How to adapt a projector lens for the Pentax 67ii?

[pkr1979]

Hi all!

Im hoping for some advise on how to adapt a projector lens to the Pentax 67ii. Not long ago I got a ISCO Ultra MC 130/2, and as far as I understand I now need to get a

• m65 to Pentax 67 adapter - anything I need to be aware of?
• m65 helicoid - but which mm?
• m65 thread adapter lens mount - but what size?

If anyone know the filter size of this lens Im curious to know what it is.

Cheers
Peter

 

[choiliefan]

Back when I was doing this sort of thing, I used a helical and a set of Pentax extension tubes in various combinations. Had I been able to afford it at the time, a bellows would have been useful as well. Its pretty amazing what one can do with black cardboard or plastic, scroll saw and a hot-melt glue gun.

 

[xkaes]

I adapted a Tamron projector lens to 35mm by measuring the diameter of the rear of the lens. A 55mm filter fit perfectly -- I removed the glass (which was in bad shape). It didn't screw on, but was a perfect fit. Then I used a reverse adapter to the Minolta bayonet.

 

[OAPOli]

You need to check the barrel diameter of the lens. Say it's 62.5mm. Then get a 62.5mm clamp to m65, m65 helicoid and m65 to P67 adapter.

Next is the issue of reaching infinity focus, if that's desired. You probably need the short 17-31mm helicoid and might need to machine the clamp adapter.

 

[pdccamerqs]

hi Peter,

As far as what size helical you will need. I would start with the focal length of the lens: 130mm. So to get infinity focus you will need to position the focus point of the lens 130mm from the film plane. The flange focal length of the P67 is 85mm, so you need 130mm - 85mm = 45mm of space between the lens focus point and the film plane of the P67. The tricky part is figuring out where the focus point of the lens is. I would point it at a distant target and just hold it in front of the the P67 and move it in and out while viewing the target on the camera's focus screen. Then measure how far the back of the lens is from the camera. This is how much space you will need between the camera and the back of the lens to hit infinity focus. To focus closer, you will have to move the lens farther away from the camera. As suggested earlier you can use a combination of helical and close up extension tubes to get to the correct distance.

Best,

Paul

 

[blee1996]

I did like @OAPOli, making sure you have enough extension tube and helicoids both for infinity and close-ups.

On the left, you have Cinelux projection lens, clamp to M65 adapter by RAF Camera, 17-31mm M65 helicoid, M65 to P67 adapter.

The battle of two 150mm homemade lens for Pentax 6x7 by Zheng, on Flickr

 

[pkr1979]

Thanks again fellas @blee1996 does your Cinelux focus at infinity? And how close can you focus?

 

[Petrochemist]

Before splashing out on hardware do some crude tests.
wedge the lens in a piece of card (to give a shadow) then see how far from a flat surface (walls & paper have worked for me) you need to be to get a focused image of clouds/sky through the lens. This will be your infinity focus. You'll never want the lens mounted closer than this, so it can be made up with rigid extension tubes (or even cardboard/plastic packaging tubes), a further 130mm of extension should give you life size (macro images) which might give you an idea of how much you need the lens to move in focusing. Most users won't want macro.

This test should also give an idea of the lenses covering power though 130mm is probably fine for 6x7

As Paul mentioned it will be 130mm from the lenses rear nodal point, but sadly that point it unknown. With projectors it should at least be withing the length of the lens (NOT the case with telephoto, or retrofocus designs)

My many projector lenses vary hugely, some only cover MFT or even less, but a few cover 5x4"
I like my 150mm/1.2 projector till it's time to move it, or control short enough exposures:
A challenging lens by Mike Kanssen, on Flickr

 

[pkr1979]

Hi all!

So - thanks for the responses guys. A little over-eager to get started I got the 17-31mm helicoid, a 52.5mm to 65mm adapter to attach the lens to the helicoid, and a 65mm to P67 adapter to attach the helicoid to the camera. All these things fits but the lens is way to far away from the camera to reach infinity... Im assuming I got focus from 0.5m to 1.5m... even if I remove the helicoid the lens is to far away from the camera. To reach infinity it seems to me the back of the lens (where is also about where the glass begins) should line up with the edge of the lens mount.

How can i fix this? As far as I can see there aren't any shorter helicoids on ebay. Id like to be able to reach infinity or at least get quite a bit closer to it then I can now.

Cheers
Peter

 

[Petrochemist]

Hi all!

So - thanks for the responses guys. A little over-eager to get started I got the 17-31mm helicoid, a 52.5mm to 65mm adapter to attach the lens to the helicoid, and a 65mm to P67 adapter to attach the helicoid to the camera. All these things fits but the lens is way to far away from the camera to reach infinity... Im assuming I got focus from 0.5m to 1.5m... even if I remove the helicoid the lens is to far away from the camera. To reach infinity it seems to me the back of the lens (where is also about where the glass begins) should line up with the edge of the lens mount.

How can i fix this? As far as I can see there aren't any shorter helicoids on ebay. Id like to be able to reach infinity or at least get quite a bit closer to it then I can now.

Cheers
Peter

It sound like you'll have to mount the lens further back in the helicoid. One of my smaller format projectors only reached infinity focus with the original rear of the lens a few mm from the sensor. Fortunately the last cm of the lens housing could be removed without effecting the optics. Even after trimming the rear of the lens came slightly out the back of the helicoid at infinity.

It is possible to use a negative diopter in front of the lens to increase it's focal length (& hence the distance behind the lens to an image) This was done in the early half of last century but I doubt IQ will be usable on today's system, so I've not tried this.

 

[pkr1979]

It sound like you'll have to mount the lens further back in the helicoid.

Thanks. If I could put the lens in the helicoid itself this would actually work. The problem is that the helicoid is a bit too narrow.

Fortunately the last cm of the lens housing could be removed without effecting the optics. Even after trimming the rear of the lens came slightly out the back of the helicoid at infinity.

The glass is pretty close to the end.

 

[OAPOli]

From the pictures I found online, it seems the 52.5mm section on your lens is pretty short. What's the dimension of the larger section? I guess it's larger than 65mm?

There are slim (1mm) P67-M65 adapters. That could gain you a few mm depending on the adapter you currently have.

Another option is using a M77 helicoid. The problem is that there are no adapters for them but I managed to 3D-print some. The pitch is 0.75 so you can combo them with step up/down rings for the threads.

 

[pkr1979]

From the pictures I found online, it seems the 52.5mm section on your lens is pretty short. What's the dimension of the larger section? I guess it's larger than 65mm?

There are slim (1mm) P67-M65 adapters. That could gain you a few mm depending on the adapter you currently have.

Another option is using a M77 helicoid. The problem is that there are no adapters for them but I managed to 3D-print some. The pitch is 0.75 so you can combo them with step up/down rings for the threads.

The large section is 71mm, so a M77 helicoid might be the way to go. However, I cant really find a M77 to P67 adapter?

 

[OAPOli]

The large section is 71mm, so a M77 helicoid might be the way to go. However, I cant really find a M77 to P67 adapter?

Like I said there are no M77 adapters unfortunately. Here's what you can do.

1) Step down ring 82-77mm. Machine out the 82mm threads, drill/tap small holes to screw it into the P67-M65 adapter;
2) 3D-print a piece with 77x0.75 male threads (or attach a 77-82 step up ring for the threads) and 71mm ID, with set screws to hold the lens.

When there's a will there's a way?

 

[pkr1979]

Like I said there are no M77 adapters unfortunately. Here's what you can do.

1) Step down ring 82-77mm. Machine out the 82mm threads, drill/tap small holes to screw it into the P67-M65 adapter;
2) 3D-print a piece with 77x0.75 male threads (or attach a 77-82 step up ring for the threads) and 71mm ID, with set screws to hold the lens.

When there's a will there's a way?

There is a way and there is will

It might also work with a 65-77 step up ring if sturdy enough to attach the M77 Helicoid to the M65-P67 adapter - as the adapter will not obstruct the narrower end of the lens.

What is 71mm ID?

 

[OAPOli]

ID=internal diameter. The idea is to make an analogous adapter to the 52.5mm-M65 you have but at 71mm-M77.

I'm afraid 65mm is not a standard filter size so I don't think you'll find such a step-up ring. In addition, filter threads are 0.75mm pitch while the m65 helicoid is 1mm pitch. The M77 helicoid has 0.75mm threads however.

 

[pkr1979]

The internal diameter is 68mm - but it might be a 1mm pitch. I didn't consider the threading... and I got a 68-67mm step down ring so I could use filters with the ISCO lens, but when I screw the step down ring on it it doesn't really go all the way.

 

[OAPOli]

I meant the ID of the adapter, which should be the OD of the lens.

You can also try to thin down the P67-M65 adapter on a lathe. If the flange is 6mm thick you could reduce it to 1mm. It won't buy you much in terms of focusing distance though.

 

[Petrochemist]

I meant the ID of the adapter, which should be the OD of the lens.

It's not a problem if the ID is bigger than the lens OD (it's easy to shim it up if needed)
Having the ID smaller than the OD is of course a big problem for mounting the lens close

 

[blee1996]

Since absolute image sharpness is not the main objective in using projection lenses on P67, you can also experiment with low-tech and temporary solutions such as gaffe tape, plastic/cardboard tube, sheet metal, etc. You can figure out what works for both infinity and close range, and play with it until you find a more permanent and elegant solution. Elegant and permanent solution might need some custom machined parts, or at the very least, 3D printed adapters.

 

[OAPOli]

It's not a problem if the ID is bigger than the lens OD (it's easy to shim it up if needed)
Having the ID smaller than the OD is of course a big problem for mounting the lens close

Yes that's correct, the ID should be a hair larger than the OD.

 

[pkr1979]

Epoxy worked just fine Focuses from infinity down to 1,4 meters.

 

[OAPOli]

@pkr1979 what did you epoxy together?

 

[pkr1979]

The mount and the 82-77 stepdown ring. Which then screws on the 77mm helicoide. And a buddy of mine 3dprintet 77mm to 70.5 mm holder for the lens.

 

[OAPOli]

Good job