(Roughly) Visualizing Camera Coverage Area Using A... Ford Ranger?

Playing around with the incomparable lens calculator in Metric Mode I happened upon this equality:

When using a 1/3" sensor*, if the distance to subject (measured in meters) = focal length (measured in millimeters) then the HFOV is ~5m (4.8m to be exact). Could this simple equality be turned into a rough estimation rule of thumb? Your thoughts and please feel to be as critical as I deserve. Here goes:

A Ford Ranger (Regular Cab),

is also about ~5m (4.8m) long. Therefore the image frame would be completely filled (across) by the truck's profile if one were using a:

  • 10mm lens when the truck is 10m away
  • 25mm lens when the truck is 25m away
  • 80mm lens when the truck is 80m away

Easy to remember. Said differently, when using a 1/3" sensor, if the number of meters to the subject is the same as the number of millimeters in the lens length, then the HFOV at that distance is always the size of a Ranger.

Ok, big deal you say, what good is that when my focal length and distance to subject are not equal, like 99% of the time? Because you can still get a rough idea of the area covered by the lens/camera very rapidly, and for multiple data points. It's also a good sanity check that you didn't make an entry error using a calculator.

So let's say you want to visualize the near coverage of a 15mm lens out into a parking lot**, one could imagine a Ranger traversely parked 15m from the lens. Make a triangle, starting at the lens, with two lines diverging to each bumper of the truck and then continuing to the horizon. Everything inside the triangle would be covered by the lens/camera.

Why a Ranger anyway, why not a Bronco? Because a Ranger is the same horizontal size in meters, 4.8, as a 1/3" sensor is in mm.

*Other formats have slightly different constants. For instance a 1/2.7" sensor would be 5.37M.

**I'm sure a large number of members can visualize camera/lens coverage areas precisely without the use of such devices, I'm not one of them, yet.

Years ago I had a "prototype" business card design that had a set of tick marks on the back. The idea was you could hold it at arms length and fit a car in between a set of the ticks, and underneath was printed a rough range guide. Essentially the same idea as what you describe here. It would be useful for doing outdoor longish-range camera guides (mostly what I was doing at the time), and made my business card something worth keeping handy.

I never ended up getting them printed in a large quantity, but it might be neat to revive that idea.