We all know two things about DoF: a) it gets shallower with longer lenses and b) it gets shallower as you get closer to the subject. Three things! We know three things! c) DoF gets shallower with lower f-number.

NOTE: I will NOT be talking about different sensor sizes and equivalence. You can all relax now.

One curious claim, though, is that a) and b) almost exactly cancel out if you make the *subject* the same size in each picture. Take a head shot with a 250mm lens at f/4 and you'll have the same DoF as the same headshot taken with a 120mm lens at f/4 on the same size sensor. Of course, the 120mm lens will have to be half the distance from the subject to get the same head size.

This can be seen from the formulas, or from geometric optics (ray tracing), but there's nothing like a demonstration! Between my own face and a Fuji lens, I picked the more attractive subject. Here are two shots at f/4 with the 250/4 and the 120/4, both wide open, and both cropped the same size.

250/4

120/4

Of course, the pictures are different, and the subject geometry is different, but it's about the same size in both pics, and the DoF, visible on the tape measure, is roughly equal.

Here are the same at f/8:

250/4 @f/8

120/4 @f/8

Now modern lenses are not classical thin lenses, so I wasn't sure that this would hold at all. I just always wanted to do this experiment. It looks pretty good.

Yes, there is nothing MF about this, except that I used a GFX 100 for the shots. I would have posted it to the Boring Lecture forum, but there isn't one. I should probably start it. Tired of Jim Kasson's careful measurements? Come to the Boring Lecture forum!

Thanks,

Matt