Printing is a secondary operation that has nothing to do with the DoF of the image on film.
- Leigh
Printing is a secondary operation that has nothing to do with the DoF of the image on film.
- Leigh
If you believe you can, or you believe you can't... you're right.
Yes, you would come up with different answers if you were trying to figure out the DoF onthe film (for theoretical purposes) or in a print (for practical ones).Originally Posted by Leigh
Thanks so much to everyone who has replied!
And thanks paulr for providing that link to that research paper-- I look forward to reading it because it looks exactly like the kind of study I wanted to do myself.
I believe we can agree the DoF calculations get us close to equivalency between formats and optically lenses would operate the same so there shouldn't be any anomalies causing different looks. It could very well be a perceived sharpness difference from the larger negative causing the large format to appear more startling compared to that of smaller formats.
Marshallarts, your numbers are wrong:
4x5" equivalent focal is 166.67mm (80x120mm) for 1.5x aspect ratio.
Also take into account that ratio is 3.3333(period). Not number four as used in the paper.
80mm/24mm = 120mm/36mm = 3.33333....
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(edited focal lenght was wrong)
Last edited by ramon; 27-Mar-2013 at 06:52. Reason: Also my numbers are wrong
That's not entirely true. Printing size will affect one's perception of what is in focus and what is not. Printing 4x5 to be printed 4x5 feet will have much less room for error than printing 4x5 to be contact printed into little handheld gem photographs.
-Chris
Acceptable Circle of Confusion (CoC) is one factor to consider. It is associated with degree of enlargement, which in turn is associated with anticipated viewing distance.
You can't make any DOF calculation without knowing the acceptable COC on the print.
No, I'm not going to agree there.
Practical usage of our cameras throw more variables at us than you are allowing for here. Isolating just DOF is impractical.
If we assume for example normal angle of view and an essentially constant subject to camera distance then both focal length and format change, that means that both DOF and film characteristics will change.
You end up, for example, comparing "35mm film with a 50mm lens" to "4x5 film with a 150mm lens" or to "8x10 film with a 300mm lens".
Different lenses also have different characteristics at different apertures, have different coatings, have different corrections, had different quality standards and each of these effects the look.
All these "variables" are inextricably linked and each imparts its own signature on the final print.
You can't depend on your eyes when your imagination is out of focus. ~ Mark Twain
DOFMaster is based on an assumed 8x10 print. But even the iPhone version allows you to select whatever circle of confusion you prefer. Generally, I select on about a half to a quarter of what they say they use for a given format in their help screen. They use a 0.1mm as the maximum acceptable circle of confusion for 4x5, which only gets a 2x magnification to reach 8x10. I use 0.025, for reasons discussed below.
Leigh is right that the circle of confusion created by a lens of a given focal length at a given aperture is the same no matter what format film is placed behind it.
But he is wrong that the depth of field is the same. Depth of field is defined as the defocus blur being slight enough so that it appears to be acceptably focused on the print. That requires defining what appears sharp on the print, and then working back through the enlargement ratio. If I don't want the fuzzy spot rendered by an out-of-focus point to be bigger than, say, 0.2mm on the print, then using a circle of confusion standard of 0.1mm works fine for a 2x enlargement--as suggested by DOFMaster.
The problem in comparing formats is that we have to make assumptions that may or may not be realistic. For example, we may assume the same print size, which means the circle of confusion we use will need to be 0.2mm divided by the enlargement factor (if 0.2mm on the print is what we want). With that assumption, and using an 8x10 print as the anticipated size, 4x5 gets a circle of confusion standard of 0.1, because .2/2x = .1. But 35mm would get 0.025, because .2/8x = 0.025.
If we assume that the enlargement ratio is the same, then we can use the same circle of confusion for both formats. But that circle of confusion will only look the same on the print if they are the same degree of enlargement. So, with that assumption, we'd be comparing an 8x10 print from 4x5 to a 2x3 print from 35mm.
For smaller prints, we assume the print size will be the same. But as print size increases, we get to the point where other factors prevent further enlargement. I would not generally want to make 16x20 prints from 35mm negatives, for example. 17x is just too much enlargement to maintain my standards. And the 0.012mm circle of confusion runs into the effects of diffraction, so that diffraction will control what looks to be in focus, not depth of field. So, while 16x20 is my standard-size print from 4x5, I just won't go that big with small format, so they can't really be compared with either of these assumptions.
For me, though, the 0.2mm fuzzy spot on the print is too big. I'd rather limit it to 0.1mm. And I standardize on a 16x20 print for 4x5--the biggest print I can make at home. So, 0.1mm / 4x enlargement = 0.025mm. I end up stopping down a lot. And at 4x, diffraction just never becomes a practical limitation, so I'll stop down as necessary. With 4x5, being out of focus is likely to be a much bigger effect than diffraction in any case.
We should also remember that it's not a hard line. It's a standard we set, but the sizes of those out-of-focus details become smoothly larger as their lack of focus increases. It may be that 0.1 is what I want for the important subject material, but not quite as important for other parts of the print, even if I still want the print to look sharp overall.
Generally, my strategy is to stop down as far as I need to for depth of field, and then print as big as the negative will go. If I needed so much depth of field that I had to stop down too far, then I just won't print as large. That happens with small format, but not so much with large format, until the prints get much bigger than I can make.
Rick "lack of depth of field and defocus blur are not the same dimension" Denney
Rick, if you were to post the first reply to every thread, this forum would be a lot more concise.
There are some calculators on this page that might be helpful to the op, particularly the first and last. Perhaps it's similar to the converter you were hoping to build?
http://www.tawbaware.com/maxlyons/calc.htm
However, that last converter seems to deal with converting infinity values, not often the case when you add in extension with larger formats for closer subjects, so there might be some additional scribbling to be done. I'm impressed by it, though I don't have the inclination to test the math. Still, it's nice to plug in some slow old large format lenses, and find that the 35mm equivalent would require a lens of f/0.6 to achieve the same result- in a kindof a peculiarly perverse sort of way...
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