Have I got this right?

[ericmark]

Thanks for confirming Exam Monday and Tuesday 22 and 23 of March. Out of interest I tried fitting my D-SLR lens on old 35mm camera. And as I zoom out you can see the round edge of lens. Fitted OK but no film so did not try taking a photo. What I do wonder is with third party lenses years ago they came with 42mm screw thread onto which one fitted the bayonet mount for your camera.
However the size of the CCD seems to vary make to make so since my Pentax is one of the larger CCD's at 23.5mm I wonder if any of the third party lens would crop some picture? Also width to hight old was 36mm x 24mm and new is 23.5mm x 15.7 which is nearly the same relationship but not sure if all are like this? Of course when working out lenses one has to go diagonal like with TV so film was 43.26mm and new CCD is 28.26mm with film the 24mm was limit as width of film but with D-SLR there is no such constraint.
Best for lens is 1:1 but surb is 1.618 (Golden ratio) so 24mm x 14.8mm would have same diagonal but follow golden ratio. Seems the ancient Greeks knew all about surb but monitor and paper manufactures haven't caught up!
Oh yes other subject I study is maths. Bet you never guessed?

[edbayani11]

the focal length remains the same. if it is 300mm it is 300mm when used in any camera. it is the camera format that changes.
like the 90mm is considered as wide angle in 4x5 view cameras, it remains a 90mm lens. what changes is the covering power of the lens like the 90mm for a 4x5 can cover more than the diagonal of the film giving room for tilts and swings
while the 90 of a 35mm camera will cover only the size of the film or ccd. that is why view camera lenses are more expensive.
focal length produces the image size.

[ericmark]

You are right and once you pointed it out it clicked. Thanks for all your help.