Focal Length question

xarqi wrote in post #9500877
And a good thing too, since the relationship is not linear!
When you double the FL, you halve the linear FoV, etc.
Here's the graph you want:

log-log saves the day

Bob_A wrote in post #9500921
The change in angle is linear, but would the % of frame be linear (... looking for my protractor)?

My trig calcs are shown below the change in angle. Depending on the subject distance, it'll vary a bit, but it's pretty darn close to the same for most normal focusing distances.

Jman13 wrote in post #9500940
My trig calcs are shown below the change in angle. Depending on the subject distance, it'll vary a bit, but it's pretty darn close to the same for most normal focusing distances.

... and as expected your data plots as a straight line with a log-log plot

Jman, here is the graph with the values for diagonal FoV you just supplied. I tried the calculator supplied by gasrocks and it is pretty similar.
Log-log is the solution to the problem, and the relation is not linear.
Thanks xarqi!

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The graphs will do little more than confuse the newbies.

The simple fact is that if you double the focal length, the in-camera size of the image of something in the scene will double in height and in width compared to the shorter focal length.

Conversely, if you use a lens that's half the focal length, the size of the in-camera image of something in the scene will be half what it was with the longer lens.

These statements are true when the camera stays in the same position relative to the subject. Once you start moving the camera, then perspective changes. For information on perspective, please read our "sticky" (now found in the General Photography Talk forum) tutorial titled Perspective Control in Images - Focal Length or Distance?.

SkipD wrote in post #9503426
The graphs will do little more than confuse the newbies.

The simple fact is that if you double the focal length, the in-camera size of the image of something in the scene will double in height and in width compared to the shorter focal length.

Conversely, if you use a lens that's half the focal length, the size of the in-camera image of something in the scene will be half what it was with the longer lens.

These statements are true when the camera stays in the same position relative to the subject. Once you start moving the camera, then perspective changes. For information on perspective, please read our "sticky" (now found in the General Photography Talk forum) tutorial titled Perspective Control in Images - Focal Length or Distance?.

I think we got caught in the numbers involved. An excellent way for a newbie to understand this topic is the tool provided in juzaphoto... or better yet, go shoot and see what works for you!
Thanks to all who contributed to the discussion. It is very enriching to read opinions from the pros.