rb67 precise focusing

Discussion in 'Medium Format Cameras and Accessories' started by nwilkins, Sep 14, 2012.

  1. nwilkins

    nwilkins Member

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    I have found that getting good focus and depth of field with the RB67 takes a little practice. I am pretty good with it now, but if I could change anything about the RB it would be finding some way of calculating focus distance more accurately on the lens, so that both hyperfocal focusing and precise depth of field control would be easier. Would it be possible to calculate the focusing distance for each lens based on the length of bellows extension? Has anyone tried this?

    Thanks!
     
  2. djhopscotch

    djhopscotch Member

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    Why not use the scale on the side of the camera?
     
  3. nwilkins

    nwilkins Member

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    because it is very imprecise, especially with the shorter focal length lenses.
     
  4. photo_griz

    photo_griz Member

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    I just bought an RB this week and have not shot a single frame with it yet, so take this with a grain of salt because this is all theory and not a lick of field experience.

    I have a decent background in physics and optics. The DofF, hyperfocal distance, and resolving power (circles of confusion vs diffraction) are completely predictable and are independent of film size or format or lens. They depend on focal length (or focus shift on view cameras) and aperture. A high quality lens may give better results, but both the high quality lens and the low quality lens will have the same focus characteristics if they have the same focal length and aperture.

    All this is to say, you can buy a focal length calculator app for you phone or iPod, and they work pretty great and are dead on accurate. I use one for 35mm and find I get better results than looking through the viewfinder. This is with Nikkor AI-S primes. I anticipate being able to replicate this on the RB, but like I said I haven't used it (I don't even have a focusing screen yet!!).

    The app I use most often does it even one better. I tell it what what I want the DoF to be and what focal length I'm using, and it tells me the aperture will give me that DoF with the smallest blur spot. The blur spot takes into consideration CofC and diffraction. (smaller blur spot=higher resolving power=sharpest image). For instance, if I'm shooting a scene where I want every thing from 20' to infinity in focus with a 50mm lens, it tells me I should use an aperture of 1/3 stop smaller than f11 and it will give me a blur spot the size of about 25 microns and 18 line pairs/mm on a 10" wide print.

    That is a lot of information from a $6.99 app and it has dramatically improved my photography. It also has prevented my from taking shots that just wouldn't work if not adequately sharp. No use in wasting film and processing and scanning if it is only going to yield 5 lp/mm on a 10 inch print.

    Lots of these calculators are free and you might want to check some out to see if they help.

    Chris
     
  5. paul ron

    paul ron Member

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  6. MattKing

    MattKing Subscriber

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    I could be wrong, but I think the problem that the OP is struggling with is not calculating settings to get the best depth of field, but rather using the relatively imprecise distance scale on the RB67 to either measure distances or scale focus.
     
  7. paul ron

    paul ron Member

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    Regardless....

    Why calculate the DOF when you can preview it?

    If you want to mark the standard for bellows extension at each ƒ-stop that's possable. I think there was a gizmo that does exactly that, but I have no idea where to find one. You can make something along that line by using the side panel scale but placing a white tape n making your own ticks.

    .
     
  8. photo_griz

    photo_griz Member

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    It can sometimes be very difficult to preview DoF and focus on the focus screen. For instance, a shot needing f22 at twilight is very dark when you preview DoF. I use the focus distance on my 35mm lenses to focus instead of the viewfinder because I have found this method to give me more consistent results and the sharpest possible pictures. I think the OP's frustration is that, unlike with prime 35mm lenses, the focus scale on the RB is not precise enough to determine actual DoF. Is there a workaround to this? I am new to the RB and would also like to know.
     
  9. paul ron

    paul ron Member

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    Here is a calculator you can make that will give you an idea what each ƒ-stop can do for your lens....

    http://lightingmods.blogspot.com/2008/05/diy-depth-of-field-calculator.html

    then there is the "measure the bellows extension" I described in this quote.

    Pick your poison.

    I use the tape on the standard method with ticks myself.
     
  10. nwilkins

    nwilkins Member

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    thanks for the responses everyone. My original question was "Would it be possible to calculate the focusing distance for each lens based on the length of bellows extension? Has anyone tried this?" And it turns out the answer is yes, although with perhaps some difficulty at further distances because you would presumably be working with fractions of a mm.