Crop Factors: How Your Camera Sees Your Lens

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Do you know what size sensor your camera has?  A quick and informal survey of our readers leads me to believe that the great majority of you are using digital cameras that feature cropped sensors.  Most of the affordable cameras on the market feature the same, though there are some of you that might be fortunate to have a full framed sensor.  If you aren’t certain – and if your camera did not cost more than $2,000 USD – it’s safe to assume that your camera’s sensor is cropped.  The size of your sensor is an important detail to be aware of and every photographer using a digital camera should know what they’re using.  Most importantly, you should be fully aware of your camera body’s crop factor.   The crop factor affects more than you may realize.

What Is a Crop Factor?

This can all be very confusing and I do not wish to complicate more than I must.  So before I go on, I need to take a little aside to better define – in layman terms – what a cropped sensor really is.  For the sake of simplicity, I am not going to get into the physics behind it all nor am I going to address the specific design considerations as these aspects will not affect your photography.  First, we need a frame of reference as we do with all units of measurement.  In the case of the sensor’s size, we use 35mm film as the industry standard to determine the size and ratio of a full-sized frame (a full-framed sensor).  Professional grade cameras feature full-framed sensors, but the technology is costly and prohibitive for a consumer grade camera.  The solution is to reduce the size of the sensor to save costs.  This is possibly an over-simplification, but again, I do not wish to cause any confusion.  So the consumer grade cameras have a smaller sensor –a cropped sensor – but what serves as the basis for that format?  Most manufacturers chose to use another pre-established industry standard as a basis for their sensor size:  The APS-C format, which was released by Kodak in 1996 under the name of “Advantix”.  As each manufacturer designed their consumer level digital cameras, they made subtle adjustments as necessary, and we now have several crop factors available on the market.  In addition, there are a few out-of-the box thinkers, like Olympus and their four-thirds sensors, who chose a size and ratio completely unrelated to previous standards.  Canon uses a larger (but still cropped) APS-H sensor for many of their pro-level bodies.

For those of you with fixed-lens cameras (point and shoot, pocket cameras or even camera phones), you can probably ignore the rest of this article.  These cameras do have cropped sensors, but in most cases the crop factor is insignificant as the published focal lengths already take the sensor into account.  To that end, the size of your fixed-lens camera’s sensor should not impact your photography.

Now, for the rest of you, I have created a little chart that should help you to understand the crop factor of your camera’s sensor.  This is meant as a rule of thumb on a per-brand basis, but there are exceptions.  Ideally, you should do some research on your camera to verify its crop factor.

  • Full-Framed Sensors (Pro bodies):  35mm (the basis for all crop factors)
  • Canon APSH-H (pro-level DSLRs):  1.3
  • Leica M8 and M8.2:  1.33
  • Nikon, Pentax, Konmica Milolta, Sony Alpha and Fuji APS-C:  1.5
  • Canon consumer APS-C:  1.6
  • Foveon X3 (Sigma SD):  1.7
  • Four Thirds (Olympus):  2.0

How Does Crop Factor Affect Your Lens?

Crop Factors

Figure 1: A comparison of a full-framed sensor vs. an APS-C sensor. Note how much of the image circle gets thrown away.

Now that we know what our crop factor is, we can now apply it to our lenses.  I ask that you first refer to our hypothetical scene in figure 1, shown here, which is a diagram showing the relative size of both a full-sized sensor and an APS-C sensor (in this case, Nikon’s which has a factor of 1.5).  The circle represents the image circle, the image that is cast by your lens.  If you had a full-framed sensor, all that gets lost is the rounded extras left over as the image is squared off.  But if you had a smaller sensor, like the APS-C Sensor, a lot more of the image circle gets thrown out.  Manufacturers design lens specifically for these smaller sensors in an effort to minimize the waste.  But a great deal more of the image circle is wasted with a smaller sensor.

The Result of a Full Frame Sensor

Figure 2: The Result of a Full Frame Sensor

The result of a cropped sensor is that the resulting image appears as though you were zoomed in closer.  It is for this reason that many people mistakenly call this a zoom factor.  This is not truly the case, however.  Your lens is not actually zooming closer, and the angle of view hasn’t changed.  All that’s happend is the sensor simply crops out more of the image circle.  The result is merely a trick – the very same trick that is used by consumer cameras to achieve a “digital” (not optical) zoom.  As you can see from the resulting shots in figure 2 (an image from the full-framed sensor) and figure 3, there is a significant difference in what fits into the frame.  Additionally, between two photos at the same resolution, there is some quality loss with the smaller sensor.  The camera is stretching those images a little bit further and so the pixels suffer a little.

Bearing in mind that the resulting image does not see all the benefits of a longer focal length, such as angle of view, there is a way to predict how a lens will behave.  To do so, we simply need to use the following formula:

[ Lens Focal Length ] x [ Crop Factor ] = [ Adjusted Focal Length ]

The Result of an ASP-C Sensor

Figure 3: The Result of an ASP-C Sensor

So, for example, if I were to use my 50mm lens on my Nikon APS-C sensor, which has a crop factor of 1.5, I would simply multiply the focal length of my lens by 1.5 and I can expect that lens on that camera to act like a 75mm lens.  Even looking through the lens, it would appear as though less would fit into the frame.

The reason this really matters is that it will affect your planning.  In practice, you likely aren’t dead set on a focal length when you’re out on a nature hike.  But when you’re planning a photo shoot – such as a portrait session – you’ll want to plan out what you’re shooting.  A head and shoulders portrait might call for a 75mm lens.  You’ll be packing your 50mm lens instead.

Final Thoughts

This all sounds like bad stuff that you’d want to avoid and you’re probably scratching your head about why anyone should even consider anything but a full-framed sensor.  Despite the shortcomings, there are several advantages to using a cropped sensor.  First and foremost is the equipment costs.  Like it or not, cropped sensors have really brought down the price of cameras, and most of you wouldn’t be using the camera you have if it weren’t for these.  From a shooting perspective, the longer adjusted focal length does help get the most out of your lenses by giving it a little extra boost.  With the quality of today’s sensors, the image quality degradation isn’t significant as compared to the full-framed counterparts.  And if it means getting a 25% boost on your focal length, it’s worth it.  Professional sports photographers prefer using cropped sensors for this very reason.  The inverse, however, is that it’s harder to get a good wide-angle lens for your cropped sensor.  Macro photographers may prefer to have and use a full-framed sensor for this reason.  As for the degradation, keep in mind that the image is going to be sharpest at the center of the image circle.  The closer you get to the edges, the more poor the optical quality (that’s just physics, not the manufacturer’s fault).  So think of it as the best quality portion of the image circle is that which will be saved.

In the end, knowing your camera’s crop factor doesn’t significantly impact your photos.  But knowing such information will help you to better plan for a shot – and that will make you a better photographer.

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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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