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05-22-2014, 08:38 AM   #16
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QuoteOriginally posted by gofour3 Quote
This is from a Pentax Lens guide for the f2.5 & f3.5 maximum aperture lenses. I'm assuming Pentax followed the same pattern for the f1.2 & f1.7 & f1.8 lenses. The unmarked click stops are never full stops.

Phil.
That's a valid assumption however it doesn't hold up in real-life testing with the SMC-A F1.2 and F1.7. It may very well be true with some of the K, screwmount and some M series lenses.

Obviously two data points is not enough to make a valid assumption.So let's compromise and go with this

1.2 - 1.4 - 2.0 (as tested on "A" lens and per Pentax support)
1.7 - 2.0 - 2.8 (as tested on "A" lens)
2.5 - 3.5 - 4.0 (as per Pentax literature)
3.5 - 4.8 - 5.6 (as per Pentax literature)

F1.8,2.2,2.4,4.5 and non-A 1.2, 1.7 unknown until such time as Pentax literature is found or testing done.

Perhaps someone with lenses of the maximum apertures, primes preferred, could do some tests. The non-linear nature of the aperture settings linkage on pre-"A" lenses (with some exceptions on the M series) may present a problem. but I would think stop-down manual meter via DOF preview would use light measurements for metering.


Last edited by Not a Number; 05-22-2014 at 10:31 AM.
05-23-2014, 04:52 AM   #17
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Wow - thanks guys... It's obviously not an easy one to answer. My attempt at confirming anything via metering wasn't a success as my conditions/equipment weren't accurate enough. I have got another 50mm 1.4 which I'll compare it to (DOF etc)... But it probably won't be able to make any 'scientific' observations there either lol.

@allthumbs - perhaps there was a variation between versions of the lens? My version (K 1458262) has half stop clicks.

But the most important thing is the fact that I absolutely love this lens... It's instant art
05-23-2014, 11:55 AM   #18
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QuoteOriginally posted by sapateiro Quote
. . .
@allthumbs - perhaps there was a variation between versions of the lens? My version (K 1458262) has half stop clicks.
Right. My K 50 f/1.2 also has half stops. When I said my lens had no half stop between f/2 and f/2.8, I was referring to my M 50 f/2.


QuoteOriginally posted by sapateiro Quote
. . . But the most important thing is the fact that I absolutely love this lens... It's instant art
Indeed!


QuoteOriginally posted by pacerr Quote
So . . . can you tell the difference between a 1/2-stop and a 1/3-stop in the typical images you view? . . .

Probably not. For me though, that's not really the point. My lenses are tools but they're also toys. I just enjoy playing with them and trying to understand how and why they work the way they do.
Gear Geekism, I suppose.

Last edited by .a.t.; 05-23-2014 at 05:21 PM. Reason: typo
05-23-2014, 10:18 PM   #19
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All of my lenses from pentax seem to follow a simple rule. The first and last clicks are full stops, everything else is 1/2 stop increments.

So I would think F1.7/8

But here is a way to tell

Find a grey wall. (Concrete blocks) or perhaps a paved roadway, and set it up to shoot manual mode perfect (histogram exactly centred ) exposure at F2.8

Then leave the shutter where it is, and take shots from F1.2 at every click through F5.6 with all neutral JPEG settings

Using a photo editor, measure the greyscale value of a selection of perhaps 10-20% in the middle of the frame.

Each full stop for about the first +/-2.5 stops is about 45 greyscale

It will be easy enough to figure out the stops, but as i said, the standard for pentax is the first click is 1 full stop, all others until the last are 1/2 stop

05-24-2014, 09:19 PM   #20
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QuoteOriginally posted by Lowell Goudge Quote
All of my lenses from pentax seem to follow a simple rule. The first and last clicks are full stops, everything else is 1/2 stop increments.

So I would think F1.7/8

But here is a way to tell . . . . .

Lowell,

I tried your experiment with my K 50 f/1.2 and K-7. A gray card was the subject and all exposures were 1.5 seconds. The histograms are from GIMP.


f/1.2



f/1.?



f/2.0



f/2.4



f/2.8



f/3.4



f/4.0



f/4.8



f/5.6




Here are the mean value differences between half stops (going backwards):

f/5.6 to f/4.8 : 17.7
f/4.8 to f/4.0 : 20.9
f/4.0 to f/3.4 : 23.5
f/3.4 to f/2.8 : 23.0
f/2.8 to f/2.4 : 21.8
f/2.4 to f/2.0 : 21.9
f/2.0 to f/1.? : 23.8
f/1.? to f/1.2 : 26.7


If I'm interpreting the data correctly, it appears to me the stop between f/1.2 and f/2.0 is (effectively) closer to f/1.7 than f/1.4.
What do you think?
05-24-2014, 10:28 PM   #21
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Yes that is correct it's f/1.7. In the mid 1970's Pentax would not have made an unmarked click a full stop like f/1.4. That would go against the way any lens, flash or handheld meter was marked.

Example if you made a handheld meter reading and the meter indicated the exposure should be half way between f/1.4 and f/2.0 for the shutter speed you wanted to use. I would assume if you moved the aperture on the K50/1.2 one click between f/2.0 and f/1.2 I would get f/1.7 and get the correct exposure. If the unmarked click was really f/1.4 that would be one full stop from f/2.0 and I would be out a half stop. So you see the confusion that would cause? It has to be f/1.2, f/1.7 (unmarked) and then f/2.0.

Unfortunately the person at “Hoya Pentax” made a mistake indicating the unmarked click was f/1.4 in the email you sent them in 2011. I would only trust an answer from someone who worked for the Asahi Optical Company back in the 1970’s or 1980’s.

Phil.
05-24-2014, 10:33 PM   #22
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QuoteOriginally posted by all thumbs Quote
Lowell,

I tried your experiment with my K 50 f/1.2 and K-7. A gray card was the subject and all exposures were 1.5 seconds. The histograms are from GIMP.


If I'm interpreting the data correctly, it appears to me the stop between f/1.2 and f/2.0 is (effectively) closer to f/1.7 than f/1.4.
What do you think?
It sounds spot on, and is consistent, in terms of my other pentax lenses, the first click and the last click are full stops, all others are 1/2 stops.

One thing to note is that as you pugs the histogram up to the top, it starts to be non linear at the top end. The real tell tale is between the unmarked stop, and F2. It shows as almost exactly 1/2 stop using the 45 greyscale per stop. Note the F2-F2.8 gap for example,
05-24-2014, 11:55 PM   #23
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QuoteOriginally posted by gofour3 Quote
Yes that is correct it's f/1.7 . . .
QuoteOriginally posted by Lowell Goudge Quote
It sounds spot on . . .

Thanks, guys.

05-25-2014, 10:20 AM   #24
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Here's the SMC-A 50mm f/1.7. I measured off an old Kodak grey card, front lit from diffused light through window. Color balance set from white side of card.

Shot using the aperture ring and eDial in 1/2 EV steps.



Looks closer to f2.0 than f2.4 to me.
05-25-2014, 11:21 AM   #25
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A simple question , Not a Number. What did you meter on, and did you shoot JPEG. Your numbers seem a little off and I doubt that the first click, is an unmarked full stop

You did not for example have highlight correction enabled
05-25-2014, 07:14 PM   #26
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I shot both pure RAW (PEF) and pure Jpeg mode. The numbers were all within 2%. I centered the metering on the grey card at the stop between f4 and f2.8 because it is exactly 3 clicks to either 5.6 or 1.7. The K10D has no setting for highlight correction. Tone is set to normal, sharpness, saturation and contrast set to neutral.

I manually meter this lens on two camera bodies and the click stop between f1.7 and 2.8 meters as around 1/2 stop - not one. I do the histogram bit which again shows this click stop is less than 1 full stop, When I set the ring to stop between 1.7 and 2.8 and then switch to "A" mode and the eDial I get the same reading on the histogram a f2.0 not the full stop up at f2.4.

If you don't want to believe me fine. The world is flat and the moon landings were faked too.

I'm done with this thread.
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