I'm a little confused about focusing loupes...help!
I just received my Wista 4X loupe from B&H, and I'm not impressed. Without bashing Wista, I thought I was buying a loupe that would allow me to pre-set the focus down to the ground glass plane as opposed to focusing on what my eyes think is in focus. This loupe seems to only have the ability to adjust focus for near and far sightedness.
So, maybe the concept escapes me. I've used my rectangular 8X Nikon loupe for both 35mm slide critique and groundglass focusing, but I have read several different posts both here and abroad that discuss the virtues of a so-called 'focusing' loupe, or one that has the ability to adjust the focus to different depths, where the argument seems to be that the focus should be on the grains of the groundglass side of the image, and not focus on the image the eye sees. Apparently, this concept is chiefly supported by the idea that if it is focused as such, it is therefore registering where the film will soon be, and is therefore the most critical location of focus. The usual method is where the loupe is resting on the photographer side of the ground glass, and therefore the focus being adjusted to that point is in fact wrong, in that the smooth side of the glass is 1.5-2mm farther away from the film sheet, and so out of focus. But that confuses me a little, because the image seen from the smooth side of the glass is simply the image that is being projected onto the groundglass side, where the image formation stops....right???
Unless of course, I am totally confused about what people are trying to convey.
Another curiosity I notice is that some people are using the term 'focusing loupe' loosely, in that some call it by that name to indicate it's name and use: A loupe meant for focusing, while others seem to take a different tack; calling it a focusing luope because it is a loupe that has the ability to shift and lock focus for the idea mentioned about the ground glass.
Could I get some clarification on this?
I also note that I have been sucessful thus far by using the Nikon 8x loupe on the groundglass, with what appears to me to be satisfactory results. I must also confess that I am quite new to 4x5 and have only processesed a handful of negatives and enlargements. But for me, the whole idea of large format is twofold: The sharpest enlargement possible for my budget, and the best perspective control. So focusing is truly a critical consideration.
The loupe is just a diopter for your eye. It is not an image-forming optic (in its intended usage scenario) and therefore doesn't have a specific focal plane or anything. Its effect is to make the focal length of your eye much much shorter so that for a given fixed extension (depth of your eyeball), you focus much closer and get a greater magnification.
With a loupe in your eye, focusing is still primarily up to the muscles in your eye. The loupe just makes you really, really insanely shortsighted, which means that you can focus on the ground-glass from 2" away when you could never do such a thing without the loupe.
As for it being a "focusing loupe", that's an adjustment of the distance between ground-glass and the loupe's lens elements. If you have an old inflexible eye with poor focusing range, you may need to use this focus adjustment on the loupe to obtain a focused image of the GG. If your eyes are any good, it will have little effect other than a small change in magnification.
Think of GG -> loupe -> eye-lens -> retina as being an optical system equivalent to subject -> closeup dioptre -> objective lens -> film. Changing the focus-adjust on the loupe is equivalent to changing the distance between subject and closeup dioptre, which (to retain a focused image in the film) requires that the objective be refocused. Where the objective is your eye, it will refocus automatically, unless it hits its focusing "stops". At that point, moving the lens/subject distance around may allow you to obtain focus again.
I will be interested to see what more experienced (and articulate) people will say to this. I read your post several times and still feel confused as to what you are saying. Ah words, they are such devils sometimes!
The easiest part of your post for me to comment on is the second paragraph. AFAIK, focusing on the groundglass does in fact mean that the image is in focus on the "grains of the groundglass side of the image", as you say "where the film will soon be". The slight thickness of the glass is not very important as the image is being projected through that thickness, _not_ onto the outer surface (the "smooth side"). Does that help?
I speculate that some of your semantic problem lies with marketing- a loupe doesn't focus in the active sense- it simply assists you in seeing where you have placed the focus. The idea that "it is a loupe that has the ability to shift and lock focus" to me sounds like a marketing rep anthropomorphizing a bit too greedily.
keitho at strucktower dot com
I think your question is a good one and polyglot has answered almost all of it.
You are correct in that the image is formed on the GG side of the GG. You are looking through the un–ground thickness of glass at the formed image on the GG. This is where your film plane is and where the film in your holder will end up when inserted behind the GG. This is why many, including myself, have foregone GG focusing loupes altogether for high diopter (3.5) reading or magnifying glasses – something less than a dollar at the 99¢/1$ stores.
Try reading Adams', The Camera, and searching this subject on LFPF if still confused.
He said the loupe is a magnifying glass.
Originally Posted by keithostertag
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Slam the loupe on the GG and focus, the loupe lets you focus more accurately by magnifying it. How hard can it be? I use a cheap $3 loupe I stuck on a lanyard for 8x10 and shoot wide open most of the time.
And if you are shorted sighted (wear negative Dioptre glasses) you can just take them off for improved close up vision.
Originally Posted by ROL
Because that's all it is.
Originally Posted by Aristophanes
i can offr this:
Originally Posted by arizonafilm
Take the lens off, or point the lens at a featureless surface. Put the loupe against the GG and focus the thing on the GG texture. Lock or tape the adjustment on the loupe. That's all it takes
That suggestion greatly simplifies the quandry. I'll try that.
Originally Posted by E. von Hoegh