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03-15-2011, 10:40 PM   #1
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F1.4 on a K 50mm F1.2

Hi folks,

Recently, a K 50 f/1.2 fell into my lap (love that Craigslist!). And I'm wondering about the unmarked half (?) stop between f/1.2 and f/2.

In the Forum Lens Reviews I see references to this stop as being f/1.4. Can anyone confirm this? How do you know for sure? Is it documented somewhere?

In the lens booklet / user manual that came with my lens, it mentions unmarked half stops for "lenses with a non-standard maximum aperture", but doesn't include the 50 f/1.2. For example, on a K 135 f/2.5, the stop between f/2.5 and and f/4 is f/3.4. And on K lenses with a maximum f/3.5 aperture, the next click is f/4.8.

Based that info, I'd think the f/1.2's next stop would be closer to f/1.7..?

Anyhow, yes, as a practical matter it makes very little difference one way or the other. But, the lens geek in me needs to know!

Thanks,
Tim


Last edited by .a.t.; 03-22-2011 at 05:48 PM. Reason: clarity
03-15-2011, 11:56 PM   #2
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I have one too, and I don't know for sure, but it must be between f/1.4 and f/1.6 - not f/1.7. That would be too far toward f/2 for an f/1.2 lens. You can always try leaving it between the detents if you want finer control - however, I don't know if this causes any extra wear on the detent mechanism over the long run.
03-16-2011, 12:38 AM   #3
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Thanks for your input, DSims. I guess it's all academic for me since I usually keep the aperture ring on full, marked stops anyway. It's just something I'd like to know - simply for the sake of knowing.

Tim
03-16-2011, 01:24 AM   #4
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You can calculate it by checking what kind of automatic exposure you get. If f/1.2 gives a 1/50s shutter speed for a given scene, and then that second click gives 1/25s, youīll know itīs a full stop of difference, ie f/1.7.

03-16-2011, 04:44 AM   #5
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You can also leave it in between the clicks. Just check out the opening. It is a noticeable size difference in aperture opening between f1.2 and f1.4, and so between f1.4 and f1.7. I have all sizes for 50mm range.
Automatic exposure mentioned by Jonathan Mac is better.
03-16-2011, 08:43 AM   #6
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According to my tests and comparison to K50/1.4, the unmarked click on K50/1.2 should be f1.4.
This would go in line with the old tale which says that with Pentax glass that is full stop wide open, the first unmarked is half stop. With glass that is half or third stop wide open, the first unmarked is full f stop.

Regards
03-16-2011, 02:23 PM   #7
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Thanks for the replies, all. I took Jonathan Mac's advice and did a little test. I shot an indoor scene with consistent lighting. I made the exposures as equivalent as I could. For what it's worth, the results were:

f/1.2, 1/20 second
f/1.?, 1/15 second
f/2.0, 1/10 second
03-16-2011, 03:00 PM   #8
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The lens manual of fa m50 1.4 states, the dot represent 1/2 stop

03-17-2011, 05:54 AM   #9
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QuoteOriginally posted by all thumbs Quote
Thanks for the replies, all. I took Jonathan Mac's advice and did a little test. I shot an indoor scene with consistent lighting. I made the exposures as equivalent as I could. For what it's worth, the results were:

f/1.2, 1/20 second
f/1.?, 1/15 second
f/2.0, 1/10 second
Strange, that suggests that f/1.2 lets in only double the light of f/2, it should be 1.5 times the amount.

Donīt try to guess equivalent exposures, just set the aperture in M mode & use the green button to meter, let the camera guess the exposure. Setting the camera to 1/3 stop increments rather than 1/2 stop will make it more precise, if not already done.
03-17-2011, 10:04 AM   #10
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It should 1.5 but it's not. I came to the same conclusion as OP when doing my controlled lighting shoot out. Despite huge differences in OOF between f1.2 and f2 the light transmision isn't all that big, or at least as big as the f stop numbers would have you believe... Anyway, the first unmarked f stop on K50/1.2 is f1.4. I'm pretty confident at saying that.
03-18-2011, 01:07 AM   #11
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Re Techmulla: Agreed, 1/2 a stop makes sense on a f/1.4 lens, but I wonder if that applies to a f/1.2? Well, since I'm not as experienced as you guys, I'll take axl's word that the mystery stop is f/1.4. But, Jonathan Mac's suggestions made me want to try another test.

I had a light tent already set up, so I thought I'd use that for consistency. My K10D was on a tripod and the 50 had a lens hood on. Metering was center weighted, 1/3 EV steps and I used the green button.

The results surprised me. I would've expected more difference in the exposure times. Have I overlooked something? As far as I can tell, both my lens and K10D are working properly.

Tim
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03-18-2011, 01:15 AM - 1 Like   #12
axl
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not really surprising,
Just look at this thread of mine: https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/131614-pentax-...-50-1-4-a.html
it was shootout between K50/1.2, K50/1.4 and FA43ltd.
exposure times for the first two lenses: @f1.2 = 1/80, @f1.4 = 1/60, @f2 = 1/40. You can scroll down to compare crops sided by side to compare exposure.
03-18-2011, 01:46 AM   #13
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Thanks, axl. I'm sorry to see you're forced to sell your 1.2.

Tim
03-18-2011, 02:25 AM   #14
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no worries,
I try not to think about it... at the end of the day, I still have the Ltds so I should do just fine
03-22-2011, 05:45 PM   #15
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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."
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