Adapting a projector lens

IgorZ · 07-24-2018, 07:28 PM

I would like to try to adapt a Colorplan 90mm f2.8 projector lens. I got an adapter to mount the lens on a focusing helicoid. The problem is that I am not sure what kind of an M42 helicoid to get. They are all of different sizes (e.g 12-35mm). In order to maintain infinity focus, does it have to be around 46mm at the shortest distance to the sensor?

**GUB** · 07-24-2018, 08:00 PM - **1 Like**

Pentax lens registration distance is 45.46.
Flange focal distance - Wikipedia
But you also need to work out how far the elements need to be from the sensor. Theoretically of course 90mm here but telephoto design makes that not hold true. So you need to try and work out infinity focus distance. Point the lens at the sun (infinity) and measure the distance to a piece of paper that is now on fire because you have it focused!!. Subtract the 45.46mm (From the flange at the bayonet) and you have some idea of the construction. Better to be a little short rather than long - it just means it will focus beyond infinity.

Medex · 07-25-2018, 01:26 PM

Colorpan 90 mm is not a big lens. Use smaller diameter helicoids with m42 thread and m42 to PK adapter (chinese silver color is OK) as well as adapter to hold the lens inside helicoid. The longer adapter, e.g. 35-85 enables you to have more extension (50 mm) or smaller 25-55 - 30 mm. In case of longer helicoid you will be able to make close focus pictures

IgorZ · 07-26-2018, 01:08 PM

Originally posted by GUB

Pentax lens registration distance is 45.46.
Flange focal distance - Wikipedia
But you also need to work out how far the elements need to be from the sensor. Theoretically of course 90mm here but telephoto design makes that not hold true. So you need to try and work out infinity focus distance. Point the lens at the sun (infinity) and measure the distance to a piece of paper that is now on fire because you have it focused!!. Subtract the 45.46mm (From the flange at the bayonet) and you have some idea of the construction. Better to be a little short rather than long - it just means it will focus beyond infinity.

Thanks! I will try that trick with sun and paper. It's been a while since the last time I used a lens to set paper on fire... Good old days...

---------- Post added 07-26-18 at 01:10 PM ----------

Originally posted by Medex

Colorpan 90 mm is not a big lens. Use smaller diameter helicoids with m42 thread and m42 to PK adapter (chinese silver color is OK) as well as adapter to hold the lens inside helicoid. The longer adapter, e.g. 35-85 enables you to have more extension (50 mm) or smaller 25-55 - 30 mm. In case of longer helicoid you will be able to make close focus pictures

Thanks for the advice. I got an adapter to hold the lens inside the helicoid. It has a M42 thread, so I need m42 to m42 focusing helicoid, just wasn't sure what size. I`ll try 35 - 85mm. Would love to try close focus photography with color plan...

07-24-2018, 07:28 PM	#1
IgorZ Veteran Member Join Date: Aug 2015 Location: Ottawa Posts: 1,735	Adapting a projector lens I would like to try to adapt a Colorplan 90mm f2.8 projector lens. I got an adapter to mount the lens on a focusing helicoid. The problem is that I am not sure what kind of an M42 helicoid to get. They are all of different sizes (e.g 12-35mm). In order to maintain infinity focus, does it have to be around 46mm at the shortest distance to the sensor?

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07-24-2018, 08:00 PM - 1 Like	#2
GUB Loyal Site Supporter Join Date: Aug 2012 Location: Wanganui Photos: Gallery \| Albums Posts: 5,760	Pentax lens registration distance is 45.46. Flange focal distance - Wikipedia But you also need to work out how far the elements need to be from the sensor. Theoretically of course 90mm here but telephoto design makes that not hold true. So you need to try and work out infinity focus distance. Point the lens at the sun (infinity) and measure the distance to a piece of paper that is now on fire because you have it focused!!. Subtract the 45.46mm (From the flange at the bayonet) and you have some idea of the construction. Better to be a little short rather than long - it just means it will focus beyond infinity.

07-25-2018, 01:26 PM	#3
Medex Pentaxian Join Date: May 2013 Location: Vilnius Posts: 1,021	Colorpan 90 mm is not a big lens. Use smaller diameter helicoids with m42 thread and m42 to PK adapter (chinese silver color is OK) as well as adapter to hold the lens inside helicoid. The longer adapter, e.g. 35-85 enables you to have more extension (50 mm) or smaller 25-55 - 30 mm. In case of longer helicoid you will be able to make close focus pictures

07-26-2018, 01:08 PM	#4
IgorZ Veteran Member Join Date: Aug 2015 Location: Ottawa Posts: 1,735 Original Poster	Originally posted by GUB Pentax lens registration distance is 45.46. Flange focal distance - Wikipedia But you also need to work out how far the elements need to be from the sensor. Theoretically of course 90mm here but telephoto design makes that not hold true. So you need to try and work out infinity focus distance. Point the lens at the sun (infinity) and measure the distance to a piece of paper that is now on fire because you have it focused!!. Subtract the 45.46mm (From the flange at the bayonet) and you have some idea of the construction. Better to be a little short rather than long - it just means it will focus beyond infinity. Thanks! I will try that trick with sun and paper. It's been a while since the last time I used a lens to set paper on fire... Good old days... ---------- Post added 07-26-18 at 01:10 PM ---------- Originally posted by Medex Colorpan 90 mm is not a big lens. Use smaller diameter helicoids with m42 thread and m42 to PK adapter (chinese silver color is OK) as well as adapter to hold the lens inside helicoid. The longer adapter, e.g. 35-85 enables you to have more extension (50 mm) or smaller 25-55 - 30 mm. In case of longer helicoid you will be able to make close focus pictures Thanks for the advice. I got an adapter to hold the lens inside the helicoid. It has a M42 thread, so I need m42 to m42 focusing helicoid, just wasn't sure what size. I`ll try 35 - 85mm. Would love to try close focus photography with color plan...