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07-03-2015, 10:11 AM   #1
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Question about aperture ring marking

I have a SMC Pentax-M 28mm f:2.8 lens that has a white dot on the aperture ring - between the f:5.6 and the f:2.8 inscriptions. Can anyone tell me what this is for?

07-03-2015, 10:13 AM   #2
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F4 I think historically the dot has been used for the closest whole aperture stop.

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07-03-2015, 11:02 AM - 1 Like   #3
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The original Pentax manual for A and M lenses states it this way:

"The white dot next to the lens' full open value indicates an aperture which is one full stop slower."

Not exactly a definitive statement as the illustration uses whole stops, eg., 2.8 and 5.6, leaving one to presume the DOT indicates 4.0 -- leaving me to wonder what the answer would be if it was an f/2.4 lens. would it indicate f/3.4 (one full stop increase) or f/4.0 (next full stop).

In either case it simply indicates there wasn't room for two digits an' a decimal point on the scale and with modern match needle or AE exposure it really isn't too significant anyway.

Given a fixed level of light/illumination like the clear mid-day north sky you could explore reciprocal exposure increments using ISO or shutter speed to make your own determination.

The manual also states that the intermediate click stops are 'half stops' and that there is no click stop at either end of the scale which presumes to be the mechanical maximum and minimum values for that lens.

And while we're at it, that white/yellow plastic bump to the right of the scales is just a touchy-feely tactile indicator for the red lens latch dot on the mount.

And the red tic line just to the left of the red diamond index line is used to re-set focus for IR film after focusing normally with the optical VF.

[@ Adam: reproductions of some of the old lens and accessory manuals would be a useful reference sticky for this site.]
07-03-2015, 02:58 PM   #4
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QuoteOriginally posted by pacerr Quote
[@ Adam: reproductions of some of the old lens and accessory manuals would be a useful reference sticky for this site.]
I won't get a notification with a space after the @ sign, so you got lucky I saw this Scans, manuals, etc. would all be perfect additions to our article section, where they could easily be browsed and found by users. A lot of them are already there alongside more detailed posts.


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07-03-2015, 05:58 PM   #5
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Heck, I didn't even know that @-no space was an alert protocol - learn sumthun every day.

Perhaps it would be more referable as a separate, organized section in the Reviews area? Maybe in Accessories?
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