The Canon MPE-65 macro lens
This is a unique and sometimes frustrating macro lens.
It has an electronic diaphram, but is entirely manual focus.
It will not focus at infinity, it is useful only for macro work,
and extreme macro work at that. Most "regular" macro lenses
will work to 1:1 magnification (and some only do that with an
additional extension tube). This lens begins at 1:1 magnification
and continues to 5x magnification. In 2012 this lens was selling
for $1000 new.
Although it is specified to have a focal length of 65mm, it is not a
fixed focal length lens. It has floating elements and would appear to
have an effective focal length of 65mm at the 1:1 setting, but at the
5x setting, it acts as though the focal length has decresed to something
like 40mm. This can be seen by just measuring the length of the lens.
If was a fixed focal length lens of 65mm, at 5x it would require 325mm of
extension, but it does not have such a long extension.
Depth of field, effective aperature, and diffraction
I was initially frustrated by this lens because I was attempting to gain
depth of field by stopping the lens down. At the magnifications for which
this lens is used, depth of field is a major concern. The problem with
stopping the lens down though is that the effective aperature is given by
the nominal aperature multiplied by the magnification. This means that at
5x and f/22 you are working at an effective aperature of f/110 !!
Diffraction effects become severe at such miniscule aperatures and
significantly reduce resolution. It turns out that the best way to use
this lens is to use it wide open (or nearly so) and to use focus stacking
to achieve depth of field. In cases where this is impossible (such as
photography of moving subjects), you have the option of stopping down and
sacrificing resolution.
Best aperture
As is discussed in detail in some of the links below, there has been a
fair bit of discussion about the best aperture for the MPE-65.
Rik Littlefield has found that f/2.8 gives him the most resolution,
whereas Charles Krebs has found that f/4.0 gives him the best results.
This probably reflects the properties of the particular lenses owned by
these two men. In fact it may depend on the combination of lens and
camera being used. The best thing to do is to perform tests with the
actual equipment you will use to take photos.
At 5x, diffraction effects are noticeable at f/5.6 and
quite significant at f/8. However Charles Krebs says that if he had to
take a single image and could not use stacking, he would probably use f/8.
More information
Feedback? Questions?
Drop me a line!
Tom's Mineralogy Info / tom@mmto.org