aperature question re: Tamron 90mm macro...

[Kodan_Txips]

This may be useful too, but it downloads so slowly that I gave up before it was fully loaded

http://www.kent.k12.wa.us/staff/TomRobinson/physicspages/PhysOf99/Cam-Atkins/thecamera.html

 

[rob1116]

robhesketh: Thank you, but as I mentioned earlier, I understand aperatures values listed in f/number syntax... that's the reason I can't picture how a lens with a variable focal length can have a fixed max aperature.



Kodan_Txips: Ahhhhh... I think you may have hit on the 'missing piece' I was looking for earlier! Concave/Convex elements! No time just now, but I'll be sure to read both those pages thoroughly. This also explains why my 70-300mm lens' physical length isn't nearly 30cm. I'm also assuming this is how lenses could technically be fixed max aperatures... because you're essentially saying that they don't have to change the physical focal length to achieve a different perceived focal length... right?

... I'll check those sites out

 

[Kodan_Txips]

I got it backwards, actually. I was talking about the concave lenses helping to give the lens a shorter physical length than its apparent focal length - but you seem to have understood my waffle even when I explained it badly. Maybe that it why I fail so often as a teacher - I am only understandeable when I get it wrong

And the aperture can remain constant by moving it backwards and forwards within the light beam, as it changes in its "spread angle", in such a way that the iris is cutting off the same percentage of the beam area.

Something like that, anyway.

And to add to the poignancy of my situation, I have an MD Rokkor 100-200 mm zoom lens that is actually LONGER than 200mm at full zoom, especially when you add in the distance from the lens mount to the film plane. At 200 mm closest focus it is 24 cm plus camera depth, and at 100 mm infinity focus, its shortest length, it is an amazing 17.5 cm plus camera depth. Weighs like a lead brick, too.