Depth of Field1

I spent a couple of glorious hours down by the sea this morning and came back with several candidates for today's blip. Then I got to thinking.

Arising out of my focus stacked butterfly of a couple of days ago, several people have asked for help in understanding the concept of altering depth of field (dof) at the shooting stage. I have even been receiving and sending e.mails on the subject. So, I thought why not show an actual sample today.

There are three factors which determine the dof in any given shot.

1. The focal length of the lens. The longer the focal length (ie a telephoto) the shallower will be the dof and the shorter the focal length (wideangle) the greater the dof.

2. The Aperture in use. The wider the aperture the shallower will be the dof and the smaller the aperture you use the greater the dof will be.

3. The Focused Distance. The closer the focused object is to the lens, the shallower the dof will be and the further away the sublect is then the greater the dof will be.

All three above are dependant on all other things being equal. In other words you do not alter any of the other settings. By juggling the above together, you ought to be able to do pretty well anything with the dof. This of course is one of the great advantages in having a DSLR and a collection of lenses.

So, looking at today's blip.

Wach picture was set at the same exposure setting of 1/250th sec. at f2.8. The camera was tripod mounted, so it didn't move at all and the same lens was used for all three shots : a Canon 50mm f1.8. This lens is the equivalent of 80mm focal length when used on my DSLR so therefore is a moderate telephoto. The ONLY thing which changed with each shot was THE FOCUSED DISTANCE, as mentioned in point 3 above. On the left the nearest chess piece was focused on. In the centre shot it was the centre piece and in the right hand shot it was obxiously the furthest piece. Note carefully what has happened and look at the squares on the board too. Three entirely different photos, same position, same focal length of lens and same aperture. Only the point of sharpest focus has changes. I switched off the auto focus by the way to avoid thelens choosing its own focus point by accident. All focusing was done manually. Convinced?

I might follow this up with some more examples over the next few days.