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Thread: Problems with Field Curvature and DOF on 6x17

  1. #1

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    Problems with Field Curvature and DOF on 6x17

    Hello!

    Since a long time now I'm struggelingbto calibrate 75mm lenses on my Fotoman 6x17. The biggest problem is Field Curvature. I noticed in all lenses I tried so far, that I have to focus beyond infinity to get the edges into a good focus. Of course I then loose DOF. First I had a SA 75mm 5,6 that had a decentering defect. After that I tried a Nikkor SW 75 4,5 but the Flange focal distance of that was too short for the camera (although not by much). Now I have an older SA 75 5,6 again. I can calibrate the Helical focusing mount that behaves quite good at infinity and at the corners but I have the feeling, that it has clearly less DOF than the others. Also I'm gaining quite less DOF when stopping down to 22. If I stop down I only can move the focusing ring a few millimeters to the mark of 8 on the Cameras scale and the objecta at infinity get unsharp again.


    Is that a normal behavoiour? Are there other tips for calibrating the lens?

    Best regards, Sebastian

  2. #2

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    Re: Problems with Field Curvature and DOF on 6x17

    Hello from France

    Actually the only thing that you can do, provided that the optical axis is perfectly set perpendicular to the film-plane, is to define the best infinity stop not at full aperture but at the actual f-stop used to take pictures e.g. f/16 or any f-number that will be your actual working f-number. And yes, this is a pain since doing so, you won't be able to rely on what you see on the ground glass for focusing at full aperture. May be that you'll have to re-label your distance scales, since aiming at the best trade-off at the working f-number for best sharpness on the whole field at every subject distance will yield something different from what the manufacturer has engraved for you!

    If there is field curvature you'll have to make a compromise between the centre and the edges, nothing to do much about that, except using a curved film holder; a cylindrical film path is not a sphere, but this trick was used in the good old days with cheap rollfilm 6x9 amateur cameras fitted with a meniscus lens to somewhat improve image sharpness in the edges.

    At least by defining at the real working aperture what is the best mechanical infinity stop, you'll get the best possible trade-off in the final image for distant objects, if you have some "best focus" shift between full aperture and f/16.
    With ultra wide angle lenses, even modern ones, we have to forget about the Gaussian model of paraxial rays: actually the best focusing setting for far distant objects is no longer the fixed, gaussian focal point, independant of the actual f-stop setting, but another point located close to it and which changes when the aperture changes. And if the manufacturer has engraved the distance scales according to classical gaussian formulae (Newton's formulae, by reference to the gaussian focal point) then those scales could be wrong ...

    Using a 75 mm focal length on a 6x17 format means that you are reaching the limits of the 100-105° of angle of classical designs like the super angulon.
    I do not know what is the actual image size delivered by your 6x17 film back, but if we assume 56x170 mm, the diagonal is close to 180 mm ; this makes a total angle of 100° in the corners so you are not far from the absolute limits of the image circle of a classical 100-105° 75 mm wide-angle view camera lens.
    With a 90 mm lens and a maximum angle of 90°, the problem would certainly be less critical, but this suggestion does not solve your real problem

    And regarding DOF problems, as you know, DOF is not intrinsic to a given lens design, a given focal length nor a given film format.
    DOF is actually defined by the final sharpness criterion you define yourself for the final print.
    DOF is not defined by others for you and is certainly not defined by DOF calculators where you have to use some pre-defined settings that you can not change.
    Whether you'll show contact prints of size 5.6 x 17 cm, of murals of 56 cm by 1m70, your definition of DOF will not be the same!

    Now comes and interesting question: if I use a classical DOF calculator anyway, which setting or which format should I use?
    If I us a circle of confusion (CoC) similar to the one in use for the 13x18 cm film format [Europeans like you and me never use formats in inches, natürlich ], hence I'll choose something like D/1720 where D is the diagonal of the image. In 13x18cm this diagonal is 210 mm but here we have only 180 mm; so let's go for c = 180/1720 = around 100 microns, 0.1 mm.
    Now let have a closer look at our mural print, 56x170 cm. Actually if I look a this image, my eyes will scan the format horizontally and I can consider that I have in fact 3 square images stitched together side to side, like if I had stitched 3 images taken with a 6x6 camera.
    Hence the other approach for the choice of c is to take the same value as for 6x6 photography. On the Rolleiflex TLR the good old masters in Braunschweig have chosen c=50 microns for the DOF scales that one can see near to the Rolleiflex fucusing knob. And using a 75 mm you are using the same focal length as all 3.5 Rolleiflex cameras! 75/1720 yields c=43 microns.
    So it means that even the estimation of what computed DOF can be fuzzy in the 6x17 format, with a degree of freedom of at least a factor 2 in the choice of the circle of confusion!!!

  3. #3

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    Re: Problems with Field Curvature and DOF on 6x17

    Quote Originally Posted by Emmanuel BIGLER View Post
    Hello from France

    Actually the only thing that you can do, provided that the optical axis is perfectly set perpendicular to the film-plane, is to define the best infinity stop not at full aperture but at the actual f-stop used to take pictures e.g. f/16 or any f-number that will be your actual working f-number. And yes, this is a pain since doing so, you won't be able to rely on what you see on the ground glass for focusing at full aperture. May be that you'll have to re-label your distance scales, since aiming at the best trade-off at the working f-number for best sharpness on the whole field at every subject distance will yield something different from what the manufacturer has engraved for you!

    If there is field curvature you'll have to make a compromise between the centre and the edges, nothing to do much about that, except using a curved film holder; a cylindrical film path is not a sphere, but this trick was used in the good old days with cheap rollfilm 6x9 amateur cameras fitted with a meniscus lens to somewhat improve image sharpness in the edges.

    At least by defining at the real working aperture what is the best mechanical infinity stop, you'll get the best possible trade-off in the final image for distant objects, if you have some "best focus" shift between full aperture and f/16.
    With ultra wide angle lenses, even modern ones, we have to forget about the Gaussian model of paraxial rays: actually the best focusing setting for far distant objects is no longer the fixed, gaussian focal point, independant of the actual f-stop setting, but another point located close to it and which changes when the aperture changes. And if the manufacturer has engraved the distance scales according to classical gaussian formulae (Newton's formulae, by reference to the gaussian focal point) then those scales could be wrong ...

    Using a 75 mm focal length on a 6x17 format means that you are reaching the limits of the 100-105° of angle of classical designs like the super angulon.
    I do not know what is the actual image size delivered by your 6x17 film back, but if we assume 56x170 mm, the diagonal is close to 180 mm ; this makes a total angle of 100° in the corners so you are not far from the absolute limits of the image circle of a classical 100-105° 75 mm wide-angle view camera lens.
    With a 90 mm lens and a maximum angle of 90°, the problem would certainly be less critical, but this suggestion does not solve your real problem

    And regarding DOF problems, as you know, DOF is not intrinsic to a given lens design, a given focal length nor a given film format.
    DOF is actually defined by the final sharpness criterion you define yourself for the final print.
    DOF is not defined by others for you and is certainly not defined by DOF calculators where you have to use some pre-defined settings that you can not change.
    Whether you'll show contact prints of size 5.6 x 17 cm, of murals of 56 cm by 1m70, your definition of DOF will not be the same!

    Now comes and interesting question: if I use a classical DOF calculator anyway, which setting or which format should I use?
    If I us a circle of confusion (CoC) similar to the one in use for the 13x18 cm film format [Europeans like you and me never use formats in inches, natürlich ], hence I'll choose something like D/1720 where D is the diagonal of the image. In 13x18cm this diagonal is 210 mm but here we have only 180 mm; so let's go for c = 180/1720 = around 100 microns, 0.1 mm.
    Now let have a closer look at our mural print, 56x170 cm. Actually if I look a this image, my eyes will scan the format horizontally and I can consider that I have in fact 3 square images stitched together side to side, like if I had stitched 3 images taken with a 6x6 camera.
    Hence the other approach for the choice of c is to take the same value as for 6x6 photography. On the Rolleiflex TLR the good old masters in Braunschweig have chosen c=50 microns for the DOF scales that one can see near to the Rolleiflex fucusing knob. And using a 75 mm you are using the same focal length as all 3.5 Rolleiflex cameras! 75/1720 yields c=43 microns.
    So it means that even the estimation of what computed DOF can be fuzzy in the 6x17 format, with a degree of freedom of at least a factor 2 in the choice of the circle of confusion!!!


    Fabulous, I love that explanation. thanks

  4. #4

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    Re: Problems with Field Curvature and DOF on 6x17

    I use several cameras to fotoman (4x5 "and 6x6) with several focus cones for several lenses .

    each cone comes from the factory set bad adjust and should use 3 small screws there over the focus ring to move the ring where the lens fits and adjust the focus.

    My cone 65 mm Super Angulon again focused to 1.5 m when it stood at infinity position. With other cones distance was another ... but not infinite.

    If you fit the cone changing the position to infinity, was not right, there were errors .

    I decided to adjust the focus distance is measured with a digital rangefinder and made ​​adjustments to 3 meters. Now focus properly at infinity and at lower distances.

    Only Super Angulon XL cone 47mm does not need any adjustment. But 7 other cones were not properly calibrated.

    Changes very fast fotoman employees and cones and cameras designs. ( I have attended at least 6 different people of Fotoman Chine, who have made mistakes , did not know about the photo and knew no gadgets , they just arent ... different cameras bodies with other older cameras models cones ) That produces many personal mistakes ... inexperienced and parts that are other models of previous designs that do not fit .

    I had many problems with them, then I ought to resolve on my table.

    once they adjust all cameras are very good .

  5. #5

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    Re: Problems with Field Curvature and DOF on 6x17

    Hello.

    in some lenses, when the diaphragm mechanism closes a curious phenomenon occurs . the position of the diaphragm blades is not in a plane.

    curve into the camera (usually , though sometimes in other lenses I've seen out) ... the appearance is of a funnel.

    that produces differences in the image plane , changes between the center and the corners. The focus is not on the same plane and a curvature of the imagen seems .

    this phenomenon occurs more frequently in the shuter mechanism (diaphragm) of angular lenses ( other lenses also occurs ... is only an effect of the diaphragm shutter mechanism , and occurs much more in angular lens , is likely to be due to lower image scale )

    the same shutter , other tele lenses montagen on same is curved ... but the effect is less spherical aberration, and also his sense of flat image and that feeling is more curved image field is smaller.

    anyway , the design Fotoman 6x17 camera , is imperfect ... is not flat, and the film is not very flat on both sides of 6x17 ... although if this plane in the center.

    I've tried Linhof , Fuji and Fotoman ... with lenses of 90 and 105 mm. This problem is only in short lenses ( 75 mm, 90 mm ​​) with fotoman . Not so with Fuji, not with Linhof .

  6. #6

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    Re: Problems with Field Curvature and DOF on 6x17

    sorry my bad English, I am old and many years ago that no English read.

    before I could read and study books in English and French language. (scientiphic and photographic technical books)

    but I've forgotten this ability. Use google translator and it is really very bad.

    anyway, the forum has an automatic corrector that alters many phrases and modifies the translation of google.

    It is very complex for me to write. I hope they bear with me. thanks

  7. #7
    ic-racer's Avatar
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    Re: Problems with Field Curvature and DOF on 6x17

    Quote Originally Posted by Francisco J. Fernández View Post
    I have to focus beyond infinity to get the edges into a good focus
    Try increasing your front-rear cell spacing.

  8. #8

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    Re: Problems with Field Curvature and DOF on 6x17

    My 6x17 is the Fuji camera, I have no problem with it.

  9. #9
    IanG's Avatar
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    Re: Problems with Field Curvature and DOF on 6x17

    Sebastian, are you focusing on a screen or using a focus helical.

    I use a 75mm f8 Super Angulon as my standard (and now only) lens on my Gaoersi with no issues. When I bought the camera the company were quite particular wanting to know exactly what lenses I would be using with the two cones I bought. The second lens was a 90mm f8 SA but I never used it.

    Remember these lenses are optimised for f22. Also they are often shimmed to give their best performance and sometimes the shims are lost, lenses fitted to different shutters etc.

    Ian

    Quote Originally Posted by sdzsdz View Post
    Hello!

    Since a long time now I'm struggelingbto calibrate 75mm lenses on my Fotoman 6x17. The biggest problem is Field Curvature. I noticed in all lenses I tried so far, that I have to focus beyond infinity to get the edges into a good focus. Of course I then loose DOF. First I had a SA 75mm 5,6 that had a decentering defect. After that I tried a Nikkor SW 75 4,5 but the Flange focal distance of that was too short for the camera (although not by much). Now I have an older SA 75 5,6 again. I can calibrate the Helical focusing mount that behaves quite good at infinity and at the corners but I have the feeling, that it has clearly less DOF than the others. Also I'm gaining quite less DOF when stopping down to 22. If I stop down I only can move the focusing ring a few millimeters to the mark of 8 on the Cameras scale and the objecta at infinity get unsharp again.


    Is that a normal behavoiour? Are there other tips for calibrating the lens?

    Best regards, Sebastian

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