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05-23-2008, 06:21 PM   #1
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Initial click-stops on an F2.8 M lens

On a M28/F2.8 lens there are two click-stops per F-stop increase except for the first one. For example, on a M28/F2.8:

It starts in the middle of "2.8" with the aperture fully open
Next click is the large white dot to the left of "2.8"
Next click is just to the right of "5.6"
Next click is the middle of "5.6"

That's 3 clicks for a 1 F-number increase in this first interval. After that it's 2 clicks per 1 F-number increase.


While on M50/F1.7 lens it's:
Centre of "1.7"
Half-way between "1.7" & "2.8"
Centre of "2.8"

That's 2 clicks for a 1.5 F-number increase in this first interval. After that it's 2 clicks per 1 F-number increase.

What's going on in this first interval and why the white dot on the F2.8 lens aperture scale?


Last edited by dosdan; 05-23-2008 at 06:41 PM.
05-23-2008, 06:55 PM   #2
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I've wondered that too... I only have a Tokina EL 28 1:2.8, and it just registers fulll stops from 2.8->4->5.6->8->11->16.

It sounds like yours is going 2.8, 4, 4.5, 5.6. I checked a photo and there is no '4' on the ring.

Note that 2.8 to 5.6 is two stops, not one. Therefore I suspect there's only one click to f/4, then two clicks onwards like the rest of the ring.

So you're missing the half stop f/3.5.
05-23-2008, 07:16 PM   #3
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CSoars, you're right. But if the 3 clicks in the first 2 f-stop range are equally placed (2/3 stop intervals) it could be:

2.8, 3.5, 4.5, 5.6

as the 1/3-stop (with 2/3-stop number bold) sequence is:

2.8, 3.3, 3.5, 4, 4.5, 5.0, 5.6
05-23-2008, 08:38 PM   #4
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Non-standard aperture lenses

QuoteOriginally posted by dosdan Quote
CSoars, you're right. But if the 3 clicks in the first 2 f-stop range are equally placed (2/3 stop intervals) it could be:

2.8, 3.5, 4.5, 5.6

as the 1/3-stop (with 2/3-stop number bold) sequence is:

2.8, 3.3, 3.5, 4, 4.5, 5.0, 5.6
Getting us closer, but not all the way there yet.

From the 8/81 SMC Lenses mini-brochure (came with M lens).

"The first half click stop of non-standard maximum aperture
The click-stop between f/4 and f/2.5 of the SMC Pentax 135mm f/2.5 lens represents f/3.4, and the click-stip between f/5.6 and f/3.5 of all lenses with maximum aperture of f/3.5 represents f/4.8."

I know that doesn't directly answer the question RE; 28/2.8, but does it offer some insight?

05-24-2008, 06:34 AM   #5
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the pentax way

I looked at several of my K mounts. I don't have the 28mm but I have 50mm F1.4, 100mm F4 macro, 105mm F2.8 and 135 F2.5

It appears many of the pentax lenses do not have a 1/2 stop increment between maximum apature and the next apature setting.

my 50mm goes F1.4, F2
my 100mm F4 goes F4, F5.6
My 105 goes F2.8 F4

The only exception is the 135 F2.5 which has an intermediate lpoint between F2.5 and F4 which I have determined is F2.8 but it is not marked.

On your lens, I suspect the F4 marking is the white dot, as there was not room to center the "4" on the apature scale
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