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Old Mar 18, 2010, 5:56 PM   #1
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Default 'Focal Length' and 'Crop Factor'

The 'Focal Length' of a lens is a measure of how strongly a lens can bend light. A lens with a shorter focal length bends light more than a lens with a longer focal length, so it has a wider angle of view. 'Focal Length' is a physical property of a lens.

The actual angle of view depends on the size of the image sensor. A larger image sensor has a wider angle of view than a smaller one. The term used to refer to the different sizes of image sensors is 'Crop Factor' (sometimes referred to as 'Focal Length Multiplier'). Since the most common and longest lived image sensor is 35mm film, an exposure that is 36mm by 24mm (the size of a 35mm film exposure) is used as the basis for comparison, and therefore has a 'Crop Factor' of 1. Since smaller image sensors do more cropping, they have larger crop factors, and larger image sensors have smaller crop factors.

For instance, Nikon and Sony both make dSLRs with image sensors that are the same size as a 35mm film exposure (commonly referred to as 'Full Frame'), as well as dSLRs with image sensors that are 1/3 smaller (24x18mm). Since a 'Full Frame' image sensor is 1.5 times larger than the smaller image sensor, the smaller image sensor is said to have a 1.5X Crop Factor. Fuji and Pentax only make dSLRs with these smaller, 1.5X crop factor image sensors. Canon makes dSLRs with three different size image sensors. Their top-of-the-line models use 'Full Frame' image sensors, some have 27.5x18.5mm image sensors (1.3X crop factor), and others have still smaller image sensors (22.5x15mm, or 1.6X crop factor.) Sigma's dSLRs use an even smaller image sensor (21x14mm) which has a crop factor of 1.7X. Leica, Olympus and Panasonic dSLRs use still smaller image sensors (17.3x13mm, or 2.0X crop factor.)

(Image courtesy of Wikimedia Foundation)

P&S Digicams have even smaller image sensors, and so have even larger crop factors. On the other hand, traditional medium format cameras can use digital sensors as well, and those sensors are typically 60x45mm, giving them a crop factor of 0.6X.

Fuji's, Pentax' and Sony's 1.5X crop factor image sensors, and Canon's 1.6X crop factor image sensors, together, are often referred to as 'APS-C', because their size closely matches the earlier 'Advanced Photo System' "Classic" film exposure. Similarly, Canon's 1.3X crop factor image sensors are often referred to as 'APS-H' for the 'Advanced Photo System' "High Definition" film exposure. Leica, Olympus and Panasonic dSLRs are members of the Four Thirds System, and so, together, are referred to as Four Thirds, or 4/3 cameras.

'Crop Factor' is useful if you need to move back and forth between two or more different size image sensors, so you can relate the angle of view you'd get with a lens on one camera, to what you'd get with it on a different camera. For instance, a lens with a 30 angle of view on an APS-C dSLR would have a 45 angle of view on a 'Full Frame' dSLR. (1.5 X 30 = 45).

This 'Crop Factor' has given rise to the use of a '35mm equivalent focal length.' For instance, a 100mm lens on an APS-C dSLR would have the same angle of view as a 150mm lens on a 'Full Frame' dSLR. So, on an APS-C dSLR, a 100mm lens would have a '35mm equivalent focal length' of 150mm. It's important to understand, however, that the actual focal length of a lens doesn't change. Its angle of view will change with the size of the image sensor, and that makes the '35mm equivalent focal length' useful for people that move lenses back and forth between dSLRs, but the lens' actual focal length remains the same. The '35mm equivalent focal length' is also useful in determining the angle of view in images captured with P&S Digicams. Because of their much smaller image sensors, and their large crop factors, the actual focal length of a lens in a P&S Digicam and the '35mm equivalent focal length' are very different.

If you never need to move back and forth between two dSLRs with different crop factors, then 'Crop Factor' doesn't mean anything to you. It's mostly a crutch used by people with considerable experience with film SLRs, so they can relate what they know about lenses on their old cameras to what they can expect on their new ones. But if you only have one dSLR, or all your dSLRs have the same size image sensor, then 'Crop Factor' is of little value to you.

One last thing. Aperture is also a physical property of the lens and is unaffected by 'Crop Factor'. An f/2.8 lens on a 'Full Frame' dSLR is still an f/2.8 lens when it's mounted on an APS-C dSLR.
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Old Mar 18, 2010, 6:03 PM   #2
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Also using the 35mm equivalent allows us all to have a standard description for field of view, as it can get very confusing especially with P&S cameras where the lenses are very short but have a comparatively narrow field of view.

So there is a place for talking about the 35mm equivalent, but there a lot of times where it doesn't help so it is a case of using this knowledge to best talk about what is going on.

Thanks TCav for putting this together.
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