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.

Keith

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.

keithostertag said:

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!

Click to expand...

He said the loupe is a magnifying glass.

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.

ROL said:

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.

Click to expand...

And if you are shorted sighted (wear negative Dioptre glasses) you can just take them off for improved close up vision.

Aristophanes said:

He said the loupe is a magnifying glass.

Click to expand...

Because that's all it is.


Steve.

arizonafilm said:

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.

Click to expand...

i can offr this:

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

E. von Hoegh said:

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

Click to expand...

That suggestion greatly simplifies the quandry. I'll try that.

I think you have received excellent answers so far. I have just a couple of points to make about focusing using a loupe.

First, the "focusing" part of a "focusing loupe" (i.e., one with "adjustable focus") is just a distance adjustment. As Polyglot stated, if your eye cannot focus on the subject of your loupe, you have to change the loupe-to-subject distance. For those of us whose eyes don't adjust much anymore (read 50+), it can be a real convenience to have the loupe always at the right distance from the frosted side of the ground glass, and, since eyes are not all the same in this regard, clever manufacturers have built a distance adjustment into better loupes. That's all there is to it.

That said, it is often devilishly hard to see the corners of an image on the ground glass with loupes of this type, since they are (with the exception of a very few) placed at right angles to the ground glass. This difficulty is especially a problem with shorter focal-length lenses. I mostly use an 8x or 10x loupe designed for viewing 35mm negatives (like your Nikon loupe) reversed, that is, looking through the "bottom" of the loupe. This means I never touch the ground glass with the loupe, and can therefore easily adjust the angle of the loupe to the plane of the ground glass to be able to see the brightest image at the corners. This also means that I have to find the proper focus point by adjusting the loupe-to-ground-glass distance myself. I find this to be no problem.

I personally find 4x loupes to be too weak for fine focusing, although there are many who use them and prefer them to higher-power loupes. FWIW, I use a pair of 3.5 to 4-diopter reading glasses for ground glass viewing and rough focusing (that in addition to my slight near-sightedness) and do the fine focusing with an 8x-10x loupe as described above.

Also, even though we all want to have our loupes at the proper distance from the ground-glass image so that what we see is sharp, even at the wrong distance, "sharpest is sharpest." That means that even if you have your loupe at a distance that does not allow your eye to focus perfectly on the image, when you adjust focus on the camera, the image will appear more or less sharp, even with this compromised resolution. When it is its sharpest (least blurry if you prefer), then it is as sharp as it gets (other small focusing errors excluded). In practice, this means you don't even have to have a really sharp image with your loupe to find the correct camera focus.

Also, since you are new to 4x5 and the attendant focusing techniques, let me just mention that, for most of us who use movements a lot and are interested in exactly what is in or out of focus, fine focusing with the loupe is usually reserved for just a couple of key image points to place the plane of sharp focus and then check depth-of-field (using whatever method you like).

I would recommend you read the article on focusing the view camera here: http://www.largeformatphotography.info/how-to-focus.html , and the one on finding optimum aperture here: http://www.largeformatphotography.info/fstop.html . These are a bit daunting and may seem overly complex at first, but are really not that difficult. The time spent mastering these techniques will save you time and wasted materials and is well-worth spending.

One last observation: I use a viewing frame, not the ground glass, to compose my images. Before I set up the camera, I know where the borders of my image will be and, since lens focal length is directly related to the distance of my viewing frame to my eye, I know approximately what lens I will be using. This often means that I don't spend a lot of time viewing and working with the image on the ground glass. I set up, make sure the borders are where I want them (if the lens is slightly wider than the critical area, I just make sure that the image is "in there somewhere") and then check the few critical focusing points with the loupe (while applying movements), stop down to the optimum f-stop and "click." In other words, if you already have composed your image and know you have your camera pointed correctly with the appropriate lens mounted, focusing with the loupe becomes just an aid to adjusting camera movements and determining the proper f-stop for the desired depth-of-field, and can be confined to the few critical points needed for these.

This got longer than I intended, but I hope it helps some.

Best,

Doremus

arizonafilm said:

That suggestion greatly simplifies the quandry. I'll try that.

Click to expand...

You can focus the loupe in the grid marks. I have vision issues which don't really allow me to se a sharp image on the gg under any circumstances. I merely look through my loupe, rack back and forth past focus and lock when the image is the smallest..i call it the in and out..never misses..don't overthink it...EC

eclarke said:

You can focus the loupe in the grid marks. I have vision issues which don't really allow me to se a sharp image on the gg under any circumstances. I merely look through my loupe, rack back and forth past focus and lock when the image is the smallest..i call it the in and out..never misses..don't overthink it...EC

Click to expand...

The grid maks are sometimes on the outside of the GG. In which case focussing on the grid marks will lead to focussing error.
To iterate, focus the loupe on the ground surface of the glass. It really is that simple.

Thanks for taking the time to explain your ideas. I got the feeling after awhile that I was either a) overthinking this or b) misinterpreted or got bad advice. With all of you folks' input, I am far more confident. I am now working each suggestion posted and will surely resolve this issue. Thanks again!

Simple procedure...that's the answer I needed! Thanks.