fecaleagle
11-Aug-2012, 13:03
This is a bit of a cross-post between here and the Collodion.com forum, so I will start by posting my original question and then try to elaborate a bit:
I have looked up and down the net seeking information about determining the correct position of a DIY iris/stop in a brass lens (petzval) barrel. *Most lenses I've seen that are cut for Waterhouse stops seem to have the slot (somewhat) centered between the front and rear lens groups, give or take 5-10% forward or back, but I've been warned multiple times against cutting a slot arbitrarily.
The thing is, nobody has been able to tell me how to determine exactly where the stop needs to go. *I am going to start by constructing a washer stop (a ring with a hole in it) that I can slide into position along the barrel, but how will I even know when the stop is located in the right position? *Are there any telltale signs that I've got it located correctly or aberrations I should look for to tell me that I've got it located incorrectly?
Does anybody here have any experience with this? *Because I'm feeling a little lost.
Thanks!*
Basically, I am finding that there are simply too many unknowns to try to calculate the front and rear nodal points of this lens, and that I'm going to have to use my eyes to produce any useful information. The lens is in Dallmeyer configuration, and it doesn't have an internal stop to clue me in as to the proper location of a stop.
I have been told that the "Principal Point Separation" represents the distance between the two nodal points and is the space within which a stop will need to be located.
I was hoping there was some formula I could apply to find the nodal points, but that seems not to be the case. *As I'm moving the stop, what are the characteristics of the image formed that I should be looking for? *Besides "looks sharp here", that is. *For instance, will the range in which I can move the stop without a discernable decrease in sharpness represent the location from the first to second nodal point? *Then should I just locate the stop between those two points?
I found a couple of relevant posts on the internet about determining lens nodal points (for shooting panoramics), but there's some confusion/misunderstanding about the difference between the nodal point/s and the entrance pupil of a lens. *Either way, maybe these are a good place to start.
http://teocomi.com/detect-the-nodal-point-of-your-lens-how-to/
http://www.thomas-schwenger.de/index.php?ch=kh&sub=sub_tt&pg=npe
Anyway, the gist of the problem is that I want to place a washer stop and don't understand a damn thing about lenses or even the name of the proper iris location. Any help is really appreciated!!
I have looked up and down the net seeking information about determining the correct position of a DIY iris/stop in a brass lens (petzval) barrel. *Most lenses I've seen that are cut for Waterhouse stops seem to have the slot (somewhat) centered between the front and rear lens groups, give or take 5-10% forward or back, but I've been warned multiple times against cutting a slot arbitrarily.
The thing is, nobody has been able to tell me how to determine exactly where the stop needs to go. *I am going to start by constructing a washer stop (a ring with a hole in it) that I can slide into position along the barrel, but how will I even know when the stop is located in the right position? *Are there any telltale signs that I've got it located correctly or aberrations I should look for to tell me that I've got it located incorrectly?
Does anybody here have any experience with this? *Because I'm feeling a little lost.
Thanks!*
Basically, I am finding that there are simply too many unknowns to try to calculate the front and rear nodal points of this lens, and that I'm going to have to use my eyes to produce any useful information. The lens is in Dallmeyer configuration, and it doesn't have an internal stop to clue me in as to the proper location of a stop.
I have been told that the "Principal Point Separation" represents the distance between the two nodal points and is the space within which a stop will need to be located.
I was hoping there was some formula I could apply to find the nodal points, but that seems not to be the case. *As I'm moving the stop, what are the characteristics of the image formed that I should be looking for? *Besides "looks sharp here", that is. *For instance, will the range in which I can move the stop without a discernable decrease in sharpness represent the location from the first to second nodal point? *Then should I just locate the stop between those two points?
I found a couple of relevant posts on the internet about determining lens nodal points (for shooting panoramics), but there's some confusion/misunderstanding about the difference between the nodal point/s and the entrance pupil of a lens. *Either way, maybe these are a good place to start.
http://teocomi.com/detect-the-nodal-point-of-your-lens-how-to/
http://www.thomas-schwenger.de/index.php?ch=kh&sub=sub_tt&pg=npe
Anyway, the gist of the problem is that I want to place a washer stop and don't understand a damn thing about lenses or even the name of the proper iris location. Any help is really appreciated!!