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Thread: Choice for the Circle of Confusion?

  1. #31

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    Re: Choice for the Circle of Confusion?

    Quote Originally Posted by Pere Casals View Post
    Of course, a good spot meter is a perfect choice, but I've not one.

    Using a DSLR/SLR in the back of a view camera is not for everyday, IMHO, but it allows to calibrate and check many things. In fact a (near for free) Nikon F65 in the back is a perfect Probe meter accounting all, lens transmission, aperture calibration, fall-off, bellows extension... It has a viewfinder so it's very convenient to navigate in the framing with the shift-rise.

    Also I can use the DSLR/SLR (+zoom) as if it was a Director's Scope that will tell if framing fits in the negative (Hor and Vert eq focals in the zoom), so when I haul the cambo 8x10 and my insanely heavy Bilora is just to shot, this way I avoid dragging around all those irons to find the good shot.

    Also, as DOF rules are the same, in the zoom I may place the same focal and aperture, and I can check what will happen in the 8x10 shot. So I focus to the intended distance, and with focus blocked I can explore and (DSLR) shot the rest of the field, by zomming in the rear screen of the DSLR I see how the rest will be compared to the object in the focus plane (if no tilt).

    I'm not saying that this has to be the regular procedure, at all, just it's useful for a learner like me.





    At all, we can make top notch art with an smartphone in auto mode.


    An artist may take advantage from softness, from sharpness or from both at the same time. But there is nothing wrong in mastering DOF management, if one wants that.
    As a learner you excel at making things far more difficult and complicated then they really need to be!

    For decades their have been very easy and accurate means of metering very small spots to the entire gg. One common system was the Prontor Probe with a Gossen Profi Six or Lunapro, as well as some other meters that metered in the film plane, this probe is what Sinar and Linhof sold.
    Or by metering through the gg and, if in place the Fresnel, with most any meter that accepted either a microscope adapter or a gg adapter.

    They were easy to use and dead accurate.

  2. #32

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    Re: Choice for the Circle of Confusion?

    Quote Originally Posted by Bob Salomon View Post
    As a learner you excel at making things far more difficult and complicated then they really need to be!
    Sure !!!

    A noetic approach usually fails, but it also can deliver inventions.


    Quote Originally Posted by Bob Salomon View Post
    For decades their have been very easy and accurate means of metering very small spots to the entire gg. One common system was the Prontor Probe with a Gossen Profi Six or Lunapro, as well as some other meters that metered in the film plane, this probe is what Sinar and Linhof sold.
    Or by metering through the gg and, if in place the Fresnel, with most any meter that accepted either a microscope adapter or a gg adapter.

    They were easy to use and dead accurate.
    Of course, but what I had at hand was slr and a dslr cameras. For negatives I had no problem, but with velvia is different. I adjusted my exposure style making bracketings with a Nikon F5, by placing the F5 in the back I could adjust the metering style for the view camera.

    Don't think that placing say a $15 Nikon F65 in the back is difficult, it's just a plate with a macro extension or reverse ring bolted. The F65 spot meter is absolutely dead on, and you have a viewfinder with an eyecup, you don't need the darkcloth. That solution is easy and totally accurate, technically good, but of course it would be a total surprise to see John Sexton using that, it would be viral !

    Well, this is testing TTL metering in a view camera for $15

  3. #33

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    Re: Choice for the Circle of Confusion?

    This has all been very interesting, and I've gained a better understanding of circles of confusion. (This name really kills me.) I appreciate everyone's contribution to this thread. With that said, I'm thinking of the following.

    It becomes apparent that choosing a practical circle of confusion comes down to determining the viewing distance from the print and knowing the human eye's angular resolution.

    I like aspects of Tuan's article.

    https://www.largeformatphotography.info/fstop.html

    For one thing, it gives a reasonable derivation of c=0.033, which has been used historically for 35mm film. It's based on making a 6"x8" enlargement, and then viewing it at a "reading distance" of about 10". Kingslake [Lenses in Photography, 1963] has suggested that a "normal" viewing distance of a print is approximately the diagonal of the print. 10" is exactly the diagonal of a 6x8 print. This is also consistent with what Emmanual has suggested, that we view an 8x10 contact print at about 12". In fact, that's very close to the diagonal of the image area of an 8x10 negative.

    As to the angular resolution, Tuan's article states that we can perceive at best, 0.2mm difference in two lines at a reading distance of 25cm. (About 10".) This corresponds to an angular resolution of 1/1250 radians. In comparison, Emmanual has suggested an angular resolution of 1/1720 radians, which he states is quite a stringent value. Kingslake has suggested that, under research conditions, the angular resolution is 1/2300 radians. But he counters that, practically speaking, it's closer to 1/1000 radians. So, I tend to lean towards 1/1250 radians as being somewhat conservative, but not stringently so.

    It's been implied in this thread and in Tuan's article, that the circle of confusion should depend on print size. But, I'm uncomfortable with this. Taken to the extreme, that would suggest that we would view a 16x20 print at the same distance that we view an 8x10, or a 6x8 print. I think that, as we view larger and larger prints, we tend to step back. Of course, one can view any print up close; but, I don't wish to base a circle of confusion on this type of close examination. On the other hand, it seems that circles of confusion should definitely be based on film format size.

    Given an angular resolution of 1/1250 radians and a viewing distance of 25cm, one gets a working circle of confusion of 0.2mm. Using multipliers for different formats, I can now construct DOF tables that give near and far distances of acceptable sharpness for each lens at each each f-stop. Each table would consist of three pairs of numbers, near and far for medium format, for 4x5, and for 8x10.

  4. #34

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    Re: Choirce for the Circle of Confusion?

    Quote Originally Posted by Tin Can View Post
    Eyesight is requiring point and shoot LF as I learned yesterday shooting in high glare conditions. Even my 20X reversed Nikon 50 mm lens that works in studio was useless in the field.
    Randy, since this came up a couple of times: a 50mm lens is not a 20x loupe. It's a 5x loupe. The magnification of a loupe is defined as 250mm/focal length. (The idea being, I guess, that a person with good eyesight can focus the unaided eye at 250mm, and the loupe magnifies X times that.) If the 50mm lens isn't close enough for you, maybe try an 8x or 10x loupe.

    You might be thinking of diopters: diopters are 1000mm/focal length, so a 50mm lens is a +20 diopter.

  5. #35
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    Re: Choirce for the Circle of Confusion?

    Thanks for the correction. You are correct I contused diopters.

    Going through boxes searching for a lesser 50mm to use. Tiny Repronar 5cm with a spacer tube may work.


    Somewhere I have a couple 50mm enlarging lens to try. The good one stays in the enlarger.





    Quote Originally Posted by reddesert View Post
    Randy, since this came up a couple of times: a 50mm lens is not a 20x loupe. It's a 5x loupe. The magnification of a loupe is defined as 250mm/focal length. (The idea being, I guess, that a person with good eyesight can focus the unaided eye at 250mm, and the loupe magnifies X times that.) If the 50mm lens isn't close enough for you, maybe try an 8x or 10x loupe.

    You might be thinking of diopters: diopters are 1000mm/focal length, so a 50mm lens is a +20 diopter.

  6. #36
    Drew Wiley
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    Re: Choice for the Circle of Confusion?

    It's amazing how much confusion can be packed into a tiny little circle! I can understand the concept of circle of confusion if a halftone image, along with text, is printed in a book intended to be read from a given distance. But never once in my own photographic work have I seen any need whatsoever to engage that nebulous concept in order to efficiently manage either depth of field issues or detail. Just one more unnecessary convoluted hurdle.

  7. #37

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    Re: Choice for the Circle of Confusion?

    Quote Originally Posted by neil poulsen View Post
    1/1250 1/1720 1/2300 1/1000 radians.
    It should be noted that eye resolving power is not constant.

    > Of course it depeds on the subject, a 20/20 subject has good eyes.

    > It depends on the target contrast, an slide may show 1:4000 contrast, a paper may show 1:100, but in an image our fine detail may have 1:2 contrast.

    > It depends on illumination, our eye varies the f/ aperture in the iris depending on light, the eye also has an optimal aperture.

  8. #38

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    Re: Choice for the Circle of Confusion?

    From Neil:
    ...that the circle of confusion should depend on print size. But, I'm uncomfortable with this.

    Hi Neil.
    You should not be uncomfortable with the idea of varying diameters for circles of confusion.

    Another historical example. For decades, amateurs used 6x9 [cm] rollfilm cameras with a 100-105 mm lens.
    If you reverse-engineer DOF scales on those venerable cameras (see just below), you'll find that very often DOF scales were computed for a circle of confusion of 100 microns, i.e. D/1000 or f/1000 (those cameras had a fixed standard lens with f=D).
    And this did make sense, because most often images were only contact prints of 6x9 cm size for the family album, viewed at a distance of about 30 cm.
    If you use a modern 100 mm view camera lens, with resolutions limits well above 60 cy/mm, those 100 microns are really fuzzy with respect to what you can record with a 100 mm lens. If you allow your prints to be scrutinized to the the tiniest details, then you should choose your CoC according the to the ultimate lens resolution capability :-)
    But if you only make 2"x3" contact prints, 100 microns will be the good CoC and you'll get plenty of DoF!


    --------

    Reverse-engineering an engraved DOF scale is straightforward
    Choose the smallest available aperture e.g. 22, rotate the focusing ring so that the infinity sign is facing "22".
    In the middle of the DOF scale you can read the hyperfocal length H and in principle facing the other "22" marking you can read H/2.
    This allows you to find H(N) the hyperfocal distance for the f-number N.
    Coming back to basic maths of DOF scales with H(N)=f2/(Nc), you get
    c = f2/(N H(N))

    I have measured the following values for "c" (sorry this is off-topic medium format, but illustrates the above demonstration)

    Rolleiflex T TLR lens tessar 3.5-75mm H(22)=5m ==> coc=51 microns

    Zeiss Hasselblad Planar CF100 model 1982 : H(22)=6m coc=75microns strange value; not very stringent !
    Zeiss Hasselblad Distagon C 50 Sync-Compur: H(22)=2.2m coc=50microns standard classical for 6x6
    Zeiss Hasselblad Sonnar C 250 chrome Synchro-Compur model ~ 196x : H(45)=24m ==> coc=60microns

    From Wildi's book :
    Zeiss Hasselblad Planar 2,8-80 (C?)/Wildi : H(22)=4.9 coc=60microns OK but lens is credited to pass about 100 cy/mm ...

  9. #39
    Jac@stafford.net's Avatar
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    Re: Choice for the Circle of Confusion?

    What a bundle of silly, irrelevant key-board metrics this has become. You have a lens, make a few exposures at various apertures, print and decide. Real life experience - what a concept!

  10. #40
    Tin Can's Avatar
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    Re: Choice for the Circle of Confusion?

    I can no longer trust what I see. Nothing is sharp...



    Quote Originally Posted by Jac@stafford.net View Post
    What a bundle of silly, irrelevant key-board metrics this has become. You have a lens, make a few exposures at various apertures, print and decide.

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