For example my lens with 240mm coverage, when used on a 8x10 will it show a black vignetting circle or is it a soft degradation of the image toward the corners of the film?
A lens projects a circle of image. But only the central part is photographically useable wide open.
what is the effective f-stop of the lens below when used at infinity?
Is it as simple as (170mm / 76mm) = 2.2?
Coverage is 240mm. Nothing to calculate.
What does that mean?
I think the op wants to know at what focus distance it will cover 8x10.
It means simply that the poster didn't fully understand the question. Indeed, at infinity focus, the coverage of the lens in question is a circle with a diameter of 240mm, as per the manufacturer's specifications.What does that mean?
Well, OP asked for the coverage of the lens. He also stated the image circle is 240mm. That leaves very little to calculate.
However, I interpreted the question like this:
What does an image circle of 240mm mean and when? That's the equivalent of 9.45 inches.It means simply that the poster didn't fully understand the question. Indeed, at infinity focus, the coverage of the lens in question is a circle with a diameter of 240mm, as per the manufacturer's specifications.
What needs calculation is how that coverage grows when the lens is extended further from the film and focus moves from infinity to closer objects. At some distance, since the lens projects a cone of light, the resulting circle will cover 8x10. The question is, at what distance. That has been more than adequately answered by reddesert and others.
Don't let this confuse you.
Doremus
What does an image circle of 240mm mean and when?
Also, what is meant on the charts when it says image circle is let's say 212mm @ f22?
Here's a question that's in the same ballpark: what is the effective f-stop of the lens below when used at infinity?
Pentax 67 SMC 300mm front group used as a barrel lens in a speed graphic board. The complete lens had an aperture stop behind what is now the rear element and an additional negative element behind that.
View attachment 365463
Is it as simple as (170mm / 76mm) = 2.2? Or does the odd rear node and protrusion of the lens into the camera necessitate some additional considerations? Being a telephoto design, my naïve intuition is that any additional light afforded to the film plane by the protruding rear section is being 'eaten' by the optics thus rendering the simple calculation valid.
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