So I have two Contax CY lenses converted to M42.
A 50mm F1.4 and a 50mm F1.7.
I want to use them on a PZ-1 body and was testing them side by side wide open and at F2.
I noticed at F2 the Contax 50mm F1.4 is taking in a lot more light than the 50mm F1.7 lens at f2.
Now the question is, I always thought that the aperture diameter was a fraction of the lens length. IE the aperture diameter of a 50mm
lens at F2 should be 25mm.
The Contax F1.4 at F2 is 20.8mm vs the 50mm F1.7 at F2 is 18.3mm.
50mm F1.4 Wide open the aperture orifice is 24.3mm which would make it about a F2 in reality instead of a F1.4.
The 50mm F1.7 Wide open the aperture orifice is 18.3mm which would in reality make it a F2.7 instead of a F1.7.
I did find the formula area = pi (FL/2*N)squared but even with that wouldnt the aperture opening be the same diameter at F2 ??
These are really nice and sharp lenses but I am just having a problem getting my head around the difference.
Can anybody shed some light on what is going on or is there a multiplier I am missing or something.
These lenses are so far apart metering WO and at F2 its not funny.
Thanks
Last edited by tammons; 07-28-2012 at 05:49 PM.