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03-13-2013, 08:13 AM   #16
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QuoteOriginally posted by demp10 Quote
“A” lenses have 6 pins, 5 to communicate min/max aperture (actually they tell the wide open aperture and the number of f-stops that can be closed down)
Actually the encoding is for the minimum aperture (not maximum) plus the range of stops. And the lens itself generally doesn't have a corresponding pin or contact at every position, usually just the bare mount in place of conducting pins, with non-conducting nubs where needed.

03-13-2013, 08:45 AM   #17
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Thanks gents

Jim I'm sure I did that before on my K-7 on Newarts' advice, but for whatever reason I couldn't get it to work for me. Can't think why though, but looking back I wonder if I used thick enough foil. It would seem like a great solution. I can't help but wonder why Pentax don't just provide a simple optional override on this, bizarre
03-13-2013, 09:58 AM   #18
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QuoteOriginally posted by baro-nite Quote
Actually the encoding is for the minimum aperture (not maximum) plus the range of stops. And the lens itself generally doesn't have a corresponding pin or contact at every position, usually just the bare mount in place of conducting pins, with non-conducting nubs where needed.
You are correct. Only the "*" contact must be a pin to move in and out. The others can be pins or just use the mount to read "1" or a rubber insert to read "0". Most "A" lenses I have do not use pins indeed. All the F, FA and DA lenses I have use pins for "1" and rubber inserts for "0".

The 2 pins on either side of the "*" pin define the minimum (as you pointed correctly) aperture: 0*0=F16, 1*1=F22, 0*1=F32 and 1*0=F45
03-13-2013, 10:24 AM   #19
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QuoteOriginally posted by THoog Quote
'A'-series lenses don't have digital pins.
Sorry, I missed that the OP was refering to A lenses specifically.

03-13-2013, 01:12 PM   #20
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QuoteOriginally posted by Nass Quote
To clarify, the vignetting is because the maximum diameter possible with f/1.7 is 29.4mm
Where did you get that number?

The image circle for the A 50/1.7 is more than adequate for 35mm film and should not vignette on APS-C. At least mine doesn't.


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03-13-2013, 01:23 PM   #21
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50/1.7=29.4

But, the aperture blades (iris) has probably a smaller diameter at setting 1.7, because its deep in the optics.
03-13-2013, 03:02 PM   #22
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QuoteOriginally posted by stevebrot Quote
Where did you get that number?

The image circle for the A 50/1.7 is more than adequate for 35mm film and should not vignette on APS-C. At least mine doesn't.


Steve
Yes, 50/1.7, that's correct, and works fine because the diaphram is at the point where the rays cross over as they usually are so the width is actually small. (You're quite right btw glasbak, I just measured it, it's actually ~22mm @ 1.7) But the systems I use use an infinite tube to focus light in parallel onto the sensor, so it's a different gig. I've done a diagram to explain it better. Normally the iris is at point A so the cone of light rays is small and a diaphram opening of diameter d suffices. In my case the iris is at point B so the same diaphram of diameter d isn't big enough because as you can see the infinite rays are bigger than the opening. That's my problem. Err one of them


Last edited by Nass; 03-13-2013 at 03:16 PM.
03-13-2013, 11:41 PM   #23
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QuoteOriginally posted by Nass Quote
Yes, 50/1.7, that's correct, and works fine because the diaphram is at the point where the rays cross over as they usually are so the width is actually small. (You're quite right btw glasbak, I just measured it, it's actually ~22mm @ 1.7) But the systems I use use an infinite tube to focus light in parallel onto the sensor, so it's a different gig. I've done a diagram to explain it better. Normally the iris is at point A so the cone of light rays is small and a diaphram opening of diameter d suffices. In my case the iris is at point B so the same diaphram of diameter d isn't big enough because as you can see the infinite rays are bigger than the opening. That's my problem. Err one of them
Thanks for the explanation. I understand.

Steve

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