How big is the moon? Or, more to the point, how does the moon appear to change size so drastically? If you go outside at night and hold a ruler out at arms length, the moon will measure about 3/8″ in diameter. It will fit on the nail of your little finger.
Just as in the photos below, one silhouetting a hawk and the other an 8 foot statue, when shot with a camera it’s apparent size will vary according to the focal length of the lens it’s shot with.
Take sign letters. An Art Director once gave me the chart below which is used to estimate the visibility of letters on signs, depending on the size of the letter and how far the viewer is standing away from it. Obviously contrast plays a big part here, so this formula works best with red or black letters on a white background. Different colored letters or a darker background can reduce this distance by at least 10%.
These numbers, of course, only hold true when viewing them with the human eye. It’s obviously a different story if you’re shooting with a wide angle or long lens.
I’ve created a formula for applying the above data depending on the focal length of the lens being used. These focal lengths are equivalent to Super 35 or a 35mm size digital sensor.
To use the chart, just multiply the letter size by the multiplication factor. For instance, if you have a 24″ letter (or object) and shoot it with a 35mm lens, it will appear the same size as a 10.3″ letter would to the naked eye. The number in parenthesis is the division factor.
Please note these are rounded factors and the true result will vary according to the camera and sensor size as well as the lens. To convert the focal length of another sensor size, refer to the previous post on focal length conversion.