BJL thanks and to continue a bit.
Example: if with the 135mm on the 6x7 we shot an image and in the middle of that image is a" for sale" sign and it measures 22x16mm in the film and you shoot the same image from the same spot with a dslr and zoomed in to cover just that "for sale" sign so it to would cover the 22x16 area of your sensor, what would the telephoto mm be on the dslr? (I sometimes flip relationships and ratios) would I need about 400mm on the dslr.
If I know that the diagonal of the apc sensor in about 1 inch and the diagonal of the 120 film is 3.5in and then if I multiply 3.5 x the 135mm that will or will not give me the mm of the apc lens needed(aprox 400mm)
This might be a bit convoluted, so thanks for the any help you might give. Wyndham
As BJL said, the only thing you need to think about is what ACTUALLY gets recorded
"The only format that counts in this comparison is the height and width of the part of the image that you use (16x22mm), not any part of the image formed by the lens that is either not recorded by the film or sensor, or is recorded but then gets cropped away."
AKA Don't think that the 16x22mm part of a 6x7 film frame behaves any differently than the 16x22mm part of an 8x10 view camera sheet or the 16x22mm part of an apc sensor.
For any given lens (such as your 135mm) and camera position, if the for sale sign in the distance fills the 16x22mm
part of the 6x7 film frame, it MUST also fill the 16x22mm
part of the 8x10 or the 16x22mm
apc ( for simplicity we're not quibbling about aspect ratio differences here). The lens will be the same distance from each of the three capture surfaces no matter how big they are . So, when you look at the ACTUAL focal length on the DSLR lens after framing the for sale sign, it has to be the same as on the 6x7 i.e. 135 mm - you do NOT need the 400.
In your example where you think about a fixed 16x22 mm image area
, you are confusing yourself by thinking about diagonal ratios. You only need those concepts when you think of the "For sale" sign FILLING the 6x7 or 8x10 sheet. Example : If your camera position is fixed and you want to capture apc, 6x7 and 8x10 images of only the sign, THAT's when you need different focal length lenses for the different formats.