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07-28-2012, 05:37 PM   #1
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Weird lens question - manual lens

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.


Last edited by tammons; 07-28-2012 at 05:49 PM.
07-28-2012, 06:42 PM   #2
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As these are fast lenses they likely are not symmetric and you may be measuring the back end of the lens--which will be wrong, and will give too small an aperature. (Are you?) And visually (lightness/difference in light readings) presumably is real, and likely one or both lenses are off. A difference of about 1/4 stop on each could thus be 1/2 stop difference.
07-28-2012, 06:48 PM   #3
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Original Poster
I took them apart to measure.
Had them apart a few times to convert them so no big deal.

I measured the actual aperture opening. Those are the actual dimensions.
I am selling the 50mm F1.4 anyway and keeping the F1.7.
I was just curious why they were so far apart.

I was getting 1/500 on the F1.7 lens and 1/8000 on the F1.4 lens, both WO, so about 3+- stops apart.

If it would have been 1 stop difference I would not think its a big deal.
I was just curious.
07-28-2012, 06:55 PM   #4
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Two other things.
First you are correct FL/Aperture Dia=f/dia= fstop. Thus 50mm lens with 25mm aperture has f/dia=50/25=2.
The other thing is lenses have so called T stops which accounts for the actual light transmission, and lenses with lots of elements, or possibly uncoated or singly coated surfaces will have less actual light transmission (T<fstop)--but with relatively modern lenses this usually is not apparent--except in mirror optics--where it is often about 1 stop difference.

07-29-2012, 05:32 AM   #5
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Location: Queens NYC
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f/2 should transmit an equivalent amount of light on both lenses.

Since these are conversions, a variation in flange to film distance
might account for the difference as measured by your PZ-1.

Fractions of a millimeter matter in these cases.
Do both lenses focus properly at all distances?


Last edited by ChrisPlatt; 07-29-2012 at 07:57 AM.

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