Crop Factor

[My old Digital Rebel is a crop body camera (sold already)]

Ray wanted me to write some blog entries in English couple of years back. I didn't feel very comfortable about it. But recently when discussing things with Jess and Jon, they've never picked on my poor English so that's big encouragement. I also thought, if my English is really hard to understand, Ray can be the translator, he must understand my Chinese-style English.

I actually wanted to discuss Canon lens lineup in the very beginning, but in the middle of it, I found out it's very hard to talk about lenses without talking about digital camera bodies first.

You can classify camera bodies from many different points of view. One way is to base on how big the camera sensor it is. In this context, the reference point is the size of a single frame from the traditional 35mm film. If the digital sensor has the same dimension as of a frame of 35mm film, we call that camera body a "full frame" camera. If the sensor has a smaller sensor than 35mm film, we call that camera a "crop frame" camera (more often, people just say "crop body") camera. So full or crop frame really only make sense in the context of digital cameras.

For crop body, there is always an attribute attached to it: crop factor. This is a ratio of the dimensions of a camera's imaging area compared to a reference format (35mm film). This is hard to understand. So an easy way to think about it is: Considering you take a picture with an apple in the middle of the frame. No matter you are using full frame or cropped body, the apple will be in the middle. However, the surrounding area that would be included in the image will be different. Full frame of course include more surrounding things. Now think about the image captured by the crop sensor. The effect will be like you first frame the image you want(full frame image) and then you zoom in a bit more (so you exclude some surrounding things), right?

That's why people told you when you have crop body, your 50mm lens become "longer". With Canon, the crop factor is 1.6x, so your 50mm lens becomes 80mm lens.

So far only 5D family and 1Ds family in Canon lineup are full frame cameras. Other than that, 1D (crop factor is 1.3x), 7D, X0D, XX0D (Rebel family), and XXX0D (XS) are all crop cameras. Nikon uses FX and DX to refer to their full frame and crop cameras. For DX, it's crop factor is 1.5X (since Nikon use a slightly bigger sensor than Canon's).

It's all about the application, full frame body is not necessarily better than crop body. When covering the indoors performance events, in order to gain more reach, I used my 30D (crop factor of 1.6x), so my 200mm lens becomes 320mm right away. Nice, eh? Crop body are also cheaper since the bigger the sensor, the more expensive the body.

Next time we will talk about Canon's lens lineup. You can decide which one should be your next lens to own. Since we talked about crop body vs. full frame, you probably figured out something already. Yes, some lenses can only be used for certain bodies...