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this was also posted in lf forum...this is based on the laws of perspective and projection..
I've done so many of these calculations one at a time, I finally made a table of equivalents...and, when all the information is all tablulated together, it is extremely useful to see what is going on when you compare the "equivalent lenses" for different formats. Note that the equivalent lens CHANGES based on the magnification required--by the visual field that is covering the image area. This is what always seems to confuse people because it doesn't change in any seemingly predictable way and it is responsible for the non-intuitive concept that short lenses are "ok" for close up portraits for larger formats. The attached table was made for an 18" lens (457mm) for the 11x14 format. for the format characteristic lengths, I didn't use the diagonal, but rather the square root of the area...so for square hasselblad, the characteristic length is 2.25 inches = sqrt(2.25*2.25)....for 8x10 it is sqrt(8*10)=8.9 inches....don't worry...it's the RATIOS that matter with format characteristic lengths..and that is what is used in these calculations.
You'll see that for the 11x14 format column, the equivalent length is all 18" for every magnification....this is, of course true, since the table is for 18" on 11x14....
the very interesting thing is the smaller formats--you'll see that the 18" lens at magnification of zero (infinity focus) is equivalent to a 43mm lens on 35mm or an 83mm lens on hasselblad 2.25...this is like "normal" lens coverage for infinity focus....
NOW....you'll see that as you move down the table to magnification of 1.0 for the 11x14 format (very tight head shot on 11x14), the equivalent lenses are 79mm for 35mm and 140mm for hasselblad---THESE ARE PORTRAIT LENGHTS FOR THOSE FORMATS.....
so you see that the 18" on 11x14 BEHAVES like you having a zoom from 43mm to 79mm for 35mm or a zoom from 83mm to 140mm for hasselblad format.
So THIS is a better explanation of what is going on when you are talking equivalent formats.....I saw this all tabulated and a light went off....AHHHHH!!!!!! THAT's it!!!!
so you see, all you need for 11x14 is your 18" lens...shoot landscapes with it at "normal" coverage and get close up and automatically get portrait lens angle of view!!!! This should help IMMENSELY when selecting a lens and camera for your new format of choice.....See attached printout for various magnifications on 11x14 format with the 18" lens and all of the equivalents from 35mm to 11x14....
I've done so many of these calculations one at a time, I finally made a table of equivalents...and, when all the information is all tablulated together, it is extremely useful to see what is going on when you compare the "equivalent lenses" for different formats. Note that the equivalent lens CHANGES based on the magnification required--by the visual field that is covering the image area. This is what always seems to confuse people because it doesn't change in any seemingly predictable way and it is responsible for the non-intuitive concept that short lenses are "ok" for close up portraits for larger formats. The attached table was made for an 18" lens (457mm) for the 11x14 format. for the format characteristic lengths, I didn't use the diagonal, but rather the square root of the area...so for square hasselblad, the characteristic length is 2.25 inches = sqrt(2.25*2.25)....for 8x10 it is sqrt(8*10)=8.9 inches....don't worry...it's the RATIOS that matter with format characteristic lengths..and that is what is used in these calculations.
You'll see that for the 11x14 format column, the equivalent length is all 18" for every magnification....this is, of course true, since the table is for 18" on 11x14....
the very interesting thing is the smaller formats--you'll see that the 18" lens at magnification of zero (infinity focus) is equivalent to a 43mm lens on 35mm or an 83mm lens on hasselblad 2.25...this is like "normal" lens coverage for infinity focus....
NOW....you'll see that as you move down the table to magnification of 1.0 for the 11x14 format (very tight head shot on 11x14), the equivalent lenses are 79mm for 35mm and 140mm for hasselblad---THESE ARE PORTRAIT LENGHTS FOR THOSE FORMATS.....
so you see that the 18" on 11x14 BEHAVES like you having a zoom from 43mm to 79mm for 35mm or a zoom from 83mm to 140mm for hasselblad format.
So THIS is a better explanation of what is going on when you are talking equivalent formats.....I saw this all tabulated and a light went off....AHHHHH!!!!!! THAT's it!!!!
so you see, all you need for 11x14 is your 18" lens...shoot landscapes with it at "normal" coverage and get close up and automatically get portrait lens angle of view!!!! This should help IMMENSELY when selecting a lens and camera for your new format of choice.....See attached printout for various magnifications on 11x14 format with the 18" lens and all of the equivalents from 35mm to 11x14....