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01-23-2014, 03:54 AM   #1
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Modern lens + teleconverter... how do you fool the SR mechanism?

Hey,
Hopefully this is the right place to ask this, and not the "general troubleshooting section".

Anyway. I'm looking for a replacement option for my Sigma 170-500mm, which essentially is my walkaround lens. Recently it caught some salt water splashes causing it to become a bit hazy, and the focusing ring also is now occasionally getting stuck. It's just old and worn, and I need something new; my K-3 is rather unforgiving when it comes to showing how soft that lens is at the long end of the zoom range. Given how the new Tamron 150-600mm won't be made in a PK-mount, I'm essentially stuck with two options within my budget... either a new Sigma 150-500, or a DA* 300mm. I'm strongly leaning toward the latter option, but for most of my purposes, 300mm is ridiculously short, meaning I will have to use a 1.7x or 2x TC most of the time. Judging by the IQ of images taken with that lens and decent TCs, that should be no issue. However, since I usually photograph whatever flies past me on hikes or bike rides, I will not be using a tripod most of the time, there's just no time. I need the SR mechanism.

Problem is that most TCs have passive contacts; my K-3 will therefore underestimate the effective focal length, limiting the usefulness of the SR mechanism. This leads to my following questions:
a) Is there a way to manually input the focal length when using a modern lens (i.e., a lens with a data contact)?
b) If not, would insulating the data contact on a teleconverter do the trick, or would I lose some other functionality, like AF? In other words, does the K-3 simply revert to using the other lens contacts, as if it were an A-lens, and "assume" it's a manual focus lens?
c) Am I perhaps overlooking even more considerations here?

Thanks for any insights you may be willing to share.


Last edited by OJGoreng; 01-23-2014 at 03:59 AM. Reason: Overlooked some typos.
01-23-2014, 05:04 AM   #2
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You can 'tell' the camera what focal length you have manually.
01-23-2014, 05:11 AM   #3
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I know that works on lenses without a chip, but when I attach a modern lens, the "input focal length" menu option is gray/disabled. Is there some way to reactivate it?

Last edited by OJGoreng; 01-23-2014 at 05:30 AM.
01-23-2014, 05:21 AM   #4
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QuoteOriginally posted by JohnX Quote
You can 'tell' the camera what focal length you have manually
Well, you cannot do that if it is a TC with "passive" electrical contacts that merely transmit the lens information (unchanged) to the camera. With such a set-up the option to manually input FL is blocked on the camera.

I believe the best chance for the OP will be to look for the Pentax-F 1.7X AF-adapter: SMC Pentax-F 1.7x AF Adapter Reviews - Pentax K-mount Teleconverters and Adapters - Pentax Lens Reviews & Lens Database Here, one will be asked to input FL and AF has been reported to work with the DA* 300mm lens. But if I recall correctly, AF isn't terriby fast..........

I have experimented myself with blocking some contacts on a Kenko Pz-AF TC. Perhaps it can be done, but for me the only result was that while I was asked to input FL, I could only set my lens wide open and/or fully stopped down.

01-23-2014, 05:29 AM   #5
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Thanks. I actually own an AF adapter, and it would be a workaround indeed, but the forced "focus limiter" (i.e., the thing only working within a small sub-range of the lens' focal range) has proven to be a bit cumbersome when I try it with other lenses.

Did you block the contact by insulating it, or by shorting it to the mount ring? I was thinking perhaps that could be a factor as well.
01-23-2014, 06:03 AM   #6
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QuoteOriginally posted by OJGoreng Quote
Thanks. I actually own an AF adapter, and it would be a workaround indeed, but the forced "focus limiter" (i.e., the thing only working within a small sub-range of the lens' focal range) has proven to be a bit cumbersome when I try it with other lenses.

Did you block the contact by insulating it, or by shorting it to the mount ring? I was thinking perhaps that could be a factor as well.
I just insulated the contact with tape. Someone with better understanding of the contact patterns might come up with some ideas.......?
01-23-2014, 12:22 PM   #7
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For lack of other people with suggestions (or patience on my part), I just took the plunge and tested both methods with a 1.7x MC4 Pz/Af converter (mine's badged Soligor, but it's the same one sold under the Vivitar, Kenko, and many other names) and a DA 18-55mm. I get the same results when I insulate the data contact and when I short it to the lens mount... no AF, no automatic aperture (camera reverts to Av mode). That means the DA* 300mm is no longer a valid option. Shame.
01-23-2014, 01:44 PM   #8
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I have bitched to pentax about this point until I am blue in the face.

There are multiple issues with a TC and SR is the least of the worries, for that, simply double the shutter speed in Tav mode for what normally gives a stable image for you and shoot. BUT there are other needs for the camera to know that a TC exists. First is general metering. Today's cameras need to know the native aperture to meter correctly. The metering has errors, that appear to be native aperture dependant(at least pentax has this issue) and the cameras are set to between F4 and F5.6 because that is the native aperture of a kit lens. Second is flash exposure, as this needs to know the true aperture for P-TTL flash to correctly determine / calculate the flash power. Without correct aperture you need to add exposure compensation. In auto exposure modes you have a problem with the MTF curve because the camera follows the wrong few oval length and aperture

So the long and short of it is, the camera really needs to know there is a TC.

By the way, the SMC-F 1.7x AF converter does calculate aperture correctly

01-23-2014, 02:45 PM   #9
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My experiences with the AF adapter have been rather troublesome. I have to say, after just reading a pretty glorious test of the new Tamron lens, I'm feeling smacked in the face with buyer's remorse after getting the K-3 only a month ago, especially since a D7100 is about 30% less expensive. It looks like my only option that won't involve selling various organs is the siggie 150-500, which appears to be only marginally better than my 170-500 and vastly inferior to the new Tamron, which is hardly more expensive. I guess I'll hang on to the 170-500 for now and hope Sigma responds to the Tamron with something more exciting (or Pentax getting into bed with Tamron to have the new lens produced under their own badge, like they did with that other zoom). There is a point where brand loyalty becomes silly.
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