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Thread: Optimum f/stop

  1. #1

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    Optimum f/stop

    Hello there,

    Is there a simple tool/method to know the "optimum f/stop" for an image? Defocus vs. diffraction. I read the info from the largeformat.info website but find it somewhat complicated, plus my camera (Ebony 45SU) which has Assymetrical tilt is different than traditional view camera when talking about how far my rail travel to focus between near & far, as it is a matter of focus on one axis, tilt untill the whole scene is in sharp focus. But how do you know that you get the right f/stop?

    Thanks in advance.
    Mohsen. Saudi Arabia.

  2. #2
    Athiril's Avatar
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    If you dont plan on stopping down past f/32-f/45, then I'd say use whatever f-stop you need for exposure and be happy..

  3. #3

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    Bite the bullet and go back to the largeformat.info website and spend the time you need to understand it all. Your Ebony, with or without asymmetrical movements has a focus spread between near and far objects just like any other camera. When using movements, you just need to really be sure which objects are the near and far points of focus. Sometimes it is a little tricky.

    That said, most LF lenses work best at f/22 or so. For scenes with very little focus spread, I would use f/22 for best results. Once you get a couple of millimeters of focus spread, you need to start using smaller apertures. f/45 is not too extreme if you make enlargements of less than 16x20 inches.

    Using the info from the website above, you can make yourself a sticker for your camera. If your camera doesn't have a focusing scale, you can make one of those too, so you can accurately measure focus spread.

    Good luck,

    Doremus Scudder
    www.DoremusScudder.com

  4. #4
    Ian Grant's Avatar
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    Zeiss stated that the optimum aperture for LF Tesars was f22, wider than that there's noticeable fall off in resolution at the edges and corners as the lenses are opened up. More modern designs like Symmars, Sironars etc are better at wider apertures but sill optimised for f22.

    Much of what's written about the effects of diffraction at smaller apertures f45/f64 etc is not borne out in practice, as I posted in another thread often the calculations don't take into account lens designs, shape of aperture etc and one design may suffer loss of quality due to diffraction a stop sometimes two earlier than a similar focal length lens. The Maths often used when people make calculations are not accurate enough formulae to cover all variations. Practical testing is better.

    Ian

  5. #5

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    This is the important part to understand from the article....

    ''If you don't want to read the explanations and wade in the math which follows, here are the important points explaned in more details in this article:

    1. If you stop down a lot diffraction will degrade resolution.
    2. However, if the subject is very "three-dimensional" you might have to stopping down a lot to have enough depth of field. In this case, do not worry about point 1, since the effect of not having enough depth of field will be more detracting than the loss of resolution due to diffraction, which is not that significant with large format.
    3. There is a very simple and practical way to find which f-stop you need to use, taking both point 1 and 2 into account. It is due to Paul Hansma. Make your movements first. Then focus on far, focus on near, read the distance "D" in millimiters between the two positions on your rail, refocus so as to split the distance on the rail, and use the following table that I recommend you carry with you all the time. "F" is given in decimal f-stops, as on a hand-held meter, for example 16.6 is 16 and 0.6 (aka between 1/2 and 1/3) of a f-stop. See here for details''.

    D(mm) F
    1 16.6
    2 22.6
    3 32.2
    4 32.6
    5 32.9
    6 45.2
    7 45.4
    8 45.6
    9 45.8
    10 64

    You just need to find an effective way of measuring the focus spread in millimeters on your particular camera to determine the spread of focus.

    I did it by making a paper dial that is wrapped around the focusing wheel of the camera. I dont have to measure the spread of focus with a ruler on the camera bed, that's done via the paper dial. Simply turning the focusing wheel to establish the far and near points of focus tells me the optimum fstop in any given scene.

  6. #6
    ic-racer's Avatar
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    There is a tool available to calculate DOF when lens tilt is involved. This is one of a series of tables the online book Focusing the View Camera by Harold M. Merklinger. (Available free on Harold's website here). If you know (or can estimate) how far your subject extends below or above your scene focal plane you can choose the correct f-stop table. This one is for f22.

  7. #7

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    I don't mean to be flip, but why calculate or bother people when you can ask your equipment?

    My first camera, a Nikkormat FTN, came with a little brochure that explained what the controls did and suggested exercises to make what the words meant clearer. One of the exercises involved shooting at every even f/stop (1.4, 2.0, 2.8, ... ) to fully stopped down, adjusting shutter speed to keep the meter needle centered, to find out what happened. I know, I know, my little Nikkormat was a 35 mm camera and you're shooting 4x5 but you have the same problem I faced years ago. Go shoot systematically and you'll find out how to get your equipment to do what you want.

  8. #8
    Martin Aislabie's Avatar
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    Using a Lupe, focus on the nearest object you want to be sharp.

    Note the Focus Wheel position or Bed Extension

    Repeat on the far object you want in focus and again note the Focus Wheel/Bed Ext.

    Move the Focus Wheel/Bed Ext to half way between the two positions and lock down focusing mechanism

    Slowly stop down the lens until both near and far objects are in focus, note the aperture and add an extra stop for "just in case"

    If you run out of lens stops then start looking at Swing/Tilt, as they bring their own complications.

    If you look at Schneider and/or Rodenstock Web Sites you will see they have optimum F Stops quoted for all their lenses.

    Use it as a starting guide but don't get too hung up about it - just use what you need

    Martin

  9. #9

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    Quote Originally Posted by Doremus Scudder View Post
    Bite the bullet and go back to the largeformat.info website and spend the time you need to understand it all. Your Ebony, with or without asymmetrical movements has a focus spread between near and far objects just like any other camera. When using movements, you just need to really be sure which objects are the near and far points of focus. Sometimes it is a little tricky.

    That said, most LF lenses work best at f/22 or so. For scenes with very little focus spread, I would use f/22 for best results. Once you get a couple of millimeters of focus spread, you need to start using smaller apertures. f/45 is not too extreme if you make enlargements of less than 16x20 inches.

    Using the info from the website above, you can make yourself a sticker for your camera. If your camera doesn't have a focusing scale, you can make one of those too, so you can accurately measure focus spread.

    Good luck,

    Doremus Scudder
    www.DoremusScudder.com
    +1. What Doremus said.

    Understanding how optics work is a skill developed over time, with effort. At least two things are needed. First, an effort to actually learn the material. Second, an effort to apply what you think you've learned -- get out there and expose some film, then make some prints! It is only through this "feedback loop" that's you'll actually understand. It's not necessarily easy, nor is it necessarily quick. But it is necessary.
    Bruce Watson
    AchromaticArts.com

  10. #10

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    I've heard repeatedly about the Rayleigh criterion and the diffraction problems at small aperture. However, experience tells me it just isn't so, at least for low magnification from large format negatives. I have a APO Ronar 480mm on my 11x14 view camera. I wanted to get a picture of a wall in the foreground (20ft away) and a waterfall in the background (600 ft away), both in focus. I stopped my lens down to the smallest f-stop (I think f/128). The negative will make a very sharp 30"x40" enlargement, with no signs of the fuzziness you would expect from diffraction. I have read similar claims from Michael A Smith that he uses high f-numbers without bad effects, but I think he makes only contact prints. In my case I'm only doing magnifications of a few X.

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