Hi everyone... I have been a lurker for some time, glad I finally joined!


I have a magnification question regarding magnifiers / reading glasses, etc. I think what I present below is accurate, but I have been fooled many times by "relative magnification" issues, so any input or confirmation would be greatly appreciated.


I am trying to reconcile these two scenarios.

1) I am using a 50mm fl film loupe to view photographic slide film on a light-box. The film is 55mm square, the loupe sits right atop the film. Pretty simple...

2) This film is then digitized, and eventually must go back to film, which is not an easy process. So as an intermediate close-inspection step, the original 55mm sq. image is printed on back-lit film on an inkjet printer. The proposed print size is 10" square. It will be viewed on a light box through a 2" wide round pipe. They eye to print distance is 5.7" and the person inspecting the image will obviously need reading glasses. This exercise is to allow the user to inspect the image at a greater magnification than the first scenario above.

So, I am trying to reconcile - How does the magnification of these two scenarios compare to each other? Here is how I calculated such...

Scenario 1 - Using the 25cm reference magnification distance, I will conclude the film loupe is magnifying the 55mm square film 5x (250/50mm fl). The film loupe (magnifier) sees the entire film area without moving the loupe.


Scenario 2 -

10 inches sq. inkjet print / 55mm film image (2.1") = 4.7x magnification of the original film.

The user views a 2" diameter circle of this 10" square image, through a 2" wide pipe at 5.7" eye to image distance.

So 10"/5.7" = 1.75x added magnification vs. 10" view distance.

These two magnifications combined, 1.75 x 4.7 = 8.3x total magnification.


So my conclusion is, when the user is looking through the film loupe at a given area for close inspection, they will see a given area at 5x magnification, but when viewed through the 2" wide pipe at 5.7" eye to image distance, they will see the same area at 8.3x. So the inspection process has added 1.7x added magnification (8.3 / 5).

The person viewing would require the following diopter reading glasses...

10" / 5.7" = 1.75x

4 diopter reading glasses will allow a person to focus at 10", so I am assuming, 7 Diopter reading glasses will allow the person to focus at 5.7" (4 x 1.75). Is this right?

I am also assuming that these 7 diopter reading glasses will provide NO additional magnification, as they do NOT act as a true magnifier, but rather magnify through proximity, i.e., I have accounted for their added proximity magnification by the 10" / 5.7" component above. Do I have this right?

If I wanted to place an achromat doublet atop the 2" pipe, so people do not have to wear reading glasses, I am assuming it will require a 143mm fl, 250/143 = 1.75 magnification. Is this accurate?

Some may wonder why we are looking through a round pipe - this is another long story, which is not very Germain to the magnification comparison issue, so I figured it was best to avoid confusion. TYIA

Any other "gotchas" or "bewares" please feel free to comment.