"Which size lens would be good for portraiture and for telephoto (landscape)? What is the difference with the lense physically? I currently have with my Chamonix 4x5 45H-1 camera... The longest lens is 350mm at infinity"
You might find it helpful to read this brief article which gives some helpful formulas: with a given prime lens (not a telephoto design) and available bellows draw, how close can we focus, how much magnification can we get, etc ?
For example...
If we consider a portrait as head and shoulders, that's a box around 20x25 inches, roughly 1/5 or 20% magnification onto 4x5 film.
Given a 210mm lens where the desired magnification is 1:5, how much bellows extension is required ?
M = (B-F) / F
where M = magnification ratio, B = bellows extension, F = focal length
M = 1/5, F = 210mm
1/5 = (B-210) / 210
1/5 * 210 = B - 210
42 = B - 210
Bellows Extension = 252mm
So a 210mm lens will work for a head-and-shoulders portrait on a 4x5 camera, as long as we have at least 252mm of extension. Your camera provides that, with room to spare. You can play with the formulas to find out what is the longest lens you can use with 350mm of bellows draw, to get the magnification you want for portraits.
If my calculations are correct, with a 300mm lens and 350mm of bellows extension, we can get (350-300)/350 or a 1:7 magnification, a rectangle around 28x35 inches. Maybe not an intimate close-up, but a fine portrait can made as such.
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