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05-20-2014, 04:27 AM   #1
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SMC 50mm 1.2 aperture stops

Greetings all.

I'm lucky enough to be the owner of a K-mount 50mm 1.2 and have a question about the aperture stops.

From 2.0 upwards, it seems to display full stops, and half half-stop clicks between them (2.0, unmarked 2.4, 2.8, unmarked 3.3 etc etc). But there is a single unmarked click between 1.2 and 2. Does anyone know what this is? 1.4? 1.7? Relevant photo here - SMC Pentax-A 50mm F1.2 Reviews - A Prime Lenses - Pentax Lens Reviews & Lens Database

Any advice would be appreciated.

Cheers,

Gary.

05-20-2014, 08:08 AM   #2
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I have the same question about the SMC-A 50mm f/1.7 which goes from 1.7 to 2.8 with a single click between. Is it f2 or 2.4? By manual metering on my K10D it appears to go from 1.7 to 2.0 to 2.8. In the manual metering mode it show 1/3 or 1/2 EV difference, depending on EV step setting. If it were f2.4 it would show as 1 full stop.

F2.0 would make sense as this is the speed of the next slowest lens in the 50mm stable. Odds are that the same relationship holds on the f/1.2 and it is f1.4. You can probably check this in the same manner with manual metering and using the aperture ring. F1.7 would meter at a full stop difference.
05-20-2014, 08:34 AM   #3
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QuoteOriginally posted by sapateiro Quote
But there is a single unmarked click between 1.2 and 2. Does anyone know what this is? 1.4? 1.7?
I would guess it would be the next full stop up, so f/1.4.

QuoteOriginally posted by Not a Number Quote
I have the same question about the SMC-A 50mm f/1.7 which goes from 1.7 to 2.8 with a single click between. Is it f2 or 2.4?
I would guess it would be the next full stop up, so f/2.0.

Phil.
05-20-2014, 08:51 AM   #4
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Here's some help from wiki - full 1/3 and stops!

http://en.wikipedia.org/wiki/F-number#Standard_full-stop_f-number_scale

05-20-2014, 02:40 PM   #5
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Thanks guys, but unfortunately I'm still not sure...

@gofour3 - I see what you're saying - the next full stop up is 1.4, but then that's inconsistent when it then jumps a full stop further to f2, then only half stops from there forward?

@jimr - thanks for that, but according to that half-stop scale, there would be 2 unmarked 'clicks' between 1.2 and 2.

@notanumber - good idea with seeing if the manual metering will shed some light, I'll try that myself.

Does anyone know if there's any archive of old manuals/documentation that would confirm this?

Cheers,

Gary.
05-20-2014, 04:40 PM - 1 Like   #6
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QuoteOriginally posted by sapateiro Quote
Thanks guys, but unfortunately I'm still not sure...

@gofour3 - I see what you're saying - the next full stop up is 1.4, but then that's inconsistent when it then jumps a full stop further to f2, then only half stops from there forward?

@jimr - thanks for that, but according to that half-stop scale, there would be 2 unmarked 'clicks' between 1.2 and 2.

@notanumber - good idea with seeing if the manual metering will shed some light, I'll try that myself.

Does anyone know if there's any archive of old manuals/documentation that would confirm this?

Cheers,

Gary.
OK I was able to check my Pentax lens guides and these are what the unmarked clicks are when a lens has a non-standard maximum aperture. Pentax treats the non-standard maximum apertures as the next full stop up and then carries on with the regular half/full stop rotation.

Example a lens with a maximum aperture of f3.5 is treated like the full stop f/4 then the unmarked click is f4.8, then the full stop f5.6.

Lens with a maximum aperture of f/1.2: 1.2, 1.7 (unmarked), 2.0, 2.4 (unmarked), 2.8
Lens with a maximum aperture of f/1.7 or 1.8: 1.7/1.8, 2.4 (unmarked), 2.8, 3.4 (unmarked), 4.0
Lens with a maximum aperture of f/2.5: 2.5, 3.4 (unmarked), 4.0, 4.8 (unmarked), 5.6
Lens with a maximum aperture of f/3.5: 3.5, 4.8 (unmarked), 5.6, 6.7 (unmarked), 8.0

Phil.
05-20-2014, 11:11 PM - 1 Like   #7
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For what it's worth... here's a link to a similar thread:

https://www.pentaxforums.com/forums/10-pentax-slr-lens-discussion/136932-f1-4-k-50mm-f1-2-a.html


And just for fun, here's a look at a lovely lens :



05-21-2014, 02:36 AM   #8
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Thanks very much for your help guys, much appreciated.

PS. Lovin' the creamy goodness of the 50/1.2

05-21-2014, 06:14 AM   #9
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So the thread linked to above gets the basically the same results on an f1.2 that I did on an f1.7 - the unmarked stop is less than 1 full stop from the lowest marked f-stop. And the reply from Pentax confirms that is 1.4.

So it appears that the first unmarked stop is the next highest standard/full f-stop or 1 full stop less than the 2nd marked stop. Unmarked emphasized:

1.2 -->1.4-->2.0
1.7/1.8-->2.0-->2.8
2.5-->2.8-->4.0
3.5-->4.0-->5.6

The lens guide referenced above may have meant the next highest full stop e.g 1.4, 2.0,2.8,4.0,5.6 and so on. "Full" would mean the stops normally indicated on the ring.

Does the 50mm 1:2 (f/2) lens even have a click stop between 2.0 and 2.8?
05-21-2014, 09:11 AM   #10
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QuoteOriginally posted by Not a Number Quote
So the thread linked to above gets the basically the same results on an f1.2 that I did on an f1.7 - the unmarked stop is less than 1 full stop from the lowest marked f-stop. And the reply from Pentax confirms that is 1.4.

So it appears that the first unmarked stop is the next highest standard/full f-stop or 1 full stop less than the 2nd marked stop. Unmarked emphasized:

1.2 -->1.4-->2.0
1.7/1.8-->2.0-->2.8
2.5-->2.8-->4.0
3.5-->4.0-->5.6

The lens guide referenced above may have meant the next highest full stop e.g 1.4, 2.0,2.8,4.0,5.6 and so on. "Full" would mean the stops normally indicated on the ring.

Does the 50mm 1:2 (f/2) lens even have a click stop between 2.0 and 2.8?
That's incorrect and not what the Pentax lens guide shows. See my above post.

Phil.
05-21-2014, 09:59 AM   #11
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QuoteOriginally posted by Not a Number Quote
. . . . .

Does the 50mm 1:2 (f/2) lens even have a click stop between 2.0 and 2.8?


My M 50 f/2 goes straight from f/2 to f/2.8. No half stop.
05-21-2014, 10:12 AM   #12
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So . . . can you tell the difference between a 1/2-stop and a 1/3-stop in the typical images you view?

if you accept the metering solution does it make a difference?
05-21-2014, 10:25 AM   #13
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QuoteOriginally posted by gofour3 Quote
That's incorrect and not what the Pentax lens guide shows. See my above post.

Phil.
Read this thread. Especially the last message. Like me they measured the readings at the various stops:

https://www.pentaxforums.com/forums/10-pentax-slr-lens-discussion/136932-f1-4...ml#post1437297

QuoteQuote:
From the Horse's Mouth
I emailed Pentax to ask about the stop between f1.2 and f2. Their reply:


"Dear Customer,

Thank you for contacting Pentax.

The unmarked stop is indeed f1.4."
05-21-2014, 04:10 PM   #14
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QuoteOriginally posted by Not a Number Quote
Read this thread. Especially the last message. Like me they measured the readings at the various stops:

https://www.pentaxforums.com/forums/10-pentax-slr-lens-discussion/136932-f1-4...ml#post1437297
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.
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05-22-2014, 07:25 AM   #15
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As a double check I did some readings using a Program Plus (Program A) body as well. This body is contemporary with the "A" lenses and has the aperture settings linkage.

Here's how I tested:
Camera mounted on a tripod lens facing a white painted wall about 2 feet away. Diffuse lighting from a window, early afternoon sun on a cloudless day. Curtains open, windows shut, no trees or plants blocking sun. Manual metering used on both bodies: Program Plus and K10D. Lens tested SMC-A 50mm 1:1.7 (f/1.7).

Program Plus
In manual mode the Program Plus shows shutter speed and EV +/- from 0 to 3 in one EV steps. Display blinks if over +/- 3. So if I "zero" the meter at f/5.6 and then change the aperture to the half stop between 5.6 and 8 the meter shows no change. When I set the aperture to f/8 the meter shows -1 EV. Now when I zero the meter at f/1.7 and change to the first unmarked click stop between 1.7 and 2.8 the meter does not change. If it were one full stop the meter would indicate a -1 EV.

K10D
In manual mode and not using the A-mode setting on the lens stop-down metering is used. The camera settings for this on the K10D is to enable use aperture ring and DOF preview set to optical. You then activate the meter, activate the DOF preview. This closes down the aperture and activates the meter bars. EV steps can be set to 1/2 or 1/3 in the setup. Shutter speed is set using the eDial, aperture from the ring. So once again the meter is zeroed at f/5.6. When changing the aperture to the unmarked stop between 5.6 and 8 the meter shows one or two bars minus EV for 1/2 or 1/3 steps respectively. The two bars on 1/3 steps should actually be 1.5 bars but there are no half-bars. After zeroing the meter when the aperture is set to f/1.7 changing to the unmarked stop between 1.7 and 2.8 shows a change of -1 or -2 bars, depending on step setting. If it was one full stop up from 1.7 the change would be 2 or 3 bars depending on the step settings.

So once again on both the K10D and Program Plus the unmarked click stop between 1.7 and 2.8 measures to less than one full stop.
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