Part 1 - DOF and Aperture

If you haven't read my photography disclaimer, take a look.

Before we start, here are links to one or two external DOF articles:

http://www.mir.com.my/rb/photography/fototech/htmls/depth.html

This link shows some of the math behind the graphs that I plot here. I find it useful to see the graphs plotted in this article to visualize these relationships. You may find the tables they have handy.
http://www.conent.com/ConAdv/Encyclopaedia/Photography/CNQ_CAPhotography001.asp 

Continuing from where we left of (Part 1 - DOF and Aperture), lets take a look at the relationship between DOF and distance to the object being focused on a little more closely. In the previous graphs we could get some insight into this relationship. Here we will plot distance on the x-axis and DOF on the y-axis.

The two graphs above show the DOF changes for the fixed aperture value f/4 and focal length of 50mm. The first graph shows the variation of DOF over a relatively short distances of <5m. The second graph shows how this varies over greater distances and show the non-linearities in the DOF when greater focusing distances are involved. From a distances of about 20m DOF start shooting up and by ~30m you get infinite DOF.

Lets throw in some more aperture values and see what we get. f/1/4, f/1.8, f/2.8, f/4.0, f/5.6

Nothing very interesting happening with the shorter distances. The larger distances show the larger fstop numbers curving off sooner. This is consistent with the idea that the greater the aperture (i.e. lesser the f-stop value) the lesser the DOF. If you have a lens that can provide f/1.4 you get a rather nice DOF control even upto distances of about 50m! The Canon 50mm f/1.4 prime that costs about 300$ should do that. The much cheaper Canon 50mm f/1.8 prime would be expected to follow the green curve - rather sweet for a 80$ lens.

Lets look at some larger f-stop values.

At f/22 even in distances under 5m we see the DOF curving up. From roughly 5m to 20m for the aperture range of f/5.6 to f/22 we start to get infinite DOF.

Now lets vary the focal length a bit, starting with some wide shots.

These are plotted for a f/4 aperture. Within 5m each of these DOF graphs curve upward to infinity. At 10mm, if you are focusing on something that a little over a meter away you get infinite DOF! If you are wondering how 10mm is relevant, the Rebel line of cameras support the EFS mount and currently the widest (non-fisheye) lens you can get if the Canon 10-22mm lens - its a beauty.

Lets look at some medium range lengths.

25mm curves upward at roughly 6m. The longer the focal length, the narrower the DOF. A 50mm would curve upward at about 30 meters.

At small distances all these focal length's are very well behaved. You get very narrow DOF and with a good lens, you should get a great bokeh! Each of these focal lengths only reluctantly yield to large DOFs. So if you need to zoom into something thats 50m away and need an infinite DOF, you need to stop down the aperture. :)

There you go. The graphs should show roughly the real world values that you should expect to see with your equipment. You might want to take some approximate readings off these graphs and then go out in the field and see how these values work for you.

In the next part (whenever I get down to writing it), we'll plot the remaining combination  - DOF against focal length.