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Fujinon 12.5mm f1.4 CF12.5A modification

zcream

New member
Finding a wide angle fast lens for the GH1 is a problem. There are simply none available. Vivek from GetDpi played around with the Fujinon 12.5mm f1.4 and modified it to get full coverage. This is my attempt to do the same.

This attempt was referenced in these places.

http://forum.getdpi.com/forum/showpost.php?p=230992&postcount=39
http://forum.getdpi.com/forum/showthread.php?p=234790#post234790

Since I cannot post a gallery, I have put up the pics and the description over here.
http://www.mishra.tv/wordpress/?p=150

Tools used. DIY Rubber tool for unscrewing recessed lenses and a set of lens spanners. Not shown here but also used is a small screwdriver from a 2$ jewellers set of tools from DSE.

Rear element. The rear element was removed with a lens spanner. The glass element is not attached to anything and will fall off, so be careful.

Front element. The front element was first loosened with the lens spanner and then unscrewed with the rubber tool.

Lens body and focus ring/hood. There are 3 small screws in the focus ring that come off with the jewellers screwdriver. Then the 2 parts easily separate.

Top view of hood/focus ring and lens body. This view shows the brass helicoid (??) that is involved in the conversion. We can also see the shutter in the lens body.
 
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zcream

New member
Now this attempt is stuck and I will wait for Vivek to answer my questions as I have absolutely no clue what he did.
 

zcream

New member
Just for me to note down my query. I can reduce the distance between the front element and the aperture, or the rear element and the aperture.
This is what I am waiting for - since I'm not sure what is ideal.
 
V

Vivek

Guest
Move the entire front group closer to the diaphram. You need to rehouse the front group and get rid of the old metal housing. This is not an easy task at all...

.. and I do not mean that lightly.
 

zcream

New member
Hi Vivek. Great to hear back from you. My brother is actually biking around India at this time on an Enfield Bullet.

I think I can file down 2mm from the housing to move it closer - thats the part above the threads. I dont think I have the expertise to make a new housing.
I can try do it tomorrow. I also bought a sampling of other C-mount lenses, 8mm, 8.5mm and 6.5mm and I will try my approach on all of them one by them to see how it works out.

Thanks for the tip!
Move the entire front group closer to the diaphram. You need to rehouse the front group and get rid of the old metal housing. This is not an easy task at all...

.. and I do not mean that lightly.
 
V

Vivek

Guest
An Enfield Bullet is an antique rarity now (at least in the South). :)

Everyone I saw has been riding a Honda or something...

Good luck with your modification. But, IMO, it is perhaps better to wait for the rumored 14/2.5 of Pana that should show up within a couple of months.
 

zcream

New member
Problem is that I really need the extra speed - I am shooting a film outdoors at night. I have no lighting and only work with street lights - so f1.4 is barely enough for my requirements.
About the Bullet, there are 5 of them in a group. The bike does not work half the time, so I dont know the appeal anyway.
I took out the front hood and now I have gone through 2.5mm. there is 2mm of thread left where the front element screws in. I will try some more - but its promising, there is a VERY small vignette left.
 

zcream

New member
I screwed up. I got impatient while reassembling the lens and broke the aperture blades. 100 bucks down the drain.
Vivek,did you sand down the black metal housing all the way to the brass part ? I can temporarily use super glue to hold the front element in front of the helicoid, but I do not see any way to disassemble the part in front of the aperture any more. The helicoid and the housing seem to be glued together or screwed in a clever way that prevents disassembly.
 

zcream

New member
I ripped out all the aperture blades as some were cracked. Now I have an image which looks like the ones from the X-Ray lenses with strange glows all over. Perhaps I did not reassemble the lens properly. can someone who owns the lens let me know if the rear element is concave or convex ? And once I have that, I can work with the inner 2 elements for proper reassembly.
 

Brian S

New member
Have any of you tried putting a negative element in front of the lens? That should increase the back focus, allowing the entire lens to be moved out from the sensor. You would need a small extension tuve. Chromatic aberration will increase.

I just picked up one of these. I might play with it. The vignetting is not so bad. The 12.5mm F1.4 is stated for use with 1" diagonal sensors, at least in literature. It seems to come close to the 4/3 EP2, which is larger than 1" diagonal sensor.
 

Brian S

New member
Okay- just discovered that 1" C-Mount format is computed by adding two sides, and not using the diagonal. So, the image circle for a 1" C-Mount lens is 16mm.

The Negative Element in front did not work. Moving internal elements around is going to change the focal length.

I suspect the rear group would have to be replaced with one with a much larger diameter, which is the difference with some half-frame vs full-frame lenses. Specifically, the CZJ 5cm F1.5 Sonnar in Contax mount vs Arriflex mount. But that's "just a guess" to increase coverage on this lens.
 

zcream

New member
Vivek has had full coverage by moving the front element to the iris. And I could see the coverage increase as well.
I treated the lens roughly while unscrewing it and then not reassemble it properly - but I can confirm that the image circle increases by moving the front element to the rear.
 
V

Vivek

Guest
Just for the records:

1. I did not move any elements (that would have been a disaster). I moved the entire front group.

2. The FL did not increase, from what I could see.

3. The lens' speed also did not become less.
 
R

raydeng

Guest
Vivek, want to give a more detailed account of how you did that?
 

zcream

New member
I should edit - I moved the front group (of at least 2 elements) towards the aperture blades.
The front group threads into the lens body. You need to remove the hood and the focus ring.
I filed down the lens body almost all the way.
Vivek possibly did away with the the threaded holder completely and built a new housing.

Look at my pics linked in the first post. I got full coverage and moved ~3-4mm closer to the body.
Wish I had bought 2 lenses to play with. Now I have to wait for another lens to come through.
 

zcream

New member
vivek commented on the computar. There wasnt enough separation between lens groups to bring them together
 

zcream

New member
I have the Hi-G lens. It has a custom mount and is difficult to take apart. Not enough coverage.

BTW I got the elements right and the 12.5mm works brilliantly. Nearly full coverage. I used the original assembly and just filed some thread away.

Brilliant!
Buying a few more off ebay..
 
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