### determining point focused on from bellows extension

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I can I tell what point I am focused on from my bellows extension. The only lens I have is a 150MM. It appears that I need to know this to use any of the depth of field tables

-- marc fleischman (marcfleischman@msn.com), December 30, 1999

### Answers

Are you trying to determine focus distance using bellows extension as a yardstick? That can be tough; it's usually easier to measure with a tape measure (at closer distances) or guesstimate (at greater distances) the distance to the portion of the subject that's in focus.

Perhaps you could rephrase the question? The "I can I tell" phrase is particularly mysterious. . . .

-- Simon (fourthpres@aol.com), December 31, 1999.

I have answerd my own question. The formula I was looking for is: 1/focal length = (1/object distance) + (1/image distance)

object distance is the lens plane to real world object length

image distance is the lens plane to film plane distance (in other words the bellows extension

Thus with a 150mm lens and the bellows extended approximatly 6.3" you are focused at 10'

-- marc fleischman (marcfleischman@msn.com), January 02, 2000.

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