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11-05-2010, 02:04 PM   #1
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Teleconverter and f number reported

Hi,

I tried a P/KA teleconverter with my pentax 50mm f2 lens. Pictures seem to be properly exposed, however the F number reported in the view finder for auto, M, Av, and P modes is not correct. Sometimes it will report an f number faster than f2.0.

I was wondering if doublers (Tokina) do this, or should the F number reported in the viewfinder be 2 times the F number of the actual aperature setting on the lens?

thanks

11-05-2010, 02:09 PM   #2
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Lowell Goudge's Avatar

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what you see will depend on what lens and exactly what teleconverter you use

Most PK-A teleconverters simply pass through the lens data, so if you have an A lens you should see the apretrue range available with the native lens. BUT if one or two of the contacts are defective, you night see apertures different than the lens native apertures.

you may want to look here
http://www.robertstech.com/matrix.htm

to figure out which pins might be defective.

just look at the range of apertures you are getting, and look at the table to see the contacts that would need to be made to get that aperture range. Then look at the contacts for your lens's range, and the difference shows what is deffective

they may be just dirty
11-05-2010, 02:14 PM   #3
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QuoteOriginally posted by Hiroshi Quote
I was wondering if doublers (Tokina) do this, or should the F number reported in the viewfinder be 2 times the F number of the actual aperature setting on the lens?
None of the current TCs reports the correct aperture and focal length of the combo lens + TC. The TC just passes the data as is from the lens to the camera.

The only exception is the Pentax F 1.7X AFA, which reports the correct aperture, but blocks the focal length data.
11-05-2010, 06:29 PM   #4
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Thanks to Lowell and SOldBear. I tried this with 3 lenses, 2 of them (both Pentax) report the F number correctly, at least one that is not faster than the lens. The 3rd one is a Sigma and it is the one that reports f1.2 on a f2.8 lens. I appreciate the quick tutorial.

11-05-2010, 07:06 PM   #5
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One thing I have noticed about a couple of sigma lenses is the aperture pins are not drilled necessairly in a circular pattern but in a couple of lines roughly along the circle

They may not contact the pins correctly
11-05-2010, 07:51 PM   #6
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SIGnificant MAlfunction?
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