How to determine the magnification provided by an unmarked loupe?

[Maris Rusis]

I've seen a convention for calculating "magnification" of a loupe as: 250/f + 1 where f is the focal length measured in millimetres. As Emmanuel BIGLER points out the 250/f formula is also used. I think which formula applies depends on whether the magnifier is used very close to the eye, add +1, or away from the eye, don't add +1. The dioptre power of the eye itself is a confounding factor and the magnification marked on a loupe is but a guide.

[Jac@stafford.net]

Put a dime, a penny and a nickle side-by-side and view them using your loupe.
It will not give you a numerical answer, but a better functional impression.
.

[Emmanuel BIGLER]

Thanks to Maris for the other definition of the "intrinsic magnification" of a loupe.

It means that the engraved magnification like "4X" is something conventional and may depend on how the manufacturer defines it!!

And Jac is right, the actual magnification depends on how you compare the apparent size of the image seen through the loupe to the apparent size of the object that you can actually see with the naked eye.

The definition "250/f" refers to the following experiment:
Assume that you are able to accommodate down to 250 mm either with your naked eye or with your ophtalmic glasses.
Look at a small object with the naked eye from this minimum distance, e.g. 250 mm = 10 inches.
Then look through the loupe, assuming that your eye is perfectly released and sees sharp at infinity.
In this situation, image at infinity, the object has to be located at the focal point of the loupe.
If the object is shown 4x bigger (in angular terms) through the loupe, with respect to what you see at 250 mm, then the actual magnification of the loupe is 4x.

My feeling (I did not make the calculation) for the other formula is that the proper image distance to see sharp through the loupe might be defined closer than infinity (many eyepieces for reflex cameras are set-up for one meter of image distance, not infinity).
And the minimum reference viewing distance with the naked eye might be different from 250 mm (mine is now probably twice as much as this reference distance

[Doremus Scudder]

I'm starting to understand that you consider DOF a "more is always better" phenomenon. Not everyone does. In fact, some people spend big money on lenses to get less DOF. So it's not always about "enough" DOF but about the "right" DOF, and this can be hard to visualize on different ground glasses that will be enlarged different amounts.

Aha! I'm beginning to understand that you might want to be able to better judge where the boundary and transition areas are between sharp and unsharp portions of a final print... I might submit that that also depends on viewing distance as well as the size of the final print.

As far as using some kind of magnification for the ground glass to somehow better visualize where the sharp image transitions to unsharp I might recommend some very strong reading glasses in the 4-6 diopter range. Using various loupes of known magnification (however that's arrived at now...) might work also. However, I think you will have to do some empirical testing to find what works for you as far as which viewing magnification corresponds to which final print size and print viewing distance.

If you really are just trying to optimize shutter speed and get an optimum DoF for a given size print (and viewing distance), then near-far focusing and basing aperture on the desired CoC for whatever size final print might work better for you.

My approach is to optimize for the greatest enlargement possible for a given focus spread.

My impression at the beginning of this thread was that you were addressing focusing... I see that that's not the case. Sorry for any confusion.

Best,

Doremus

[Emmanuel BIGLER]

Hi again !

I did a small calculation on the back of a piece of recycled paper (I had no used envelope handy) and found a very simple and very interesting explanation for both formulae defining the "intrinsic magnification" of a loupe.

M = 250/F assumes that when you look inside the loupe, your eyes are unstrained and see sharp at infinity;
M = 1 + 250/F assumes that when you look inside the loupe, you are able to accommodate down to 250 mm, exactly like when you look at the reference object located at 250 mm = 10 inches from your naked eye.

If you are able to see sharp at one meter through the loupe, then M would be = 250/F + 0.25

So as a conclusion, the actual image magnification M for a loupe of focal length F is somewhere in between 250/F and 1 + 250/F

On the large format forum, you learn every day, and when you learn from a LF friend living exactly on on the other side of our planet, learning becomes a real international pleasure
Many thanks to Maris Rusis!!