View Full Version : Question about lens measurements

2006-Aug-09, 04:26 AM
I'm trying to find a term that's used to measure a lens. Basically, it's the total height of the lens if you place it on the ground. So if I put the lens flat on a table, how height would it reach into the air? It's not the same as thickness, because you could have a very thin lens that is highly curved. I think it has something to do with "sagittal height", but that seems slightly different.

2006-Aug-09, 09:51 AM
Focal Ratio.
Focal ratio, or f number, is the focal length of a lens, or mirror, divided by its diameter. A focal ratio of 8 would be written as f/8.

An f/8 telescope is "slower" than an f/4. Fast telescopes give wider, brighter images with a given eyepiece than slower ones. In general, the slower the telescope, the more forgiving it is of defects in the objective / mirror and eyepiece.
From: http://www.r-clarke.org.uk/telescope_glossary1.htm

See Also : http://en.wikipedia.org/wiki/F-number

2006-Aug-10, 01:28 AM
Thanks, but I don't think that's actually what I'm looking for. I'm not looking for something to do with the focal point or anything like that, just the physical dimension that basically corresponds to the diameter, but in the other direction. So in other words, if you were to put the lens in a box, the length and width of the box would be the diameter of the box; how tall would the box have to be? It may be that the word I'm looking for is simply "height".

If it helps, the manufacturer (a Japanese company) uses ET for "edge thickness," CT for "central thickness," and the acronym "DH" for this measurement. I guess it would be something like "drop height"?

Edited to add part about acronym.

2006-Aug-10, 03:23 AM
Wouldn't it just be the center thickness (http://www.optipedia.org/wiki/index.php/Centre_thickness), with the apparent exception of negative power lenses (whatever that is)?

2006-Aug-10, 03:39 AM
No, because the thickness is the actual thickness of the lens itself. If I can give you an analogy, suppose you have a frisbee sitting on the ground. The center thickness would be the thickness of the plastic at the center of the frisbee. But I'm not looking for that. I'm looking for the thickness of the plastic plus the air that's under it (because of the rim).

2006-Aug-10, 04:00 AM
He means this:

No idea if there's a specific name for lenses or anything, but "saggital height" is the name for the height of the "cut" of a circle.

2006-Aug-10, 06:31 PM
Is that a "negative power lens"?

2006-Aug-11, 01:17 AM

Thanks a lot for the diagram. It's exactly what I mean. The value of d.