# Digital back on an Arca-Swiss 4x5?

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• 14-Jun-2010, 03:35
Emmanuel BIGLER
Re: Digital back on an Arca-Swiss 4x5?
How did you calculate the distance of the focus travel for the lens?
Ooops ! yes, Gordon, I simply use Newton's formulae ; the displacement of the image of an objet locaced at a distance D, image moving with respect to the focal point (where one finds the image of infinity), is equat to (f*f)/D or f^2/D where f is the focal length and D the finite distance from the lens to the object.
If we take one metre / about 3 feet as the reference distance D, the formula is simply
displacement in mm = (focal lenth in mm)^2 / 1000
e.g. for a focal length of 100 mm you find 1 cm or travel from "infinity" setting to "one metre" ; this is very comfortable with a classical knob, 2 cm per one full turn.
With a 55 you get (55x55)/1000 = 3 mm this is without question accessible to the same focusing knob, 3 mm is 1/6 of a turn 54 degrees of rotation.
With a 35 mm you get (35x35)/1000 = 1.2mm this starts to be difficult @20mm per knob turn,
and with a 28, you get about .8mm, this is really problematic with a conventional rack and pinion@20mm per knob turn, but easy to achieve with an helical , at least if you know where the focus is located.
• 20-Jun-2010, 14:26
Gordon Moat
Re: Digital back on an Arca-Swiss 4x5?
Merci Emmanuel! Great explanation of how you got to those figures. This is one answer I will be bookmarking. I have a degree in Fine Art, not mathematics, so I never knew of that formula. I'm more of an artist with a camera, than a technical photographer.

What I hope to do with this is create a template I can print onto adhesive backed paper, and then place onto my 4x5. I am hoping that I could fit focus distance marks for a 90mm, 135mm, and 180mm all onto one tape, or at most two different tapes.

Ciao!

Gordon Moat Photography
• 21-Jun-2010, 00:07
Emmanuel BIGLER
Re: Digital back on an Arca-Swiss 4x5?
Regarding Newton's formulae as mentioned above : the amount of travel at rear, required to focus to a certain object distance, depends only on the effective focal length, and is totally independant from the lens design. Those formulae are really simple and universal !
In other words, a 360 mm Apo Ronar or a 360 mm Telephoto need exactly the same amount of travel with respect to the focal point, although their flange-to-focal distance is completely different : about 360mm for the apo ronar and 210 mm for the Tele Arton telephoto lens. The extra amount of travel required to focus from infinity to the same object distance is the same for both lenses.
• 21-Jun-2010, 23:54
Gordon Moat
Re: Digital back on an Arca-Swiss 4x5?
Awesome! You are one of the great resources in photography. Merci!
• 22-Jun-2010, 08:18
RK_LFteacher
Re: Digital back on an Arca-Swiss 4x5?
All of the above are correct. However, there can be a problem for standard bellows cameras when coupled with wide angle lenses and digital backs. You would need to know exactly when you are at infinity. And lenses do vary slightly. And also where the nodal point lies to measure for infinity. With this information you can do an accurate focus adjustment.
The Arca RM3d/RL3d with wide angle lenses are even more precise verses even their great rail cameras. Partially because it is very hard to accurately focus by eye with lenses such as 35mm, 47mm, and the shorter 23/24/28mm objectives. The focus is set at the factory by having an integral helicle in the body on the R cameras.
The Arca rails from about 55mm and longer are very accurate for digital and super accurate for film. Remember that film is thousandths thick and digital is only 1 micron! This means that you must be a minimum of 4 times more accurate at focus to be sharp. Schneider has a great article on their site and copied to my site as well, on this phenomenon called 'Why choose Digitar'. It has great info in general on the difference between focusing for film verses digital. So, in my opinion if you are looking at short lenses for digital and want the most accurate focus the RM3d/RL3d is the way to go. For standard lenses, you can go with traditional cameras and obviously for long focal lengths they are way better. The longer focal lengths have narrower depth of focus and therefore are easier to focus as they pop in and out of focus while you are viewing making it far easier to get sharp focus optically.
Hope this adds information.
Rod
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