Originally posted by wildman Based on my experience I don't see any significant difference in DOF if FL length is the only factor __and the subject is at the same __**apparent** distance.

Case in point;

Two shots one taken very close up (about a foot?) with a FA35mm and the other taken with my 560mm, plus extension to bring the focus closer, at about 15 feet or so. Both about the same size on the same bush. The only difference is FL and distance between the lens and the subject. However, and I don't know how much difference this makes, but the 35mm was at f5.6 while the scope has a fixed wide open aperture of f7.

I've looked at the formulae for depth of field, and for a given circle of confusion, your hypothesis is practical. Image size is almost directly related to focal length over quite large ratios.

The standard DOF formula commonly used states that depth of field is related to the aperture, subject distance, focal length and the desired apparent circle of confusion. The formula is be shown as

where

*t* is the total depth of focus,

*N* is the lens aperture

*c* is the circle of confusion,

*v* is the image distance, and

*f* is the lens focal length. The magnification of the image varies with v/f, and that indicates that for a given subject magnification, v/f is identical for both lenses, the 35 mm and the 560 mm, and the depth of field is identical in both cases. Going from 35mm to 560mm gives a relative change in focal length of about 16.5X, so you would need to be 16.5 times farther to obtain the same approximate image size, reducing the final term in the formula to the same value.

Quote: As matter of fact I have a Sigma 105mm macro but find the 560mm is better on butterflies than the 105mm because I can track their abrupt random movement better on the scope with the gun sight mounted on it. After all for any given lateral movement of the subject the angular movement is much smaller on the scope at 15 feet than on the 105mm at 12 inches.

My M 400/5.6 has a "gunsight" on it as well. It is a really simple but effective way to find the bird in the viewfinder.

Quote: Anyway take a look at these pics. Darned if I can see any significant difference in DOF between the two given the huge difference in FL.

If anyone else has any other theories about this I'm willing to listen.