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p.14 #5 · p.14 #5 · Sony FE 35mm 1.4 VS. Voigtlander 35mm 1.7 Big Bronco Test  
artur5 wrote:
Fred, if by longer you mean that the focal length increases then no, it’s actually the opposite. When you add a positive diopter element to any lens, the focal of the combo becomes shorter. It’s a basic law of optics. The simplified formula is : F3 = F1*F2/(F1+F2). Where F3 is the final length, F1 is the focal of the original lens and F2 is the focal of the PCX filter.
In this case : F3 = 35*5000/(35+5000) = 34.76
That’s why you’re able to focus nearer when you use this PCX on your CV 35/1.7. The focal goes from the original 35mm to 34.76mm. To practical effects, it’s like adding a 0.24mm. tiny extension tube. Therefore you can focus nearer than with the ‘naked’ lens. Of course that means also that, in order to reach infinity, you need to shorten 0.24mm. the length of the adapter or remove internal shims from the lens.
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Math is math and we can't argue with it.
In practical terms though, I didn't see any different in FL at infinity (with and without the PCX). If there is a difference, it's minimal and definitely not wider.
The CV 35/1.7 Ultron is a bit longer than my Loxia 35 at infinity to begin with.
Here are two crops showing the extreme edges at infinity. The left image is without the PCX and the right with it. I don't see a noticeable change in FL at this distance.
TopLeft Corner: Normal (left), PCX (Right)
BottomRight Corner: Normal (left), PCX (Right)
