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Thread: What COC is best with a 4x5?

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    Alan Klein's Avatar
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    What COC is best with a 4x5?

    What circle of confusion size do you use when figuring DOF and f stop in a 4x5?

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    Re: What COC is best with a 4x5?

    Quote Originally Posted by Alan Klein View Post
    What circle of confusion size do you use when figuring DOF and f stop in a 4x5?
    Personally, I never got all that complicated with it. I ensure the focus plane is where I want it, then stop down while watching the ground glass. Once I hit the aperture where everything I want in focus is in focus, I generally stop down 1 more stop for good measure. The view on the ground glass will, of course, get very dim as you stop down, but I've found that with judicious use of the dark cloth I can still see the image well enough.

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    ic-racer's Avatar
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    Re: What COC is best with a 4x5?

    Quote Originally Posted by Alan Klein View Post
    What circle of confusion size do you use when figuring DOF and f stop in a 4x5?
    You need to look back at your own prints to see. Just to the algebra on the parameters for acceptable prints of your own and figure out what your own acceptable size is.
    If you want to know my number it is 0.15mm.

    In 35mm photography, I usually use two hatch marks inward from the marked F value on the lens barrel.

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    Peter De Smidt's Avatar
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    Re: What COC is best with a 4x5?

    Quote Originally Posted by ic-racer View Post
    In 35mm photography, I usually use two hatch marks inward from the marked F value on the lens barrel.
    I do that as well, including with my medium format Fuji rangefinders.
    May tomorrow be a better day.

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    Re: What COC is best with a 4x5?

    You need to work that backwards from how much enlargement you are going to do and your own tolerance of what’s acceptable.

    The best thing approach in my mind is, take a few shots that you care about (meaning not just bricks, but representative scenes of things you typically photograph) of the same scene with your most used focal length, and take a few shots at various F-Stops. Then print them to your desired size (you can print the same crops on a single page) and inspect them at the desired viewing distance. Ask friends or family for their opinion.

    You be the judge. There’s no one size fits all. A few sacrificial 4x5 sheets will save you greater pains later.

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    Re: What COC is best with a 4x5?

    Quote Originally Posted by Alan Klein View Post
    What circle of confusion size do you use when figuring DOF and f stop in a 4x5?
    Alan,

    There really isn't one standard CoC for all applications. What you might desire and what you end up using are also two very different things, which are dependent on the desired depth of field, the 3D elements of the scene, the amount of enlargement, viewing distance, etc.

    I think, rather, in terms of "optimum CoC." In other words, I try for the smallest circle of confusion I can get while still juggling the other aspects of the photograph I'm trying to make, e.g., diffraction vs. depth of field.

    In an ideal scene, everything falls within the depth of field provided by f/22 of whatever lens I'm using and results in a CoC of less than 0.2mm in the final 20x24-inch print. (Note: everything I discuss here relates to 4x5-inch negatives; different formats have different optimum apertures).

    Such ideal scenes rarely present themselves to me... So, I compromise. I always choose adequate depth of field over diffraction degradation, so, in a scene with lots of distance between near and far focus, I'll need to stop down past the ideal aperture of f/22. I can still make fairly large prints (e.g., 16x20) from negatives made at f/45, but with apertures smaller than that, diffraction rears its ugly head and limits the maximum print size I can make. So, I make smaller prints, or, if the image is still really great large, I just plan on living with the diffraction degradation and plan to have the print viewed from greater distances.

    That's why I recommend and use the method of choosing the optimum f-stop by Paul Hansma and described by QT on the LF home page here: https://www.largeformatphotography.info/fstop.html (yes, I've linked to this before).

    Note that this method yields an optimal f-stop compromise between diffraction and depth of field, but that the resulting CoC from diffraction and depth of field combined grows in size as the focus spread and desired depth of field increase, which calls for a smaller aperture. We don't stop down all the way to get a small CoC calculated from just the depth of field since doing so would result in diffraction causing an even larger CoC (actually a "spot" of diffraction, but we can think of it as a CoC for simplicity's sake) than if we used a larger aperture. The sweet spot is when the CoC calculated from the aperture/depth-of-field and that caused by diffraction are equal in size. This combined CoC size, however, grows with the need for more depth of field and smaller apertures.

    This means that you might not be able to make a really large, acceptably sharp print from a negative made at a really small aperture. However, a smaller print size may be just fine, especially if viewing distance is greater than the 25-30 cm we usually use to calculate acceptable CoC.

    Keep in mind that a 4x enlargement of a 4x5 negative is already a 16x20-inch print and that an aperture of f/45 will still yield an acceptable CoC (i.e., 0.2mm or less) even when enlarged to that size. Note that QT, in the article I linked to above, figures that f/53 gives a 0.2mm CoC at 4x enlargement. A 6x enlargement (24x30 inches!) needs the smaller aperture of f/35. In other words, you can make a negative at f/32 and still enlarge it to 24x30 before worrying about either diffraction or a too-large defocus CoC.

    Here's QT's table from the article giving maximum magnification for various f-stops while keeping the CoC of 0.2mm:

    M = magnification. D = focus spread (on the camera rail in mm). N = f-number

    M........D(mm)......N
    2...........30.........106
    3...........13..........70
    4..........7.5..........53
    5..........4.8..........42
    6..........3.3..........35

    Note that you can still make an 8x10-inch print with an acceptable CoC and use f/106!

    I don't know about you, but I'm happy limiting my shots made at f/45 to 16x20 inches or smaller and those made at f/64 to 11x14 or so .

    Best,

    Doremus

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    Peter De Smidt's Avatar
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    Re: What COC is best with a 4x5?

    I also agree with Doremus. I use the same focus spread method for large format work.
    May tomorrow be a better day.

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    Nodda Duma's Avatar
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    Re: What COC is best with a 4x5?

    Here’s a link to a thread on Photrio I started about this topic. You can see what blur diameters I used for the calculations there.

    https://www.photrio.com/forum/thread...ling-f.169727/

    As you read the thread (and as stated above), consider that your choice of CoC depends on what you want to do. Scan, print, and what size and/or resolution etc...
    Newly made large format dry plates available! Look:
    https://www.pictoriographica.com

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    Re: What COC is best with a 4x5?

    Quote Originally Posted by Nodda Duma View Post
    ... consider that your choice of CoC depends on what you want to do. Scan, print, and what size and/or resolution etc...
    My approach is similar, but from the opposite direction. My subject and lens choice dictate what f-stop I need to get everything I want within the limits of depth of field. For me, depth of field is king and everything else falls into place behind it.

    So, whatever CoC results from the choice of f-stop I make limits the size of the final print I will eventually make.

    Also, I'm not allergic to compromising a bit; I'll live with a little overall unsharpness due to diffraction in a large print if the image still speaks at that size. Sharpness, per se, is not my goal; making an expressive print is.

    Best,

    Doremus

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