Originally posted by Parallax Equivalence is a way to understand things, but its usefulness is questionable. Every day there are people buying cameras that have never taken a single photo on 35mm film, so knowing the equivalent FL, or DOF, or anything else as it relates to that format is useless/meaningless to them. My Ram pickup has a cargo capacity of about 2,000lbs. How useful is it to know how many Ford Model A pickups would yield the equivalent carrying capacity? I know that if I hit my thumb with a hammer, it hurts. What's the benefit in calculating the equivalent force needed to yield the same amount of pain by hitting it with a rock?

Seriously, if I have an APS-c format camera and a 645Z and I decide that the 645Z is the best body to use for a particular subject or scene, (due to pixel count, DOF, whatever) I only need to know what lens to put on it to capture the scene as I wish to capture it. The comparative, or mathematical relationship between that and what I would need to do to get a similar result in the other format is completely superfluous.

As my sig line reads: Life gets easier once you forget the concept of "crop factor" and "full frame equivalent"

Do you also question the use of Coordinated Universal Time (UTC), asking "Why do I need to know the time in Greenwich, or how many hours offset any given time zone is from it? I don't live in Greenwich and I don't plan on going there. I can see the clock and sky just fine. If I ever want to know what time it is anywhere in the world I can do that by going to that place and looking at a clock."

Now I have a compact camera right in front of me, whose focal length as reads on the lens is 5.8-23.2mm. Do you understand what it looks like, and whether this range of AOV is useful for you? It is very easy to have a single standard against which to compare any number of formats to understand what the AOV on any of them will look like without even seeing the standard (i.e. you don't even need to know what 35mm looks like). Say you have formats A, B, C, D, E and X (e.g. 1.7", 1", m4/3, APS-C, 645D, 35mm). There are four ways you can understand what the AOV of any given sensor/lens combination looks like:

If you use X as a standard you can then learn exactly 5 conversions to understand what the AOV on any of them looks like: A <--> X, B <--> X, C <--> X, D <--> X, E <--> X. This is useful even if you don't know what X looks like, as long as you know any single one of them, because it is a simple 2 step math conversion from any format to any other format: e.g. D -> X -> A. Adding additional formats increases the combinations linearly (e.g. adding 1 format increases the conversions needed to learn by 1).

The second way is to memorize the conversion between each and every one, which leads to 15 combinations which you must figure out. Adding more grows the combinations exponentially (e.g. adding 1 format increases the conversions needed to learn by 6).

The third way is to not learn AOV equivalence at all. Now you can just "figure it out" once you can look through the camera and lens in your hands, in which case good luck buying a compact camera, or figuring out what lenses you need to make a new system viable before you buy into it.

The final way is to learn the formula for AOV in degrees/radians (which is analagous to your 2000lb truck capacity example, which uses the standardized units of "pounds"). This is the most universal, unambiguous way to understand AOV for any sensor/lens combination in existence. I have that handy dandy formula right here for you to use in your everyday life:

AOV = 2 arctan(d/(2f))

where

*d* is the sensor length in millimeters in the dimension you're interested in (i.e. horizontal, vertical or diagonal), and

*f* is the focal length in millimeters. Have fun doing that in your head.