Originally posted by GUB Can you clarify one thing? From common sense I have always presumed the front glass had to measure at least the Focal length divided by fastest aperture. Does that still hold true with what you say?
Focal length divided by the aperture gives you the speed of a lens, expressed in f/ number. The diameter of the front element is another thing, and of course it makes no sense to have lens barrels shaped like... barrels
That is, with the stop wider than the front glass.
Usually the front element is somewhat larger, as can be easily seen in many lenses. Some projector optics have no stop, what in layman's term we can call the "center" of the optic is as large as the front. Others have a fixed step, a disc if you prefer, that cuts out peripheral rays.
You experiment with a 110 lens doesn't show that the lens is recycled from a larger format, and of course there were no "cosmetic" reasons to to make the lens this way.
The size, curvature, refractive index of the front element are due to the characteristics the designer wanted to get, like speed, design typology and field of view.
The internal iris, wide open, is smaller because of one main reasons. It's not coverage though. It's aberrations.
Light rays coming from the periphery of the lens have to travel a longer path (go through more glass) cause they come from a wider angle, so they don't focus on the sensor plane the same way as light rays at 90 degrees (or close to it) do.
The two main problems are chromatic and spherical aberrations.
The former was (for the most part) solved long time ago with the introduction of the so called achromatic lenses.
Spherical aberration is compensated, more or less, by the optical "tricks".
The simplest is to reduce the influence of peripheral rays with a stop, fixed or adjustable (diaphragm).
I'm sure that placing a washer in front of the lens, as you have done, made the image a little sharper.
I don't remember much of optics, so I might be wrong, as far as I remember doing something like that, in presence of an internal step, should affect aberrations (sharpness) and only marginally speed.
Please correct me if I'm wrong
Cheers
Paolo