Lens modification for reducing vignette

R

rondo

Established
Local time
4:36 AM
Joined
Sep 4, 2008
Messages
132
View My Gallery
I am experimenting with a Nikon 3x teleconverter on Ricoh GR.
You can see a sectional diagram of the Nikon lens here:
https://ibb.co/fjmOqF
Is there mechanical modification that I can do on the Nikon to reduce the vignette?
Reducing the space between the fixed lens of the camera and the rear glass of the converter will help to an extent: I hope to manage to remove the rear ring holding the rear glass together, shave it off, and reinstall. that should help a little, but not eliminate it altogether.
I was wondering if reducing the space between the front and rear glass groups would help at all?
That's doable mechanically, but I am not aware of its optical consequences: will the lens stop focusing for example? Or will the magnification ratio change? But I feel like reducing the length of the tube will reduce vignetting. Does that make any sense at all?
 
Changing the distance between any elements or groups of elements in a lens means that it will no longer focus. Even a very small amount, 0.1 mm will already lead to "soapy" focus.
 
popavvakum said:
Changing the distance between any elements or groups of elements in a lens means that it will no longer focus. Even a very small amount, 0.1 mm will already lead to "soapy" focus.
Click to expand...
Thanks!
Yes, but this is a afocal lens. Basically a magnification glass in front of the camera lens. So I'm not sure if focusing will be as critical as a regular lens. I don't know. That's why I raised the question.
 
Actually: I think you're correct. Moving the converter lens back and forth as a single entity, doesn't effect focusing capabilities. It just reduces the image size, increase the vignette etc. basically, just like a magnifying glass in front of your eye. But rearranging the glasses within will most likely damage the focus. I don't know...just thinking out loud.
 
Update: I have another teleconverter where the glass groups are threaded together. I separated and tried several hand held tests. Seems like there is no focus problem when the distance between groups changes for few millimeters . Of course this was a very quick and dirty test. Not sure if corner sharpness or some other parameter (such as lens distortion) changes. But it didn't look completely hopeless to me.
 
Lens design is made to optimize focal length, aberrations and aperture ,while keeping physical dimensions reasonable.
If you do what you are asking about, you will screw up all 3 to redce length. If you get lucky it will indeed increase the clear aperture but definitely won't be afocal anymore, and aberrations will make the image worse.
 
Rob-F said:
The vignetting is occurring with the teleconverter, or with the Ricoh?
Click to expand...

Ricoh GR is a fixed lens camera. No problem with GR lens. The teleconverter, on the other hand, is designed for small sensor Nikon compacts from early 2000s. It seems like a sharp lens but with a small image circle when coupled with Ricoh.
That's how I started with a Fuji teleconverter lens on the GR but managed to make it work with GR, hence this exercise. Although this one seems to be much more challenging.


https://www.dpreview.com/forums/post/55926904
 
Use a center gradient filter like Schneider or Heliopan make. I use my Xpan center filter on my 47mm Super Angulon on my Veriwide and it works great. Also have on on my 105 f8 Fujinon on my G617. I had them for my 47mm XL SA and 58mm SA XL too and they work perfect. No reason why you can't use one on your setup.
 
Maybe vignetting was wrong way of putting the problem. It is small image circle when the 3x converter is combined with GR.
 
Rob-F said:
The vignetting is occurring with the teleconverter, or with the Ricoh?
Click to expand...

I think I have misunderstood your question.
I believe what you asked me was if the Ricoh lens was not extending far enough to reach the rear element of the Nikon and therefore causing vignette vs the length of extension lens causing the vignette?
My guess is it is a combination of both.
I will start by removing the rear retaining ring on the nikon, so that the Ricoh lens can get closer to the rear glass. That should enlarge the image circle, but not sure if that would be enough. By the way my Ricoh GR 2 is a modified camera-the dpreview link I gave above has some pictures. With the original camera the vignette caused by Ricoh would have killed the project from start!
The fuji teleconverter is almost perfect on modified Ricoh. But I very much doubt this will turn out to be as good.
 
Can't eliminate vignetting with this combination. I did manage to reduce it, but on Ricoh GR all I can get is a 5 MP crop (140 mm equiv)
 
You must log in or register to reply here.
Back
Top Bottom