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10-01-2011, 09:59 PM   #1
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Front-Focus issue with old Zenitar 16mm Fisheye

Just wonder if anyone else had noticed a similar problem with older MF lenses on the K-r body? I had no issues using this lense on my old *-ist DS body, but it does not play nice with the K-r. I'm guessing that the Pentax Designers have actually changed the distance from the imaging sensor to the rear element, perhaps to accomodate either something with the design of the anti-shake or dust redux features, or because of recent lense design changes to accomodate the SDM technology. Either way, my Zenitar has been rendered useless beyond 8-10 feet or so. The firmware which allows an adjustment to the focus plane is disabled for MF lenses, (unless some helpful Electrical or Software engineer among the user community has figured a work-around?)

Does anyone know if the +/- 10 increments of adjustment for the focus plane is set in stone? (Ie; the limitation is mechanical, not due to firmware design)

Has anyone considered starting a database of "compatible" lenses for the K-r body?

Looks like I'll be in the market for a fisheye soon.

Tim in Minnesota

10-01-2011, 10:47 PM   #2
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You can't really say that a manual focus lens has front focus, because the AF confirmation with MF lenses is more or less approximate for every lens. Plus, with a fisheye, don't you just leave it at infinity most of the time and let the DOF do the work?

AFAIK there shouldn't be any major problems with using old MF lenses on the K-r. Some wide-angles can be problematic when it comes to metering (the A 15mm is a good example), but it's nothing you can't compensate for manually.

Adam
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10-02-2011, 01:24 AM   #3
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QuoteOriginally posted by Adam Quote
Plus, with a fisheye, don't you just leave it at infinity most of the time and let the DOF do the work?
Nope! As an owner of the Zenitar 16/2.8, I can attest that accurate focus is as critical with this lens as with any other, just more difficult to attain. The focus throw is fairly short and it is difficult to tell when you are in focus for moderately close subjects. (Everything is sooo tiny in the viewfiender!) Remember that for a given subject size on the focal plane, the working distance is much less with the Zen than with a longer lens. As a result, DOF is not really that much better. I have plenty of out-of-focus shots that demonstrate the principle quite nicely.

To the OP, I can suggest that an aftermarket focus screen with split image and/or microprism may be a good solution. I know that it makes all the difference for me. Your post indicates that you get poor results beyond 8-10 feet. Do you mean closer or beyond? If beyond, you may want to double check that infinity focus on your Zen is properly set up. My lens, when I received it from Russia, could not focus past about 10 feet. This is an extremely common issue. Correction involved adjusting the infinity stop for the focus ring. The adjustment is easy to do, but is best done using a body equipped with a horizontal split image focus screen and a distant vertical object.


Steve

Last edited by stevebrot; 10-02-2011 at 01:40 AM.
10-02-2011, 03:30 AM   #4
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I also have a Katzeye-clone screen that is very helpful with well-lit subjects. Even with others, it gets me into the focus neighborhood. With my rotting eyes, I depend on CIF and/or focus confirmation (FC). My procedure: Watch the screen to home in on focus; watch for FC to fine-tune; hit it with CIF when focus is achieved.

Or just hold down the shutter and let CIF take care of it.

10-03-2011, 02:39 PM   #5
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My sears/rikenon 50/1.4 front focuses. While the green hexagon is wrong in viewfinder, live view lets me get it right. I may just sell it & get a f/1.7; my f/2 rikenon is fine. Other MF lenses have been ok but i should check more closely.
10-03-2011, 02:47 PM   #6
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Maybe my 'center' focus point is closer to the bottom of the marked square? I should try a few oblique af shots to check this.
10-03-2011, 07:11 PM   #7
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The term focus "point" is deceiving anyway. It's more of a focus zone. It's generally accepted that when using old manual lenses that the focus confirmation is more of a guide, and won't be as accurate as AF using an AF lens. I've noticed this myself with a couple of my old m42 lenses, but now that I know what to expect, I just compensate visually.
10-10-2011, 04:39 PM   #8
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Focus zone is a good description of what's happening with the AF confirmation on a lens like this with deep depth of field. I'm not familiar with the lens in question, but a if you can get "out of focus" at both ends of the focus zone, you may be able to determine the two points at which the AF confirmation goes off as you approach from the front, and then from behind your subject. To center the depth of field around your subject you'll probably want to be somewhere close to the middle between these points.

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