The Illusion of Crop Factor

It took me a long time to understand this...

Your digital camera has a crop factor. It's likely that when you bought it, the salesperson said something like "it makes your 200mm lens like a 300mm lens."

And that's the big misconception - even for sales people. That person likely thought they were telling you the truth. But they weren't.  There's not another piece of glass between your lens and the sensor, and that's the only way to increase the focal length of a lens it add more glass, like a teleconverter. (Note that Sony does have a digital teleconverter in some of their cameras, but it does not related to crop factor.) 

You're 35, 50, 85, 105, 70 to 200mm zoom and whatever else is still just that, no matter what camera you put it on. 

Why the misconception? The decrease in image area on a cropped frame sensor camera gives the illusion that your lens is zoomed in closer to the subject.

That's it.

It's an illusion. 

This picture is a quick snapshot measuring 4" X 6". The yellow border represents the full frame, and the white inset frame a crop sensor. Note that in either case, the off spring is the same size. If a cropped sensor actually increased zoom, she'd be bigger.

This picture is a quick snapshot measuring 4" X 6". The yellow border represents the full frame, and the white inset frame a crop sensor. Note that in either case, the off spring is the same size. If a cropped sensor actually increased zoom, she'd be bigger.

Don't believe me? Find an old 35mm film slide or 4 X 6 photograph. Those are considered to be full frame shots.  (Or tear a picture out of a magazine.)

  • If you're using a slide, consider removing 12mm from the long side and 8mm from the short side (roughly 1/3 of each side). 
  • If your using a photograph, consider removing about 2 inches from the long side and 1-1/3" from the short side (again, roughly 1/3 of each side).  (Yeah, I know it's not exactly a 4 X 6 print, but you'll get the idea.)
  • If you tore the picture out of a magazine, just cut 1/3rd off a long and short side.

Now, look closely - are you zoomed in or is the picture just smaller? 

It's smaller and that's crop factor. It's not zooming in closer than the actual focal length of your lens, the sensor is just capturing a smaller portion of the scene. (Most crop sensors only capture around 1/2 of scene area that a full frame sensor captures.) 

And that's what gives the cropped sensor the illusion of adding zoom.

(If anything, your photographs are pre-copped in camera.)