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# Thread: Calculating Optimal F-stop for 8x10

1. ## Calculating Optimal F-stop for 8x10

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. gb,
i am not sure this is exactly what you are looking for but it has a great table that you can use to calculate all kinds of DOF stuff.

http://www.cambridgeincolour.com/tut...h-of-field.htm

eddie

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

5. 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. 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. 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. 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"

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 2¼ 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.

Ian

9. 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. Originally Posted by George Losse
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.
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.

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