, then set your priority to the lowest setting your lens / camera will allow (somewhere in the F/1.4 to 5.6 range). If you want everything to be in focus, then set your aperture as high as it will go (somewhere in the F/18 and up area – but beware of this producing blurry images from slow shutter speeds – tripods are handy).
But I can’t say how nice it is to have everything digital so you can take a picture and see full-sized results in seconds. You can learn this so much more quickly than before. The best advice by far is to set aside some time with what you want to do and just play around.
If you want to go further, look into the alternate, but related, topics of bokeh and tilt-shift…