Depth of field and point of focus - how they work together.

From left to right:  135mm FX body at f/1.8, f/8, and f/16

If you haven't looked at the above image in full size (it is a HD image), you will be missing something of what I am about to say.  There are three factors that you have control over regarding depth of field on your camera, and all of them have to do with your lens.  These include aperture, focal length, and point of focus.

I use the above images to display the relationship between depth of field and point of focus.  In all three of the above pictures, I focused on the second bollard (post) but changed the aperture throughout (I have seven shots, each having the aperture used a stop apart).  You can see clearly that the first image has very little depth of field while the last is sharp from the first post all the way to the gangway.

As the aperture gets smaller, depth of field increases.  The point here though is to look at HOW the depth of field changes.  Clearly, it is not equidistant from the point of focus.  After the point of focus the distance in focus increases almost exponentially while before the point of focus there are moderate gains.  The relationship is about 1/3 increase before the point of focus and 2/3 increase after.

This has many implications for the photographer, especially if you do landscapes and are trying to get as much depth of field as possible.  Hyperfocal focusing is an excellent technique to master; I have written many blogs on the subject before.  You can search for them using the word "hyperfocal".

I was out today with my new 135mm f/1.8 lens and will be writing about it in the next few blogs.  I got some nice shots.

Thanks for reading.

Eric Svendsen     www.ericspix.com