Large Format Lens Inquire

[filmbase]

With large format lenses, is there a way to find out what the

minimum distance from the lens is when the lens is focused at

infinity? I�m looking for a mathematical formula not a tape

measure. :-)

[robert_davis2]

"the minimum distance from the lens is" Define the. If you mean the lens from the film then a normal lens will be basically the an amount equal it's focal length. Telephoto lens are different.

[filmbase]

Good point, Robert. More info probably would be better.

 

I'm looking to find out how many feet away my subject has to be,

to be in focus when my lens is at infinity. Does that make

sense?

 

I have a Schneider APO-Symmar 210 f5.6, Nikon SW 120 f8 and

a Nikon M 300 f9.

 

Thanks for the help, Robert.

 

-- Jeff

[eugene_singer]

Jeff, are you asking about the distance between back edge of the mount for the rear element and the film? I don't know if there is a formula for that measurement. It depends on the individual lens design. The nodal point of lens (approximately where iris diaphram is located) to film distance is usually equal to the focal length of the lens, except on tele designs. Perhaps you should post your question on the Large Format forum. You can probably get the answer you are looking for. That's where the LF lens formula experts hang out.

[eugene_singer]

Jeff, now that you have elaborated, I understand which distance you were inquiring about. Post your clarified question on the LF forum and be prepared to learn about hyperfocal distance and circles of confusion.

[stephen hazelton]

"I'm looking to find out how many feet away my subject has to be, to be in focus when my lens is at infinity."

 

That's a depth-of-field question, look up depth-of-field here or elsewhere. Of course, if you want the best focus, you don't focus at infinity, but somewhere between the subject and infinity.

[filmbase]

Thanks everyone for the help. Im new here and didnt realize I

was posting in the wrong catagory. Sorry about that.

 

-- Jeff

[robert_davis2]

This is the formula I have:

 

1/focal length = 1/distance + 1/bellows extension

 

Which of cource doesn't work for infinity-))

 

But you can get close by adding just a little to the bellows extension number.

 

I think the rule of thumb is 200x the focal length but working with the formula I get close to 300X.