I own a speed graphic with a few lenses, one 100mm an others 13,5cm. Could I use those lenses in a 5x7 with a 4x5 back? What's the minimum mm lens I could use in the 5x7 with the 4x5 back?
Sure, you can do that. The minimum focal length just depends on the design of the 5x7 camera.
On a different topic: whether your 4x5 lenses will also cover 5x7 depends on how closely you focus. Many 4x5 lenses will not cover 5x7 at infinity focus, even at f/22 and beyond. But just about any lens will cover 5x7 if you focus closely enough... and if you have long enough bellows
Whether a lens will cover 5x7 depends on the lens design, not the camera design. Most 75mm lenses won't even cover 4x5, but the right one will cover 8x10. Press camera 100mm and 135mm lenses typically won't cover 5x7 at infinity, but they might at portrait or close-up distances where the coverage increases as the bellows extends.
you would not have trouble using those lenses with the 4x5 back on the camera,
it will operate just like a 4x5 camera, only a little bit bigger ...
that said, you may need a recessed lensboard for your 100mm or if you
get a 90mm, just as you would need a recessed lensboard on a regular 4x5 field camera ..
True but I think the question was what is shortest 'mm' (FL?) that can be used with a 4x5 reducing back on a 5x7. If I understood correctly.
Now, I have two 5x7 field cameras that would not be appropriate for 4x5 ultrawides and reducing backs simply because the baseplate would be in the field of view. A monorail 5x7 is not going to have this problem and you can probably go as wide as you like. Obviously you don't care about the image circle because you're still shooting to 4x5.... I was just pointing out, as an unsolicited free bonus that a lot of lenses will cover bigger formats at close focus.
The reducing backs also sometimes move the film plane back a bit but it's probably not going to be an issue. I've used 3 different kinds of reducing backs on 5x7s (cambo and rittreck) and saw no issues apart from the bed coming into view with lenses like a 55 or 65 on the rittreck field.
an Agfa Ansco 5x7 with a 4x5 back. I also own a few fidelity plastic film holders, will those go into the Afga jAnsco as well and old models like that? I am basically looking for a cheap option to a $500 4x5 nicer camera.
The 135mm should be okay. My Ansco is in storage plus I don't have a 4x5 back so I can't measure for you. I think it should be fine at infinity but you won't be able to get any/much movements with the camera that closed up.