Crop Factors

Started Sep 20, 2008 | Discussions thread
Sean Nelson
Sean Nelson Forum Pro • Posts: 12,532
Re: No such thing...

Guerito wrote:

I would describe it as having a 35mm focal length equivalent of X.

...and in fact this is called the "Focal Length Multiplier". In other words, a 7.4mm lens on my Canon A650 with a sensor that is 1/5 the size of a 35mm film frame has a "Focal Length Multipler" of 5X, meaning it has a 35mm-equivalent field of view of 35mm.

And, in fact, there is NO DIFFERENCE between "Focal Length Multiplier" and "Crop Factor". They are both derived by dividing the 35mm film frame diagonal by the size of the smaller sensor. The result is the same number, and the number is used in the same way.

I wouldn't describe a Canon 5D as having a crop factor any more than I would
a Pentax 645 or an 8x10 sheet camera if they are using the lenses designed
for their native sensor/film size

And yet a Canon Digital Rebel with a kit lens designed specifically for that camera's sensor size is still described as having a 1.6X crop factor. And users refer to that crop factor when they say that the 18mm wide end of the lens is equivalent to a 29mm lens on a 35mm camera.

My Pentax DSLR has an APS-C sensor with a crop factor of 1.5X. The camera has the same crop factor whether I use a 100mm lens designed for an APS-C sensor or if I use 100mm lenses designed for 35mm or 6x7 cameras. If I glued the lens to the camera so that it couldn't be removed the crop factor would still be 1.5X. If I built a camera with a non-removeable lens with the same APS-C sensor then the same crop factor still applies. It has nothing to do with lens removeability or coverage and everything to do with the relative size of the sensor compared to a 35mm film frame.

(Crop factor can use other sensor sizes as the "reference size" as well - for example Pentax 645 users speak of the crop factor of that camera compared to a 6x6 or 6x7 film frame. But 35mm is the "de facto" standard and if people are using something else as a baseline then they're always careful to state that).

I've cited references and explained to the best of my ability. I'm apologize if you don't get it, but I've done my best.

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