I'm curious about how my "area of acceptable focus" changes when the focal length of the lens I'm using changes as I zoom (or switch lenses). In particular, I'd like to know how the front & back focal planes change, thus changing the depth of field and the minimum focus distance.
While it is a fact that changing focal length from shorter to longer reduces DOF and using a smaller (less light) aperture in will increase DOF (providing format is identical) however there is a simpler way to think of it.
DOF decreases the larger the subject is in the frame regardless of the lens and increases with smaller apertures.
Example: If you shoot the same photo, say a headshot, with a 200mm lens and, at the same distance, with a 35mm lens. Then take the image from the 35mm and crop it to match the image from the 200mm you will find the DOF/image identical.
Of course this is an example assuming that the resolution would not be factor. Which is WHY we change lenses and don't just crop.
Depth of field is a function of the relationship between image magnification and diaphragm opening.
Lens focal length has nothing to do with depth of field.
The misconception arises because, from a given subject-camera distance, a short focal length lens gives a smaller degree of image magnification and consequently more depth at a given distance. The depth comes from the image size, and not from the shorter lens. If the images are of like magnification and the f-stop is identical, then the depth of field is identical, regardless of focal length.
There is tool to calculate DOF by putting Focal length and f-stop here: http://www.dofmaster.com/dofjs.html
With result you can create an interesting graph
If you stand still
The depth of field quickly gets narrower as you zoom in.
If you keep subject magnification constant
If the depth of field is large (comparable to the focusing distance), then it gets somewhat narrower when increasing focal length. If it is already narrow, then it is practically independent of focal length.
Front and back depth of field
When it's narrow, the depth of field is practically symmetric relative to the plane of best focus. As it gets wider, and specifically as it reaches the order of magnitude of the subject distance, it gets more and more asymmetric (more depth of field behind the subject than in front of it). At one point it reaches infinity, then things are sharp from half the focus distance up to infinity.
A simple rule that is probably more useful than my previous paragraph: the depth of field is always practically symmetric when read from the lens' focus scale.
Usually this question is asked primarily in terms of "how do I make the background more blurred relative to my subject". That question is answered in depth at http://www.bluesky-web.com/dofmyth.htm for instance. The tl;dr of it is:
- Assuming you have the same aperture, taking a photo of the subject from closer with a wider lens, or from further away with a shorter lens, will not change how blurred the background objects are relative to themselves. For instance, if a tree is visible in the background and you can clearly make out its leaves with a wider/closer shot, you will also clearly make out its leaves with a telephoto/distance shot. However:
- Taking a picture with a wider lens, closer to the subject, makes the background objects much smaller, so a blur which was very noticeable in the telephoto picture will be much less noticeable in the wide angle. At the same time:
- If you have consumer-grade gear, you will likely be able to use a much more "open" aperture in the wide end of the zoom than in the tele end of the zoom, so the effective blur illusion with the wide end may be the same or better than that with the telephoto end.
Specifically with regards to "what is in focus" rather than "what is distracting", if you are using a telephoto lens which causes your subject to fit top to bottom in the frame, your depth of acceptable focus off that focal plane will generally be the same as if you compose the same shot in a wide angle. That is, if the ears are out of focus in one shot they will also be out of focus in the other (they will just appear smaller, and the nose bigger, with the wide angle).
Which brings me to the unsolicited piece of advice: choose your zoom for the desired distortion, not for focus, unless that zoom stops you from using the proper aperture (ex, on a consumer-grade lens which is f/3.6 at 70mm and f/5.6 at 300mm). Telephoto zooms are often used in portrait photography because they make the face appear "smaller" which is generally considered more attractive. The "background blur" effect is also beneficial here in that everything behind the focal plane is made "bigger", which almost by definition is a less cluttered background.