Depth of Field3

Bringing to an end these three days of depth of field blips, we come to the third thing which I mentioned as having a direct bearing on dof. The Focal Length of the lens in use. Assuming that the camera to point of focus distance doesn't change and that the f stop remaind the same, what happens if we use a different lens or a different setting in the zoom range? Here's what happens. Ignore the centre photo for a few minutes. I'll come back to it.

In Dublin today, I came across my first daffodil of the year in Stephen's Green. Another few days and there will be hundreds of them. In celebration I decided that it would be my blip for the day despite already having shot a couple of others.

In the left hand shot I used a shutter speed of 1/90th sec at f8 with the zoom set at 70mm. You will notice that the depth of field extends from roughly the edge of the path, to a point a couple of feet beyond the daffodil. A distance of about 5 feet.

In the right hand shot I stayed in exactly the same position and again shooting at 1/90th sec at f8 I racked the zoom out to 240mm. Look what has happened to the dof. The clump of green daffodil leaves in front of the flowering clump is no longer sharp and the dof doesnt extend over any more than about two feet now.

The only thing which has changed has been the focal length. Remember rule 3? The longer the focal length, the shallower the dof and the shorter the focal length the greater the dof. Here you have the photographic proof.

Oh yes, the centre photo. Well this is simply a crop of the left hand one. If you find that one on the left too small to see what I am talking about, I have pulled it up to roughly the same size as the one on the right See what I mean. I don't intend to do any more of these demo shots but I may just come back to the subject in words tomorrow just to mention a couple of other matters to watch out for concerning dof.