AF Nikkor 20mm f2.8 damaged front.

[Seapy]

Not sure if this is the right place or under 'Lenses' but decided on balance here.

My 20mm took a tumble while attached to my D3, D3 is fine, lens not so good.  It crunched the front rim and it rattled, sheared a screw inside.  I tried to unscrew the front to remove the front but just made it worse, the screw head was jamming the AF and since I needed the lens I dug a hole in the already damaged front to allow the screw head to escape.  The damage is ugly.



Now I have my lathe up and running, I have decided to make a new front for the lens.  I have found by removing the rubber grip on the focus ring and focusing at infinity I can access the three screws which retain the front piece of the lens.   The front piece is very simple and only about 10mm long by about 67mm diameter.   I don't have a lens hood for this lens but I believe the correct hood is very shallow.  Looking at my 16mm fisheye it's lens hood is built in, with a slip on metal cap which is very handy and quite robust.

I am considering making a new front which has a similar built in lens hood with a slip on metal cap.  This provides some protection for the front element and perhaps helps shade the lens somewhat from direct sunlight.  My preferred stock is 25mm thick so that would allow for a hood about 15mm deeper than the original front of the lens.  Is that worth doing?  I *could* use different stock and have a much longer hood but the 20mm lens is pretty wide on FX, I anticipate some vignetting or even loss of image at the corners if I go too long.

[Erik Lund]

The usual approach is to cut a piece of thin cardboard, draw a line while looking through the viewfinder for 1/4 of the circumference and copy and mirror it around the lens for a full version, trim cardboard template until satisfied, test stopped down f/22 at close and far, CRC,,, transfer template to metal,,,

[richardHaw]

I can try and ask for replacement parts
I am going to the Nikon warehouse soon anyway

[Erik Lund]

I'm pretty sure this is not a case of path of least resistance,,, 

[Erik Lund]

This is a shortcut to creating a template:


http://www.lenshoods.co.uk/hoods/Nikon-AF-Nikkor-20mm-f-2.8-D.php

[Seapy]

I'm pretty sure this is not a case of path of least resistance,,, 

  Not looking for a standard solution!

Want to spend a few hours making swarf.

Thank you for the link Eric, good starting point, it will be a good guide for the cut outs, I think I will keep the flat top profile similar to the 16mm f2.8 fisheye, and parallel so I can make a and fit a matching slip on cover.  I have lot's of nice aluminium. 

I had been fretting about cutting the filter threads, two threads 62mmØ at  0.75mm pitch my lathe can do metric but only 'officially' down to 1mm pitch.  However after spending a few hours with Excel spreadsheets I discover that ~43 threads per inch is mighty close and I can find a combination from my change wheels that gives me that... But I don't really need a filter thread, I don't use filters usually and if I make an integral lens hood with slip on cap, I don't even need a pinch lens cap.

[Seapy]

I can try and ask for replacement parts
I am going to the Nikon warehouse soon anyway

Many thanks for the offer Richard.

On this occasion I don't feel that's my path.  Maybe one day I would really need something special?

[Seapy]

Well, my 20mm f 2.8 D has sat on my desk long enough, I took some measurements and headed for the shed...

I cut a blank then realised it had a hole in a bad place so cut another.  Faffed about mounting it in the lathe so it could be reversed and retain accurate alignment, it stays within 3 thou about 0.09mm that's good enough for me.  I have roughed it out and pretty well finished the back of the part.  I have decided to make the lens shade 75mm OD and pretty shallow.  I will assemble it and if need be I will relieve it as required to avoid any corner vignetting.

Mounted on a length of scrap shaft, roughing out the outside.



The original black plastic lens front propped behind the new part.



This is the lathe cutting metal into swarf! 

[Erik Lund]

Looks like s good shape is already coming to life


Remember to make the inside rippled or do you plan to use flocking,,,


I also prefer the flat top and bottom like on the Fish


Nice when you put the lens front end down

[Akira]

Robert, now you started to make us wonder again!

[Seapy]

Looks like s good shape is already coming to life


Remember to make the inside rippled or do you plan to use flocking,,,


I also prefer the flat top and bottom like on the Fish


Nice when you put the lens front end down

Plan to put grooves in like the plastic original,  then hopefully anodise mat black.  I would like to make a Fish cap like the 16 with the Nikon logo... Face down it won't fall over, that's for sure!

I am told it's best to store these lenses face down to help avoid oil on the blades, is that right?

[Seapy]

Robert, now you started to make us wonder again!

That's the plan Akira! LOL   

[Matthew Currie]

Nice recycling of dead ball bearings there too.

[Seapy]

LOL, throw nothing away...  Well keep handy bits of metal anyway.

I needed a 14mm spacer and that bearing is 14mm bore, perfect.

[Erik Lund]

.....I am told it's best to store these lenses face down to help avoid oil on the blades, is that right?

I would say it doesn't matter