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05-29-2010, 04:06 PM   #1
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A* 135mm/1.8 reporting 1.7 wide open on K-7?

Hola all, My 135/1.8 has arrived, and I'm seeing something unusual. The lens reports 1.7 as wide open to the K-7.

I can still select 1.8 as an aperture through the camera, and when I do so, the lens is ever so slightly stopped down (Used Bulb exposure mode to see it stop down).

I love the idea that this lens is secretly 1.7, but I somehow doubt that is the case.

The lens seems to stop down fine from there all the way to f22.

To those of you that have this lens, is this just a bug with the lens in general, or does my copy have some circuitry that's not working quite right?

05-29-2010, 06:05 PM   #2
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Who cares? You have an A* 135mm f/1.8!!! :P
05-29-2010, 09:08 PM   #3
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I have a Sigma 28/1.8 lens that does the same thing. I think the answer is halfway down this page:

Features and Operation of the Ka Mount

With KA lenses, the camera figures out the maximum aperture by the pattern of contacts on the lens mount. Note the table shows possible values in only half-stop increments. Your lens is probably telling the camera it's an f1.7 because it can't say f1.8.

I don't get any aperture blade movement at f1.7 or f1.8 on the Sigma. That shouldn't be happening but it's not worth fixing. You can always set f1.7 if you really need it to be wide open, and live with the 1/6-stop metering error.

When Pentax switched to AF, the lenses had chips that the camera could read, so the camera uses that data instead.
06-02-2010, 05:19 PM   #4
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Moving from another thread:
QuoteOriginally posted by photogerald Quote
Yes, my copy does the same thing - that is, reports f1.7 wide-open. I didn't know that at f1.8 it is slightly stopped-down - I never bothered to check mine that closely. But I will say that wide-open (f1.7 or f1.8 - I don't remember which), my copy underexposes very slightly (maybe by 1/3 stop) compared to the other aperture settings.



I agree with you here - while it gets even better when you stop down, it is by no means considered soft at wide-open. FWIW, my A*135 wide-open looks sharper when viewed at 100% than my FA*200/2.8 @2.8.

I'll also say that you'll find excuses to shoot with this lens, even when it is not the most convenient (in terms of weight and/or FL) to do so. Enjoy!



Last edited by Clinton; 06-02-2010 at 05:30 PM.
06-02-2010, 05:20 PM   #5
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Moved from other thread:
QuoteOriginally posted by photogerald Quote
It would be interesting to compare the indicated f1.7 vs. f1.8 settings closely to see what the difference in exposure is, and also to see if there is a noticeable difference in the rendering of the bokeh (at f1.7 the aperture should be perfectly circular).
Agreed. It's also interesting that the k-7 when you stop down goes from 1.7 to 2.0 directly, but when you open up, it goes from 2.0 to 1.8 to 1.7. Which is to say the stop is only available when opening up.

As for exposure time, The camera picked the same shutter speed for both 1.7 ans 1.8 when I was playing with it.

Maybe I'll ping Pentax support tomorrow about this bug.

Last edited by Clinton; 06-02-2010 at 05:30 PM.
06-02-2010, 05:21 PM   #6
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Moved from other thread:
QuoteOriginally posted by Pentaxor Quote
I dont think it's a bug. it's rather that the Sigma 28/1.8 has an approx. aperture value of f1.7. it is probably playing in between 1.7 and 1.8 but rounded off at 1.8 instead. there are some lenses like this IIRC.

Last edited by Clinton; 06-02-2010 at 05:31 PM.
06-02-2010, 05:29 PM   #7
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I talked with Jeffrey R from Pentax support today, and he said he would pass this info and the link to this post (hence the moving of items from the other thread) along to the product team and get back to me with what they say.

His reaction is that the K-7 firmware doesn't handle half stops (F1.75), and so it rounds down to the nearest full number so that wide open is still wide open.

His other thought is that when this lens was made, the 1.8 stop was common, but 1.7 was unusual so it was labeled 1.8.

In either case, he said the product team would likely get back to him tomorrow and he'd forward the response on to me to post here.
06-02-2010, 07:08 PM   #8
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Look at the resourse sites such as the following

Pentax Lens Modifications for Matrix Metering - Mark Roberts Photography

the aperture coding does not include F1.8 but F1.7, because that is what pentax sold on the 50mm, as opposed to canon and nikon at F1.8

so for lenses without the data pin (introduced with the SMC-F lenses and the autofocus, there is no way to code F1.8 into the 5 contact pattern.

06-02-2010, 08:00 PM   #9
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wow! A*135/1.8! How much did you pay for it?
06-04-2010, 03:18 AM   #10
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When I first got my A50/1.2 it showed 1.0 on the camera. A good clean of the contacts fixed the discrepancy.
06-04-2010, 05:08 AM   #11
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QuoteOriginally posted by Light_Horseman Quote
When I first got my A50/1.2 it showed 1.0 on the camera. A good clean of the contacts fixed the discrepancy.

I would have kept it that way, if it shows up in the EXIF you could turn a few heads


on a more serious note, some of my A series lenses do the same thing. I really don't worry about it.
07-25-2010, 09:43 AM   #12
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So, I'm a little slow in sharing the response from support, but here it is:

QuoteOriginally posted by Pentax:
If he is using the “A” setting on the aperture ring and selecting the aperture on the camera body, the camera is getting the aperture information from the lens. To display the selected f/stop, the camera must perform a calculation involving the lens’s maximum and minimum apertures and the different lens information contact point on the lens mount which is in use. In any case, one of the lens contact “patterns” (how the lens’s aperture range is encoded) covers a range of f1.7 to f32, so it might be possible for the lens to display f1.7 instead of f1.8 at it’s wide-open position. Other separate lens aperture ranges include f1.4 to f22 and f1.2 to f22. "
So it would sound like either answer is plausible. These lenses may be slightly wider than 1.8 and hence display at 1.7, or it may be possible that our modern bodies just read these lenses at 1.7.

I'd kinda hoped Pentax would have a clearer answer. <shrug>
07-25-2010, 01:35 PM   #13
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My A*135 and K-7 is stopping down a little bit if I select f=1.8.
At f=1.7 it is wide open.
07-25-2010, 02:46 PM   #14
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QuoteOriginally posted by Digitalis Quote
I would have kept it that way, if it shows up in the EXIF you could turn a few heads
I'm with you on this one, I would have had some fun with this.

The Limited Pentax Noctilux

Last edited by Kerrowdown; 07-25-2010 at 03:13 PM.
07-25-2010, 02:56 PM   #15
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so did you take any pictures or just messed around with the menus?
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