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03-31-2010, 04:29 PM   #1
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Is f2.4 possible on the A 1.7?

I'm asking because I noticed that the f stops on the camera, when using the A setting, are different than on the aperture ring of the lens (A 1.7). So what I am asking is can the camera force the lens to f2.4 when the ring says it goes from 1.7 2 2.8...

03-31-2010, 04:38 PM   #2
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The aperture ring should click in half or even one-third stops as well as the full stops marked down on the ring. So the camera can set an aperture at any place there is a distinct click.
03-31-2010, 04:46 PM   #3
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Yes, I know but the camera (Kx) has more stops available than the lens does.

Kx- 1.7, 2.0, 2.4, 2.8, 3.5, 4.0, 4.5, 5.6

SMC A 50mm f1.7- 1.7, halfway, 2.8, halfway, 4, halfway, 5.6

So with that said, where does it stop to? 2.0 or 2.4? I figured 2.0 using the rationale of the 2.0 lens which doesnt make sense even to me...
03-31-2010, 04:50 PM   #4
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That's normal,
If you watch the metering it changes for every click of the camera, so it's definitely changing the f-stop even if it isn't stamped on the lens.

03-31-2010, 04:59 PM   #5
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QuoteOriginally posted by The Kurly One Quote
Yes, I know but the camera (Kx) has more stops available than the lens does.

Kx- 1.7, 2.0, 2.4, 2.8, 3.5, 4.0, 4.5, 5.6

SMC A 50mm f1.7- 1.7, halfway, 2.8, halfway, 4, halfway, 5.6

So with that said, where does it stop to? 2.0 or 2.4? I figured 2.0 using the rationale of the 2.0 lens which doesnt make sense even to me...
Apologies I just checked out my A lens and it has one half way click between labelled stops on the aperture ring and 4 intermediate steps on the camera. I'm interested if the camera actually uses all of these intermediate steps too.

Edit: I was just using the depth of field preview and looking at the aperture closing on the lens and it looks like it does actually go through all the intermediate steps
03-31-2010, 05:10 PM   #6
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And I'd assume if you set your camera to work in 1/3 stops instead of 1/2, you could get the lens to do that, too.

BTW, my copy of the A50/1.7 doesn't have a "2" marked on it at all. The markings go right from 1.7 to 2.8. There isn an unmarked click between them that I've never tested *since it's an lens, I don't use the aperture ring), but on the M version of the same lens, that unmarked click is really f/2.4, not f/2, as far as I can tell.
03-31-2010, 07:01 PM   #7
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Marc thanks for the reply, like I said I was assuming (if you read my post that it started at 2) again I don't know thats why I am asking... you have the same lens, and maybe the same camera (I dunno), so that unmarked click is an unknown, right? So with that there does the Kx provide me with an extra stop or is it just a frivolous setting?
03-31-2010, 08:40 PM   #8
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The aperture adjustment on most lenses is a smooth, continuous mechanical function as you can observe by looking through the lens and turning the ring manually. The click stops are merely for your convenience and standardization.

Modern electronic systems are able to adjust the iris regardless of clicks (shutter and strobe speeds too) but a continuously variable readout simply wouldn't be useful, necessary or practical if displayed in extremely small increments.

Half an f-stop was splitting hairs not too long ago and I don't remember ever planning an exposure in less than full stops. Well, maybe once or twice, but . . .

H2


Last edited by pacerr; 03-31-2010 at 09:54 PM.
03-31-2010, 09:34 PM   #9
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QuoteOriginally posted by The Kurly One Quote
Marc thanks for the reply, like I said I was assuming (if you read my post that it started at 2) again I don't know thats why I am asking... you have the same lens, and maybe the same camera (I dunno), so that unmarked click is an unknown, right? So with that there does the Kx provide me with an extra stop or is it just a frivolous setting?
I disagree with Marc about the unmarked click; I think it's supposed to be f2. He is right about his lens, I am sure about that, which is the only reason I mention it. It doesn't matter what it's supposed to be, it matters what the lens you have in your hand does.

You can test your lens in just a few minutes. Set up your camera on a tripod with the lens, with a really boring evenly lit subject. A white wall in artificial light is perfect. Daylight is fine as long as the light level is constant. Focus is not important. Take a test shot in M mode with the lens wide open. I like to adjust the shutter speed until the peak on the histogram lines up with one of the divisions on the display. A white wall will only have one big peak.

Let's say your exposure is 1/125 seconds at f1.7. You should get the same exposure at 1/45s and f2.8 - 1.5 stops smaller aperture, 1.5 stops slower shutter speed. The histogram peak will be in the same place as the test shot. Make sure that works, to test the theory, setup and lens. Then try the unmarked setting, and change the shutter speed to get a histogram that matches the two test shots. If the shutter speed is 1/90s, the mystery setting is f2.0; 1/60s means f2.4.

You can do the same thing for your M version. I think the mystery stop is supposed to be f2 here as well. Careless reassembly can get the M50/1.7 aperture off by a lot.

The aperture mechanism has two parts. When you use an A lens on the A setting, it uses a different mechanism than using the aperture ring. Because of this, using the aperture ring to set f4 may produce a slightly different exposure than using the A setting and the wheel to choose f4 on the camera.

I don't think I've come across any lenses with aperture rings in third-stop increments.
03-31-2010, 10:02 PM   #10
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QuoteOriginally posted by Just1MoreDave Quote
I disagree with Marc about the unmarked click; I think it's supposed to be f2. He is right about his lens, I am sure about that, which is the only reason I mention it.
Well, it's been a while since I tested, but what makes you say think it's *supposed* to be f/2? One bit of circumstantial evidence that it isn't: at least two other Pentax lenses I own have an white dot marking the unlabeled stop, and someone told me this to to indicate that the unlabeled stop really was a "full" stop. Since the 50's lack a white dot, I might conclude it was not a full stop.

QuoteQuote:
You can test your lens in just a few minutes.
I had done a much lamer version of this test with no tripod, no checking of histograms, etc - just eyeballing things. Also just checked to see what shutter speed the camera suggested when I green buttoned each click. Both methods had suggested a full stop between f/1.7 and the mystery click. I just tried a slightly less lamer version of the test with my "A", though, and you're right - only half a stop from f/1.7.

QuoteQuote:
Careless reassembly can get the M50/1.7 aperture off by a lot.
My M50/1.7 was a beater, that's for sure, and it seemed pretty clear someone at some point had taken it apart- or tried to, anyhow. So that could well be.

Anyhow, I'm happy to be mistaken on this.
04-01-2010, 10:28 AM   #11
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QuoteOriginally posted by Marc Sabatella Quote
Well, it's been a while since I tested, but what makes you say think it's *supposed* to be f/2? One bit of circumstantial evidence that it isn't: at least two other Pentax lenses I own have an white dot marking the unlabeled stop, and someone told me this to to indicate that the unlabeled stop really was a "full" stop. Since the 50's lack a white dot, I might conclude it was not a full stop.
It's kind of like interpreting the Constitution or a religious text: what did the original author really mean? Just like those interpretations, we have inconsistent evidence.
  • K300/4 - dot marking f5.6
  • K135/2.5 - unmarked stop between f2.5 and f4, f2.8 on mine
  • M50/1.7 - unmarked stop between f1.7 and f2.8, f2.0 on mine
  • M135/3.5 - unmarked stop between f3.5 and f5.6, f4.0 on mine
  • K55/1.8 - unmarked stop between f1.8 and f2.8, f2.0 on mine
All we need is a charismatic leader for each side and a fragment of Japanese text from the Asahi Opt. factory and we could have a war. "As you can see, Captain, Lokai is black on his left side."

QuoteQuote:
My M50/1.7 was a beater, that's for sure, and it seemed pretty clear someone at some point had taken it apart- or tried to, anyhow. So that could well be.
I took some photos to show my theory about this. First, the M50/1.7 with the name and filter ring removed:



The red arrows mark three screws holding in an assembly of the aperture blades and all the glass.



The assembly is removed, and the arrow marks a fork that connects to the aperture ring and lever mechanism.



This is the assembly and the arrow marks the chrome bar that goes into the fork above. The bar moves the aperture blades. (With a little work, Pentax could have put weather seals on this lens.)

If the three screws in the first photo are loosened, the assembly can be rotated in place. Then the chrome bar moves while the fork doesn't, so the blades move too. The assembly can shift enough to make the aperture ring settings wrong without causing any other assembly problems. If I do this just right, I can get that unmarked stop to be closer to f2.4 than f2.0. The other stops are not quite right either, but close enough so that you'd only notice if you did back-to-back controlled tests with another lens. And why would anyone ever own more than one of these?



(The big one is the F version. Aperture rings all set to the unmarked stop.)

I have no opinion on how this affects dynamic range.

BTW, I just noticed my last post said "I don't think I've come across any lenses with aperture rings in third-stop increments." A few lenses start on third-stops, like the K135/3.5 or the K55/1.8.
04-01-2010, 05:39 PM   #12
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This appears to be a question that is nigh unanswerable with any certainty, I was just curious LOL!
04-02-2010, 08:48 AM   #13
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As far as I know. On all Pentax lenses which have half or third as a wide open f stop the first unmarked click between wide open and next marked full f stop is always full f stop and all other unmarked clicks half stops.
So in case of 50/1.7 the stops would go 1.7 (marked) 2 (not marked) 2.8 (marked).

If you are using A setting than all of above is irrelrvant as it's camera and not aperture ring that decides the f stop. What f stops are available then entirely dapends on the fact if you have your camera set in 1/3 or 1/5 steps...

my 2p

PS: just1moredave's findings on his lenses seem to confirm this theory
04-03-2010, 09:05 AM   #14
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In AV mode my A 50 1.7 will go from.
Dialing up.
1.7
2.0
2.2
2.5
2.8
Dialing down.
2.8
2.5
2.2
2.0
1.8
1.7

So, 2.4 is not available. And 2.2 seems to be the unmarked position between 1.7 and 2.8.
I don't know where the 1.8 is coming from when increasing aperture.

Hope this helps.
Cheers, Mike.

Last edited by Ex Finn.; 04-03-2010 at 09:34 AM.
04-03-2010, 09:46 AM   #15
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QuoteOriginally posted by Ex Finn. Quote
In AV mode my A 50 1.7 will go from.
Dialing up.
1.7
2.0
2.2
2.5
2.8
Dialing down.
2.8
2.5
2.2
2.0
1.8
1.7

So, 2.4 is not available. And 2.2 seems to be the unmarked position between 1.7 and 2.8.
I don't know where the 1.8 is coming from when increasing aperture.

Hope this helps.
Cheers, Mike.
Your camera is set to third stops. If you change it to half-stops you can get f2.4. I don't know exactly why it skips f1.8 (a third-stop increment)on the way up but not on the way down. You could use the DOF preview and see if the blades move at f1.8. I have a Sigma 28mm f1.8 that tells the camera it's f1.7, but the blades don't move at all at either f1.7 or f1.8. I guess in certain modes there will be a 1/3 stop exposure error as the camera assumes it can set a non-existant aperture.
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