Quote:
Originally Posted by inkista
Well... technically that's not true. Focal length doesn't actually change the DoF. But in the image it looks like it does, so it's generally taught that way. You'll notice I was weasely and wrote "to increase out of focus blur," not "to decrease DoF". 
You'll note, however, that 6x is also the crop factor for a 1/2.5" sensor.
The best explanation I've seen of sensor size and its effects on noise, field of view, dynamic range, DoF, and diffraction effect is the one on Cambridge in Colour. |
The article you linked makes me stabby. The premise is almost completely worthless because the author demonstrates an entirely different physical property of light in order to make his premise.
Your statement, "Focal length doesn't actually change the DoF" is 100% false without the weasel words the author includes: "if the subject remains the same size".
That's because the depth of field is entirely the result of three variables, for any given sensor size:
A: Distance to Subject
B: Focal length of lens
C: Aperture size
The author is pretending focal length doesn't matter by changing the distance to subject. That's a perfectly fine thing to do, but only if you KNOW that's what he's doing. I repeat again: He's holding his depth of field constant when changing one variable ONLY by a change in another.
For the question that was asked, it's a VERY important bit of information because with a point and shoot you ABSOLUTELY WILL change your depth of field by zooming in and out. That's why with a point and shoot you absolutely want to use the maximum zoom that will contain your subject from the distance you're at if you want to minimize your depth of field.
All the article you link says is that if you zoom in, and then back up, or zoom out and then move forward, you'll cancel out the change, which is true and good to know.