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09-18-2020, 11:49 AM   #1
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mirror lens mount advice sought

Hello Everyone,
I have acquired a CENTON 500mm f8 mirror lens that has a practica pb mount. I thought would be interesting to find out how good/bad this is on my K50 but cant because of the mount. Then I discovered that the pb mount unscrews and was wondering whether this means I can get a PK version of the mount?
If so what would it be called (a T2 adapter?)
Any advice would be appreciated.

Rob

09-18-2020, 12:46 PM   #2
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Flange focal distance for Praktica B is only 44.4mm, meaning that a simple mount swap is not a solution.

Edit: I was assuming use of a screwdriver to remove the PB mount. If T-mount, as noted below, that would be another matter.


Steve

Last edited by stevebrot; 09-18-2020 at 02:50 PM.
09-18-2020, 12:53 PM - 1 Like   #3
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If I had to guess it is probably a T mount lens with the T->pb mount adapter.

The T-mount is a 42mm X .75mm thread so it has a thread pitch of .75mm and a diameter of 42mm. This actually produces an easy test to see if it is T-mount if you have a M42 to K mount adapter (the good pentax one works). If it looks like the screws might fit the M42 screws and the threads on the lens don't just slide through (M39 threads will) but when you try and screw it on with very light effort it only turns 1/4 to 1/2 a turn it is very likely T-mount. Do not force it since the thread pitches are different and you don't want to mess them up.

The thread pitch for M42 is 1mm so M42 mount is 42mm X 1mm with T-mount being 42mm X .75mm.
09-18-2020, 12:58 PM   #4
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The mount unscrews how? Does it thread off the rear of the lens or do you have to remove screws? If the former it may be a T-mount. Measure the diameter of the thread on the rear of the lens. If it is 42mm it is probably a T-mount. All you would need is a T/T2 mount for Pentax K mount.

09-18-2020, 12:59 PM - 1 Like   #5
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QuoteOriginally posted by stevebrot Quote
Flange focal distance for Praktica B is only 44.4mm, meaning that a simple mount swap is not a solution.
While true (I didn't check but assume that is true) a lot of those mirror lenses were T-mount sold with a T->whatever adapter. It sounded like this was the case since it was mentioned:
QuoteOriginally posted by Rob Payne Quote
Then I discovered that the pb mount unscrews and was wondering whether this means I can get a PK version of the mount?
Which to me would be a good indication that it was T->PB adapter on it originally so getting a T->K mount adapter would be all that is need.
09-18-2020, 02:47 PM   #6
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QuoteOriginally posted by MossyRocks Quote
While true (I didn't check but assume that is true) a lot of those mirror lenses were T-mount sold with a T->whatever adapter. It sounded like this was the case since it was mentioned:

Which to me would be a good indication that it was T->PB adapter on it originally so getting a T->K mount adapter would be all that is need.
I took it to mean that the mount could be removed using a screwdriver. If it is T-mount, that would be another matter. I did some Google work and at least some reports would indicate a lens with T-2 mount, so a change of adapter would work. Centon was the house brand for Jessops.


Steve
09-18-2020, 03:11 PM - 1 Like   #7
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Something else that may be an issue: A problem with some mirror lenses is that the back of the mirror case has a large diameter which prevents it from mounting close to a camera body with an overhanging pop-up flash housing. An extension ring can get around the mount distance issue. But to get infinity focus it may be necessary to adjust the focus stop. If you need to adjust it, this may help:

https://www.pentaxforums.com/forums/58-troubleshooting-beginner-help/173922-...ml#post1807361
09-19-2020, 12:55 AM   #8
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QuoteOriginally posted by MossyRocks Quote
If I had to guess it is probably a T mount lens with the T->pb mount adapter.

The T-mount is a 42mm X .75mm thread so it has a thread pitch of .75mm and a diameter of 42mm. This actually produces an easy test to see if it is T-mount if you have a M42 to K mount adapter (the good pentax one works). If it looks like the screws might fit the M42 screws and the threads on the lens don't just slide through (M39 threads will) but when you try and screw it on with very light effort it only turns 1/4 to 1/2 a turn it is very likely T-mount. Do not force it since the thread pitches are different and you don't want to mess them up.

The thread pitch for M42 is 1mm so M42 mount is 42mm X 1mm with T-mount being 42mm X .75mm.
Yes the diameter of the mount base is 42mm so it looks like you guys are right in that it is a T mount.

So I'll get a T-Mount to PK adaptor. What is the difference between a T1 and T2?


EDIT: I've just found the answer to the difference between T/T2. In this case it makes no difference but maybe the T2 gives you a bit more flexibility for using it with other lenses.

Thanks to everyone who chipped in with their knowledge.


Last edited by Rob Payne; 09-19-2020 at 01:06 AM.
09-19-2020, 01:51 AM   #9
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Decades ago, I bought (used) my Centron 500mm mirror lens and have used it with both T2-Practica Bayonet and T2- Pentax K adapters (although that might have actually been T2-M42 and then a M42-K adapter!) - as previously stated, you just manually unscrew one from the lens and then screw on the "other" one (a 20 second job).

OTOH, the lens is fairly sharp, but has had little use because of the limited number of situations in which it is a practical option (e.g. static targets at medium distances), and I now have other lenses that are both sharper and more practical to use.

PS: that lens needs a very deep hood on it because the front element is almost flush with the filter ring.

Last edited by jeallen01; 09-19-2020 at 02:00 AM.
09-20-2020, 01:17 AM   #10
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QuoteOriginally posted by jeallen01 Quote
Decades ago, I bought (used) my Centron 500mm mirror lens and have used it with both T2-Practica Bayonet and T2- Pentax K adapters (although that might have actually been T2-M42 and then a M42-K adapter!) - as previously stated, you just manually unscrew one from the lens and then screw on the "other" one (a 20 second job).

OTOH, the lens is fairly sharp, but has had little use because of the limited number of situations in which it is a practical option (e.g. static targets at medium distances), and I now have other lenses that are both sharper and more practical to use.

PS: that lens needs a very deep hood on it because the front element is almost flush with the filter ring.
Hi jeanlen01,
I've ordered a T2-PK adaptor and am waiting to give it a go.
Thanks for the tip about a hood.

I suspect that, like you, I may find few practical applications to use it - maybe with a teleconverter it might do a reasonable lunar picture?
I like compression in composition so will try it in 'townscapes' initially.

Thanks for the advice
09-20-2020, 01:01 PM   #11
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I have this lens. I bought it used with a K mount.

Years later I realised that the basic lens barrel ended at the back end in a cylindrical stub with a groove around it. A K mount adaptor was fixed to this by means of three recessed radial pinch screws 120 degrees apart. I have just taken mine apart for this photo, for which a jewellers screwdriver is needed. You can see one of the screw recesses at the top of the adaptor. The inner ends of the screws pinch into the groove to make a firm attachment, and in my photo you can see a tiny bright witness mark that a screw has made.

I don't know anything about the Adaptall or T-mount systems - is this it?
QuoteOriginally posted by stevebrot Quote
Flange focal distance for Praktica B is only 44.4mm, meaning that a simple mount swap is not a solution.
.... While Pentax is 45.46mm. But with the structure of my lens, the Pentax adaptor would simply need to be 1.06mm deeper than a Practika one, so I would not see a problem (unless you envisage frequent swapping back and forth). I guess the whole point of Centon making it this way was to use the same lens barrel with different simple adaptors for different camera brands. Of course, at F8, this lens has no diaphragm control to complicate things.
QuoteOriginally posted by PJ1 Quote
A problem with some mirror lenses is that the back of the mirror case has a large diameter which prevents it from mounting
Not a problem on my K-1. In fact there is more clearance than with my 24-70mm F2.8 DFA lens

09-20-2020, 02:29 PM   #12
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QuoteOriginally posted by Lord Lucan Quote
I have this lens. I bought it used with a K mount.

Years later I realised that the basic lens barrel ended at the back end in a cylindrical stub with a groove around it. A K mount adaptor was fixed to this by means of three recessed radial pinch screws 120 degrees apart. I have just taken mine apart for this photo, for which a jewellers screwdriver is needed. You can see one of the screw recesses at the top of the adaptor. The inner ends of the screws pinch into the groove to make a firm attachment, and in my photo you can see a tiny bright witness mark that a screw has made.

I don't know anything about the Adaptall or T-mount systems - is this it?


What your picture appears to show is a partially disassembled T2 mount. On "most" T2-mount lenses the "v-groove" part, where the screws locate, unscrews from the lens itself, that is to say it's part of the mount, not part of the lens. If this is the case it's simply a case of screwing on a different T2 mount to suit the new camera. The three screws are there simple to allow the lens body to be rotated with respect to the mount so everything lines up nicely when it's mounted on a camera.

Unfortunately not all T2-type lenses follow the rule, making the "v-groove" actually part of the lens … it's cheaper to machine a groove than a fine thread! This is unfortunate, because, although T2 mounts in their entirety are interchangeable, the component parts of the mount may vary between manufacturer. For example, the "v-groove" part may be slightly bigger (or smaller) from one manufacturer to the next.

Not a problem if you've got a lens which needs a complete mount, but some mount "outers", the part with the camera bayonet on, may not fit a fixed "v-groove", necessitating trying several different mounts to hopefully find one that matches the lens (bin there, done that!)


Good luck
09-20-2020, 04:00 PM   #13
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QuoteOriginally posted by kypfer Quote
On "most" T2-mount lenses the "v-groove" part, where the screws locate, unscrews from the lens itself, that is to say it's part of the mount, not part of the lens.
Yes, mine is somewhat like that. The rear circular closing plate, with the grooved stub on it, is held with three tiny cross-head screws. I have just taken it off out of curiosity. I have not taken another photo, but the rear lens element came with it together with a matt black light baffle in the form of a partial cone tapering towards the object end.
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