Why aren't macro lenses made with P>1?


Having a pupillary magnification greater than one does provide a potential advantage in resolution at the detector compared to symmetric and telephoto lenses. So, why aren't all lenses designed this way? It turns out that there are lots of reasons that lenses aren't or can't be designed this way.

As you will recall, having a P>1 means that the lens is a retrofocus design. With most SLR's this design is necessary with shorter focal length lenses in order to allow infinite focus. The necessary displacement of the principal planes toward the detector characteristic of retrofocus lenses will cause you to have to give up some working distance. This is because the front principal plane also tends to be displaced toward the back of the lens with a retrofocus design. Since object distance is measured from the front principal plane, you will have more lens in front of it and thus less working distance. With non-macro work, this displacement makes little if any difference in the use of the lens. With macro imaging you may find that the object you are trying to photograph is running into the front element of the lens at a relatively low magnification. A good working distance is a highly prized feature for a macro lens.

A telephoto lens design (P<1) is just the opposite. These lenses tend to have extra working distance, but do pay a small penalty in resolution capabilities. Symmetric or nearly symmetric lenses are a good compromise between these competing factors. You may not get the same resolution capabilities inherent to a retrofocus design, but you will retain a reasonable working distance.

I honestly don't know how all of these issues factor into lens design. I think that a typical SLR lens is designed either retrofocus or telephoto out of necessity. The differences in resolution and working distance mean very little or nothing at non-macro focus distances. At high magnification, these factors become much more important and have to be considered in the design process. The question is: do you want working distance or improved resolution? Most of the time, working distance is preferred.

A small point to end this entry. The resolution difference between lenses of various pupillary magnification is really pretty small. The improvement can be easily shown mathematically, but in the real world the difference is generally minor. That's why many choose to ignore this factor.