Hyperfocal Distance for D70?

Your assumption is wrong. Hyperfocal distance is a factor of focal length not sensor size.

But doesn't the smaller sensor size change the effective focal length (by maybe 1.5)?

For these purposes, no. Think of it as a crop instead of a change in focal length and you'll be pretty much there. In other words, a 24mm lens has the same hyperfocal properties, but you only see the angle of view you'd see on a 36mm lens (if there were such a thing).

Given all the complaints about increased depth of field with crop factor digital SLRs you would think it clear that the sensor size affects the depth of field.

First of all depth of field (and hyperfocal distance) are not absolutes. The depth of field is based on a a concept known as circle of confusion. The circle of confusion is roughly the largest spot on the sensor that will be seen as a point on the final print (and hence is related to the size of the print). Fixing a print size the digital image must be enlarged more to get the same size print. Thus the circle of confusion for the digital is smaller than for film.

The hyperfocal distance is where the depth of field is infinite. It depends on the circle of confusion, the focal length, and the f stop.

The usual optic formulae for simple lenses gives H = F^2/ (f*c) where F is the focal length, f is the f stop and c is the diameter of the circle of confusion. Since the circle of confusion (for a fixed spot size in a fixed size print) is smaller for digital this means the hyperfocal distance is longer. The diameter of the circle of confusion is 1.5x smaller for the Nikon so the hyperfocal distance is 1.5x longer.

It is true that you crop but then to get to the same sized print as before you enlarge more. This makes previously "sharp" areas appear "out of focus" and you no longer have infinite depth of field.

Bob Atkins has an excellent tutorial on depth of field.

Why compute it when you can wear it?
http://www.cafepress.com/nikonians/383701

-- Jon

Good answer Jon !

Alistair, Thanks for your good explenation. You conclude that "The diameter of the circle of confusion is 1.5x smaller for the Nikon so the hyperfocal distance is 1.5x longer." but is not the hyperfocal distance shorter for the D70? A little math:

H=F^2/(f*c) can be rewritten to H*c = F^2/f.

When comparing two systems with the same lens we get:

H1=H2*c2/c1 where H1,c1=35mm system, H2,c2=D70

There the circle of confusion should be 1.5 times smaller for the D70 c1=1.5*c2

Thus H1=H2*c2/(1.5*c2)

H1 = H2 / 1.5

in words the Hyperfocal distance the D70 (H2) is 1.5 times shorter than that of a 35mm camera (H1)

Paul

AFAIK the hyperfocal distance formula is H = f^2/Nc+f where f is the focal length, N the f number and c the circle of confusion (CoC).
On this basis if one divide the CoC by 1.5 the hyperfocal distance must be multiplied by 1.5 if i understand well.
Then if the D70 CoC is 0.02 against 0.03 for a 35mm camera (0.03/1.5= 0.02), its hyperfocal distance is 1.5 times longer IMHO.
Best,
LCT

... But to compare apples to apples we must take into account the different field of view (FoV) of the D70 and the 35mm camera.
For example, we have to mount a 50mm lens on the D70 to get the same FoV as a 75mm lens on the 35mm camera.
Then at f/11 for instance we get an hyperfocal distance of 37.4ft for the D70 + 50mm lens against 56.1ft for the 35mm camera with 75mm lens on the grounds of the above formula.
This way the D70 hyperfocal distance is finally 1.5 times shorter that that of the 35mm camera.
Best,
LCT

Great, now I am really confused!

Let's forget about the math and be practical. If I take one of my lenses which show the DOF markings, what do they mean when I put this lens on the D70?

My feeling tels me that if I use these markings, the final photo will not be as sharp as I wanted. right?

Paul de Vries wrote: "If I take one of my lenses which show the DOF markings, what do they mean when I put this lens on the D70?"

You'll get a good approximation if you look at the DoF markings of the nearest slower f stop of your lens i.e. the f/16 markings when you choose f/11 for instance.
Best,
LCT