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12-18-2013, 05:31 PM   #1
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Aperture Display and F Lenses

Just before work, I received a used F 35-70 lens I bought to use with my new K-5.

I only had a minute to try it out.

It focused and took a picture.

But the Aperture figure does not display.

The friends I asked both said try cleaning the contacts.

But long story short, should the Aperture figure be displayed?

It says "F--" that's all. Maybe that's all it's gonna display?

I did "enable aperture ring" in settings as instructed.

I know this answer is probably here somewhere, but I'm at work and I just want to know if no display means the lens is bad or not.

Thanks.

Pentax-F 35-70mm f/3.5-4.5

12-18-2013, 05:36 PM   #2
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You have to put the aperture ring in A on the lens.
12-18-2013, 05:46 PM   #3
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Put the aperture ring on 'A' -- should lock in with the little button. And leave it there forever.
12-18-2013, 05:47 PM   #4
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Right - I got that.

But should the top and viewfinder displays indicate the aperture? Or is that info not communicated between this type of lens and the K-5.

Thanks.

12-18-2013, 05:49 PM   #5
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If the lens is on 'A', yes. If not, no.

The communication is all one-way with simple A lens -- the pins simply tell the camera the aperture range, that's it. Then when you select an aperture (on the body, not the lens), it will know how far to move the mechanical lever to stop it down. If it is working properly, there will basically be no reason to ever take the lens off the A setting and physically turn the aperture ring.

Which brings up a question I have -- does this lens communicate differently when zoomed in since it is a variable aperture? I think on most of them, even though at the long end the aperture is slower, it will still probably read 3.5 instead of 4.5. (Whereas a modern DA lens will change the aperture range as it zooms on a variable aperture zoom).

Last edited by vonBaloney; 12-18-2013 at 05:56 PM.
12-18-2013, 05:52 PM   #6
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QuoteOriginally posted by vonBaloney Quote
If the lens is on 'A', yes. If not, no.
Thanks. I will check it out later after work!

Really appreciate the quick answers everyone. You guys are like the eye in the sky!

Global support!

Dave
12-18-2013, 05:54 PM   #7
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QuoteOriginally posted by davids8560 Quote
But should the top and viewfinder displays indicate the aperture? Or is that info not communicated between this type of lens and the K-5.
Your lens is not successfully communicating its presence to the body. This is done through the "A" contact on both the mount and the lens. The "A" contact has continuity (signals as present) when the aperture ring is on the "A" setting. When the "A" contact is detected the camera will show the aperture in the viewfinder and LCDs and will also control the set aperture from the body.

You can test the "A" contact on the lens with an ohmmeter if you have access to one. To identify which pin, see diagram at Features and Operation of the Ka Mount about halfway down the page (identified with *). With the aperture ring in the "A" position, there should be continuity between the pin and the mount flange.


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12-18-2013, 06:02 PM   #8
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Would it make a difference if I moved the aperture ring to the A position while the lens is mounted to the K-5? Or does this need to be done before the lens is mounted?

Thanks Steve and everyone else too!

12-18-2013, 06:07 PM   #9
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QuoteOriginally posted by vonBaloney Quote
Which brings up a question I have -- does this lens communicate differently when zoomed in since it is a variable aperture? I think on most of them, even though at the long end the aperture is slower, it will still probably read 3.5 instead of 4.5. (Whereas a modern DA lens will change the aperture range as it zooms on a variable aperture zoom).
Just checked with my F 35-70 and the aperture display changes with focal length. Put it on f/3.5 @ 35mm and zoom to 70mm and the display changes to f/4.5 and back if you zoom in.
12-18-2013, 06:10 PM   #10
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QuoteOriginally posted by davids8560 Quote
Would it make a difference if I moved the aperture ring to the A position while the lens is mounted to the K-5? Or does this need to be done before the lens is mounted?

Thanks Steve and everyone else too!

Nope, you can freely move the aperture ring regardless of it being on or off the camera.

That's how you would switch between full auto and aperture priority back in the film days (A = auto, click from A to an aperture and it would be Aperture priority)
12-18-2013, 06:14 PM   #11
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As soon as you click into 'A', you should see the aperture display with a number, as soon as you take it off of 'A', the display will go to "F--"...
12-18-2013, 06:39 PM   #12
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Awesome! You guys are the best!

I would like to be able to check it out more thoroughly but it'll have to wait until after work.

Hopefully it's just another "new used lens" glitch and not altogether a bum used lens.
12-18-2013, 11:39 PM   #13
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1:33am EST

If you look on this lens there is a green dot and a red hash mark. They're right next to each other.

I lined up the "A" with the green dot before. And got nothing.

Just a minute ago, I lined it up with the red hash mark. The aperture setting jumped right up on the screen!

So thanks everyone, and by the way, what is that little green dot for?

Anyway. Feeling pretty dumb, but happy my used $44 eBay F lens works okay.

Sigh...

Last edited by DavidSKAF3; 12-19-2013 at 01:00 AM.
12-19-2013, 01:04 AM   #14
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And incidentally this little lens seems to have a bit going for it!

How can it be so small, yet almost as equally bright on the long end as the 16-45?

What elements determine the overall mass of a lens?

Is bigger in fact better?

No idea if the 35-70 was considered a good lens when it first came out, but for $44 it seems like a pretty good deal! Might not be as wide as the current kit lens, but it is definitely smaller and a little bit brighter. Does anybody know if any famous celebrities like Jimi Hendrix or Andre the Giant ever owned one of these? How about Broadway Joe Namath? Raquel Welch? Tiny Tim?

Lol

Last edited by DavidSKAF3; 12-19-2013 at 05:41 AM.
12-19-2013, 01:51 AM   #15
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QuoteOriginally posted by davids8560 Quote
1:33am EST

If you look on this lens there is a green dot and a red hash mark. They're right next to each other.

I lined up the "A" with the green dot before. And got nothing.

Just a minute ago, I lined it up with the red hash mark. The aperture setting jumped right up on the screen!
Yeah, the hash mark is pointing at the selected aperture number or the 'A'. You want it on the A. There should also be a little button there on the ring that you press while moving it which will keep it locked into or out of A. (Or at least it should click into A a bit more rigidly.) Keep it locked in A and forget about it.

I'm guessing the green dot has something to do with the "macro" mode on that lens -- must be zoomed to 70, or possibly past 70 into the "macro zone" or something like that. (Don't force anything, but some lenses are like that when at full zoom, then they will slide into a close-focusing mode.) Anybody know?
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