[X] My Assistant

[Tman]

If there already was a discussion about, then I would gladly ask: where is it? Otherwise does someone know something about the object size in the pics, getting from Pancam and Navcam, in comparison with "seen by human eyes"? For example, probably the object sizes getting from Navcam are smaller as we would see it with our own eyes at the same position, but how much approximately? And how about Pancam?

Primarily I would like to know the "real" size of sun by the sunset seen from Spirit on sol 489 (Pancam): http://www.unmannedspaceflight.com/index.p...indpost&p=10917

[djellison]

Ahh - I thought it was more complicated that that

Doug

[dvandorn]

Ahh - I thought it was more complicated that that

Doug

It is a little more complicated than that.

I hate to disagree with the explanation above, but the way *I* always heard it was this:

The first factor in the ratio is, indeed, distance from an eyechart. The *second* factor is point size of type.

Someone with 20:20 vision can't resolve type smaller than 20-point from 20 feet away. Someone with 20:40 vision can't resolve type smaller than 40-point from 20 feet away. If you have 20:200 vision, you can't resolve type smaller than 200 point from 20 feet away.

There are some people with 20:15 and even 20:12 vision. This is extraordinarily good vision -- it means they can resolve 15-point or even 12-point type from 20 feet away.

As you can see, this allows the ratio to actually measure visual acuity. If it were simply a comparison between better and worse vision, as suggested above, it would have no absolute value and therefore would be a rather meaningless measure.

The "20:20" thing doesn't have anything to do with apparent size of objects, anyway. That's governed by the degree of curvature of the camera lens. I know that in regular photographic equipment, objects appear in fairly normal perspective (i.e., as large and small, by distance, as they would to a human eye) when you use a 50-mm lens. A wide-angle or "fisheye" lens, of 35-mm or less, makes objects appear farther away than they are (smaller at a greater rate with distance than we see with the eye). A narrow-angle, 75-mm up to 500-mm or more, makes distant objects appear much closer than they are (smaller with distance at a lesser rate than seen by the eye).

-the other Doug