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11-04-2016, 05:11 PM   #1
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Force aperture wide open in M mode without aperture ring?

Hi,

(Background first, skip to end for question. And disclaimer, I've tried to search but it's hard to find relevant threads among related but other areas)

I have been shooting manual lenses for a number of years and thought I had it all figured, didn't know what all the fuss is about regarding K vs M vs A lenses etc...except for AF and A-in-camera-setting they all worked the same to me, but I'm starting to understand it's because I've always used M mode, so using the aperture ring was no biggie or something I saw as a drawback barely...

Now I got a Pentax T6-2X converter, and also recently got a Tammy 70-200 f2.8. I also have a K400 5.6, and I thought if the Tamron@F2.8/200mm + converter = 400mm/F5.6 was as good as they K quality-wise, I could sell it. (The K is ok but no champ, while the tamron is very sharp and the converter seems ok from an IQ perspective, so the idea not that far-fetched..).

So my thought was that, ok, the tamron without aperture ring will always be 400@F5.6 when on the converter, but fine, I'll work with that for those cases. (I also have F*300 F4.5 when 600mm@F9 is needed, and then with an aperture ring in that case for full M-control).

But it doesn't work like that! When in M-mode and using the converter, it's full closed at F22!

But if I use AV (which all the other talk has been about that I never understood), then it's wide open, but then I can't set the shutter time. TV, P TAV etc just say "AV" when selecting them, I guess that's because of the lack of electrical contact with the lens?

So, in my head I'm thinking this is a software problem, but that may not have a solution, but better to ask if someone wiser may know how to do it:

My question is:
It seems the body CAN "choose" wide open or full closed for these cases depending on the selected mode. Is it possible to tell it "I know you don't get aperture info, but always use wide open and let me set shutter speed and worry about exposure"? Is this possible with some combination of P or user modes or electrical short of pins or other tricks?


Thanks for any help!
[EDIT: Before all "why not use AV mode?"-suggestions, my use-case here is for astro so manual exposure is critical, and in-camera won't get it right looking at a dark sky...]

11-04-2016, 05:29 PM   #2
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Are the linkages coupled?

Don't know if this is your problem but when I use a teleconverter put the converter on the lens prior to mounting to camera and you can check if the aperture linkage is coupled by gently pushing on the linkage, then put the completed lens assembly on the camera.

Last edited by gmans; 11-04-2016 at 05:42 PM. Reason: Added opening question
11-04-2016, 05:43 PM   #3
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Thanks for the suggestion, but no, it's linked alright when pulling it.

I think it works "as designed", since one wouldn't want the camera to open up the aperture just because it's unknown in M-mode, but I guess my use case is not that common and not what it was designed for, even if it would be possible to do it in theory, but hence my question if anyone knows if the camera can be set to always open the aperture (even though unknown) while also letting me select shutter speed.
11-04-2016, 05:55 PM   #4
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The default is the aperture wide open on the camera, for focusing etc. Then when you take the shot it will stop down for exposure. if coupled correctly or in the case of manual K and M lenses push the green button.


Last edited by gmans; 11-04-2016 at 05:58 PM. Reason: add last sentence
11-04-2016, 06:12 PM   #5
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If there is no "A" contact the mode defaults to Av (except for M, X and B) so the camera will always meter K-mount lenses at wide open and you can't select the shutter speed. The auto-stop down lever will not actuate to close down a non-A lens except in M. Since the TC has no pass through contacts the Tamaron is treated the same as a series K or M lens (or a with the ring off the "A" setting).

If you short out the "A" pin you would have also block the appropriate pins so the camera would get the min and max apertures for the lens and restore "A" functionality for the lens mounted on the TC.

You may as well get a TC with the contacts which would be a lot less trouble than some stop gap solution.

I suppose you could wedge the lever on the Tamaron to stay wide open with a matchstick or something. If it works loose it would fall into the lens or TC and hopefully not the camera body.

Last edited by Not a Number; 11-04-2016 at 06:20 PM.
11-04-2016, 06:27 PM   #6
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QuoteOriginally posted by Igor123 Quote
Hi,

(Background first, skip to end for question. And disclaimer, I've tried to search but it's hard to find relevant threads among related but other areas)

I have been shooting manual lenses for a number of years and thought I had it all figured, didn't know what all the fuss is about regarding K vs M vs A lenses etc...except for AF and A-in-camera-setting they all worked the same to me, but I'm starting to understand it's because I've always used M mode, so using the aperture ring was no biggie or something I saw as a drawback barely...

Now I got a Pentax T6-2X converter, and also recently got a Tammy 70-200 f2.8. I also have a K400 5.6, and I thought if the Tamron@F2.8/200mm + converter = 400mm/F5.6 was as good as they K quality-wise, I could sell it. (The K is ok but no champ, while the tamron is very sharp and the converter seems ok from an IQ perspective, so the idea not that far-fetched..).

So my thought was that, ok, the tamron without aperture ring will always be 400@F5.6 when on the converter, but fine, I'll work with that for those cases. (I also have F*300 F4.5 when 600mm@F9 is needed, and then with an aperture ring in that case for full M-control).

But it doesn't work like that! When in M-mode and using the converter, it's full closed at F22!

But if I use AV (which all the other talk has been about that I never understood), then it's wide open, but then I can't set the shutter time. TV, P TAV etc just say "AV" when selecting them, I guess that's because of the lack of electrical contact with the lens?

So, in my head I'm thinking this is a software problem, but that may not have a solution, but better to ask if someone wiser may know how to do it:

My question is:
It seems the body CAN "choose" wide open or full closed for these cases depending on the selected mode. Is it possible to tell it "I know you don't get aperture info, but always use wide open and let me set shutter speed and worry about exposure"? Is this possible with some combination of P or user modes or electrical short of pins or other tricks?


Thanks for any help!
[EDIT: Before all "why not use AV mode?"-suggestions, my use-case here is for astro so manual exposure is critical, and in-camera won't get it right looking at a dark sky...]
From your many words, it is not clear to me that you do understand the difference between the original K-mount and the KA-mount.
To be clear on how the original mechanics work, you should probably review Adam's article
https://www.pentaxforums.com/forums/54-pentax-lens-articles/110657-how-use-me...-k-30-etc.html
11-04-2016, 06:49 PM   #7
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Long story short, if you have a lens without an aperture ring and use a teleconverter that prevents electronic aperture control but uses an automatic diaphragm (like yours), the lens will always shoot at its smallest aperture. In theory, the camera could control the aperture reliably if it could accept user input for the aperture parameters, but this is currently not possible (and probably never will be).

If you absolutely want to use a TC, your best solution would be to use an A-type teleconverter. There's the current DA 1.4x from Pentax, and a number of 1.4x and 2.0x options from Tamron, Soligor (etc), and Sigma that can be found second-hand.

The Pentax TC will probably deliver better detail/image quality than any cheap 2.0x converter, so it's worth getting IMO.

Alternatively, you could trim the part of the teleconverter's aperture lever that interfaces with the camera and glue the remainder in place at the wide-open setting. A bit of a hack, but you'd get accurate metering in M mode.

Adam
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11-05-2016, 02:45 AM   #8
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Thanks for the replies! Then it seems as I suspected that there is no way (without physically altering the converter, or jamming the lens lever) etc to do it.

Adam: Good suggestion, but I'd rather avoid trimming it since I want it functional for use with other manual lenses.

NaN: You might be on to something, I'll try blocking/shorting pins according to the table on bojidars page: Features and Operation of the Ka Mount, but I guess while blocking is easy, the "on/1" might be hard if there is a certain voltage required..

I'll get back with results in a few days...

11-05-2016, 04:30 AM   #9
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QuoteOriginally posted by Adam Quote
Alternatively, you could trim the part of the teleconverter's aperture lever that interfaces with the camera and glue the remainder in place at the wide-open setting. A bit of a hack, but you'd get accurate metering in M mode.
Thinking about it, sorry, my previous comment doesn't make any sense regarding this, lenses with aperture ring should work when an aperture is set regardless of the lever so the only thing I'd lose is perhaps some resale value, and would work as I want it to...hmm...may go this route if taping "selected" pin-areas on the converter doesn't work. Thanks again!
11-05-2016, 02:50 PM   #10
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It worked!

I thought (according to the table in Features and Operation of the Ka Mount) that I would have to tape a more "complex" pattern on the converter metal, but on the first try, just to see if it makes any difference, just by taping all pins from the "*"-pin and away from the red dot was enough! (leaving the two pins closest to the red dot in contact with the converter metal)

When shooting in M-mode, now the aperture doesn't move, and when setting the same shutter time as AV uses for a given framing, it gives me the same exposure, indicating that it's now operating wide open as I wanted.

So now on to test/compare the IQ of K400/F5.6 wide open vs Tamron 70-200/F2.8@200 + T6-2X to decide if to keep the K400 or not....

(Sure, the K has the advantage of possibility to stop down which the converter combo can't, but the tamron can zoom as an advantage, and when needing 400mm, usually even more is even better so then I would still have the F*300+TC (600mm/F9) that can also stop down more if needed, so in the end i feel the IQ will be the deciding factor)
11-05-2016, 10:47 PM   #11
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Are you shorting the "A" pin? It is recessed on the camera mount side so you need to put a ball of metal foil or something in it. Otherwise you still get the F- - in the viewfinder and the eDial won't set the aperture.
11-06-2016, 01:39 AM   #12
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Hi,

sorry, no that was never the intention - the converter has no pin contacts and can't relay the "signal". All I wanted was to make the camera think/behave like in AV-mode - not closing the aperture (more than F2.8 at least), but leaving it wide open instead even in M-mode, thus allowing me to set shutter speed (which I couldn't do in AV-mode of course). (I could set shutter speed before as well in m-mode, problem was that it would always close aperture to F22 then..)

Thanks
11-06-2016, 06:32 AM   #13
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A report from a quick comparison test of the above if anyone finds the thread in the future and is interested:
Tamron 70-200@200@F2.8+Pentax T6-2X (~400mm@F5.6) vs Pentax K400@F5.6 (both lenses wide open). The test is a bit lacking as it was done indoors/short focal length (~10m) and not long, but this is what turned up for me:

Center sharpness: More or less equal, not great in either case but clearly usable if not cropping heavily or trying to sell the images. Maybe the TC combo wins by a little, or maybe it's just fringing from the K throwing me off.
Border sharpness: Pentax wins by a bit
Aberrations: Lots of purple fringing with the pentax K while none with the TC-combo. So bad with the K in fact that lightroom can't clean it all up (using just the "quick" defringe+width tools, I know there are some more settings if having time to fiddle..). The last pixel remains purple for some reason and in thin objects a couple of pixels wide it doesn't even consider it fringing and leaves purple it as is.

But what struck me as a unexpectedly large difference after the test; Although I knew and expected the K to be easier to focus, the difference in manual focusing ease is stunning...for astro or static subjects maybe it doesn't matter very much but if using the TC combo and being forced to use manual focus in "everyday" situations, I'd say the tamron+tc manual focus is almost unusable. Or perhaps if catch-in-focus works well, but otherwise very hard with that razor-thin DOF and miniscule throw of the tamron focus ring.

So all in all, on one hand the TC combo has worse IQ overall if i value global sharpness a bit higher than the monstrous fringing of the K. The K can also be stopped down which the combo can't, and the K focuses more easily. But if stopping the K down to say F8, in many cases I'm probably more likely to go for TC+F*300 ~ 600F9 (good IQ, easy focus, longer, stoppable) or if I need a bit more speed just plain F*300 without TC and crop.

So I guess I'll sell the K anyway since the F*300 (possibly with TC) covers most usecases of the K, and the initial plan for Tamron+TC-400@F5.6 just barely covers one usecase, but doesn't leave a lot of usecases left for the K400 as is.

(One could consider having the K enabling me to sell the TC, but it is smaller, lighter and gives me a bit more with the 300mm...and I'm trying to downsize some LBA...)
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