crop factor

Some photo stuff: Crop factor.

Crop factor seems to come up a lot. Let's start from the beginning.

Crop factor is caused by the fact that the sensor in many cameras is smaller than 35mm film was.

If the sensor is APS-C (NEX-3, NEX-5, Nikon D7000, Canon 5xxD, 7D, x0D, all Pentax dSLRs) then the crop is generally around 1.5x, as the sensor is 2/3 the size of full frame.

With me so far? good.

The issue is that the viewpoint and field of view of the LENS does not change, but because of the smaller sensor size and the therefore restricted view shown to the sensor, it appears as if the lens "got longer". It didn't, it just looks that way. As part of the package, you get effects on depth of field, too (smaller sensor = more of the pic in focus).

Positive: long lenses "get longer". So you'll see a smaller cutout but it will still be detailed. This is good.
Negative: short lenses also get longer. So really wide angle is no longer possible, as you're always working longer than on full frame.

What this is useful for is one thing, and one thing only in my book: comparing what lenses from different systems will do in terms of one another.

Pancake 16mm on APS-C is FF-equivalent of 27mm or so. Pentax' 24mm --> ca. 36mm.
On Micro-4/3 (so a GH-1), it's Pancake 20mm --> 40mm as the sensor is smaller. This is *really* nice to know as otherwise you might be buying what you take as being a "wide" lens and it's really not that wide at all. Back in film days, 28mm was considered "normal" - so that's around an 18mm in crop terms - and anything lower than that was wide. It's not changed that much.

I quite like the Pentax Q way of dealing with this issue, removing the mm numbers from their lenses. I presume the idea is to reduce confusion, but whether it will work or not is another story....

If you want to take, say, car pictures from up close and don't mind distortion, you'll need something shorter than 18mm. 10mm will distort, anything much lower than 14 will visibly distort. If you can live with this, go for it.

If you need examples, go check out pbase and search on the lens. You'll see exactly what it's capable of. I'd strongly recommend this for any prospective purchase.

Another decent article on this (including pics): http://en.wikipedia.org/wiki/Crop_factor

late addition: http://www.kenrockwell.com/tech/crop-factor.htm


- Bret