Ever have something that you just accepted and took for granted suddenly become crystal clear to you? I know that most of you will groan and flip to the next page on this - I should have too, I've been a photographer for over 20 years, but large format only for the last three or four.
I'm getting ready to get my first 8x10 soon and I was thinking about a 4x5, or other smaller format reducing back to go with the camera. I got to wondering, what if I have a 300mm lens mounted and set up for an 8x10 photo, then change the back to 4x5. What else will need to change if I don't want to change the lens? And also, how will all my 4x5 lenses, which mostly won't cover 8x10, work with the reducing back? Exactly the same?
Suddenly, the "Big Light" came on about the relationship between focal length and format, or image area. I never intuitively understood why "effective" focal length always increases as the format gets smaller (or vice-versa) as in, "why does my Nikkor 50mm become an 75mm on an APS size camera?" I just accepted that it did and carried on.
So it just occurred to me why this is:
Let's say, just for fun, I have a house that happens to have a window smack in the center, and I have an 8x10 all set up to have the whole house completely fill the frame. The FL of the lens is 300mm. Then, I leave everything else the same and change to a 4x5 reducing back. Now, with the smaller format and the same FL I've lost most of the house and "zoomed in" on the window in the center, just as though I had increased the FL of the lens!
The actual projected image, or image circle, of the 300mm FL is still all there and never changed, but now only the center part of the image area is being "used" by the little 4x5 frame and the effective FL of the lens has now became longer! And it's become longer by exactly the same as the difference, or the ratio between the two format sizes! Ha ha!
I know this is relatively simple if not obvious, but I never really understood it. Maybe, by chance, there is someone else reading this who gets it now, too.