How to determine the magnification provided by an unmarked loupe?

[BetterSense]

I like the theory that one should use a loupe with magnification such that the result looking through the loupe matches the intended print...if the intended print is 16x20, then the correct loupe is 4x for 4x5 and 2x for 8x10.

I have only one loupe and I don't know its magnification. How can I find out?

[Doremus Scudder]

A loupe for focusing is to achieve the best focus possible, and that should be your goal always. The idea that one can (or should) focus less critically for smaller prints is simply illogical and would be, IM-HO just sloppy workmanship. You want your best possible focus, always. And, what if you focused poorly for a small print and then later wanted to make a larger one? The more I think about it, the more the "theory" seems like pure nonsense.

To focus well, you need a loupe that will do that job for you. I personally like an 8x or 10x loupe when doing fine focusing. Others like less magnification because they find that the grain of the ground glass at higher magnifications hinders their focusing. Still, they aren't intentionally focusing less-well than they can by choosing a 4x or 6x loupe, but doing what is optimal for them.

Whatever power you choose, it should be one that allows you to feel confident that you are focusing as well as possible all the time.

Doremus

[BetterSense]

The idea that one can (or should) focus less critically for smaller prints is simply illogical and would be, IM-HO just sloppy workmanship

Doremus

The entire concept of depth of field is defined in terms of magnification and perception. Any "sharp" photo only appears sharp if you don't look harder. This is is very apparent in microscopy.

Depth of fied comes with a price. Sacrificing motion blur and diffraction trying to achieve dof above what is needed for one's pictorial goals is what is poor workmanship...you may as well shoot pinhole all the time.

Anyway I shoot both 2x3 and 8x10, so I need to use a loupe which is 4x or 6x more powerful on 2x3 unless I intend to have different goals for print sharpness. But I only have one unmarked loupe and I don't know the magnification. It is easy to determine the focal length; how do I convert that to "X"?

[mdarnton]

Depth of field is an illusion. There can only be one plane in best focus. Why don't you want to do your best?

[Doremus Scudder]

The entire concept of depth of field is defined in terms of magnification and perception. Any "sharp" photo only appears sharp if you don't look harder. This is is very apparent in microscopy.

Depth of fied comes with a price. Sacrificing motion blur and diffraction trying to achieve dof above what is needed for one's pictorial goals is what is poor workmanship...you may as well shoot pinhole all the time.

Anyway I shoot both 2x3 and 8x10, so I need to use a loupe which is 4x or 6x more powerful on 2x3 unless I intend to have different goals for print sharpness. But I only have one unmarked loupe and I don't know the magnification. It is easy to determine the focal length; how do I convert that to "X"?

BetterSense,

I'm not talking about depth-of-field; rather focusing critically at the plane of sharp focus. There's absolutely no reason to compromise here. Deciding what amount of sharpness you need for a certain size print in the areas at the extremes of the depth of field is another thing entirely. There are lots of ways to do this, including maybe your magnification method. However, I wouldn't rely on anything but the optimum magnification for doing the critical focusing on the plane of sharp focus.

That still begs the question: What if you "tailor" a print for 11x14 and then later want to make a 20x24? If you've comprised on optimum sharpness in the negative, you're out of luck. I realize that compromises must be made for moving subjects, low light, etc. but we should still work for optimum.

Again, I'm not advocating anything concerning depth-of-field; the camera will focus sharply somewhere. It's up to us to do our best to place that "somewhere" where we want it.

Best,

Doremus

[Denny]

BetterSense,

The easiest way to find out the magnification of your loupe might be to walk into a camera store and compare your loupe to a few in the store. Alternatively, you can look up Magnification on Wikipedia and find several formulas relating magnification and focal length.

Hope that helps,
Denny

[BetterSense]

What if you "tailor" a print for 11x14 and then later want to make a 20x24?*

There is no way to have the same DOF for two different magnifications, except to shoot two negatives, so you have to tailor DOF for one size or the other. That's just physics. If you want a certain look at 11x14, then that's what you want. It's the choice you make when composing every photograph ever.

I understand that more loupe mag is better for hitting actual focus points. I have been looking at 50x toy handheld microscopes wondering if they could be used on that little bitty 6x9 ground glass.

One can always shoot the format that matches the desired print and contact print! It does simplify things.

[Doremus Scudder]

There is no way to have the same DOF for two different magnifications, except to shoot two negatives, so you have to tailor DOF for one size or the other. That's just physics. If you want a certain look at 11x14, then that's what you want. It's the choice you make when composing every photograph ever.

I understand that more loupe mag is better for hitting actual focus points. I have been looking at 50x toy handheld microscopes wondering if they could be used on that little bitty 6x9 ground glass.

One can always shoot the format that matches the desired print and contact print! It does simplify things.

BetterSense,

I look at the entire system from a different angle. There is, unequivocally, an optimum aperture for a given depth of field that optimizes the degradation introduced by lens aberrations on the one hand and diffraction on the other. The variable is the print size: the greater the depth of field, the smaller the maximum size print from that negative can be. I'm always trying to get a negative that will yield the largest possible acceptable print. If I make a smaller print than "maximum acceptable size," then the "extra" DoF is just wasted. However, I wish to have the option of making a larger print later.

Sure, if you have a lot of trouble with subject or camera movement degrading the image and you need to choose an aperture based on shutter speed constraints, then you're just stuck with making smaller (or very small) prints for subjects that need a large DoF in order to keep the circles of confusion at the extremes below the resolution ability of the eye. The shutter speed here determines maximum print size.

And, depending on how you wish to define depth of field, I can certainly have "the same DoF" for two different size prints: Any size print I make at or below the "maximum acceptable size" will be acceptably sharp and have the required depth of field.

Depth of field is, in any case, a flexible concept, totally dependent on what we as photographers or viewers of photography consider acceptable in a final print. A negative is an intermediate step, only valuable for the information it contains that can be expressed in a print. Therefore, an acceptable circle of confusion size on the negative depends on the degree of enlargement and the subjective demands of the viewer for apparent sharpness. If you want to work backwards from a variety of different final print sizes to find acceptable apertures for a given shutter speed or speeds, that is certainly a viable approach. I just don't think that you can do that repeatably and accurately by viewing the ground glass with loupes of different magnifications. You might want to adapt the focus-spread technique outlined here: http://www.largeformatphotography.info/fstop.html . You could do the calculations for shutter speed vs maximum acceptable print size for your chosen CoC and come up with a table.

Note that none of the above has anything to do with focusing...

Best,

Doremus

[BetterSense]

And, depending on how you wish to define depth of field, I can certainly have "the same DoF" for two different size prints: Any size print I make at or below the "maximum acceptable size" will be acceptably sharp and have the required depth of field.*

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.

[Emmanuel BIGLER]

..
I have only one loupe and I don't know its magnification. How can I find out?


Hello from France!

The "intrinsic commercial magnification" of a loupe used for visual inspection is equal to 250 divided by the focal length of the loupe measured in millimeters.
Hence a 4X loupe has in principle a focal lenght of 62.5 mm, a 3X loupe has a focal length of 83 mm.

For most loupes you can estimate the focal length by measuring the distance between the center of the glass elements and the image of a distant object.
A windows frame located a few meters away (10 feet or longer) is projected as a sharp image very close to the focal point.

[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!!