Lens Conversion Factor
Table of Contents1. What is Lens Conversion Factor? 2. Why do I Need to Know Lens Conversion Factor? 3. Bottom Line
Of course there have always been 'lens conversion factors' but before, in the days of film, no-one really talked about them. Those who needed to know knew and those who didn't got on with their lives in blissful ignorance. It has only become a talking point now because we have switched from one very popular format, 35mm film cameras, to another very popular format, the APS-C sensor.
What is it?
OK here comes the sciency bit. The 35mm film surface area was 36mm x 24mm which meant that a standard lens had a focal length of 50mm. A standard lens is defined as one that gives a view through the camera equivalent to how we view the scene with our naked eye. In other words we do not appear to be closer or further away from our subject when we view it through the lens, objects in front of us look about the same size.
The APS-C sensor that we have in our DSLR cameras (except the most expensive ones) is quite a bit smaller (16.7mm × 25.1mm) than the 36mm x 24mm of the old film. This means that the standard lens (as defined above) will be a shorter focal length, somewhere around 31mm. The difference between these two numbers is called the lens conversion factor. So, if the camera has a standard lens of 31mm, then compared to a 35mm camera it has a lens conversion factor of 1.6.
In fact it gets a bit more complicated because different manufacturers have different sized sensors. So the conversion factor I mentioned above,1.6, is for Canon cameras. Nikon have a lens conversion factor of 1.5 which means their sensors must be slightly larger than Canon's, and Olympus have a lens conversion factor of 2.0, which means that their sensors are the smallest.
Why do I need to know?
If you never used the old film cameras you probably don't really need to know unless you splash out one day on a full frame camera like the Canon 5D, where you will suddenly notice that your 40mm lens is no longer a telephoto, it is now a wide angle. For those of us who cut our teeth on 35mm cameras and are used to the focal lengths and what they look like through the viewfinder, the new cameras come as a bit of a shock at first. Suddenly our 200mm lens is behaving like a 300mm lens and, in order to get a decent wide angle, what used to be 28mm, now needs to be a 17mm lens. A focal length almost unheard of in the old days is now the lower end of a mid range zoom.
So the number to remember is 1.6 if you are a Canon owner or 1.5 for Nikon. If you have a different brand, look it up in your manual. Then apply these conversions to the focal length you think you need to arrive at what you really need. If you're coverting from 'old money' to new, divide by 1.6, to get back to the old days from the new focal length, multiply by 1.6.
Old 200mm lens would be 200/1.6 = 125mm on a new APS-C camera.
New 200mm lens would be 200 x 1.6 = 320mm in old 35mm thinking.
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What you need to know when choosing a new lens.
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What you need to know before you go shopping.
More of a 'why you need a tripod' than a buyers guide, but it does include some tips on buying and using a tripod.
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