quesiton about article "Understanding DOF"

[monkeybiz]

The original article url is Understanding depth of field, where the author said:

"This means that a normal lens for a format that small is as short as 15mm. A 15mm lens at f/5.6 has Depth Of Field from about 2.5 feet to infinity. Not too much opportunity for selective focus, is there?"

But ealier in the article, it has been stablished that focal length does not change DOF. So does this paragraph say otherwise?

Thanks.

[Jonathan Wienke]

The original article url is Understanding depth of field, where the author said:

"This means that a normal lens for a format that small is as short as 15mm. A 15mm lens at f/5.6 has Depth Of Field from about 2.5 feet to infinity. Not too much opportunity for selective focus, is there?"

But ealier in the article, it has been stablished that focal length does not change DOF. So does this paragraph say otherwise?

Focal length has no effect on DOF if you adjust the subject size in images shot with different focal lengths to match. But if you do not do this, DOF is inversely proportional to the square of the focal length. The other thing to consider is that shorter lenses have smaller apertures than longer ones (f/4 on a 20mm lens is 5mm, f/4 on a 200mm lens is 50mm), and ratio of aperture diameter to subject distance vs. sensor size is the ultimate determinant of DOF.

[howiesmith]

From: www.cambridgeincolour.com/tutorials/depth-of-field.htm

"Note how I did not mention focal length as influencing depth of field.  Even though telephoto lenses appear to create a much shallower depth of field, this is mainly because they are often used to make the subject appear bigger when one is unable to get closer. If the subject occupies the same fraction of the viewfinder (constant magnification) for both a wide angle and a telephoto lens, the total depth of field is virtually unchanged.  This would of course require you to either get much closer with a wide angle lens or much further with a telephoto lens, ... " [Emphsis added]