Question regarding focal lengths of lenses

[Tobychrome]

I have a 4x5 speed graphic that I am going to use with a roll film back. Currently fitted is a Graphex Optar 135mm lens, this of course, when used with a roll film back covers a smaller area than 4x5 film and has a sort of telephoto effect. I was thinking that I would get a smaller focal length lens perhaps a 90mm, but that got me thinking, when I use 4x5 film, I can extend the lens out from the infinity stops and increase the focal length. So anyway, if I got a 90mm lens, could I extend it to the infinity stops of the 135mm lens and use it as such? I'm sorry if this is a silly question but I am curious to find out.
Regards
Tobias

 

[markbarendt]

No.

A 90 mm lens focuses at infinity when it's 90 mm from the film.

As the lens moves farther from the film the subject Focus Point moves closer to the lens.

So when your 90 mm lens is 180 millimeters from the film the subject that's in Focus will also be 180 mm from the lens.

 

[markbarendt]

What will work as you describe though is a pinhole.

 

[Tobychrome]

Thank you for that, Now I just need to find a 90mm lens for a reasonable price and I'll be happily off to make some photographs.

 

[Prof_Pixel]

If you extend a 90mm lens to the 135mm infinity stops, you won't be able to focus the 90mm lens to infinity.

 

[shutterfinger]

Moving a lens farther from the film plane than its infinity point is close focusing. The close focus point for a 135mm lens set at infinity will be around 6 feet before a bellows extension factor needs to be applied. A 90mm lens set at infinity will close focus at 3.5 feet before a bellows extension factor will need to be applied.

Another characteristic of lens is that the 135 when focused at 6 feet may be 150mm to 160mm from the film plane but to focus at 5 feet 11 inches may require the lens to be moved an additional 50mm to 60mm from the lens plane. Moving the lens far enough from the film plane becomes macro photography.

The advantage of using a smaller format with the same lens that covers 4x5 is that you have more movement latitude available.

 

[RalphLambrecht]

I have a 4x5 speed graphic that I am going to use with a roll film back. Currently fitted is a Graphex Optar 135mm lens, this of course, when used with a roll film back covers a smaller area than 4x5 film and has a sort of telephoto effect. I was thinking that I would get a smaller focal length lens perhaps a 90mm, but that got me thinking, when I use 4x5 film, I can extend the lens out from the infinity stops and increase the focal length. So anyway, if I got a 90mm lens, could I extend it to the infinity stops of the 135mm lens and use it as such? I'm sorry if this is a silly question but I am curious to find out.
Regards
Tobias

remember that moving a lens back or forward will change focus but not the focal length of the lens

 

[darkroommike]

If you have a Pacemaker model you can actually get a second set of infinity stops, the stops made for the Pacemaker flip down when your now using them, you could have two sets on the rails of your camera. A good cheap 90mm might be a Wollensack or Schneider Angulon f/6.8. If you plan on using the focal plane shutter it wouldn't even need a good leaf shutter. Either would just cover 4x5.