Looking at my trusty little "the f stops here" lens chart, I see a 210mm on a 5x7 is equivalent to a 150mm on a 4x5. But picking nits, the two formats have slightly different aspect ratios and I wonder how that works. Specifically, assuming the above mentioned lens combinations, will the 5x7 show as much as the 4X5 PLUS a tiny bit on the sides, or nearly exactly the same field of view as the 4x5 with a bit on the top/bottom chopped off? Thanks for any help in this. Believe it or not, I'm contemplating a specific shot for which this slight distinction will matter.

...and I wouldn't be bothering anyone else about it if I had both my cameras available to check it out for myself.

That's one of those things where you just have to see how it works, or to be able to move the camera so you can get the framing you want with the lens you have. If you are on a PC, look up a shareware program that performs many useful photographic calculations on the net called F/calc. It will give you the vertical, horizontal, and diagonal angle of view for any lens in any of the common formats up to 11x14", and that should tell you what you need.

Thanks, David. I found f/calc and will download/explore it shortly. I'm sure you're right that only real testing will give the full picture ...so to speak.

poco, just measure the diagnal of the film size and you will see that the 5x7 is about and I repeat about 210 mm. Multiply that by 25.4 and that will give you the inches and the normal lens length I am doing this from memory and don't have a ruler or a film holder handy and if I am wrong someone will surely point that out. lee\c

Given the same lens placed on a 4x5. 5x7 and 8x10 you will get the exact same picture, the only difference is that you "see" more if the image circle in the larger formats. Since the smaller format "cuts" a lot of the image circle if you want the larger format to show the exact same crop then you would need a linger focal length, this is the reason the focal length changes as the camera gets bigger. When I worked in a camera shop we had a rule of thumb to come up with the equivalent focal lengths. We would multiply by .3 or multiples to get the approximate focal length, for example: 210 x 0.3= 63 if I wanted to go from 4x5 to 35mm 210 x 0.6= 126 for medium format etc. It is not very accurate but it gets you in the ball park.