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Thread: Using DOF Calculations Bestly for Print Size

  1. #1
    Tin Can's Avatar
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    Using DOF Calculations Bestly for Print Size

    I have used DOF Master Calculator often, this one. https://www.dofmaster.com/dofjs.html

    Probably incorrectly as even I see problems in my small enlargements.
    sin eater

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    Re: Using DOF Calculations Bestly for Print Size

    The usual rule of thumb is that resolution in the final print should be at least 8 lp/mm for it to appear sharp at normal viewing distance. 8 lp/mm is usually interpreted to mean a circle of confusion with diameter 1/8 mm = 0.125 mm.

    Enlarging by x makes resolution in the print = 1/x resolution in the negative. So, to enlarge 2x would require a CoC of 0.125/2 = 0.0625 mm, and so on.

    IMO, possibly mistaken, 8 lp/mm in the final print isn't enough, more is better. But this view is not widely held.

    And now we see why enlarging film by more than 10x isn't always a good idea if (big if) a final print that looks sharp at normal viewing distance is needed. 10x requires at least 80 lp/mm in the negative. This is not easy to attain with the best equipment, film and technique. See, for example, https://www.flickr.com/photos/nesste...44296/sizes/o/ and https://www.flickr.com/photos/nesste...44224/sizes/o/

    Against this, and I have and have seen lovely prints that were fuzzy all over the frame, sharpness isn't everything.

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    Re: Using DOF Calculations Bestly for Print Size

    Quote Originally Posted by Dan Fromm View Post
    8 lp/mm is usually interpreted to mean a circle of confusion with diameter 1/8 mm = 0.125 mm.
    I guess this is not easy to interprete.

    When in graphic arts they mention 300 dpi this is 11.8 pixels per mm, so 6 lp/mm at (ideally) 100% MTF. Is this more or less than analog 8lp/mm at extintion? what if using Rayleigh criterion ?

    Ctein asks: Why does a print that resolves 20 lp/mm look less sharp than one that resolves 65 lp/mm?


    I think that I understand MTF graphs and what is lp/mm at extintion, but I get often confused by terminology around !

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    Jac@stafford.net's Avatar
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    Re: Using DOF Calculations Bestly for Print Size

    Quote Originally Posted by Pere Casals View Post
    [...] Ctein asks: Why does a print that resolves 20 lp/mm look less sharp than one that resolves 65 lp/mm?
    If the printing is optical through an enlarger despite the degree of enlargement, or even 1:1, there is an unavoidable loss of resolution. It is physics.

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    Re: Using DOF Calculations Bestly for Print Size

    Quote Originally Posted by Jac@stafford.net View Post
    If the printing is optical through an enlarger despite the degree of enlargement, or even 1:1, there is an unavoidable loss of resolution. It is physics.
    In tests I made I found that (a good) optic enlargement does not degradate Image Quality that it's in a LF negative. The enlarger lens usually works in a better situation than the taking lens:

    > Lens can usually be used at the optimal aperture.

    > Focus it is critically adjusted with a grain loupe, negative can be in a glass carrier, and the easel may keep paper very flat.

    > Lens is usually specialized in the particular job. My old semi-destroyed "no letter" rodagon 210 takes at least 145 lp/mm from the negative.


    Think that we see the grains in the loupe when focusing (if the loupe is strong enough) the LF image in the negative is less sharp than that, so little is lost.

    Then paper resolves a lot, more than the enlarged image. Paper may resolve 60 to 100 lp/mm, if the LF negative has (say) 60 lp/mm when enlarging x2 we have 30lp/mm on the easel.



    Quote Originally Posted by Jac@stafford.net View Post
    or even 1:1
    At 1:1 we may have a loss if we don't use the right lens, for 1:1 we would need a Duplication lens (Rodagon R or D). If we use a regular lens it won't take all information in the negative because the lens won't work well at 1:1, anyway that pitfall it won't be noticed in a print because it will still print more than what an human eye would see.

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    In my (limited) experience what I found is that a (good) LF optic enlargement crafts an insane amount of image quality on a paper.

    First that I found is that a EL Nikon 50mm takes from film way more than a drum scanner at 9,000dpi effective (if this exists), a LF Rodagon is not very far.
    Click image for larger version. 

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    (my glass slide is defective in the group 7, still it shows what an insane resolving power enlarger lenses have)


    Click image for larger version. 

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    That the enlargement has no loss it's easy to check, just make a sandwich with the negative and a usaf 1951 glass slide, with emulsion in contact with the pattern side to keep focus, with no calculation it's evident that we see way, way finer detail in the target than in the film, showing that the enlargement does not limit image quality.

    Click image for larger version. 

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    IMHO to challenge the enlarger lens at work we would need an extraordinary negative, something like a CMS 20 shot made with a crazy good lens that I guess it was not sold in the commercial photography market, say a 30 kg aerial lens.
    Last edited by Pere Casals; 25-Aug-2019 at 15:32.

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    Re: Using DOF Calculations Bestly for Print Size

    Quote Originally Posted by Jac@stafford.net View Post
    If the printing is optical through an enlarger despite the degree of enlargement, or even 1:1, there is an unavoidable loss of resolution. It is physics.
    That is the reason I keep going to larger formats and contact print everything. My 2 enlargers may be dying of old age,but not overuse.

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    Re: Using DOF Calculations Bestly for Print Size

    to add to above, the reason I started this practice many years ago was a test I made. I made an enlargement of a very good negative on glossy paper. Then I contact printed a very similar,but larger, negative on Azo. The superiority of the contact was obvious to everyone to whom I showed it.

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    Re: Using DOF Calculations Bestly for Print Size

    Quote Originally Posted by Jim Noel View Post
    glossy paper. Then I contact printed a very similar,but larger, negative on Azo. The superiority of the contact was obvious to everyone to whom I showed it.
    Jim, if you say that the Azo result was better I'll for sure trust that, but this is not a side by side comparison. Different papers with different tonality, different shots, different formats.

    IMO it's difficult that the enlargement (if good) degradates Image Quality that's in the LF negative, another question is what Image Quality we are able craft in the negative, and regarding this we have a DOF vs diffraction trade-off that may be different depending on the print size and depending on what we want.

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    Re: Using DOF Calculations Bestly for Print Size

    Owners of 8x10 enlargers know that image degradation from stopping the camera lens too far is visible in prints. However everything is a compromise. The math has already been done so, setting aperture based on focus spread works best for me. I have a mark on my focus knob, so, for example, one revolution = f32, etc.

    I can't tell the difference between my contacts and 1:1 enlargements onto photographic paper, other than the contact prints have Newton rings and dust.

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    Re: Using DOF Calculations Bestly for Print Size

    Y'all do realize that all this has been done and discussed in depth here: https://www.largeformatphotography.info/fstop.html , don't you?

    FWIW, I'll gladly live with some diffraction and a smaller degree of enlargement to get the DoF I need for a particularly tricky subject. How else do you get those shots anyway? Like ic-racer, I've worked out tables for optimum aperture and maximum enlargement based on my preferred CoC. Works just fine for me.

    Doremus

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