Calculating Optimal F-stop for 8x10

Discussion in 'Large Format Cameras and Accessories' started by gbenaim, Oct 18, 2007.

  1. gbenaim

    gbenaim Member

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    Hi all,

    When using 4x5, I used the defocus distance method for choosing optimal f-stop. Now that I'm using 8x10 for contacts, I've been using the same values, though I know I can be more liberal. Does anyone have a chart listing f stop values for 8x10 to use the defocus method? I'm often in need of more than the 10mm listed in the LFF article, and don't really know how much dof I'm getting beyond f64. Thanks,

    GB
     
  2. eddie gunks

    eddie gunks Member

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  3. Ian Grant

    Ian Grant Subscriber

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    eddie, I started to reply to the post earlier, but I have never heard of the defocus distance before, in 40 years of photography. I looked at QT Luongs article on the LFP website and found it overly ponderous and rambling, I think the lack of even a simple diagram is part of the reason.

    I notice the link you posted doesn't mention defocus, I assume its the shifting of the focal plane to a point between what you want in near focus and what you want at furthest distance, and usually if needs be you refer to DOF tables.

    Ian
     
  4. gbenaim

    gbenaim Member

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    So Ian, tell me how you decide which f-stop to use, forget the other method.
     
  5. Shawn Dougherty

    Shawn Dougherty Member

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    I simply focus to the best of my ability with the lens wide open, then stop the camera down while looking at the ground glass. When the image looks sharp enough to me I close down the aperture one more stop for good measure and expose the film. If you're only making 8x10 contacts diffraction won't be much of an issue. If you're enlarging the 8x10 I might find another way. Best. Shawn
     
  6. Ian Grant

    Ian Grant Subscriber

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    That's very easy I guess because I've been using LF for so many years. I don't really have to think about it at the time of shooting, and as I shoot mainly landscapes, prefer longer exposures I can afford to stop the lenses right down.

    In other cases I suppose you get a feel for how much you have to shift focus between the near & far objects you want sharp in an image, and how much you need to stop down, and refer to good DOF tables when you need to.

    I'm not debunking what your doing, what I queried was the term "defocus" because it's not a term I've come across. As I said I found the QT Luong article poor, in comparison the answer you had to this same question on the LFP forum was far better and more succinct.

    I'd be interested in reading more about the technique your using as I'm sure it would be very useful when working with my 10x8 cameras were depth of field on longer lenses is more critical. So if you have any links I'd appreciate them.

    Ian
     
  7. gbenaim

    gbenaim Member

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    Ian, I learned about it from the article on the LFF, and it's quite simple, really. You focus on near, measure the extension, focus on far, measure the extension, substract one from the other in millimeters, check the table, and you have your f-stop, regardless of focal length, format, or light available. You then focus at the halfway point, and that's it. Let's say you measure a distance of 10mm. That gives you an f-stop of 64, so you then focus 5mm in, end of story. I use a simple tape measure to check distance, but you can attach one to your camera if it allows it. The answer I got at the LFF is just what I needed, btw.

    GB
     
  8. Ian Grant

    Ian Grant Subscriber

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    Thanks GB, I looked at one of the links on QT Longs page, that was far simpler to comprehend then when I looked again at Luongs page it was quite simple.

    Except Leonard in his reply to you isn't quite sure he says "I suspect" :smile:

    Actually I have issues with DOF tables because they assume that as you move up formats you are prepared to have less sharp negatives, I enlarge my 10x8 images and I want the highest possible sharpness. I remember using calculating my own DOF tables 30+ years ago for 2ΒΌ and 5x4 because I felt the COC of the manufacturers tables were too large.

    When I get time I might do the same again but this time for 5x4 & 10x8 lenses its far easy now with spreadsheets, then compare them to the bellows extension, and the figures in these Hansma tables.

    I'd really like to see the original articles about this way of working.

    Ian
     
  9. George Losse

    George Losse Member

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    Wow, do all these people really carry all those charts into the field?

    I use the same method that Shawn described above. It work for me for most of the work I do.
     
  10. Alex Hawley

    Alex Hawley Member

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    Same here. Even though I may have perfect focus at wide open, I typically stop down to f/22 or f/32 anyway just to enhance DOF. Although none of my lenses stop down below f/45, I've never seen a hint of a diffraction problem, whether making enlargements or contact prints.

    IMO, throw the charts and calculators away. Life is much simpler without them. Use the ground glass. It doesn't lie.
     
  11. Ian Grant

    Ian Grant Subscriber

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    Alex & George, I totally agree with you that's the way I've always worked except I rarely if ever use the the lens minimum stop. It's also the way everyone else I know using LF uses.

    However there is some logic behind this Hansma system and it may be potentially useful working with 10x8 or large formats particularly when you want to work with faster shutter speeds and know you still have sufficient DOF.

    While the greats like Weston, Adams etc were capable of working exceptionally well without Hansma tables, its only a small bit of card in the backpack, and mental arithmetic.

    Ian
     
  12. Ray Heath

    Ray Heath Member

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    g'day gb
    as a non (traditional) 8x10 user i assume you are talking about techniques to increase DOF to the max, something like hyperfocal distance focusing

    regardless, shouldn't one always focus on what is important and let DOF sort out the rest?

    i would assume that at f64 pretty much everthing is in focus if you actually focused just shy of infinity

    plz explain further if i have this wrong

    Ray
     
  13. Roger Hicks

    Roger Hicks Member

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    Yes. Forget gurus' calculations. Take pictures. Do not look for more precision than exists in the system.

    Cheers,

    R.
     
  14. Shawn Dougherty

    Shawn Dougherty Member

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    Obviously there is not a "right or wrong" answer to this but in terms of my own recent work (and that's not to say it won't change tomorrow) everything in the image is important or it wouldn't be in the picture. I've also found that, with 8x10, focusing near infinity and stopping down doesn't cut it. I'm almost always using movements of some kind and just rely on the ground glass for my DOF... as Alex said, "it doesn't lie." All the best. Shawn
     
  15. walter23

    walter23 Member

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    Yeah, once Scheimpflug enters the equation it's better to just inspect the ground glass and stop down a little bit more than seems necessary. Do you really want to calculate your DOF wedge or whatever for every tilt & swing angle and focus distance and all that? Tilt movements are why I haven't bothered with DOF calculations for LF. That and the nice ground glass & focusing loupe.

    I admire those who take the time to work it out, but it's just not necessary for me. You can probably optimize sharpness a bit over what I'm doing, but my 16x20s look razor sharp when I want them to be and that's all I would ever ask from a negative.
     
  16. Roger Hicks

    Roger Hicks Member

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    Why?

    I'd only admire them if they got better pics. As far as I know, they don't.

    Rather, pity them. Photography is about taking pictures, not making calculations that have only a tenuous connexion with the real world.

    Cheers,

    R.
     
  17. sanking

    sanking Member

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    I agree with Roger. Read as much as you can about DOF and diffraction, then practice a lot. If you do this, your work in the field will be intuitive and you won't have to worry about such tedious calculations.

    I think you will also make better images because you will concentrate more on the really important things, i.e. relationship of objects in the scene and tonal values.

    Sandy King