Crop Factors: How it Affects Your Focal Length

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See-ThroughIf you are using a Digital SLR camera (or even some of the higher end range finder cameras like the Olympus Pen or a Leica), you should know your camera’s crop factor.  If you do not even know what a crop factor is, this may be a bit of a rude awakening.

A crop factor is simply a comparison of your camera’s image sensor to standard 35mm film.  Unless you’re using a full-frame camera (and even some full frame cameras have a slight crop factor), your camera’s sensor is likely smaller than 35mm.  In order to make things work on the technical side, many of your lenses may be specially marked for digital sensors.  Here are some of the typical designations:

  • Canon: ‘EF-S’
  • Nikon:  ‘DX’
  • Sigma:  ‘DC’
  • Tamron:  ‘Di-II’
  • Sony:  ‘DT’

These lenses will refract the light a special way to cast a smaller image circle onto the smaller sensor.  In theory, these lenses are optimized for the smaller sensor.  Regardless of what lens you’re using, the crop factor directly impacts its apparent focal length.  The focal length of a lens is listed based on it’s 35mm equivalent.  But on a sensor with a crop factor, it may actually increase the apparent focal length.  To figure out the effective focal length, you simply use this formula:

[focal length of lens]x [crop factor]= [adjusted focal length]

For example, my camera – a Nikon D80 – has a crop factor of 1.5.  If I’m using a 50mm lens, taking the crop factor into account – the effective focal length is 75mm (50mm x 1.5 = 75mm).  Warning, not all cameras are the same, and not all cameras of the same brand will have the same crop factor.  Most Nikons (except full frame cameras) have a crop factor of 1.5.  Some of Canon’s cameras, like the 40D or the 50D, use a 1.6 crop factor while others, like the 1D Mark III, have a crop factor of 1.3.  You will need to check your camera manual, or the manufacturers website to be sure.

A crop factor is not necessarily a bad thing.  Many professional sports photographers and wildlife photographers like using the smaller D-SLR cameras simply to take advantage of the crop factor – to get a little extra focal distance out of their lenses.  Just think, your 200mm lens would function as a 300mm on a Nikon camera body.  There may be a slight loss in image quality, but often not noticeable to the untrained eye.  As these lenses are expensive, it should save a lot of money.  But many photographers working up close may like full framed cameras for the finer detail and to get the shorter focal lengths.  A 10mm focal length is nearly impossible with a camera containing a cropped sensor.  If you plan on upgrading to a full-framed camera in the future, you need to be careful as your optimized lenses (the DX, EF-S and so on) most likely will not work on the larger sensors.

In the end, the biggest take-away of this article is that you really should know your crop factor.  As a photographer, you need to know that when you’re using your 50mm lens, it functions as a 75mm lens, or that your 35mm could function as a 56mm lens.  Knowing that fact makes comparing photos and your learning much easier.

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About Author

D. Travis North is a professional Landscape Architect, a Freelance Photographer and founder of Shutter Photo. Ever since he picked up his first SLR, his father's Nikon N2000, he's been hooked on photography. Travis likes to photograph urban environments, architectural details and has a new-found interest in close-up photography. His work can be found at D. Travis North Photography. Follow Travis on twitter: @dtnorth.

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