View Full Version : bellows extension versus flange focal length?

13-Oct-2006, 07:01
This one is for the math majors -)

If the flange distance is a fair bit different then the focal length which number do I plug into the formulas to calculate bellows needed to focus a certain distance?

If the flange distance is 570mm I'd focus at infinity at 570mm of bellows right? Even if the focal length was 600mm.

Ole Tjugen
13-Oct-2006, 07:09
The flange distance is not an optical property of the lens. The focal length is.

So with a 600mm lens with a 570mm flange distance, you add the difference needed to focus a 600mm lens to X distance to the flange distance to get the needed extension at X distance. The offset between the flange and the front focal node is constant for any given lens.

Michael Gudzinowicz
13-Oct-2006, 07:10
Calculate the total extension using the focal length in formulas, and then subtract the focal length from the required extension. That value is the extension required beyond infinity focus. Add that value to the flange distance or any other distance measured on the camera when focused at infinity.

13-Oct-2006, 07:27
Okay that makes sense.

Thanks to everybody

Leonard Evens
13-Oct-2006, 13:04
The easiest way to do it is as follows. Focus at infinity and note the position on the rail or bed of the standard you are using to focus. Then focus at the desired distance and note the new position. Find the distance between those two points.

If you want to know the total bellows extension, just add that distance to the focal length.

If you want to know the magnification, divide that distance by the focal length.

If you want to know the fundge factor used to adjust exposure, add one to the magnification. If you multiply the marked f-number by that number, you get the effective f-number which should be used in exposure calculations. Alternately, use the marked f-number and multiply the time by the square of that number.

The rear flange focal length would not be used for any of these calculations. You use it to estimate how close you can focus with a given bellows draw.