The Digital Lens Calculator
How to know the right lens for every photograph
©1995-2021 Fred Parker
One trait that separates professional level photographers from amateurs is the ability to pre-visualize a photograph. Sure, you have every focal length lens from 6mm to 1200mm. But you can't carry all of those lenses with you all of the time. As you move through your life, most of the time you will not have a camera around your neck. But, if you have the soul of a professional, you will see photographic opportunities everywhere. Many will be missed entirely because of the fleeting nature of life experiences. However, several times a week you will be standing in a location, repeating the recurring chant of the photographer: "Wow! What a great shot! I wish I had my camera!" The fact is, if the "shot" is compelling enough, you will return. You'll think about the best time of day/season for lighting, make a note of your position and come back at the perfect time.
But, wait! Haven't you forgot something? Which of your drawer full of lenses will you bring with you? If you're like most photographers, you'll bring a bag full of lenses, but not the right one. The challenge is to come back at precisely the right time with only those lenses that you need. How can you do it? Read on!
Be Well Armed WITH DIGITS
All you need to estimate the proper focal length lens is your arm and digits (fingers, that is - remember, this is a DIGITAL lens calculator). If you're a REAL stickler for accuracy, you can bring along a very small (one meter) tape measure calibrated in millimeters. You see, Nature must have had photographers in mind when she designed Homo sapiens. Not only that, she prefers the 35mm format for film. And the metric system.
I should explain. As it turns out, the long dimension of the 35mm format at various focal lengths from 50mm to 1200mm is (pretty exactly) represented by the anatomy of your hand stretched out to arms length.
For example, if you spread out your hand with your arm stretched out in front of you, the distance from the tip of your thumb to the tip of your little finger will equal the long dimension of a 100mm lens. A 200mm lens is represented by the length of your index (pointing) finger at arm's length, etc. The table below gives you hand measurements for most lenses from normal through extreme telephoto.
But there's more! At the distance of an outstretched arm, 10mm equals one degree of field of view. Since you already know the angular field of view for all of your lenses, figuring which lens to use is a snap! Just bring along a meter tape measure. What's that!? You don't know the angle of view for all of your lenses? Well….let me give you a shortcut. A 50mm lens has a 40 degree angle of view. That's all you need to know. Double the focal length to 100mm and you cut the field of view in half (20 degrees). Double it again to 200mm and halve the angular distance again to 10 degrees. And so on. You get the point. If you have very long or very short arms, please be aware that the finger measurements will be more accurate than the tape measure. That's because finger and arm length tend to scale proportionately, while a tape measure remains fixed. And, alas, 5% of the population will have long arms and small hands (or short arms with large hands). If you are in one of these groups, you'll have to make adjustments from the chart. This chart will be very accurate for 95% of photographers.
Of course, all of this works only with the 35mm film format. And a yard stick won't work (it must be metric). The table below gives data for the most popular lenses in the 35mm format. If you have a lens with a focal length that falls between the ones in the chart, fake it. Please note that this is an anatomically correct calculator. A brief lesson in anatomy will help with the chart. If you look at one of your fingers (you choose) you'll find that it is made up of three segments (except the thumb, which has two). The term for each segment is "phalanx". Collectively, they are "phalanges". The distal phalanx is the one with the nail on it. The proximal phalanx is the one closest to your palm.
Estimating Lens Angle of View