View Full Version : rails vs lens length

ryanmills

22-Feb-2013, 13:17

Is rail length required to focus a lens the same for any lens. If so is there a math formula to find out how long the rails need to be to focus at X distance with Y sized lens? In my case I have a 210mm Symmar S on a 4x5 Sinar F and I would like to focus at about 2-3 feet. I could just get an 18" rail ext but I dont really want to have to deal with that poking out. But im not sure a 6" or 8" is enough.

Bob Salomon

22-Feb-2013, 13:31

Not necessarily. It depends on if the lens design. A telephoto design requires less rail and bellows then a non-telephoto design. Telephoto in this context is the design of the lens, not just a long lens.

Also, a 210mm Symmar was never designed to perform optimally at 2 to 3 feet. That is where a macro lens would perform best. But at infinity down to about 1:20 the Symmar would perform better then a macro lens.

ryanmills

22-Feb-2013, 13:36

yea the 2-3 feet is a worst case, just wish I had a way to know how much good a 6" will do or will add.

E. von Hoegh

22-Feb-2013, 13:47

m = (v/f)-1 = f/(u-f)

where 'v' is the lens or bellows extension, 'u' is the lens to subject distance, and 'f' is the focal length.

ryanmills

22-Feb-2013, 13:59

m = (v/f)-1 = f/(u-f)

where 'v' is the lens or bellows extension, 'u' is the lens to subject distance, and 'f' is the focal length.

what unit of measure would "u" be in?

Bob Salomon

22-Feb-2013, 14:05

m = (v/f)-1 = f/(u-f)

where 'v' is the lens or bellows extension, 'u' is the lens to subject distance, and 'f' is the focal length.

Depending on lens design.

For instance:

FFL for Schneider 250mm Apo Tele Arton = 206.5mm. For 240mm Apo Symmar = 238.5mm

FFL for Rodenstock 23mm HR Digaron-S = 44.8mm. For Schneider 24mm Apo Digitar = 26.4mm

E. von Hoegh

22-Feb-2013, 14:15

what unit of measure would "u" be in?

It doesn't matter as long as you use the same units consistently. I'd suggest metric units.

E. von Hoegh

22-Feb-2013, 14:16

Depending on lens design.

For instance:

FFL for Schneider 250mm Apo Tele Arton = 206.5mm. For 240mm Apo Symmar = 238.5mm

FFL for Rodenstock 23mm HR Digaron-S = 44.8mm. For Schneider 24mm Apo Digitar = 26.4mm

Yes, Bob. The OP has indicated he's using a 210 Symmar.

Bob Salomon

22-Feb-2013, 14:25

Yes, Bob. The OP has indicated he's using a 210 Symmar.

But the formula works for some lenses and not for others. It is not an all purpose one. And the OP might think otherwise.

Brian C. Miller

22-Feb-2013, 14:52

So Ryan needs about 13 inches of bellows for focusing at 2ft with a 210mm lens?

Leonard Robertson

22-Feb-2013, 16:38

Ryan - I set up my 4X5 Sinar Norma with a 210mm G-Claron in Copal #1 shutter. Not exactly what you have, but maybe close enough to give you an idea. Neither the Claron or Symmar are telephoto formulas, so I suspect being the same FL they will give similar image size for the same extension. I used a Sinar base rail and a 6" extension with end caps on both ends. Total rail length is almost exactly 18". With the Norma I could get the distance from the ground glass to the center of the shutter to approx. 16 1/4", giving a center of shutter to subject distance of approx. 16 1/4", for a 1:1 ratio of subject size to image size on screen. The standard bellows is pretty well extended at that point. I'm not sure I could get very much more extension without adding a second bellows. Maybe I should mention neither the base rail or bellows are Norma originals. Both are later model Sinar parts, so should be much like your F parts.

Moving the camera back so the subject to center of shutter distance is approx. 24" gives a ground glass to center of shutter distance of approx. 12 1/2". The 5" dimension of the ground glass shows about 9 1/4" of the subject. These measurements may not work out exactly with the formula since I'm measuring with a cheap Stanley tape measure and sort of guessing where the center of the shutter is.

So it seems to me a 6" extension rail should be plenty to get you to 2' from your subject with a 210mm lens.

Congratulations on getting a Sinar too.

Len

ryanmills

26-Feb-2013, 02:04

Thanks, i got bored and just took the front standard off and held it in front to guess. I did not realized how fast I would run out of bellows anyway so a 6" was the logical choice but I think everything matched up to what you found. I ordered it last week so it should get here tomorrow. I will post up what I find.

I think there is an error in this equation. The second = sign should be something else, or it does not make sense.

m = (v/f)-1 = f/(u-f)

ryanmills

28-Feb-2013, 01:05

For anyone thats curious I did get that 6" rail today. For a stock Sinar F thats the max I can go with the bellows and even then I cant max the rail. I was able to focus at less than 2 feet with it so it works perfectly for me, will have to look into a 300 thou, still closer than I would like for headshots.

Leonard Robertson

28-Feb-2013, 17:59

Ryan - Thanks for posting the follow-up. Your information will probably be very useful to another Sinar F owner. I figured that due to the difference in design of the standards between the Norma and the F, the measurements I posted earlier would only be roughly the same for an F. I've read there is a Horseman bellows that interchanges to Sinar and is longer than the normal Sinar bellows, but I'm not sure of the actual length. I have an intermediate standard and a wide-angle bellows to combine with the normal bellows for more extension (I need to set that up and measure the actual length extension). It is more pieces to haul around though. I'm going to try putting a 12" FL lens on the Norma with 18" rail and see what image size I get. Also, it seems you got the rail from Atlanta pretty fast. That is great.

ac12 - I'm not quite sure, but I think the equation m = (v/f)-1 = f/(u-f) is combining two equations, m = (v/f)-1 OR m=f/(u-f). However, I was traumatized by my first algebra teacher about 50 years ago and I'm still not comfortable with formulas, even though I can plug numbers in and get an answer (sometimes a correct one).

Len

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