now don't get too excited because since GG is frosted it will not give a truly sharp image. i.e. it will be quite fuzzy when photographed with a digital camera and hence will be difficult to judge down to the levels you just calculated. Let us know how it works out.
rob, the digital camera records the target directly from the lens, no gg used now l!!!! That is the beauty of this!!
if you have a camera that can do that then yes it is by far the best but how are you going to measure focal length now you have a camera where the gg was going to be.
It sounds like you have it in mind to build your own special rig for this and I guess you will swap a GG screen for the digi back as necessary.
p.s. invoice is in the post...
yes, digital back replaces gg.
The only purpose of this exercise is pair lenses whose image heights (recorded and measured in graphics software) are within a certain % of each other... the details to be determined, but the tolerances are there.... image magnification is what is being measured, and that can be back tracked to fl...but I will forgo that, in lieu of just matching magnification.
I look forward to your invoice... :-)
rob, hold the invoice!
This gig tricked me... my lens is a magnfier, so it will not focus an image on a gg or a digital sensor, dah....arggggggg.... However, I can use a camera lens, if the nodal point of the lens can be put 30mm from the end of the viewing lens, then the camera will focus it on the digital sensor.
but I thought from what Emanuel Bigler wrote, that by reversing the "loupe" so that the doublet is at the camera/gg end it would project an image. To test that it will project an image hold the lens under a desk lamp facing down onto the desk and move it up and down to see if the bulb image is projected onto the desk. Works for my nikon lupe regardless of which round it is. Same for my horseman loupe.
rob, I am no lens expert, hence this post :-) But the optics designer that designed this lens claims, you can not turn the lens around for these tasks. yes, it will project an image, but you can't discover the properties you need to know in this regard. The light path is a one way street as for lens performance. This is specially true due to the very short fl.... I wish I could explain more.
If the lens will project an image, which I think it will, then changing the focal length will change the magnification in which case you can detect it. You don't need to measure anything about the actual properties of the lens. You only need to make comparative tests so maybe its true you can't measure focal length but you can still see comparative changes in magnification. Put that to your lens designer and see what he says.
(What follows is not in any way in conflict with what has been said about focal length determination - I just wanted to comment on your observation.)Originally Posted by bglick
Note that any lens-like optical system (regardless of construction or intended use) has a focal length (actually, the precise optical engineering term is "effective focal length", for a reason I won't bore you with right now), and it is always defined in the same way. The truly valid, generalized techiques for determination of focal length apply in any case.
Of course, when we consider the use of a lens for different purposes (such as a camera lens vs. a loupe), the use we make of the focal length parameter may be different.
In addition, in making calculations involving the operation of a lens, we in general need to take into account the locations of the two principal/nodal points of the lens (the principal and nodal points are in the same location for a lens immersed in the same medium - such as air - fore and aft, but have different definitions, so we keep diferent names for them). In the fanciful "thin lens", we assume that the two nodal/prinipal points are in the same place (namely, hafway through the lens, although since a true thin lens has no thickness, that definition really has no meaning!) When we deal with the lens in a simple (non-compound) loupe, the "thin lens" conceit serves us pretty well.
Best regards,
Doug
If I were trying to get 200 pairs of matched focal length magnifiers, I might not bother measuring the absolute focal length of any of them. Rather, I would transilluminate an appropriate test target and project it with the doublet closest to the target. A screen can be placed quite some distance from the lens. Move the screen until the image is in focus, and measure the distance between two points on the test target. You can use a yard stick for this instead of precise equipment. Note this measurement on the lens. Do this for all 400 lenses. Now match them up in pairs.
Posting Permissions
Reply With Quote
Bookmarks