Lens Magnification

Have you ever wondered why lenses for cameras don't include magnification specs?

For binoculars or spotting scopes magnification has always been a very important metric. Binoculars for example are always listed like 7x50, meaning that they magnify by a factor of 7 while having an objective lens measuring 50mm in diameter. That is the major criteria.

It's puzzling that a camera lens for interchangeable-lens cameras or a compact camera with integral lens never includes a magnification number. Especially for telephoto lenses or super-zooms that are purported to be very telephoto at their far end.

For decades I thought that such a factor is impossible to determine since a lens is not a closed system like binoculars or a spotting scope. That is, one can not easily compare a scene viewed by the naked eye with one that used a camera and some sort of presentation device. After all a computer screen may be any size, its resolution being another factor, and finally ones viewing distance all contribute to the largeness of the photographed subject.

Then I got interested in resolution, "retina displays", viewing distance, diffraction limiting, line-pair, lens MTF charts, etc. That's when I decided to go through the math myself from lens to sensor to viewing screen to eye to determine which factors carry through and which cancel out.  

My conclusion is that existing lens specs are sufficient to come up with a theoretical magnification value. When combined with the camera body we can arrive at an even more useful factor. I have used those new specs to weed out some questionable specs as well as make more informed buying decisions. I now believe that manufacturers deliberately omit magnification numbers because such would undermine their exaggerated marketing claims.

How this calculator helps you with your buying decisions.

  • Wade through bridge camera marketing hype. Is the impressive number of megapixels really necessary? Does the aperture limit the sharpness of photos in most cases?
  • Does a teleconverter really increase usable magnification? In some combinations of older low megapixel sensors it may. Keep in mind their effectiveness is maximized with expensive larger aperture lenses. More often you're better off cropping a photo (software zoom).
  • For photographers who are seeking increased telephoto options, is it worthwhile to buy a bigger telephoto lens or to upgrade to a newer, more megapixel camera body? Surprisingly the cheaper solution is to upgrade the camera body. Pay attention to modern ones which have no AA (anti-aliasing) filter.
  • Compare telephoto lenses. This may save you thousands or even tens of thousands of dollars when you can consider buying compromise lenses having smaller maximum apertures.
  • Do your favorite lens and realistic lighting conditions make full use of your camera's capabilities? For example, if you like to shoot one or two stops down to achieve maximum sharpness, is the result taking advantage of the camera's sensor resolution?

The internal formulas used are my own. I did however double check against existing calculators to ensure I was using the correct factors - visual acuity angle, wavelength of light, size of Airy disc, etc.

Here is an excellent website. It explains the theory so there was no need for me to go into the details.

Diffraction Limited Photography: Pixel Size, Aperture and Airy Disks

Revised 17 Sep 2015