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06-20-2010, 02:46 AM   #1
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Samyang 800mm + T2 -> Pentax infinity focus problem

just bought a Samyang 800mm f8 mirror lens, it needed a T2 to Pentax mount for my K7.

tried 2 mounts but all cannot infinity focus, but T2 mount to Nikon ok.

please advice... do I have to do any adjustment with the T2 mount?

thanks in advance.

06-20-2010, 07:16 AM   #2
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Is the Nikon a "full frame" dSLR? That might be the difference.

According to Wikipedia:

"Because the T-mount has a long flange distance of 55mm and most 35mm cameras have shorter flange distances, a simple mechanical adapter called T/T2-Ring is needed to adapt a T-mount lens to any camera body without optical correction to achieve infinity focus."

I am thinking that the APS-C sensor in the K-7 would need a different adapter than one used on the FF sensor.
06-20-2010, 08:39 AM   #3
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QuoteOriginally posted by bigdog104 Quote
Is the Nikon a "full frame" dSLR? That might be the difference.

According to Wikipedia:

"Because the T-mount has a long flange distance of 55mm and most 35mm cameras have shorter flange distances, a simple mechanical adapter called T/T2-Ring is needed to adapt a T-mount lens to any camera body without optical correction to achieve infinity focus."

I am thinking that the APS-C sensor in the K-7 would need a different adapter than one used on the FF sensor.
thanks bigdog

my Nikon is FF because it is a film camera

understand what you mean; my T2 mount is for FF PK mount instead of APS-C.

but it named can be used for k20d, kx, etc.
06-20-2010, 12:05 PM   #4
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No, it has nothing to do with FF and APS. The mount is the same. You simply have to adjust the infinity on focusing ring. Can't give you specific instructions for this lens but look for set of screws that will let you adjust focusing ring.

Marcin

06-20-2010, 03:35 PM   #5
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QuoteOriginally posted by bigdog104 Quote
Is the Nikon a "full frame" dSLR? That might be the difference.

According to Wikipedia:

"Because the T-mount has a long flange distance of 55mm and most 35mm cameras have shorter flange distances, a simple mechanical adapter called T/T2-Ring is needed to adapt a T-mount lens to any camera body without optical correction to achieve infinity focus."

I am thinking that the APS-C sensor in the K-7 would need a different adapter than one used on the FF sensor.
NO - that is not correct.

A T2 adapter is a T2 adapter and Pentax kept the flange distance between the 35mm film cameras and the current line of APS-C and all future cameras, that will be able to use K-mount lenses.

This is one of Pentax' main selling points!

Ben

Last edited by Ben_Edict; 06-20-2010 at 03:43 PM.
06-20-2010, 03:42 PM   #6
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QuoteOriginally posted by k7bb Quote
just bought a Samyang 800mm f8 mirror lens, it needed a T2 to Pentax mount for my K7.

tried 2 mounts but all cannot infinity focus, but T2 mount to Nikon ok.

please advice... do I have to do any adjustment with the T2 mount?

thanks in advance.
Though there is the possibility, that the lens does indeed not reach infinity focus, because the focusing ring might be misaligned, I would be hesitant with such an explanation.

First of all, you should be very(!) aware, that focusing a slow 800mm lens is anything but easy. If this is a new lens for you, I would not be surprised, if you need several days, before you even start getting the focus right!

Than you should be aware, that a mirror lens has usually not a mechanical stop at infinity - you can focus beyond infinity for several reasons - and the infinity mark on the focusing ring might also be just a rough indication and the real infinity focus may be offset from the indicator.

Also, you should consider, that if focusing a 800mm lens to infinity means, that you will have a lot of air between your camera and your target. This long distance will reduce sharpness under most circumstances significantly, due to haze, atmospheric turbulences etc.

I had a similar experience with a Pentax 500/4.5 lens, which - I thought - could not be focused to infinity. I even disassembled the focusing unit and readjusted it in a way, that I could focus beyond infinity, just to be on the safe side. But after a lot of trials, I finally could prove, that all the lack of sharpness was only due to atmospheric conditions during my initial tests.

Ben
06-20-2010, 03:51 PM   #7
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QuoteOriginally posted by Ben_Edict Quote
NO - that is not correct.

A T2 adapter is a T2 adapter and Pentax kept the flange distance between the 35mm film cameras and the current line of APS-C and all future cameras, that will be able to use K-mount lenses.

This is one of Pentax' main selling points!

Ben
Well I guess I will lose some reputation points for that suggesting it might be the APS-C vs FF.
06-20-2010, 05:25 PM   #8
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thanks Marcin & Ben, also bigdog

another thing is, afterwards attached the lens to a Canon DSLR (as you know crop factor is 1.6x instead of 1.5x), I got wider view angle and can be infinity focus...

so, Ben, I'll try again with your advice. if still not work, I think I have to search 3rd mount

thanks all

06-20-2010, 08:03 PM   #9
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Are you sure that the T2 adapter you're using is for a k-mount? I have a T2 adapter that I use on a cheap 500mm mirror lens I picked up on eBay. It works perfectly on my K10D, as well as on my Pentax 35mm film cameras.

You say that the lens can focus to infinity on other cameras, so it is probably not the lens. The only way I can think of that it would not focus to infinity on a Pentax camera is if you were using a T2/M42 screwmount adapter, along with a M42/K-mount adapter that has the large flange outside the lens mount.
06-20-2010, 11:34 PM   #10
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thanks noblepa,

since I have never seen M42 mount, can only trust the packing, it is T2...
06-21-2010, 01:14 AM   #11
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QuoteOriginally posted by k7bb Quote
thanks noblepa,

since I have never seen M42 mount, can only trust the packing, it is T2...
Could you provide some detailed images of the lens' rear? The images, you linked to, are mostly of the front element and do not give a clue about the mount.

Thanks,
Ben
06-21-2010, 09:46 AM   #12
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photos of the adapter and lens mount
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06-21-2010, 01:48 PM   #13
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QuoteOriginally posted by k7bb Quote
photos of the adapter and lens mount
O.k. - thanks for the images. Looks fine to me, with the T2 thread on the lens and the T2-K adapter. Can you screw the T2 adapter fully onto the lens threads? The lens flange and the flange of the adapter should be flush, with no space between.

If that is also thew case, the only other thing, I would like you to do,: please provide two or three images, where you tried to achieve infinity focus and couldn't. Perhaps we can draw a conclusion from those.

Ben
06-21-2010, 02:22 PM   #14
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I'm pretty sure that it's adjustment problem. I had the same with my russian lens. The fact that it works with other mounts supports it. Look for a way to adjust infinity range on focusing ring. If you are not confident enough to do it, give to professional but it should be relatively straightforward. I did it myself on my lens.
Marcin
06-21-2010, 03:44 PM   #15
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QuoteOriginally posted by marcinski Quote
I'm pretty sure that it's adjustment problem. I had the same with my russian lens. The fact that it works with other mounts supports it. Look for a way to adjust infinity range on focusing ring. If you are not confident enough to do it, give to professional but it should be relatively straightforward. I did it myself on my lens.
Marcin
What kind of adjustment problem would that be, if the same lens reaches infinity on one brand camera and not on the next, if both are connected by a T2-adapter? The T2 system should make the lens itself fully independent of the camera system, as the adapters compensate the different register distances of the different bodies.

Ben
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