View Full Version : Loupe Viewing Distance

Tony Evans
23-Feb-2011, 16:56
Hi, a question from a newbie.
I find my loupe is not in focused on the film plane when I hold it against the GG. Must raise it a delicate 1-3 mm just off the glass to get a sharp image. No problem with that, but is this how a loupe should behave? Thank you.

23-Feb-2011, 17:34
Do you wear glasses? Or possibly need them to adjust for presbyopia or other focus problems?

I'm in trifocals and if I don't get the center section as my focus through area, I've got to do the same dance with my loupe. I've had a second set of just bifocals made for just shooting. It's got a much larger mid range focusing area and regular distant focus at the top. I had them raise the mid focus range higher in the physical lens to make it easier to use the loupe.

Much easier to use the loupe with that set up. But, if my vision changes, I still have to work with the loupe distance until i get the new scrip made up.

Keep in mind that your optometrist can set your middle viewing distance power to different ideal distances. I took my whole system in for my check up to have him work out the distance I needed.

If your loupe is adjustable as in ahelixthread, maybe all you need to do is reset the distance once you've got one image really sharp. Just adjust the loupe to the really sharp ground glass image and secure it with tape as being your target focus point.

Tony Evans
23-Feb-2011, 17:41
Checked and it looks like you might be right. I think my Costco +2.75 (3 pair for $19.99) is now going to +3.00.

Emmanuel BIGLER
24-Feb-2011, 02:17
Hello from France !
The basic setting for loupe is : film plane at the focal point of the loupe, the image is sent to infinity.
Most photographic eyepieces are set-up in the so-called 'minus one dioptre' position, it means that the image instead of being projected at infinity, is set one metre ahead of the viewer. It is supposed that this setting suits most people.
Being myself short-sighted by more than one dioptre, in order to see sharp through a loupe or an eyepiece set-up at minus one dioprte is to keep my ophtalmic glasse on.
When the loupe is set closer that the focal distance, the image gradually comes closer.
If your loupe is a 4X, hence its focal length is 250/4 = 62.5 mm a 10x loupe would be 250/10 = 25 mm, etc...
If the 4X : f=62.5 mm loupe is located 4 mm shorter than its focal length, the image is sent to (62.5X62.5 / 4 ) i.e. about one metre.
Newton's formulae : small gap between the focal point and the object = (fxf) / Distance to the image.

If the loupe has a built-in helical, the best way to properly focus and avoid eye strain consists in starting from a position which his purposedly longer than the focal length and gradually shorten the distance by screwing the helical "in" until the image becomes sharp. It is better not not go past this position, otherwise your eye acting as an auto-focus lens system will accommodate and you'll get eye strain.
This applies to the proper setting of a pair of binoculars. If you do not see how the binocular moves due to internal focusing, best is to purposedly focus on a "dummy" object located close to you, then aim at infinity and gradually bring the image in focus and stop. Doing so you'll be able to look through the binoculars for a long time without eye strain.

This setting to avoid eye strain, should in principe (sigh !!) no longer really apply to people affected by presbytia ... but anyway it is always best to get to the proper focus by shortening the loupe distance than the reverse.

24-Feb-2011, 04:54
Following up what Lenser said. I also have Bifocals and you can have a pair manufactured without the near sighted correction which also eliminates the loupe focus problem. You can also have a pair of glasses made up that shows sharp focus out to around 3 feet/1Meter. The Costco solution works fine too!

John Berry
24-Feb-2011, 12:39
Checked and it looks like you might be right. I think my Costco +2.75 (3 pair for $19.99) is now going to +3.00.
Tony With the same interest, I looked at a rack once. It indicated that all diopters were figured for a 14" viewing distance. Unless you are dead center you will be tilted to look at center anyway. I prefer a little lift, and would bet it is more common than not.

William McEwen
24-Feb-2011, 13:00
Hey, Tony:

I own a couple of loupes. I've had my standard Agfa 8x for at least 30 years. Always placed it up against the slide or groundglass or whatever and got a sharp image. I had 20/20 vision then. Now I that my eyes are older and no longer 20/20, I find that I have to raise it a little to get a sharp view.

I also have a fancy brass $40 loupe that I got from a company that once existed called Light Impressions. The eyepiece-to-object distance is adjustable, so I'm able to keep it planted firmly on whatever I'm looking at.

Anyway, this is a long-winded response -- you are supposed to have the loupe right against the object.

tom thomas
24-Feb-2011, 13:21
On the subject of loupes, I found a Radio Shack folding magnifier that seems to work great. I wear the infinite bifocal glasses (no ledges between magnifications for pigeons to land on) due to farsightedness. I find this one works great. Mine is a three section folding magnifier with a 1 1/8th inch square viewing area. I can slide it gently around on the gg (hopefully not scratching it) easily to check focus everywhere.

It looks just like the one in the photo. Mine is a anodized metal though, this on now says its made of plastic. The one pictured advertises 6X mag. Mine seems to be 3 or 4X.

Try googling "folding magnifier", then scroll down the page to Images for folding magnifier. Clicking that brings up all kinds of magnifier photos, sources and prices. I did google Radio Shack for the one I have, 63-1303 (China) but they don't offer them anymore. Darn. It sure works neat and I can clearly see the image on the GG with my heavy glasses on.


kev curry
24-Feb-2011, 13:28
Pretty sure thats a ''linen tester''

tom thomas
24-Feb-2011, 16:42
You're right Kev. My son uses one to check his negs on a light board. Works great. I'd wondered what the engraved scale might be used for , counting linen lines per inch I guess.

Nacio Jan Brown
27-Feb-2011, 18:24
I use a technique I read about and it seems to work. With the lens open and the camera pointing at the sky, I rotate the diopter adjustment on the loupe until the grain of the ground glass becomes sharp, this with the loupe against the glass. I then mark the position of rotation so I can reestablish it if the diopter ring somehow gets moved. On glasses I normally wear blended bifocals but found that the near distance setting, about 17", was way too far away for viewing the image on the ground glass. I played with clip-on magnifying lenses as well as flip-down lenses but found both too fussy. I finally came up with what is for me ideal. I had a pair of clip-on lenses to wear over my regular bifocals made with the bottom half set for a sharp viewing distance of 8" which is just right when under the dark cloth. The top half offers no correction of any kind, meaning that my regular glasses provide the correction needed for distance viewing. So, no fussing with anything. I did hear one idea that might be an improvement on the above. When I am under the dark cloth I have to tilt my head back to view the image through the bottom half of the glasses. Someone on one of these forums suggested having the near focus correction be in the top half of the glasses instead so as not to have to do that. This would mean tilting my head back for distance vision instead, of course, but that might be easier than doing so under the confined space of the dark cloth.