Question
I read on a this site: Lens Buying Guide
Focal-length determines the
angle-of-view seen through a lens for
a given sensor-size. With a full-frame
sensor a lens gives the same
angle-of-view as it would on a 35mm
film camera. With a smaller sensor,
the angle-of-view becomes smaller. The
crop-factor, also called
FLM (Focal-Length Multipler), is the
ratio representing the difference. Say
a lens gives a 90° angle-of-view on a
full-frame DSLR such as the Nikon D700
Nikon D700, then the same lens would
give an angle of view of 60° on a
D7000 Nikon D7000 because its FLM is
1.5X.
Short focal-lengths show a greater
angle-of-view compared to longer ones.
Lens focal-lengths fall into broad
categories according to the
angle-of-view they give depending on
the camera's sensor size. There are no
official numbers, the table below
shows the ranges used here and typical
uses.
In layman's terms, what is angle-of-view? Is it the same as focal length?
If not, how is it different? How is it used? Why do I need to know about it?
Answer
In layman's terms (assuming a layman who knows some very basic geometry), imagine your nose as the apex of a triangle. The left side of the triangle is the left edge of your peripheral vision, and the right side is the right edge. The horizontal angle of view is simply the angle between those edges. (And the vertical angle of view is the same thing for up and down.)
For a human eye, this happens to be about 95°, but since your eyes move around unconsciously and your brain fills in the details, it feels much wider than that.
The terms field of view and angle of view are basically interchangeable — angle of view is one way of measuring the field of view. (One could also say something like "10 meters at 20 meters away"... this describes different aspects of the same geometry, and with basic trig one can figure out one thing from the other.)
As the text you've quoted says, "Focal-length determines the angle-of-view seen through a lens for a given sensor-size." This is also basic trig, and you can actually plot it out on a piece of paper and measure for yourself. Obviously with a lens this is a three-dimensional problem, but we can just consider the horizontal dimension and reduce it to two. (Imagine this as a top-down cut-away view of the world.)
Draw a line 23.6mm long — the width
of the sensor in your D7000 — in the
center at the bottom of a blank piece
of paper.
From the center of that line, draw a light
perpendicular line out from that
center dot out towards the middle of
the page, so you have an inverted T shape. (This is for
convenience. Think of it as "the line towards what you're pointing the camera at".)
Measure from your sensor along the center line you just drew. Put a dot at 35mm. Label this "35mm lens". This represents the pinhole aperture of an idealized 35mm lens.
Now measure from your sensor along the center line. Put a dot at 50mm. Label this "50mm lens". (And of course this represents the pinhole aperture of an idealized 50mm lens.)
With your straightedge, draw a line from the left edge of your sensor line through the 35mm aperture dot, and continue all the way through and on to the edge of the page. Then do the same thing from the right edge of the sensor line. This should produce a big X shape. Label both lines of the top cone of the X "35mm field of view".
Do the same thing with the 50mm lens dot. Label this, of course, "50mm field of view".
Now, you can directly see that a shorter focal length produces a wider field of view. Anything that's within those lines will be in your picture, and everything outside will be out of frame. Note that the lens may project a much wider cone light that doesn't fall on the sensor — the lines you drew ignore that, since light that isn't recorded doesn't really matter.
If you measure the angle, you should see that it's about 36.5° for the 35mm lens, and about 26° for the 50mm lens.
Then, two further experiments:
- Choose some different focal lengths (15mm, 200mm) and see what those give you.
Increase the size of the sensor line to 36mm, as in Nikon's "FX" full-frame cameras. Keep the line centered on the same dot, of course. Use your same lens dots but draw new X lines to the larger left and right edge of the sensor. It's immediately apparent that this makes the field of view of the same focal length much wider. Notice that the 35mm on your D7000 gives roughly the field of view of the 50mm on FX — this is why people talk about "equivalent" lenses.
You can see that the lines for APS-C 35mm and "Full-Frame" 50mm aren't right on top of each other, as one might expect for an "equivalent". That's because this breaks down a bit at macro distance. If you move back a few millimeters, it'll line up correctly (but change perspective ever so slightly). The lines are roughly parallel, though, so those few millimeters are still just a few millimeters across the room, where they're inconsequential. If you draw this on a really big piece of paper instead of this little on-screen one, that'll become clear. (And of course, they're not exactly parallel, because the lenses don't quite match the crop factor — 32.7777...mm and 50mm would be more exact. Ah, the real world, always getting in the way of explaining things simply.)
This neatly (I hope) answers the question of the relationship between focal length and angle of view / field of view, and also explains the effect of different sensor sizes — and, as a bonus, shows how cropping is interchangeable with zooming (if you don't mind using less of your sensor).
Update: as greg g points out, the math in the quote you've found is wrong. If you do the exercise above, you can clearly see that 1.5× crop factor doesn't turn a 90° field of view into 60°. (And you can see why, which is the beauty of actually sketching out geometric constructions.)
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