Variable Maximum Aperture Lenses

[matthew_banks1]

<p>Looks like we have many knowledgable members here and my brain hurts just from reading all the responses. Now please make my brain really hurt - if aperture changes according to focal length, how do non variable aperture lenses work????</p>

[matthew_banks1]

<p>Looks like we have many knowledgable members here and my brain hurts just from reading all the responses. Now please make my brain really hurt - if aperture changes according to focal length, how do non variable aperture lenses work????</p>

[nsfbr]

<p>Matthew,as I understand it, what happens in one of those lenses is that the constant aperture lenses are allowing light to fall either outside the image circle, or being lost elsewhere in the path through the elements, or some combination. Only at the longest focal length (I would assume) is the simple relationship held such that the focal length relates to the diameter of the final element (away from the film/sensor) such that it yields the f-stop when one is divided by the other. That was where I started in my thinking about this. <br>

My question to myself was, okay constants work this way, is it a given that variables go the completely other route or is it somewhere in the middle? <br>

As to why there is evidence of a lens going from say 75% to 100% of the maximum focal length while holding the f-stop constant I would get that it has to do with the lens prematurely losing some light at an intermediate step initially, but then eventually stopping so that at the maximum focal length it once again behaves like one would expect. <br>

I do think there is a limit that should be obvious: At any point, the maximum f-stop can not be faster than the actual focal length/the diameter of the objective lens element.</p>