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Mod-3 with M42 lenses and swappable filters

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#1 stnagy

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Posted 22 September 2021 - 02:22 PM

There are a few posts on this forum explaining how to use unmounted 1.25" filters with Mod-3 devices and larger aperture camera lenses, involving modifying a c-mount adapter with epoxy to attach an empty filter ring inside the adapter. For example here and here

 

If you are open to using M42 lenses (available on eBay and elsewhere), here is another alternative for the Mod-3 that uses swappable 1.25" filters without the need for unmounting or modifying adapters with epoxy. 

 

The connection between the Mod-3 and the lens is made as follows:

 

1. Mod-3.

 

2. 5mm c-mount spacer (to clear the manual gain knob). (link)

 

3. C-mount to M42 adapter. (link)

 

4. 10mm T-Thread (M42) extension tube.  (link)

 

5. 1.25" filter adapter for T-Thread (M42) capture. (link)

 

6. M42 lens (in my case an 85mm f1.5 lens).

 

The filter adapter has a center hole threaded for 1.25" filters and an exterior diameter just smaller than the inner diameter of the M42 threads. So filters screw into the adapter, and the adapter "sandwiches" in a T-thread male and female union, in this case between the extension tube and the lens. The 10mm extension tube is used to get the lens in the "sweet spot" where it will comfortably reach focus at infinity. I played around with different extension tube distances, and +/- 5mm from this will probably also work. 

 

For this configuration, I ended up purchasing a filter adapter for each 1.25" filter I own. The adapters fit in the filter cases, and this way it is easy to quickly swap between filters as needed by removing the lens, swapping the adapter/filter combo, and re-attaching the lens. I have taken this set up out a few nights now and it works great. Some pictures are below. 

 

20210920 182453
 
20210920 182544
 
Edit:
 
Realized I should also post what didn't work for me in case others are curious. The above set up using the TS filter changer (instead of the 10mm T-Thread extension and filter adapter) had just a bit too much optical path to focus at infinity. It was close, so a narrower M42 filter changer might work, but I'm not aware of one that's currently marketed. 

 


Edited by stnagy, 22 September 2021 - 04:48 PM.

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#2 ButterFly

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Posted 23 September 2021 - 01:54 PM

It's a shame about the drawer.  I do see screws holding the body together.  Is it belt sander "just a bit too much" or just way too much?


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#3 bobo99

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Posted 23 September 2021 - 03:38 PM

 

There are a few posts on this forum explaining how to use unmounted 1.25" filters with Mod-3 devices and larger aperture camera lenses, involving modifying a c-mount adapter with epoxy to attach an empty filter ring inside the adapter. For example here and here

 

If you are open to using M42 lenses (available on eBay and elsewhere), here is another alternative for the Mod-3 that uses swappable 1.25" filters without the need for unmounting or modifying adapters with epoxy. 

 

The connection between the Mod-3 and the lens is made as follows:

 

1. Mod-3.

 

2. 5mm c-mount spacer (to clear the manual gain knob). (link)

 

3. C-mount to M42 adapter. (link)

 

4. 10mm T-Thread (M42) extension tube.  (link)

 

5. 1.25" filter adapter for T-Thread (M42) capture. (link)

 

6. M42 lens (in my case an 85mm f1.5 lens).

 

The filter adapter has a center hole threaded for 1.25" filters and an exterior diameter just smaller than the inner diameter of the M42 threads. So filters screw into the adapter, and the adapter "sandwiches" in a T-thread male and female union, in this case between the extension tube and the lens. The 10mm extension tube is used to get the lens in the "sweet spot" where it will comfortably reach focus at infinity. I played around with different extension tube distances, and +/- 5mm from this will probably also work. 

 

For this configuration, I ended up purchasing a filter adapter for each 1.25" filter I own. The adapters fit in the filter cases, and this way it is easy to quickly swap between filters as needed by removing the lens, swapping the adapter/filter combo, and re-attaching the lens. I have taken this set up out a few nights now and it works great. Some pictures are below. 

 

 
 
 
 
Edit:
 
Realized I should also post what didn't work for me in case others are curious. The above set up using the TS filter changer (instead of the 10mm T-Thread extension and filter adapter) had just a bit too much optical path to focus at infinity. It was close, so a narrower M42 filter changer might work, but I'm not aware of one that's currently marketed. 

 

Is there anything driving the decision for M42 lenses? Reason I ask is I already have 2 types of lenses (cmount and canon fd). I'd like to preferably get this working with one type of lens, and without unmounting 1.25" filters.



#4 stnagy

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Posted 23 September 2021 - 04:07 PM

Is there anything driving the decision for M42 lenses? Reason I ask is I already have 2 types of lenses (cmount and canon fd). I'd like to preferably get this working with one type of lens, and without unmounting 1.25" filters.

After thinking about it and looking over old posts on this forum, I thought my best chance of avoiding the epoxy method was to use M42 lenses. As opposed to Canon EF and Nikon F mount lenses, M42/T-thread lenses can mate with very narrow (~5mm wide) adapters, which freed up optical path for playing around with filter holders. 

 

Edit: ... and the c-mount lens threads are too narrow for 1.25" filters, so you'd probably need two adapters to attach a filter holder (one on the lens side, one on the Mod-3 side, with a filter holder sandwiched in between). The optical path just seemed like it would be too long, and I think the c-mount lenses have much shorter back focus than M42 lenses. 


Edited by stnagy, 23 September 2021 - 04:14 PM.

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#5 GeezerGazer

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Posted 25 September 2021 - 03:17 PM

stnagy, very nice use of adapters.  It is similar to what I did for Nikon lenses, using the M42 capture ring and found it to be very satisfactory.

 https://www.cloudyni...h-nikon-lenses/

I did a light cone trace for my lenses and found the best placement for the 1.25" filter/capture ring was closest to the NVD C-mount.  Your M42 lens might not require this, but if you see vignetting in the image, you might try moving the filter closer to the C-mount.  I use a 1/16" O-ring around the C-mount side of the M42 adapter, so I did not need the 5mm C-mount extension tube.
 

Currently, I use 2" filters behind my Nikon lenses.  They become an integral part of the adapter system, but require threads on both sides of the filter cell.  



#6 stnagy

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Posted 25 September 2021 - 08:39 PM

stnagy, very nice use of adapters.  It is similar to what I did for Nikon lenses, using the M42 capture ring and found it to be very satisfactory.

 https://www.cloudyni...h-nikon-lenses/

I did a light cone trace for my lenses and found the best placement for the 1.25" filter/capture ring was closest to the NVD C-mount.  Your M42 lens might not require this, but if you see vignetting in the image, you might try moving the filter closer to the C-mount.  I use a 1/16" O-ring around the C-mount side of the M42 adapter, so I did not need the 5mm C-mount extension tube.
 

Currently, I use 2" filters behind my Nikon lenses.  They become an integral part of the adapter system, but require threads on both sides of the filter cell.  

Thankfully, I don't see any vignetting with the filters positioned where they are. In this configuration, I'd guess they are maybe around 10-15 mm from the Mod-3 housing. 

 

I am thinking of trying out the Baader UHC filter system next. It looks like it may be possible to ditch the c-mount to M42 adapter with this system by ordering a c mount plate that will work with the system from precise parts. 


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