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.

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