The .666 Factor

For photographers, “666”, this isn’t the sign of the devil, it’s a number to know when you are buying a lens or planning a photoshoot with your DSLR. You may not realize it but the focal length of your lens isn’t always what is advertised.

The 0.666 factor comes into play if you have a DX style camera. That’s a camera (both Canon and Nikon) with an APS-C sensor. Most entry level & advance amateur DSLR’s tote an APS-C sensor. An APS-C sensor is 66.6% the size than old school 35mm film – remember film? Although most people, at least the photographers I know, shoot digital the industry still uses the 35mm film size as the standard when calculating focal length for lenses.

Now, what does this mean for those photographers with an APS-C sensor DSLR? It means your 50mm F/1.8 lens actually has approximately a 75mm focal length. Your 18-70mm kit lens is actually a 27-105mm.

In many cases this is a good thing. It gives you extra reach with your lens. A mid-range zoom becomes quite long on the far end. However, it also means your wide angle lens isn’t so wide angle.

The simplest way to figure it out is to divide the focal length of your lens (again this only applies to an APS-C sensor DSLR, a DX camera in Nikon speak) by 0.666. Examples of a few common lens focal lengths:

Primes

• 28mm = 42mm
• 35mm = 52mm
• 50mm = 75mm
• 60mm = 90mm
• 85mm = 128mm

 Zooms  

• 18-55mm = 27-83mm
• 18-135mm = 27-202mm
• 24-70mm = 36-105mm

Of course, if you want to avoid all this then get a “full-frame” or FX camera. These are cameras with sensors whose size is equivalent to 35mm film. I won’t get into a discussion about which sensor is better, but at minimum you can expect to pay more for an FX camera than a DX camera. OK, personally I think an FX is better.

It should be noted many lenses are classified as “digital” and therefore are tuned for an APS-C sensor. Also, some lenses are considered a full-frame lens. I’ve used these types of lenses interchangeably, but a purest will likely call me out for the evils of my ways.

So remember, when purchasing a lens or planning your photoshoot think about the focal length you need and then plan your lenses choice appropriately. Either way, get out there and shoot!