Focus distance

[Swanick]

Is there a relationship between the focal length of a lens and the range or maximum distance the camera can still compensate for focus. The distance scale on my lens is from 4 to 50 ft with a little space before infinity. If I set my zoom lens at 28mm what is the range in feet that the camera can still compensate for focus before reaching the infinity setting. Secondly is this the point that most photographers will use the hyper focal point for focusing assuming you want the best depth of field? Changing the zoom lens to 135mm the impression I have is the camera can still compensate for focus over a greater distance than the 28mm before reaching infinity.

Is there a formula that will tell you the range or maximum distance that the camera can still adjust for focus given the lens focal length. Hopefully this is not a stupid question.

[Jon]

What do you mean "compensate for focus"? Are you referring to depth of field, by any chance (how wide the zone is that will appear to be in focus when you're focussed on a specific point)? if so, that depends on the focal length of the lens, the aperture you're using, the size of the sensor/film, and the degree of enlargement you're going to make. There's a depth of field simulator (http://www.liquidsculpture.com/dof.htm) available for you to play with if you ike. There are also sites that will calculate depth of field tables for you to print out for reference.

[Swanick]

I am not referring to the depth of field. I have found on the wed formulas for determining the depth of field given the aperture, focal length and F-stop. I have created a matrix in excel showing the near and far depth of field from 2 to 200 ft. Changing the aperture, local length or F-stop will change the values in the matrix. 200ft seems to be over kill for the 28mm but not for the 135mm. It is very interesting to see the change in depth of field using a zoom lens. If you don’t realize this parts of the pictures may be out of focus when using a longer focal distance.

[Jon]

Then, until you can better explain what you mean by "compensate for focus", I don't know how to answe your question.

[Swanick]

Reading my posting the term compensate was probably the wrong term to use. At what distance can a 28mm lens focus on before reaching the infinity point? The answer I am looking for is something like this; Up to 66 ft. after which the cameras focus is on the infinity point. If you read the second to the last sentence of the origional posting my question is better stated.

[Jon]

Oh, that's the Hyperfocal Distance. Strictly, the hyperfocal distance is that point at which the lens is focussed on when the rear depth of focus first reaches infinity. At that point, front depth of focus will be about 1/2 the distance focussed on. Many DoF charts will also provide this, and lenses with focal length scales will include a mark for this at various apertures. It varies with the aperture and focal length both. But your lens doesn't jump its focussing point in steps; regardless of the focal length, focus point varies continuously from point of closest focus to infinity, and always (with the exception of certain specialized lenses) goes at least to infinity.