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Kevin J. Kolosky
23-Jul-2013, 22:33
I own and use both a Sinar P2 and a Zone VI camera. One of the features that I really like on the P2 (actually I like them all) is the focus adjustment feature that is used after setting swings and tilts. For those that don't know how it works, you focus on the farthest object you want in focus. You then set the rear fine focus knob to zero and then use that fine focus knob to focus on the closest object you want in focus. You then set the indicated F stop on the lens, and then move the focus knob 2 stops back for optimum focus.

So I have looked in Stroebel and other places to see if I could find the formula for this. OBVIOUSLY it has to do with the amount of distance between the near and far points one wants in focus coupled with the amount of focus (lens travel) between those two points on the camera. Or to put it more mathematically, the amount of rotation of the focus knobs on the camera is a function of the amount of distance between near and far focus points in the scene. Maybe the answer has something to do with a derivative or an integral or something like that.

I was thinking of taping some scales to the Zone VI to measure the distance in lens travel, and then enter that number in to the formula to calculate know how much to move the camera back in f stops.

Anybody know where I can find this formula?

jb7
24-Jul-2013, 03:35
There's a really good explanation, with tables, by QT Luong, which is linked to on the home page of this site-

http://www.largeformatphotography.info/fstop.html

Jim Jones
24-Jul-2013, 06:53
Lenses for older 35mm cameras often had a DOF scale on the focusing mount. A similar scale can be calculated and attached to a view camera. The calculations involve knowing the degree of image enlargement and the viewer's preference in sharpness. Once the scale is incorporated into the camera, no further math is needed. Harold M. Merklinger has written extensively on focusing the view camera. Google him for more information. Some compensation may be required, depending on subject matter and the degree of enlargement. DOF is more a matter of art than of science and math. As Q.-Tuan Luong says, "In summary, the acceptable circle of confusion on the negative depends on a number of factors, and it is better to determine the one which is best for you, rather than rely on the manufacturer's which is often not critical enough."

ic-racer
24-Jul-2013, 08:19
Yes the aperture to use is related to focus spread as previously described but are you asking about knob rotation and travel on your specific camera? To do that rotate the knob once and measure the travel. Divide that by 12 and you heve the travel for each 'hour' of knob rotation etc. for example on one of my cameras I know I need f16 for 3 hours of knob rotation.

Mark Sampson
24-Jul-2013, 08:32
Having owned and used both Sinars and a Zone VI for many years, I wonder if you could actually design/build such a setup that would actually be workable on the Z-VI. But if you want to try, why not find the relevant Sinar parts and adapt them?

Bob Salomon
24-Jul-2013, 08:37
Look at the Rodenstock depth of field/Scheimpflug pocket calculator. It will calculate what you want on any camera from 35mm to 8x10 and tell if an exposure factor is required and how much of one as well! The ruler is built-in to both sides of the calculator so mm scales do not have to be added to your camera. But an easy way to determine tilt angles would be a help.

jb7
24-Jul-2013, 09:00
My new camera focuses from the rear using a handle rather than a knob. One full rotation equals 2mm travel. I can work out the required aperture using QT's table before I emerge from the darkcloth...
of course, I did need to spend a long time memorizing the numbers...

Bob Salomon
24-Jul-2013, 09:04
2mm?

jb7
24-Jul-2013, 09:06
Inline Trapezoidal spindle...

Kirk Gittings
24-Jul-2013, 09:49
Hmmmmm, I suppose it would be handy but I never found such devices necessary............

Kevin J. Kolosky
25-Jul-2013, 10:48
Hmmmmm, I suppose it would be handy but I never found such devices necessary............

I agree, not necessary. But when time starts to take its toll on one's eyes, anything that helps out is nice to have.

I like that idea of dividing the focus knob into 12ths and trying to figure out how that would work on the zone VI. I think I will get my calipers out and do some measuring.

Doremus Scudder
25-Jul-2013, 16:49
You don't need a formula or fancy tables, etc.

Just mount a mm ruler or adhesive strip one of the non-moving parts on the bed of your camera. Make some reference points on the moving rails. Use this to determine focus spread between near and far focus in mm. Set your focus at the point halfway between the extremes and use the method linked to on the large format web site to determine your optimum f-stop.

I have Wista, Zone VI, Woodman, Sinar and Graphic View cameras. I have outfitted all of them that do not already have mm scales with them. I just print out one of the easily-found-on-the-Internet scales onto self-adhesive labels and stick them to the camera bed/rail.

I have also made a sticker for each camera with my optimum f-stops, derived using the method from the web-page referred to above.

Here's my sticker: D= distance between near and far focus points in mm, F = f-stop in decimals (i.e., 16.6 = f/16 2/3). The formatting gets lost on this forum, but I think you can figure it out, it's only two columns.

D(mm) - F*
1 - 16.6
2 - 22.6
3 - 32.2
4 - 32.6
5 - 32.9
6 - 45.2
7 - 45.4
8 - 45.6
9 - 45.8
10 - 45.9
11 - 64

Hope this helps,

Doremus

Kevin J. Kolosky
26-Jul-2013, 14:38
I know I don't "need" fancy tables, etc.. As I stated in my original post, what I "want" is the formula Sinar used to design their focusing system.

Nicolasllasera
27-Jul-2013, 00:28
I would say you need to know the amount of distance the back standard moves per full turn of the DOP wheel and adapt that with Sinar fstop to your Zone wheel. It cant be that hard. I have somewhere the stickers for DOP from Sinar. I could take a picture for reference.

ic-racer
27-Jul-2013, 05:08
I know I don't "need" fancy tables, etc.. As I stated in my original post, what I "want" is the formula Sinar used to design their focusing system.
Do some measuring and you should be able to decode the table and see if it follows the formula of Hansma.

Brian Ellis
27-Jul-2013, 06:17
This sounds like a higher-tech or easier-to-use version of the method many of us have been using with many different cameras for years, i.e. focus on the near, note where the front standard is (or any other part of the camera that moves as you focus and that can be measured will work as well), focus on the far, note where the front standard is (or vice versa), then set the front standard (or whatever else you're using) half way between the two points, then consult a depth of field table to determine the optimum aperture based on the distance between the two points, etc. etc., all as explained more fully in the excellent article by QT Luong mentioned above.

All depth of field and similar tables including hyperfocal distance tables and the DOF scales on some 35mm lenses make certain assumptions, e.g. that the print you're making will be a certain size and that the size of the circles of confusion at that assumed print size will result in a print that's acceptably sharp for you. These assumptions aren't necessarily true for everyone but if nothing else they provide a good starting point for making your own determinations.

wombat2go
27-Jul-2013, 11:40
Yes, Reading Kevin's posts a couple of times and Brian's post, I think Kevin might be asking for the math functions to calculate the distance between the 2 focus points in terms of the actual length between the 2 positions of the lens so he can mark the knob. Then he might need the function used to determine the f/ to achieve a DOF between the 2 positions.
I have prepared spreadsheets for similar calcs for my home built scale focussed camera, and extra scales etc for the Speed graphic and for initial dimensioning of a neg copier using the Speed Graphic in reverse.

No substitite for good estimations by eye when taking the shot, but the math gives a starting point.

Here are the analytical functions
( I use what I think are fairly conventional symbols, and [mm])

Apply this function twice, once for near (v1) and once for far (v2) object distance
v = (u*f)/((u-f)

The difference v1-v2 will be the movement of the lens

Now determine DOF where C = Circle of Confusion and can be obtained for example by dividing frame height by lines of resolution needed, and is "personal" if the image is digitally dislayed. N is the f number eg 6.3 or 8 etc.

Near Sharp Limit = ((f*u) * (f+(C*N)) / ((f^2) +(u *C*N))

Far Sharp Limit = ((f*u) * (f-(C*N)) / ((f^2) -(u *C*N)) and is infinity if negative

The difference Far - Near is the DOF for a given aperture f number

Kevin, I have these functions in my spreadsheet which i can carve out as a block of cells. If you need that please send me a pm.

Kevin J. Kolosky
28-Jul-2013, 17:54
Ah! Something to try. Thank you very much.

ataim
29-Jul-2013, 08:11
Most of the time I use the chart that Doremus has shown. BUT I have used the app "depth of field guide" for the Iphone.