Digital Camera Crop Factor Explained

We here at The Magic in Pixels speak a lot of the “lens magnification factor” or “crop factor” or “zoom equivalent”.  All three terms mean roughly the same thing, but what the heck does it mean?  Does it really matter and should you even care?  Well, there is a very simple explanation for “crop factor” and what it means for you depends on what kind of camera and lenses you are planning on buying (or already own) and what you enjoy photographing.  There really is no right or wrong choice, but since knowing is half the battle, let’s discuss the crop factor and how to best apply it to suit your tastes.  Since I have been using full-frame cameras from the past several years, I am partial to them, but there are many factors to consider, including size and cost, and the types of lenses you might need to use.

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Crop Factors

Crop factor is based entirely on the physical size of the camera’s digital sensor and is based on a comparison of said sensor to a 35mm piece of slide film.  A 35mm piece of side film measures 36mm x 24mm and would be considered “full frame”.  A “full frame digital camera”, of which there are precious few right now, has a digital sensor that measures 36mm x 24mm.  Any given lens on a full frame digital camera yields the same field of view as that same lens on a 35mm film camera.  Therefore a 50mm lens would give the same exact picture on a full frame digital as a 35mm film camera.  A 135mm lens would give the same picture on either camera and so on.   According to B&H Photo, there are currently five (well, five and ½) different sensor sizes being produced, including full frame.

Where things get interesting and slightly more complicated is when you start working with cameras that have digital sensors that are smaller than 36mm x 24mm.   At the risk of getting overly mathematical and blasting you with information overload, let’s just say that the crop factor of sensors smaller than full frame is based on the hypotenuse of the digital sensor.   Remember your Pythagorean Theorem, kids?  (A2 + B2 = C2)?   Once we get through a brief explanation of the sensor sizes, we can discuss how they apply to real world photography.

The next largest sensor down from a full frame is based on the APS-H film size and yields a 1.3x crop factor.  An example of this is Canon’s 1d Mark III camera body (not to be mistaken with Canon’s 1dS Mark III, which is full frame).  It boasts a sensor that measures 28.1mm x 18.7mm, which is 1.3x smaller than our 36mm x 24mm full frame.  Thusly, a 50mm lens on the 1d Mark III will produce an image that is equivalent to a 65mm lens on a full frame digital or 35mm film camera.  I use the word equivalent because some people erroneously think that the crop factor is the same as adding more zoom.  It is very similar but not exactly the same for reasons that I will explain later in this piece.

Our third sensor size is the most common in digital SLR camera, APS-C which is  based on the Kodak Advantex (or Advanced Photo System).  Canon’s APS-C equivalent is 1.6x while Nikon DX, Pentax, and Sony all clock in at 1.5x crop factor.  Nikon’s new D5000 digital SLR body has a sensor that measures 23.6mm x 15.8mm which yields a factor of 1.5x.  Thusly, a 50mm lens on a Nikon D5000 would produce a similar image to a 75mm lens on a full frame digital or 35mm film camera.  Since Canon uses a marginally smaller sensor (22.3mm x 14.9mm) in their Digital Rebel and x0d lines (currently 50d), their crop factor is 1.6x.  As such, a 50mm lens on those cameras would produce a similar picture to an 80mm lens on a full frame digital or 35mm film camera.

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Digital Sensor Sizes

The fourth and fifth sensor sizes are the Sigma Foveon and Olympus Four-Thirds sensors.  The Sigma Foveon sensor measures 20.7mm x 13.8mm and yields a crop factor of 1.7x while the Four-Thirds system measures 17.3mm x 13.0mm and produces a crop factor of 2.0x.  As we have done before, a 50mm lens on a Foveon sensor produces the same image as an 85mm lens on a full frame digital or 35mm film camera while a 50mm lens on a Four-Thirds camera would be the same as a 100mm lens on a full frame digital or 35mm film camera.

Ok, so what the heck does this have to do with photography?  Well, if you are like me and into shooting buildings, landscapes, people, and many other types of things, you would need the wide angle capabilities of a larger digital sensor.  I use full frame and have a lens that goes as wide as 17mm, which produces a VERY wide field of view.  If I wanted to have a 17mm equivalent field of view with an Olympus Four-Thirds camera, I would need an 8.5mm ultra wide angle lens, of which there are precious few, if any, available.  However, the opposite rings true as well.  A 300mm telephoto lens on an Olympus Four-Thirds (~$328) gives the same equivalent view as a 600mm lens on my Canon 5d Mark2 (~$7,600).  Therefore, wildlife photographers and bird watching enthusiasts may want to go with a crop-factor camera.   That is a bit of an extreme example and there are ways to significantly lower the gap between the examples but it does show the kinds of issues that crop factor can bring to light.   Granted an Olympus E-520 with two lenses costs about $778 while a Canon 5d Mark2 with no lenses costs $2700 but there are also different classes of camera bodies.

So, if a 50mm lens on a Four-Thirds produces the same image as a 100mm lens on a full frame, the picture should produce almost the same exact image (adjusted for sensor quality, noise, build, etc.), right?  Well, not really.  As I alluded to earlier, there are several differences that need to be accounted for:

Depth of Field – The camera with the smaller sensor will have a larger depth of field that the camera with the larger sensor.  This means more of the smaller camera’s image will be in focus, given that both cameras used the same aperture, or f/stop, and the same lens from an equivalent distance from the target.

Distortion – A cheaply made lens on a full frame sensor will show all of its imperfections and shortcomings, especially at the edges and in the corners.  These shortcomings may not show on a crop factor camera, since the camera is producing its image from the very center of the lens’ glass as opposed to almost all of the glass.  Since most lenses have “sweet spots” in the center of the glass, all of the distortion and issues around the edges are excluded since the sensor is not recording those areas like it would on a full-frame.

Camera shake – The old rule of thumb needs to be increased on a crop factor camera to include the camera’s crop factor.  (Refresher – the minimum shutter speed for handholding a camera is 1/x where x is the focal length in millimeters).  Therefore, a 50mm lens on a 1.6x camera body would require a 1/80 shutter speed to ensure sharp photos.

Perspective Issues – Because a crop factor camera would require you to be further away from the target than a similarly equipped full frame (same lens), any perspective distortion created would be different between the two types of cameras.

Lens and body costs – Like I mentioned in the Distortion bullet point, full-frame cameras require better lenses to produce the best possibly images, which cost significantly more money than their lower priced counterparts.  For example. Canon’s 28-135 IS/USM costs about $390, while their 24-105L IS/USM costs $1,150.  Want to hazard a guess which lens gives better color and sharpness?  Similarly, full-frame sensor cameras are costlier than all but a very few crop factor cameras due to the size of the sensor and costs associated with producing them.

Sensor Noise at Higher ISO – Full Frame cameras are much better at handling noise at higher ISO’s and can generally go much higher than their crop factor equivalent.

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Hopefully this helped to educate and inform you as to what “crop factor” means, how to apply it to your photography, and how to make an educated decision when making your next camera body purchase.   B&H Photo, the exclusive supplier of gear to The Magic in Pixels, gives you the sensor sizes and crop factors on each camera’s specification page to help you make the best possible decision.

 Discuss this article and suggest others in The Magic in Pixels Disney Photo Forum.

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