Hyperfocal distance and circles of confusion for 120 film

Discussion in 'Medium Format' started by philip_dygeus|2, Dec 20, 2017.

  1. I'll be doing a project to document the inside of a building and shoot in spaces most of which are not very large, perhaps 3-8 metres at the longest edge. I'll shoot with my Hasselblad and mainly use the 40 Distagon.

    Since I'd like as much as possible of each scene to be in focus I've been toying with the idea of using hyperfocal distance focusing. But to determine which distance I should use I realise that I need to determine an acceptable circle of confusion.

    The charts over at dofmaster.com use 0,045mm as the CoC for 120 film but, as I understand it, that choice of CoC has to do with the expectation that 120 film negatives need not be enlarged as much as 35mm film negatives, which is why the CoC for the latter type is often set to 0,03mm (or, I have read, 0,036mm).

    So the question of use of the images is relevant, then. I intend to scan my images and use them online. That said, I wish to be able to provide large scans (4000dpi and up) which I would like to be as sharp as possible. I might later be able to put together a photo book and there might also be prints later on (of unknown sizes).

    Against this background, would I be dumb to use 0,045mm for my CoC calculation or should I instead use 0,03?

    And, am I over-thinking this and should I instead just use f/16 and focus at about 2m?

    That would, according to the DoF field on the Distagon (which is corroborated by the the dofmaster charts based on 0,045mm) give me a near DoF limit of about 1m and far DoF limit of close to infinity.

    A related question - with the Distagon 40 in particular is there a risk of image degradation due to diffraction at f/16 so I had better use f/13?

    I've shot a lot with Distagon and know it quite well but I must admit I've never thought about this.

    Thank you very much in advance
  2. Harsh truth - there is only one plane of sharp focus. Anything not on that plane will be more or less out of focus.

    A CoC sets an artificial limit to what is perceived as 'sharp' - or acceptably fuzzy - and this limit changes with viewing distance and print size or magnification.

    The scan ppi makes no real difference to this, unless it's low enough for each pixel to be larger than the chosen CoC. 4000ppi is almost an order of magnitude finer than a CoC of 0.045mm.

    So unless you plan to print a metre square and let viewers plant their noses on the print, the accepted 0.045 mm CoC will be fine.

    In any case, the relative size difference between 35mm and 6x6cm is at least 56/36 or 1.56 times. Therefore, even if you take the stricter 0.03mm CoC as the 'standard' for 35mm film, using 0.047mm for 6x6 gives you the equivalent sharpness.

    "A related question - with the Distagon 40 in particular is there a risk of image degradation due to diffraction at f/16 so I had better use f/13?"

    - Now you're really splitting hairs. You'll be lucky if the lens aperture is accurately calibrated to half a stop, let alone notice any difference in diffraction effect.
    Last edited: Dec 20, 2017
  3. Hi Joe and thank you for the reply. It's nice for a bit of perspective. I had a feeling I was over-thinking this so I much appreciate your comments :)

    I'll just set focus somewhere appropriately depending on the scene and pick an f stop that gives me long enough DoF. Who ever said this needed to be complicated? Um, that was I (sheepish smile).

    Thanks again

  4. Are the negs being scanned? Though I'm not usually a fan of "tricks", focus stacking with just a few shots can work miracles impossible by any other means. Also, AFAIK, diffraction is entirely a function of physical aperture size, and is no better or worse for any particular lens. Where (and if) a lens becomes diffraction limited is another matter. I see it with smaller formats, but doubt it's a big factor with medium format.
  5. Hi Conrad, thanks for the reply. Yes I will scan the negs. Focus stacking sounds interesting. I'm wondering how I would determine how many frames I would need and at which focus distances?

    Yes, I think my worries about diffraction are baseless actually. I should be able to shoot at f16 without meaningful degradation.


  6. I admit to being a relative neophyte at stacking, but have had good success with the ones I've done. There's a huge amount online about it. I'd think interior shots at f/16 could be accomplished with just a few frames, focused on whatever features you're worried about. If you take too many, it's no problem not to use them. I use the free program CombineZP. It's old and no update in 7 years or so, but it works well. Most people use it for closeup work where DOF is a huge problem, but it should work for anything. Some images here- Stacked images
  7. According to the physics, diffraction at f/16 gives an Airy disc diameter of around 0.02mm, which is less than half the CoC recommended for MF. Shouldn't be an issue.
  8. I've used as few as two frames for focus stacking landscapes - one for the foreground and one at infinity. The foreground is the most problematic. If it is in several planes, you need to make sure every part is in focus in at least one frame.

    You can use the focusing rings for landscapes, but most lenses tend to change magnification unless the changes are relatively small. For closeups, I have a rack-and-pinion focusing rail for macro objects and a screw thread rail for anything closer than about 1/4x. With an Hasselblad, I recommend a high-power viewfinder, like the stovepipe version (4x). A prism finder (2.5-3x) is adequate for general photography.
  9. And then there is diffraction. With diffraction, there is no plane that is sharp focus, only one that is more sharp than the others.

    If you set the CoC size to the diffraction spot size, you aren't losing much. As above, diffraction might be less than the suggested CoC size, but not a lot less.

    You don't say which film you use, but many have the MTF graph out to about 80 lp/mm, so about 0.013mm.

    Between film resolution and diffraction, 0.03mm or 0.04mm or even 0.045mm seem about right.
  10. Thank you all for the further replies (and my apologies for the delay in responding). I have begun the project to we shall see how it turns out. In a few locations I have taken two shots with a view to trying focus stacking.

    Glen, I'm shooting the project on Ektar. And Ed, you bring up something I've been thinking of which is that I really need higher magnification than what the built-in finder's magnifier offers. I currently use one of those "chimney" finders as a loupe for my lightbox so I'll try that one.


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