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What's the difference between focal length & "focus distance"?

What's the difference between focal length & "focus distance"?

What's the difference between focal length & "focus distance"?

I am building a viewer along the lines of a View Master, and I've been having an issue with various lenses I've been purchasing online.  The lenses are essentially for the purpose of being able to focus on images that are too close to the eyes to otherwise focus (as opposed to using them for magnification).  I'm using a single DCX lens for each eye (I was told that these are preferable  to PCX lenses in this usage - is that correct?).

I've purchased lenses from several places including Edmund Optics and Surplus Shed, and they of course list the lenses by focal length and diameter.  But the problem I've been experiencing is that with the lens held close to the eye, objects that are the stated focal length away from the lens are not sharply focused.

In fact, the actual distance from the lens to the image that achieves sharpest focus is always less than the stated focal length.  For example, on a lens with a rated focal length of 6 3/4", the actual focus distance is about 5 1/2"; for one with a rated focal length of 2 3/8", the focus distance is about 2 1/4".

I am measuring this by looking through the lens held against the end of a ruler, and sliding a printed card back and forth until I find the sharpest focus and measuring the distance.  I've had a number of people verify this, including children, to eliminate any possibility of it being a vision defect, and the results are always about the same.  In addition, I verified that the stated focal length is correct by imaging the sun against a white card and measuring that distance.

I currently have a mockup which was designed around a specific distance from the lens to the image, so I'm hoping to find lenses that matches that.  I have two questions:

1.  What is the explanation for this difference between the stated focal length and the distance to sharp focus?
2.  Is there any formula which would allow me to derive the sharp focus distance given a set focal length, so that I don't have to randomly purchase a bunch of lenses and test them all out?

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