Originally posted by Frogfish ...
a) even from the test shot posted there is a distinguishable difference in the DoF. At both ends of the range !
I'm happy the results came out as good as they did. DOF tests are not easy to do with limited equipment.
I think it is fair to say the result is consistent with the prediction that DoF is independent of the lens used and depends on actual f-number and magnification. The result is not consistent with the prediction that using a Raynox causes a very small DOF.
Quote: b) when you add a diopter / Raynox you have indeed changed the FL so a test based on a DoF analysis at the same FL (with and then without the Raynox) is actually not a real life scenario.
I'm not sure what you mean, I used widely different focal lengths to emphasize the point that focal length isn't the main variable involved. Do you want a test where one lens is a 50mm macro lens and the other lens has been turned into a 50mm by adding a Raynox?
Quote: In practice the DoF is much narrower because the FL has changed substantially.
That's not the whole story; the FL has changed causing the f-stop to change.
It is the f-stop change that results in the DoF change. The DoF can be modified by turning the aperture ring if you like.
This is an important point. When a Raynox or other diopter is used to increase a lens' magnification to m, the f-stops are also changed by a factor of 1/(1+m).
Say that a Raynox 250 (125mm) is added to a 125mm, F16 lens; that lens is changed to a 62.5mm, F8 lens (the "16" mark on the lens barrel now means "8")*.
Quote: I do appreciate you doing these tests though - I'm learning a lot from you.
Thanks, I enjoy trying to make theory useful.
*edit: this may in theory limit the maximum DoF obtainable. For example, maximum f-stop of the DA 55-300 is F:22 - when the Raynox is added and the magnification is adjusted to 1:1, the actual maximum f-stop is F:11. On the other hand, the Macro-Takumar's maximum f-stop is F:22, so it can in theory have twice the DoF as the zoom with Raynox (these are hypothetical numbers because diffraction problems set in at about actual_f-stop*(1+m) ~16- 20 for a K-x, therefore both lenses should have about the same DoF capability at 1:1.)