Focussing loupe ruminations

[kenj8246]

Been thinking about getting a loupe for the GG, as my eyes are, for sure, not getting any younger. What I'm doing now is using a pair of 4x reading glasses and it's working fairly well for the images I'm making. My reading and research shows folks' opinions and recommendations about loupes are all over the board.

One thing I've seen more than a few times, though, is that the 'proper' way to employ one that is adjustable is to remove the camera lens, use the loupe to look thru the GG at a light source and, when one can see the 'grains' of the GG, to lock the loupe at that point. My question is this: is there an empirical/visible difference between an image made this way and one made by not doing it this way? Isn't it kinda splitting hairs? Just asking questions here.

Kenny

[Bob Salomon]

You are focused on the grain of the gg when you properly adjust the loupe this way. Otherwise you could be out of focus by the thickness of the gg and, if a Frenel is placed on top of the gg, then also by the thickness of the Fresnel as well. That can be significant.

[kenj8246]

You are focused on the grain of the gg when you properly adjust the loupe this way. Otherwise you could be out of focus by the thickness of the gg and, if a Frenel is placed on top of the gg, then also by the thickness of the Fresnel as well. That can be significant.

I don't doubt that but at f45, can one tell the difference?

[Bob Salomon]

I don't doubt that but at f45, can one tell the difference?

At f45 on 45 you are well into diffraction so you will have other problems. But you are confusing depth of field with depth of focus.
Depth of field is the area of apparent sharp focus from nearest to the lens to the fathest from the lens.

Depth of focus is the area behind the lens where the image plane lies.

Depth of field is greatest with wide angle lenses. Depth of focus is greatest with long focal length lenses.

The plane of sharp focus on the gg is within the depth of focus, not within the depth of field. So f45 would not be critical since you usually focus wide open with the least depth of field and then stop down and swing/tilt to control depth of field and the plane of focus.

[Ari]

I don't doubt that but at f45, can one tell the difference?

Why would you want your focusing to be off?
Forget about f45 or f90, focus properly first, then get your ducks in a row.

[Luis-F-S]

I use a pair of 5x reading glasses I had made around 20 years ago. I also carry a Toyo focusing loupe to check myself, and I still stop my Dagors to f45. Luis

[Jmarmck]

I am blind as a bat. So much so that bifocals do not work. I have two pairs of glasses, well three. One for close work like on the computer and another for distance work. Guess what, neither work well with a loupe set at normal. The loupe does not adjust to the point where anything is in focus with out my glasses. I have to set the loupe to work with the distance glasses as that is what I am wearing out in the field. If need be, I just remove the glasses and stick my nose on the glass.....no kidding.

[Doremus Scudder]

Been thinking about getting a loupe for the GG, ...
One thing I've seen more than a few times, though, is that the 'proper' way to employ one that is adjustable is to remove the camera lens, use the loupe to look thru the GG at a light source and, when one can see the 'grains' of the GG, to lock the loupe at that point. My question is this: is there an empirical/visible difference between an image made this way and one made by not doing it this way? Isn't it kinda splitting hairs? Just asking questions here.

Kenny

If you want a sharp image to focus on, you need to have your loupe focused on the surface plane that the image is projected on, i.e., the lens-facing surface of the ground glass.

If your loupe is not exactly in focus, the image you see through it will never be as in-focus as possible. That said, if you are off by a bit, the sharpest image you can get will still be the one in best actual focus on the ground glass.

But, adjusting your loupe is easy, so just do it. And don't focus at f/45 please unless your lens has a focus shift (modern lenses don't).

FWIW, I use a short loupe flipped around so that I always have space between it and the ground glass. I have to move in and out to find the ground-glass surface before focusing, but I can easily tilt my loupe to find the bright spot for shorter lenses or when applying extreme movements.

I use 4x reading glasses for general framing/focus and then an 8x loupe for fine focusing.

Best,

Doremus

[Jon Shiu]

You won't necessarily have a "focus error" if the loupe is not adjusted perfectly. If it looks sharp through the loupe, it will be sharp. Note that you can move your eye closer to the loupe. So, even a cheap non-adjusting 8x Agfa loupe can be used, (or reversed as mentioned above for more working distance.)

Jon

[kenj8246]

I think I didn't make my question as clear as I might've. Language can be a bear sometimes.

Two developed/printed/scanned images taken at ANY aperture, the difference between the two being: 1) one was focused using a loupe as the primary or additional means of focus and 2) the other not employing a loupe. Can someone look with the naked eye and tell the difference between the prints/scans? Is there an observable difference between using a loupe and not using a loupe? I intend to get and use one anyway. As with most things in life, it probably depends on the intended use of the images.

Thanks for your interest and responses in this thread.

Kenny