Noob Loupe Questions - Focusing v Slide, Magnification

[Shutterbug57]

I apologize if this has been covered, but I did not find it with a search. I have recently purchased, and am eagerly awaiting, a new Intrepid 4x5 gen 3 camera. I am amassing the accoutrements necessary to use the camera when it arrives. I have 5.5 film holders (11 shots as 1 slide is cracked) and a Nikkor-W 150/5.6 lens on the way. Now I am looking at loupes.

I have a 8x loupe that I have used for years to view slides on a light table. I have read some things that indicate this may not be the right tool for focusing a view camera. I don’t want to get tools I don’t need, but I also don’t want to shoot several frames to figure out that I don’t have the right tool.

I have seen sone loupes that allow you to focus the loupe. That seems weird as it appears that you are focusing two things against each other, but, if that is the right tool, once explained, it should make sense.

I appreciate your input on this issue & in helping a LF noob out.

[Jim Jones]

A focusing loupe is convenient because it can always be focused exactly on the ground glass image. However, using a loupe that must manually be focused on the GG while one is focusing the camera becomes instinctive. Inexpensive loupes may not have the optical performance of better ones, but they have long sufficed for me. If they clearly resolve the grain of the GG, they are good enough.

[Pere Casals]

With a GG focusing loupe you remove the lens of the camera, you point the camera to a well illuminated area and then you focus the loupe until the grains in the Ground Glass are perfectly sharp.

With this procedure the loupe is adapted to your sight. This is the Pro way.

With a regular loupe without focusing feature you can do the same, making a diy spacer that separates the loupe from the ground glass in the same way for the optimum distance. This is what I do.

Or, less convenient, you can tilt a bit the loupe every time you focus until you see the grain sharp and at the same time you try to see if the image is in focus. It can be done in this way and you would also get perfect focus, but it requires playing care, it takes more time and, I reiterate, it's way less convenient. This is how I started.

[Shutterbug57]

Thanks for the responses. I will see how the loupe I have works. If I need to shim it, I will give felt pads a try as they won’t scratch the glass.

[Bob Salomon]

Thanks for the responses. I will see how the loupe I have works. If I need to shim it, I will give felt pads a try as they won’t scratch the glass.

A shim is nonsense, especially on the Agfa type loupe which is not a shining example of optical quality. Especially the Chinese made on. The Spanish ones were a little better.

First, a clear sided Loupe is for prints, nor slides or focusing. Those type loupes require opaque sides.
Secondly, you need to focus on the grain side of the gg, not the top of the system. For that you need a focusing eyepiece. During the course of time, be it a day, a week, a month or years, your eye will change, a shim does not. A focusing eyepiece does!

[Pfsor]

A shim is nonsense, especially on the Agfa type loupe which is not a shining example of optical quality. Especially the Chinese made on. The Spanish ones were a little better.



Besides, how could a felt pad serve as a useful shim is beyond me. It has no dimensional stability - the more your press it, the thinner is the shim.
If you insist on using your Agfa loupe, turn it the other side out and focus with it with a free hand. It will be more difficult to use but you will not be dependent on a wrong thickness shim. Then go and buy yourself a focusing loupe.

[Alan Gales]

An 8x loupe is fine for some. Others prefer a 4x to 6x loupe. We are all different. The focusing loupe is the best.

Try out your loupe. This will tell you plenty.

[jim10219]

I use a cheap thread counter. It works so well, I can't imagine any advantage to using a more expensive one.

Here's my problem with a focusing loupe. Unlike a piece of film, your eyes can focus themselves. So you don't need to focus the loupe on the front of the ground glass, because your eyes can do that. And unless you keep your eye at the exact same distance every time you view through the loupe, it's focus will change too. Just like how if you raise or lower your enlarger's head, it changes the focus as well as the size of the projected image. So unlike a camera lens on film, you've got two competing and constantly changing variables. Therefore, focus just need to be within a usable range, and not precise. Maybe if my near vision was bad, I'd find a use for a focusing loupe (to help compensate for that). But with my good near vision (and terrible far vision), pretty much any magnifying glass works just as well as any other.

And my real world experience backs this up. I've never had focusing issues using the super cheap 8x thread counter. Plus I like how it folds up small, and is easily and cheaply replaced should I lose or damage it.

[Bob Salomon]

I use a cheap thread counter. It works so well, I can't imagine any advantage to using a more expensive one.

Here's my problem with a focusing loupe. Unlike a piece of film, your eyes can focus themselves. So you don't need to focus the loupe on the front of the ground glass, because your eyes can do that. And unless you keep your eye at the exact same distance every time you view through the loupe, it's focus will change too. Just like how if you raise or lower your enlarger's head, it changes the focus as well as the size of the projected image. So unlike a camera lens on film, you've got two competing and constantly changing variables. Therefore, focus just need to be within a usable range, and not precise. Maybe if my near vision was bad, I'd find a use for a focusing loupe (to help compensate for that). But with my good near vision (and terrible far vision), pretty much any magnifying glass works just as well as any other.

And my real world experience backs this up. I've never had focusing issues using the super cheap 8x thread counter. Plus I like how it folds up small, and is easily and cheaply replaced should I lose or damage it.

Providing that your thread counter isn’t grossly defective it is focused on the surface that you have placed it on. With a camera that can be very far away from the surface that the image is being formed on, the grain side of your gg.

A focusing loupe is focused on the grain of the gg, not the top of the gg or on top of the Fresnel screen, if it is on top of the gg.

Yes, your eye changes during time, but once the loupe is focused on the grain of the gg it stays focused on the grain, unless you replace the gg/fresnel. But if your eyes change the loupe can alw@ys be readjusted.

Your thread counter or an Agfa loupe can not be adjusted other then by your changing the distance between where you hold it!

Lastly, a proper focusing loupe has an opaque barrel to eliminate stray light and maintains maximum contrast to your eye to make focusing easier. Your thread counter or any loupe with a clear barrel, or anytime you have clear space between your loupe and your gg you will induce flair from extraneous light and reduce that contrast!

[Pfsor]

I use a cheap thread counter. It works so well, I can't imagine any advantage to using a more expensive one.

Here's my problem with a focusing loupe. Unlike a piece of film, your eyes can focus themselves. So you don't need to focus the loupe on the front of the ground glass, because your eyes can do that.


No, they cannot. If your loupe is not correctly focused on the plane that needs to be in focus, your eyes cannot correct for that. Why? Simply because if your loupe sends the incorrect picture to your eye any focusing in your eye cannot compensate for the lost optical information of the first optical system that sends the picture (fuzzy one) to your secondary optical system (the eye). What you say is completely wrong from the optical point of view.

Were it as you say, we could easily focus a non sharp negative in the printing optical system to get somehow a sharp print - and we all know, it is not possible. Only digital system can do such a trick, the human eye cannot.

[Peter De Smidt]

If you have close up reading glasses, give them a try.

[Pere Casals]

Providing that your thread counter isn’t grossly defective it is focused on the surface that you have placed it on.

Bob, at least my thread counter is not focused on the bottom surface, but some 5mm farther, I guess this allows it to be used in more situations and perhaps to be used by some people requiring some kind of glasses and not wearing it.

...so I have to separate it a bit from the gg to see the grain in perfect focus.

To use a thread counter as a focusing loupe one has to clamp it with fingertips a bit under it, and using the fingers to adjust focus until one can see the grain in the other side of the gg, when grain is seen perfect then at the same time we have to check the image focus at the same time.

It's not convenient, it's not as fast, but it also works.



If you have close up reading glasses, give them a try.

182842

[Bob Salomon]

Bob, at least my thread counter is not focused on the bottom surface, but some 5mm farther, I guess this allows it to be used in more situations and perhaps to be used by some people requiring some kind of glasses and not wearing it.

...so I have to separate it a bit from the gg to see the grain in perfect focus.

To use a thread counter as a focusing loupe one has to clamp it with fingertips a bit under it, and using the fingers to adjust focus until one can see the grain in the other side of the gg, when grain is seen perfect then at the same time we have to check the image focus at the same time.

It's not convenient, it's not as fast, but it also works.




182842
Do it the right way, it is faster, easier, more precise and if your hand shakes it makes no difference!!

[jim10219]

No, they cannot. If your loupe is not correctly focused on the plane that needs to be in focus, your eyes cannot correct for that. Why? Simply because if your loupe sends the incorrect picture to your eye any focusing in your eye cannot compensate for the lost optical information of the first optical system that sends the picture (fuzzy one) to your secondary optical system (the eye). What you say is completely wrong from the optical point of view.

Were it as you say, we could easily focus a non sharp negative in the printing optical system to get somehow a sharp print - and we all know, it is not possible. Only digital system can do such a trick, the human eye cannot.

I think you're misunderstanding me. A biconvex lens, like in my thread counter, doesn't have a set focal point. So you can't say that it's focused on the rear of the ground glass without also defining a set viewing point. It's a simple lens system and obeys the Thin Lens Equation (the inverse of the focal length is equal to the sum of the inverse of the subject distance and inverse of the image projection distance). So in the case of the thread counter, if it is indeed focused on the top of the ground glass from where your eye is, all you'd have to do is move your eye a little bit closer to bring the other side of the ground glass into focus.

That, however doesn't take into effect your eyes ability to focus. Your eye works differently. Instead of changing the focal point by changing the relationship of the distances, it changes it's shape, and thus focal length. It obeys the same Thin Lens Equation, however.

Now anyone who has had experience with a simple lens, such as a magnifying glass, knows that there is not just one point in space that allows your eye to focus on whatever is being magnified when viewed through the glass. In other words, you don't have perfectly maintain the distances between your subject, magnifying glass, and eye in order to take advantage of it's magnification effects. They do, however, have to be within a useable range. At certain distance relationships, all you'll see is a blurry mess. But at many other relationships of distance, you can see a clear image. This is because the two lenses (the one in the magnifying glass and the one in your eye) work together. As long as the image projected from the magnifying glass is able to be focused by the lens in your eye onto your retina, you'll see a clear image. If, however, the lens in your eye can isn't capable of focusing the image from the combined lenses (magnifying glass and eye) onto your cornea, it will be blurry.

What's happening is the loupe focuses the image on a point behind the biconvex lens and your eye. Then your eye focuses on that point. As long as your eye is able to focus on that point, your image will be in focus. If you move the point of focus of the loupe lower, say the distance of the thickness of the ground glass, then the focal point behind the loupe will move closer to the lens by a small amount. All that is required for your eye to see this image in focus, is to focus a few millimeters further. If you pull the loupe slightly off the ground glass, you can still focus on the ground glass by focusing your eyes even further out, closer to the lens in the loupe. At some point, however, you'll move the point of the projected image from the loupe beyond the area where you're eye is capable of focusing. You can either move it beyond infinity, or beyond your eye's nearest focal point. But as long as the loupe projects and image within your eyes focusing ability, the image can be in focus.

[Pere Casals]

Do it the right way, it is faster, easier, more precise and if your hand shakes it makes no difference!!

Of course, a good focusing loupe it's a must have tool, a Pro would had not think it twice, but an amateur that's engaging LF has a lot of things to acquire and a good loupe it's not cheap. Perhaps it's better to buy it later but buying a good one.

[Pere Casals]

So in the case of the thread counter, if it is indeed focused on the top of the ground glass from where your eye is, all you'd have to do is move your eye a little bit closer to bring the other side of the ground glass into focus.


No... nearing the eye to the thread conuter won't modify the focus, you have to separate the loupe more or less from the ground glass to see the sharpest.

Check this, place your thread counter on a newspaper and you'll see how probably you will have to separate the base of the loupe a few mm from it to see the sharpest image.

Don't tell me that you were not doing that when focusing the view camera with the thread counter !!!

[Tin Can]

If we see the GG grain and the imposed image by any means are we not seeing usable focus?

Is it possible to find a fake focus from the user/rear side of the GG?

And how would that be possible?

[Pfsor]

I think you're misunderstanding me. A biconvex lens, like in my thread counter, doesn't have a set focal point.

...

Now I understand what you say! Your thread counting loupe is just a biconvex lens. Frankly, I did not expect that - the last time I saw a thread counting loupe, it was a composed cell lens, even apo corrected. Now I understand you - we were just speaking about two optically different things, called the same name. So it is the case where we both are right! Cheers!

[Jac@stafford.net]

I have 72 year-old eyes. My loupe is long to work under a hood. It has a diopter adjustment (if that is the correct term). When I find where it focuses upon the glass grain, I lock it in to avoid mistakes in the future. Keep it simple.

[Pfsor]

No... nearing the eye to the thread conuter won't modify the focus, you have to separate the loupe more or less from the ground glass to see the sharpest.

Check this, place your thread counter on a newspaper and you'll see how probably you will have to separate the base of the loupe a few mm from it to see the sharpest image.

Don't tell me that you were not doing that when focusing the view camera with the thread counter !!!

I think he's talking about the depth of focus of his lens and the ability to focus with your eyes on its different edge - the closer or the more distant one. And that ability is possibly dependent on the lens - eye distance. That's how I understand (or misunderstand) him.

[Pere Casals]

If we see the GG grain and the imposed image by any means are we not seeing usable focus?


Randy, of course, when we see in perfect focus the gg grain (by any means) then we see how sharp is the image. If the grain is not in perfect focus then we cannot be aware of how accurate is our focusing, but still we can get the points were are loosing focus (moving fordward and backward the camera focusing) and finding the middle point, if we do that at f/5.6 and later we stop to f/22 then sure we'll have the subject in focus, because the greater DOF will help.

Anyway the gg grain has lower resolving power that the film, so anyway we always have some room to find a middle point, IMHO.

[Pfsor]

Furthermore, you can still focus correctly even with a focusing lens that is not correctly focused on the ground side of the gg. That's because even if you're seeing a non correctly focused (by your focusing lens) gg image you're still able to judge the taking lens position that gives you the smallest fuzz on the gg. Not practical, of course but doable in a case of emergency.
(Pere, you beat me to it...:))

[Pere Casals]

Well, with a 90mm at f/22 hyperfocal is at 3.67m, so sometimes one has to be a genious to miss focus... :)

[Bob Salomon]

Well, with a 90mm at f/22 hyperfocal is at 3.67m, so sometimes one has to be a genious to miss focus... :)

A lens only sharply focuses on a single point. DOF is simply the area that will be acceptably sharp at a particular magnification.

[Pere Casals]

A lens only sharply focuses on a single point. DOF is simply the area that will be acceptably sharp at a particular magnification.

Of course, but real photographs are usually of 3D scenes, so at the end almost nothing is on perfect focus anyway.

[Bob Salomon]

Of course, but real photographs are usually of 3D scenes, so at the end almost nothing is on perfect focus anyway.

Except the point that was focused on.

[Pfsor]

So what??

[Pere Casals]

Except the point that was focused on.

Yes, manufacturers place the flat resolving target in that point to plot the MTF charts :)

While that information is quite interesting, real optical performance in a real photograph has, of course, many contributing factors. We may have not a single 1 mm2 of the print at the focused distance, for example if we have a near subject and a distant background ...and we want all in acceptable focus.



So what??

Pfsor, it's clear that, in LF, with simple recipes we obtain LF negatives that have a crazy amount of resolving power, way beyond what we may need in our print or monitor, a crazy overkill.

I we focus at f/5.6 to place the plane of focus where we want, and later we stop just under the diffraction limit of the lens (f/16 to f/22 for 4x5)... then even a rookie like me obtains impressive IQ.

______

But I've realized yet that managing LF focus is a refined art because of the amazing impact in the aesthetics. Controlling that requires a masterliness.

A proficient focus management requires understanding the film capability, the aperture impact in the focus plane vs the rest of the field, and even undertanding the nature of our owned glass, and combining that with tilt-swing. Achieving an optimal balance requires an skilled LF shooter in certain situations.

But that was the easy chapter. The difficult thing is managing defocus. A true artist may not use defocus at all, or he may (even intensively) exploit the aesthetical resources of it.

IMHO mastering the aesthetics of defocus (and the involved technique) it's something that shines in a work. To me, this is the difficult chapter of the LF learning, learning to exploit that resource in a sound way. And IMHO that's where LF it's really unique.