Here's an interesting object: a piece of coral
[PLAIN]http://img41.imageshack.us/img41/8051/dsc4663c.jpg
As I mentioned, I've been playing with focus stacking. The human eye operates between (about) f/3 and f/8.5, depending on the pupil diameter. Recall, images look most natural when using 50mm focal length lenses on a 35mm format image: you can verify this for yourself, by simply looking through the camera viewfinder with one eye and keeping both eyes open. When I do this, I almost can't tell I am looking through a camera with one eye.
Using the 50mm lens/35mm image combination, f/8.5 produces a hyperfocal distance of about 32 feet- that is, everything between 16 feet and infinity is in focus. A relaxed eye focuses on distant objects, and so we have been accustomed to seeing most things in focus.
At high magnification, low f-numbers (high numerical aperture) is required in order to resolve detail. For example, a 4X objective may have an na = 0.1 (f/4.5), while a 16X objective typically has na = 0.5 (f/1) and higher. My 63X immersion objective operates at f/0.34.
The result is that 'microscopic' imaging produces images that don't look 'normal'- the depth of field is highly restricted. For flat objects, this is not a problem. But for 3-D objects, like the coral above, going to a moderate magnification gives images like this (these are all full-frame images):
[PLAIN]http://img28.imageshack.us/img28/4584/dsc4666p.jpg
[PLAIN]http://img195.imageshack.us/img195/1466/dsc4665s.jpg
and at higher magnifications:
[PLAIN]http://img844.imageshack.us/img844/4102/dsc4668b.jpg
[PLAIN]http://img832.imageshack.us/img832/6193/dsc4667.jpg
There's nothing wrong with these, but they don't look 'natural'. I could stop down the lenses used for some of these images (the top one was taken with the 100mm luminar, the middle two with the 63mm luminar) to improve the depth of focus, but only because the magnification is low- to have the aperture begin to conflict with the resolution limit would mean I operate the lenses in excess of f/300, beyond the capability of the iris. The bottom two were taken with the 25 mm luminar, and the thin depth of field is a dominant feature of the image. I can't stop down the 25mm much past f/16, and that still only gives me a thin depth of field in addition to causing resolution problems. The 16mm luminar doesn't have an iris, so I have no ability to control the depth of field with that lens. Here's an image is of a 'thing' I happened to notice in one of the 25mm images (top right corner). I used the 16mm luminar at about 30X magnification (the lens was about 2 feet from the camera), and here's a single frame from the stack:
[PLAIN]http://img820.imageshack.us/img820/165/dsc5066.jpg
Again, the thin depth of field is so dominant, the image appears unnatural. So I turned to focus stacking when going up to the 25mm and 16mm luminars to generate images like this:
[PLAIN]http://img141.imageshack.us/img141/903/newout99994.jpg
[PLAIN]http://img710.imageshack.us/img710/1640/newout99991.jpg
[PLAIN]http://img153.imageshack.us/img153/4236/newout99992.jpg
[PLAIN]http://img18.imageshack.us/img18/4168/newout99998.jpg
There are definitely some artifacts from focus stacking, and there's room for improvement as far as contrast goes, but clearly focus stacking is a useful tool.
As an aside, does anyone know what that thing is? It's about the size of a period ("."'), and I think it's from the ocean (the coral was sitting out on a desk for decades), and organic in origin- I don't think it's a piece of netting.
