Rudy Winston
Rudy Winston

Rudy Winston has over 16 years experience with Canon USA's Pro Products team, and has been responsible during that time for training Canon's staff on new products, creating presentations for customers and dealers, numerous writing projects, and providing technical assistance to professional and amateur photographers.

Which lens do I need? Calculating lens and image sizes: Part I

August 13, 2013

Any math teacher I ever had in high school can confirm that it was a good thing I got into photography because I was always a lot better with visuals than with numbers. But no matter how accomplished you are in the art and craft of image making, there are questions that can sometimes be more simply answered by pulling out a calculator. Lens choices can present such questions. And one of the most pressing if you’re thinking about adding a new lens to your collection or just tackling one upcoming photo situation (especially for telephoto lenses) is “which lens do I need?”

Even the numerically challenged, like myself, can come up with a reasonably accurate answer with a few quick entries into a pocket calculator. We’ll walk you through some steps here to help you in our two-part series on calculating your lens, image and subject size and how to put it to use.

What can you calculate?

This is actually delightfully easy. Using a calculator and perhaps a sheet of paper and a pencil, you can easily answer the following questions:

  • Which lens do I need?

    When you know about how far away you’ll be from a given subject and the approximate size of that subject, which lens will you need to get it to take up x-amount of space on your digital image sensor (or a piece of 35mm film)?

  • With a particular lens, how far or close will I need to be?

    If you know you’ll be using a particular focal length lens and you know the approximate size of your subject, how far will you need to be to fill a given area of your final image?

  • How big was that object I photographed?

    Knowing the lens you used and approximately how far away a subject was, about what was its actual size?

Terms you’ll need to know

There are four different terms used for these simple calculations to give an approximation to answer the three questions posed above. They are:

  • Lens focal length

    Actual focal length of the lens used or expected to be used in a given photographic situation. Generally, figures can be rounded off. If your EF 70–200mm zoom was set to 192mm for a particular shot, you usually can punch-in 200mm and be close enough for our purposes. This figure is always in millimeters.

  • Distance

    The distance from the lens to subject. Because we’re not speaking specifically of close-up or macro-type situations here, you can think of this as general lens-to-subject distance. Critical shooters would likely base this on the image sensor plane or film plane to subject distance, but at normal distances, this usually won’t be vital.

  • Object size

    The actual height or width of a subject, converted into millimeters. A 6 ft. tall man (1.82 meters) would be about 1,829 mm tall. Object size always refers to the actual size of our subject or at least an approximation of that actual size.

  • Image size

    The height or width of the subject, in millimeters, as it appears on the image sensor or film. As we discuss below, a hypothetical full-length shot of a standing man might be about 20mm on a full-frame camera horizontally or roughly 12mm on an APS-C sensor digital SLR.

Conversions to millimeters

Because our lenses are classified by focal lengths in millimeters (mm), it’s really handy to enter your values in millimeters across the board. This means converting figures we normally think of in feet, inches, miles or meters/kilometers into millimeters – at least to perform these calculations. To answer questions like, “how far away will I need to be?” and “how big was that object?,” you’ll usually need to convert back to a unit of measurement like feet, inches or meters. But that won’t be hard using these following points:

  • 1 inch = 25.4mm
  • 1 foot = 304.8mm (10 feet = 3,048mm; 100 feet = 30,480mm)
  • 1 yard = 914.4mm (10 yards = 9,144mm; 100 yards = 91,440mm)
  • 1 mile = 1,609,344mm

Readers who frequently use metric measurements doubtless know that their conversions are a lot easier:

  • 1 centimeter = 10mm
  • 1 meter = 1,000mm
  • 1 kilometer = 1,000,000mm

Again, the point is that however counterintuitive it may first seem, if we convert distances and so on into millimeters, it makes what we want to figure out a lot easier because our lenses are classified in millimeters of focal length.

Understand the size of our image and image sensors

We’ll limit our discussion here to digital SLRs and traditional 35mm film SLRs because they use the same types of interchangeable lenses. The same calculations can be applied to medium or large format film and digital cameras, as well as compact PowerShot class cameras, which use much smaller sensors, but we won’t go into that in detail here.

Today’s digital SLRs typically use image sensors that fall into the following categories:

  • “Full-frame”: 24 x 36mm in Height x Width (same size as a 35mm film frame)
  • “APS-C”: approximately 22 x 15mm
  • “4/3-inch”: approximately 18 x 13.5mm (Canon doesn’t make any digital SLRs with 4/3-inch sensors as of mid-2013, but users of popular competitive models with 4/3 sensors may be reading this article)

Then, there’s the issue of how big will a subject appear on the image sensor. For example, imagine you’re taking a full-length shot of an average sized adult standing upright and want to fill most of the height of your frame with this person from head to toe.

With a full-frame camera, the final height of that subject on the sensor might be something like 20mm, allowing for a little bit of “breathing room” above his or her head and beneath their feet in the image. Likewise, if you used an APS-C camera (like a Canon EOS Rebel model or an EOS 7D), you have 15mm of total image height available — a comfortably composed, full-length horizontal shot might have the subject taking up perhaps 11 or 12mm of space on the image sensor.

Turn the camera vertically and you might be composing a full-length shot of a standing person using about 30mm of image size with a full-frame camera. That equates to 17 or 18mm of the possible 22mm on a vertical shot with an APS-C sensor camera.

Whether you’re photographing a bird in flight, a race car at a speedway or a cover shot of a baseball player for a sports magazine, ultimately, you’re trying to fit a particular sized subject into a certain area of your digital image sensor or onto a piece of 35mm film.

Check back next week for Part II, where we discuss the basic formulas to easily calculate what lens you need for a particular subject and situation, how close/far away you need to be with a particular lens from a subject or even to calculate how large a subject's actual size is after it's been photographed.

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