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.
Why aren't all macro lenses
designed with a P>1?
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