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Thread: DOF and printed image

  1. #1

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    DOF and printed image

    I downloaded to my Pocket PC the DOF calculator from LF website. It is very good but now I am trying to figure out how many lines per mm I need to get a crispy image at the final print. There I need some help:
    Let us say that I am shooting in 4x5 and the final print will be 30 x 40 cm (10 x 16 inches). Prints are supposed to be viewed at 50 cm (20 inches) distance. The subject is a good one to demand a sharp rendering. It has textures and well defined edges. Then I have two questions: for the area "in focus", what is the average lines per mm definition for a print in that situation that would make a crispy look to the average human eye? and second: As I am enlarging roughly 2.3 times the neg, is that correct to take the answer to the first question and devide it by 2.3 in order to assess (at the shooting phase) the definition in lines/mm I need in the negative and foresee what is going to be in focus in the final print using the DOF calculator?

  2. #2

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    Re: DOF and printed image

    Honestly: leave your Pocket PC at home, and concentrate on getting a good photo. Use your groundglass to evaluate your DOF, and stop down as much as you need....
    Don`t make things so complicated for yourself.

  3. #3

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    Re: DOF and printed image

    I second not worrying about the pocket PC calculation. Sharpness is about contrast as much as LPM. Are you trying to not stop down any more than necessary to keep the shutter speed up? When I am really worried about the trade-off I shoot two sheets with different F stops. I also spent some time figuring out where diffraction really starts to limit my sharpness - the theoritical limits are more strict than what you see in real life. In my case, while theory indicates that diffraction starts to be an issue beyond F32 in 4x5, I cannot see any difference in my prints at F45. I scan and work digitally - maybe I could see the difference with a drum scan, otoh, digitally sharpening really works to increase the ILLUSION of sharpness, which is as good as real sharpness in a print.

  4. #4

    Re: DOF and printed image

    The value commonly quoted for the resoloving power of the human eye is 5 lp/mm, but there isn't a sharp cutoff. If you decide that you want, say 4 lp/mm on the print that is a 2.3X enlargements, then you need 9.2 lp/mm on the negative. You have to multiply the desired resolution on the print to get the desired resolution on the negative since you are enlarging the negative. (This is assuming both no losses in enlarging, nor gains from digital sharpening.) You might want to experiment to find out the minimum value that produces acceptable results for your tastes.

    If you read the article on the main page, an easy way to calculated the f-stop is to use the focus spread in image space. Is this program using that number as the input? If so, with one number differing from photo to photo, you could prepare a simple table in advance and not have to carry the Pocket PC. Such tables are already in the article.

  5. #5

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    Re: DOF and printed image

    Recall that DOF is calculated assuming a certain maximal allowable circle of confusion at the level of the film. Alternately, this could be expressed as the minimal acceptable resolution in lp/mm, which would be the reciprocal of the coc in mm.

    The usual rule of thumb is that prints will be viewed from a distance equal to the diagonal of the print. For an 8 x 10 print, that would be about 12 inches. The usual criterion for such a print, would be a coc of between 0.1 and 0.2 mm, which corresponds to a resolution of between 5 and 10 lp/mm. Since, you have to enlarge a 4 x 5 negative by a factor of two to get an 8 x 10 print, you would have to halve the coc, or double the resolution, at the level of the film for a coc of between 0.05 mm and .1 mm, or a resolution between 10 and 20 lp/mm.

    If your final print were larger, but it was still viewed from its diagonal distance, no further change would be needed in the resolution at the level of the film. That is because any increase of resolution needed due to a larger print is exactly compensated for by the greater viewing distance for the print. If it were viewed at a different distance, you could correct the appropriate film resolution by the ratio the actual viewing distance to the print diagonal. For a 10 x 12 print, with a diagonal of about about 15 inches, viewed from about 20 inches, the ratio would be 20/15 ~ 1.33. That would mean that a coc of between 0.067 mm (resolution 15 lp/mm), and 0..133 mm (resolution 7.5 lp/mm) would be adequate in the film.

    The other advice proffered, while not addressing the actual question you asked, has some merit. Many other factors can affect how sharp a print will appear. First, a single resolution figure doesn't take into account how resolution varies with contrast. For that you would have to use MTF analsysis, which would be too complicated for practical use. In addition, the nature of the subject may influence in various subtle ways your perception of sharpness. I just saw a 16 x 20 print at our local art show made from a 4 Mpx digital camera which looked tack sharp, at least from normal viewing distance. Of course, it was sharpened in a photoeditor, but the main factor was that it actually had very little fine detail and everything depended on having well defined edges of larger structures. At the same art show, I saw a fantastic 20 x 25 print made by a 4 x 5 camera, scanned on a $50,000 scanner and printed by a very high quality (not inkjet) printer. Even close-up one could see the fine detail like barbed wire at some distance from the lens. One could never acheive this with any of the current crop of digital cameras, but even here the nature of the fine detail helped with the overall feel of incredible shaprness.

    Be that as it may, you do have to start somewhere, and if you are estimating DOF, you are going to have to input a coc. For 4 x 5, usually something between 0.05 and 0.1 mm is appropriate. Personally, I find DOF distances in object space, which is what most calculators give you, of little use. I prefer to use methods based on the focus spread, the distance on the rail between near and far points desired in focus. Such methods are described in detail elsewhere in the large format photography webpage. I usually start with a coc of 0.1 mm, the higher end (lower end for resolution), determine the appropriate f-stop from that and then stop down as far beyond that as I can without subject motion or diffraction becoming a problem. The LF page also describes a method due to Paul Hansma which tries to find the best balance between defocus and diffraction, which is not based on a choice of coc, and many people use that.

  6. #6
    Donald Qualls's Avatar
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    Re: DOF and printed image

    The rule of thumb is a minimum of 7 lp/mm on the print, for viewing at arm's length. Multiply that by the magnification factor you'll apply to get what's needed on the negative -- so a 4x5 negative intended for printing to 8x10 could get away with 14 lp/mm, which implies a COC no larger than .07 mm.
    If a contact print at arm's length is too small to see, you need a bigger camera. :D

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