This is kind of stupid but..I just "Got it"!

[Cletus]

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.

[E. von Hoegh]

The effective focal length never changes, ever. The perspective doesn't change, either, unless you move the camera. The part of the image you record on film is what changes.

[Cletus]

Well, my terminology may have been wrong then. Maybe I should have said the "Apparent" Focal Length. Anyway, the point is the same...

[E. von Hoegh]

Angle of view, which is not the same as image circle. Compare 300mm lenses on 35mm, 4x5, and 8x10.

Saying a 50mm "becomes" a 75mm will take you down the wrong road.

[Cletus]

I guess I'm still stuck on the whole "equivalent to 35mm" in the way that I understand the meaning, or better, the application, of focal length. Do you think my example is way off though?

I know that nothing actually changes with the 300mm lens in my example, but when I change camera backs from 8x10 to 4x5, with all else being the same, it's as though I've installed a lens with a longer focal length.

Angle of view, such as "a lens with a 105deg angle of view", I still don't really understand. Two lenses of different focal lengths can have the same angle of view can't they? Or am I thinking of something else?

[BrianShaw]

Cropping does the same thing.

[Cletus]

Sure it does, but that's not what I'm getting at. All I was trying to say was that I never really understood why the...I have to choose my words carefully now...why "evident", "apparent", "seems to be" focal length of any lens, on any format on any camera, seems become a "longer lens" as format gets smaller.

I have a pretty good feeling I'm just being messed with here, but that was my only observation. Like I said, most of you already know this, I just didn't really understand why that was.

[BrianShaw]

Yes, I assumed you knew that cropping gives the same effect but thought that saying it may help. How, IDK... but it had been unsaid.

Since you are talking mostly about how to express that effect in words... yuup, it is hard to express. I often say "xmm lens when used on 6x7 gives the same appearance as a yymm lens used on 4x5." That is probably wrong too but people seem to understand.

Yes, you will get "messed with" here... whether you like it or not, and whether you deserve it or not. Smile, nod, and then get even!

[Oren Grad]

You've basically got it right. Think of lenses as projecting an image circle of a given size. In that projected image you can place a sheet of film of any size you want, to capture as much or as little of the projected image as you want (or even beyond it, if you want or don't mind vignetting). That's what you're doing, of course, when you swap backs for different formats on your camera.

EDIT: As you gain more experience working with view cameras in different formats, it's easier to get away from the "equivalent focal length" thinking, which can sometimes lead to fallacies. You eventually develop your intuition for how large a piece of film you need to stick behind a lens of a given focal length to capture how wide a view of the scene in front of you.

[Oren Grad]

Angle of view, such as "a lens with a 105deg angle of view", I still don't really understand. Two lenses of different focal lengths can have the same angle of view can't they? Or am I thinking of something else?

The angle of view tells you how wide a cone of light the lens projects toward the film. The size of the sheet of film that you place within that cone determines how broad a view you actually capture of the scene before you.