Calculating Lens Equivalence and Angle of View.
It seems that one of the first questions a new large format user asks is "What lens will be the same as my 28mm on my 35mm camera?" Or any other focal length equivalent. Here's my views and a handy spreadsheet...
The answers to this question range from wrong, to arrogant insults for asking such a question, to the old standard coverage equivalents using the diagonal of the different film sizes. I went along with the diagonal thought line for quite a while. Then I thought: "Who shoots film diagonally?" My equivalence question was simple. "What lens on my 4x5 would give me a 90 degree coverage or better?"
All the answers I got were wrong, some at least close... Because I was concerned with the horizontal angle of view on the 5 inch film. This can be directly related to the 36mm horizontal on my 35mm camera where I shoot a 17-28 zoom. I don't care what the diagonal is! I care if this or that building will fit into the frame when I stand so far away. Or the interior of a room. The only thing that matters is the length of the film and the angle of view of the lens!
I did a lot of searching until I finally found a very simple formula to calculate the angle of view. Unfortunately, I have forgotten where? I am sorry I can't offer credit, though it did come from some university...
I then fitted this formula into a spreadsheet so I could enter in a lens focal length and get back it's AOV for the different formats I use. 35mm, 2 1/4 x 2 1/4, 4x5 and 8x10. Then I added a list of AOVs from an eighth of an inch (3.175mm) through sixty inches (1524mm) for all those formats so just about any size could be quickly looked up. This all prints out on ten pages of text with the chart listing. A convenient thing to have around.
So, next time you want to know what lens will give you 90 degrees of view on an 8x10 camera, look it up on this chart! It's a 127mm lens by the way, which also offers a 53 degree view on a 4x5. And that 17mm lens on a 35mm camera would be the same horizontal coverage as a 120mm in 8x10 or a 60mm lens on a 4x5...
To use this spreadsheet, just click on the version below (Lotus or Excel) to download it. Then load it into your spreadsheet. You can enter either the lens focal length in inches or mm at the top, enter in a desired angle of view, or check the chart. Or even print the thing out for the ten pages of reference. If you're interested in the AOV for vertical shots, you're out of luck ;-)
Okay, there seems to be a minor problem here for the downloading. If you simply click on the file below, it will open in a new browser window. That's okay of you let it load completely and then save the page as: aov.123 or aov.xls. It should work. A better method is to either right-click on the file and choose "save target as" or whatever your browser offers to save the file to your hard drive. Unfortunately, the filename will display as "dm.pl" and that's wrong. Type in a filename of "aov.123" or "aov.xls" and then click on save. That's a problem with my download counter... I'll see what I can do about that...
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