Is focus a plane to plane affair?

[lungovw]

234700

A theoretical and maybe silly question. Suppose I have a camera with a well corrected lens at point A and a subject at the point S far from image centre. Let us say that I don't have a ground glass and have to deal with a foccusing scale in getting S in focus. Which distance should I set in camera focus rail? L1 or L2?

I "tested" that with an Apo-Symmar 150mm f/5.6. I focused a spot on a wall 6 meters away from lens, right in front, on the lens axis. Then I checked another point at the corner in a 4x5" ground glass. My assessment was that focus could be improved there. I fine tuned it and found indeed that in several attempts I had to bring the lens closer to the film plane in order to improve sharpness at the corner. That tells me, without any math, that at least for this lens, the actual oblique distance is the one that has a better chance of being correct if I had to use the distances scale.

The reason why I am asking that is that I am planning to print scale focusing and use a wire frame finder plus the aid of a laser measure. By doing so I could, in some cases, dispense the ground glass.

[ridax]

L2.

The difference you've noticed is most probably due to the lens' field curvature. Though I have not seen a noticeable field curvature in Symmars....

[Doremus Scudder]

The premise of your question shows an incomplete understanding of how light rays pass through the lens and are then focused on the film/ground glass/whatever.

That said, the short answer to your questions is that, assuming the film plane is parallel to the focus plane and the lens axis is perpendicular to those, you can just focus at the L2 distance and any object that lies in that parallel plane will be in focus. Note that we must also assume that your lens is plano-parallel and has no curvature of field.

So, if your camera has fixed lens and film positions, like a non-view camera, you can focus anywhere on the corresponding subject plane and everything in that plane will be rendered sharp on the film. The trick, really, is determining where that plane is in space in front of the camera. I'd be willing to wager that your unsharp corners are simply due to your misjudging where that plane of sharp focus lies.

Keep in mind, the plane of sharp focus will then move around in space with the camera. Tip the camera up and the plane of sharp focus tips as well. The same applies for pointing the camera down or side-to-side movements.

If you are using a view camera with movements, then you need to learn how tilts and swings affect the position of the plane of sharp focus. There are lots of resources here and other places on the web as well as textbooks that have been written about that subject. If you really want to take advantage of movements on a view camera, a ground glass is indispensable.

Have fun,

Doremus

[lungovw]

Thanks for the clarification. So it is plane to plane, subject to image, both perpendicular to lens axis. Maybe there are lens imperfections, specially closer to borders, but anyway that is how it should be.

[Nodda Duma]

not quite. This is why lens designers speak in terms of “surface of best focus”. This surface is not inherently flat. Field curvature correctly implies that different field angles focus at different distances. It is only by designing the optics to correct for field curvature such that the plane of best focus lies sufficiently flat that the plane of the film or sensor lies within the depth of focus. This is a wordy sentence, but it exactly describes how “flat” image planes are actually just “sufficiently flat”.

If it were not for field curvature, or conversely if the film or sensor could be arbitrarily curved to lie on the sphere of best focus, then optical designs would be much, much simpler.

[Bernice Loui]

Do curved projection screens fit into this optical design reality?


Bernice





If it were not for field curvature, or conversely if the film or sensor could be arbitrarily curved to lie on the sphere of best focus, then optical designs would be much, much simpler.

[Mark Sawyer]

not quite. This is why lens designers speak in terms of “surface of best focus”. This surface is not inherently flat. Field curvature correctly implies that different field angles focus at different distances. It is only by designing the optics to correct for field curvature such that the plane of best focus lies sufficiently flat that the plane of the film or sensor lies within the depth of focus. This is a wordy sentence, but it exactly describes how “flat” image planes are actually just “sufficiently flat”.


Quite correct. I'll add that process lenses are designed for a very flat focal plane, as those lenses are made for very demanding copy work of flat artwork, so if someone is looking for minimal field curvature, that's the way to go.

[Alan Klein]

My LF camera has asymmetrical tilt on the rear standard. So I focus at the distance first. Since the axis is about 1/4 from the top of the GG (it's on the bottom but everything is upside down of course), I'd be focusing first on L1, not L2. How does that affect our discussion?

Then I'd tilt the rear standard down to get the close item in focus at the other end of the GG. So then which is the best spot to focus close? Can I use any point close-up or the one that has the line going across the GG?

[Nodda Duma]

Do curved projection screens fit into this optical design reality?


Bernice

Bernice, you’re too intelligent to ask a question like that.

BTW, I’ll be in your neck of the woods at the end of this month, at Photonics West.

-Jason

[Oren Grad]

If it were not for field curvature, or conversely if the film or sensor could be arbitrarily curved to lie on the sphere of best focus, then optical designs would be much, much simpler.

Here you go:

https://patents.google.com/patent/US4241987A/en

Popular Photography made a big fuss about it at the time, but I've never read that anything ever came of it.

EDIT: Also, the proposed embodiment is really oriented to roll film, though with sufficient mechanical insight and imagination one could perhaps adapt the ideas to a different embodiment more suited to sheet film.

[Kevin Crisp]

The excellent little 15mm lens in Minox cameras matches up with a curved pressure plate. As you advance the film, the pressure plate backs off, then reapplies for the next exposure, putting a curve in the film for the next shot. It is an incredibly sharp lens. In huge enlargements it is the shortcomings of film that limit what you can do with it.

[Havoc]

It depends from lens to lens (design). Some lenses are designed to project a plane onto a plane. Others project a curved surface onto a flat plane (as good as possible). If you look at old panoramic group shots, you'll see the people are in a curved setup just to get the best focus (and with minimal distortion) on the film plane. Some cameras do have curved film planes like the Afga Clack 6x9. And I think it was Sony that patented a curved digital sensor for this reason (of cheaper lenses).

[Darren Kruger]

Have you looked into figuring out the hyperfocal distance for your intended lens and aperture combination? I have heard stories of old press photographers setting up their speed graphics that way (with a 135mm lens set to like f8) to be increase the chance of getting their shots quickly.

[lungovw]

Darren, I understand your point about using the hyperfocal, that would be a solution. Nevertheless I would like to go a bit further. I asked this question because I am planning to make, for some old lenses RR and the like, focusing scales and wireframe viewfinders and use them for pictures in which I can keep film and lens board parallel. That would be for portraits, for instance. Having a reasonable focus and depth of field my idea is dispensing with the ground glass in order to speed up the workflow. Using a laser measure I think that could be a nice set up.

[Alan Klein]

It depends from lens to lens (design). Some lenses are designed to project a plane onto a plane. Others project a curved surface onto a flat plane (as good as possible). If you look at old panoramic group shots, you'll see the people are in a curved setup just to get the best focus (and with minimal distortion) on the film plane. Some cameras do have curved film planes like the Afga Clack 6x9. And I think it was Sony that patented a curved digital sensor for this reason (of cheaper lenses).

Wouldn't setting the people up in a curve also makes them all look about the same size? Otherwise, the people at the ends will look smaller than they are. Also, it's a way of fitting a lot of people into a less wide-angle lens.

[pgk]

If it were not for field curvature, or conversely if the film or sensor could be arbitrarily curved to lie on the sphere of best focus, then optical designs would be much, much simpler.

And lenses using curved sensors have been with us for a very long time: http://www.earlyphotography.co.uk/site/entry_L129.html and have never caught on particularly well.

But to go back to the original diagram. If the drawing is extended to include the image surface all might become clearer.

[Nodda Duma]

Oh of course. Curvature in one axis has been around since the Brownie cameras, and observatory grade Schmidt telescopes had vacuum backs for lining up film with the curved field.

Every so often I would get a sensor supplier in asking about interest in curved focal plane arrays.

Academically interesting, but field curvature varies too much from one design to the next to be useful. The curve of the sensor would need to be tailorable to the specific project. Not really feasible at reasonable cost… even for the high tech stuff I work on.


Hence the use of the word “arbitrarily” in my statement above. Technology’s not there yet. It’s much more cost-effective to correct field curvature in the optics.

[Corran]

Interesting, I never thought about how field curvature and sensor curvature would need to be tailored on a per-lens basis. But it makes sense.

I'm reminded of a very expensive fixed-lens Leica that corrected all that in-camera, even baked into the 'raw' files. Someone hacked the uncorrected files and showed the lens actually didn't even cover the sensor fully, clipping a few pixels in the corner. Easier to crop a little bit and do software corrections than any heroics in the lens design I guess.

[Bernice Loui]

Lens optical ills can be corrected in the lens design or today by software in camera or during image processing.

Leica is far from the only camera company that is applying software correction for lens optical ills..

Canon's recent EOS rf 16mm f2.8 applies both in lens and in camera or post processing software corrections that in combo results in overall optical performance not possible by either alone at a given cost as a system. Expect far more examples of these combo lens/camera/post processing systems in the future..

One of many ways to apply apodization function and much more..
https://en.wikipedia.org/wiki/Window_function


Bernice




Interesting, I never thought about how field curvature and sensor curvature would need to be tailored on a per-lens basis. But it makes sense.

I'm reminded of a very expensive fixed-lens Leica that corrected all that in-camera, even baked into the 'raw' files. Someone hacked the uncorrected files and showed the lens actually didn't even cover the sensor fully, clipping a few pixels in the corner. Easier to crop a little bit and do software corrections than any heroics in the lens design I guess.

[Tin Can]

Little me conducting the World inside a camera

[Bernice Loui]

Question is... what are the conductor's goals?
https://www.youtube.com/watch?v=Bh67gEkPmWs


Bernice




Little me conducting the World inside a camera

[Tin Can]

insight

I used to sell tools to https://www.swlewis.net/

any excuse to see his workshop

another member here makes violins & repair

love visiting his high rise shop with a superb view

i chafed at music teacher goals in the 50's

i wanted jazz.....now!

my last piano annoyed my neighbors

so it went





Question is... what are the conductor's goals?
https://www.youtube.com/watch?v=Bh67gEkPmWs


Bernice

[Alan Klein]

Lens optical ills can be corrected in the lens design or today by software in camera or during image processing.

Leica is far from the only camera company that is applying software correction for lens optical ills..

Canon's recent EOS rf 16mm f2.8 applies both in lens and in camera or post processing software corrections that in combo results in overall optical performance not possible by either alone at a given cost as a system. Expect far more examples of these combo lens/camera/post processing systems in the future..

One of many ways to apply apodization function and much more..
https://en.wikipedia.org/wiki/Window_function


Bernice

Why pay so much for a Leica lens if they're going to correct in software?

[Bernice Loui]

Leica cameras are a social status brand..
Some history about Leica camera:
https://www.referenceforbusiness.com/history2/47/Leica-Camera-AG.html

Leica Microsystems ala Microscopes is not connected to Leica cameras..

Leica Geosystems ala Surveying instruments is not connected to Leica cameras..

Leica Biosystems ala Pathology instruments is not connected to Leica cameras...

https://www.leica.com

There are folks that value owning social status symbols.
In the world of marketing this is know as Brand Identity..
https://www.investopedia.com/terms/b/brand-identity.asp


Bernice



Why pay so much for a Leica lens if they're going to correct in software?

[Corran]

Or, because it is a good product. See: their lenses that don't require copious software corrections :p. Or their film cameras...

[pgk]

Why pay so much for a Leica lens if they're going to correct in software?

Only full-frame digital rangefinder. The M lenses have very little correction applied and then, only if they are 6-bit coded or the camera is set to correct for a specific lens. The M rely on optical excellence as opposed to software adjustments. Other Leica products which have communication between lens and camera body do use software adjustments. That said combining optical and software adjustments may well produce a higher state of correction. What's wrong with that?

[pgk]

Leica cameras are a social status brand.

This record has been stuck for a very long time. The problem today is that for a brand to have status it has to be known and despite what some people think, in the world of photography a surprisingly small number of people know about Leica (although oddly enough this is probably increasing now so sombody is doing a good marketting job somewhere). I've used Ms on an off for nearly 45 years and I could count on two hands the number of times people have recognised them.

[David Lindquist]

insight

I used to sell tools to https://www.swlewis.net/

any excuse to see his workshop

another member here makes violins & repair

love visiting his high rise shop with a superb view

i chafed at music teacher goals in the 50's

i wanted jazz.....now!

my last piano annoyed my neighbors

so it went

I can see why you like to see his shop. He has a Hardinge toolmakers lathe...

David

[Alan Klein]

Only full-frame digital rangefinder. The M lenses have very little correction applied and then, only if they are 6-bit coded or the camera is set to correct for a specific lens. The M rely on optical excellence as opposed to software adjustments. Other Leica products which have communication between lens and camera body do use software adjustments. That said combining optical and software adjustments may well produce a higher state of correction. What's wrong with that?

I wonder if Leica may reduce lens quality and correct lens aberrations in their software rather than producing better lenses as in the past.

[Alan Klein]

This record has been stuck for a very long time. The problem today is that for a brand to have status it has to be known and despite what some people think, in the world of photography a surprisingly small number of people know about Leica (although oddly enough this is probably increasing now so sombody is doing a good marketting job somewhere). I've used Ms on an off for nearly 45 years and I could count on two hands the number of times people have recognised them.

The same thing could be said about Maseratis. The people who buy them know about them.

[pgk]

I wonder if Leica may reduce lens quality and correct lens aberrations in their software rather than producing better lenses as in the past.

Certainly true of Leica lenses which can 'talk' to the camera, but not so much for M lenses which can only relay their focal length.

[Drew Wiley]

Unlike our malleable curved retina, film is flat. That fact is not lost on optical designers, and in fact, controllable curvature surfaces do exist, especially in very sophisticated astronomical mirrors. But that kind of thing is way way beyond ordinary photographic budgets and realistic bulk and weight requirements. So the necessary correction has to be made to the lens itself, as best as possible given the likely photographic application.

It must have been fun to be an actual optical engineer like Jason. I once worked with a man who engineered the optics for the Pioneer satellite program, and now his son works for NASA in his stead. But I'm too stupid at physics for much of anything except a torque wrench; and torquing lenses themselves just results in shattered glass, so I don't carry one around.

[Nodda Duma]

I’m not dead yet, Drew. And I still design optics. ;) This time as head of the engineering department at the country’s largest optical shop.

[Tin Can]

Agree

but not with the spec on your 8X10 plate holder some of us bought

No worries, I will still buy your coated plates

I appreciate your dedication to REAL things

and willingness to lead with visible example


I’m not dead yet, Drew. And I still design optics. ;) This time as head of the engineering department at the country’s largest optical shop.