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Thread: Flat Field Focus on LF Lenses?

  1. #1

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    Flat Field Focus on LF Lenses?

    So I recently bought a laser rangefinder for measuring distance, in order to perfectly get the hyperfocal distance and other uses. (And to use in conjunction with focus calculator apps like Lumariver DoF).

    However, I'm not sure if I should use the rangefinder in Line-of-sight mode (the straight line distance) or the 'true range' mode that tells you just the horizontal distance (using an angle sensor and basic trigonometry).

    I know modern lenses are designed to be 'flat field' so the focal plane is actually a flat plane parallel to the lens plane. But naturally, an uncorrected lens will actually create a focal field that is curved like a sphere. But I'm not sure if this is the case for older lenses like those for large format.

    Does anyone know if most large format lenses are corrected to have a flat focal plane or not?

    This would only make a difference if photographing things that are well above or below the camera when it's being pointed horizontally, which could be the case if photographing a building, or from on top of a hill.

    Specifically, the lenses I have are:
    • Nikon 90mm f/4.5 NIKKOR-SW (4x5)
    • Schneider 240mm f/5.6 Apo-Symmar (8x10)


    Update: After doing some additional reading, it seems like most lenses should be corrected for field curvature. There are lenses that are specially designed to be extremely flat, called 'flat field' lenses but I don't think they really exist for large format photography.

    This wiki page describes what I'm talking about: https://en.wikipedia.org/wiki/Petzval_field_curvature

    A notable thing from there is that apparently stopping down the lense will reduce curvature, so if you're stopping down to like f/32 or f/64 that should help. Also it says that shorter focal lengths have a lot more curavture. Though I'm not sure if that's in absolute terms, or 35mm equivalent focal lengths. If not, it would seem the longer focal lengths used in LF would make it not as much of a problem.

    Finally the best evidence I found is from Nikon's old brochure of large format lenses: https://galerie-photo.com/manuels/ni...%20cameras.pdf

    On page 3 where it talks about Nikkor-W lenses, it says "Distortion, curvature of field and chromatic aberration are corrected to the high standards you'd expect from nikon". The way I read that, is that means some degree of field curvature is designed into basically all their lenses, but it's pointed out as being particularly good in that range of lenses.

    Considering most large format lenses often have just as many lens groups as other more modern lenses, there surely must be field curvature correction in there.
    Last edited by Angstrom; 25-Jan-2021 at 14:36.

  2. #2

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    Re: Flat Field Focus on LF Lenses?

    Roger Ciala gave some method of determining it yourself. Must be somewhere on the lensrentals blog I think.

    I have no clue what you are going on about the line of sight and true range distance. An optical rangefinder working on the principle of matching 2 images with a given baseline will always work with trigonometry and it will always give you the distance when viewing along the plane formed by the straight line through baseline and along the main optical axis.
    Expert in non-working solutions.

  3. #3

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    Re: Flat Field Focus on LF Lenses?

    Quote Originally Posted by Angstrom View Post
    Does anyone know if most large format lenses are corrected to have a flat focal plane or not?
    Um, er, ah, lens designers aim for flat fields. Some lenses are better than others. For practical purposes, all of the lenses you mentioned have flat enough fields.

    For flatter fields, use a good grade of process lens.

  4. #4
    ic-racer's Avatar
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    Re: Flat Field Focus on LF Lenses?

    Quote Originally Posted by Angstrom View Post
    So I recently bought a rangefinder in order to perfectly get the hyperfocal distance a]
    You realize that you will also need a ruler to perfectly position your eye from the print for it to be that exact.

    I focus at infinity because the prints look better to me that way. Otherwise you can read about it here too:

    http://www.trenholm.org/hmmerk/TIAOOFe.pdf

  5. #5

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    Re: Flat Field Focus on LF Lenses?

    Quote Originally Posted by Havoc View Post
    An optical rangefinder working on the principle of matching 2 images with a given baseline will always work with trigonometry and it will always give you the distance when viewing along the plane formed by the straight line through baseline and along the main optical axis.
    I should have been more clear, I don't mean a rangefinder camera, I mean a laser rangefinder, like that used in shooting.


    Quote Originally Posted by ic-racer View Post
    You realize that you will also need a ruler to perfectly position your eye from the print for it to be that exact.

    I focus at infinity because the prints look better to me that way. Otherwise you can read about it here too:

    http://www.trenholm.org/hmmerk/TIAOOFe.pdf
    Hm that looks like a book I'll have to definitely read through. I guess I don't mean 'perfect' hyperfocal, but if the calculator said something like I'd need to focus at say 18 meters, I could do it much more easily with a laser rangefinder than eyeballing it (and I'd probably focus a bit past it for safety).

    Basically I'd plan to use it when I want to focus "not quite" at infinite and at least get a bit extra closer in focus than normally.

  6. #6

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    Re: Flat Field Focus on LF Lenses?

    Quote Originally Posted by Angstrom View Post

    Update: After doing some additional reading, it seems like most lenses should be corrected for field curvature. There are lenses that are specially designed to be extremely flat, called 'flat field' lenses but I don't think they really exist for large format photography.
    You can get enlarging lenses mounted in shutters which are quite flat field. Most process lenses also fall into this category. Also, too much reading will only confuse you.

    L

  7. #7

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    Re: Flat Field Focus on LF Lenses?

    Quote Originally Posted by Luis-F-S View Post
    You can get enlarging lenses mounted in shutters which are quite flat field. Most process lenses also fall into this category. Also, too much reading will only confuse you.

    L
    You might have to reverse mount enlarging lenses for best results as a taking lens.

  8. #8

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    Re: Flat Field Focus on LF Lenses?

    Quote Originally Posted by Bob Salomon View Post
    You might have to reverse mount enlarging lenses for best results as a taking lens.
    Wrong. Enlarging lenses are optimized for big print in front of the lens, small negative behind the lens. Taking lenses are optimized for big subject in front of the lens, small negative behind the lens.

    Big front, small behind in both situations.

  9. #9
    Drew Wiley
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    Re: Flat Field Focus on LF Lenses?

    Not necessarily if you classify duplicating lenses among enlarging lenses, since they are potentially mounted on enlargers, and tend to be tweaks of similarly named series, for example, Apo Rodagon D (duplicating) versus Apo Rodagon N. But that's the exception rather than the rule. And there are symmetrical process lenses which basically do it all quite well, regardless of 1:1 clear to out infinity, potentially usable on either an enlarger or copy camera, or as a taking lens on a view camera. And there are macro and close up optimized view camera lenses. Both G Clarons and Fujinon A's are close-corrected plasmats also excellent at infinity.

    Unrelated : In today's technology, a pro laser distance meter will be more accurate than an optical rangefinder. Just don't buy a toy one at Cheapo Depot. The good ones can be found at survey equip suppliers. They can be small enough to fit in a shirt pocket, no bigger than a light meter.

  10. #10

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    Re: Flat Field Focus on LF Lenses?

    Yeah, yeah, yeah, but what sensible person would use an Apo Rodagon D as a general purpose taking lens? Now that I've posted this, I expect that a few creatures will come out of the woodwork to announce that their Apo Rodagon Ds are the best general purpose taking lenses they've ever used.

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