Photographing at the µmeter scale

Started Feb 6, 2007 | Discussions thread
MOD Marianne Oelund Veteran Member • Posts: 7,868
How far can you go? Let's see . . .

Tom Christiansen wrote:

The shot was at 1/15s at ISO 400 with an offboard SB-600. The
prime lens
was a 70-300VR zoomed to 300mm @ f/5.6, with a a 50mm f/1.4 @ f/11
mated to the front of the prime using a reversing ring to allow for
male-to-male coupling, except where prohibited by law.

Since reproduction ratio is the quotient of the focal length of the
prime lens divided by that of the close-up lens, this gives

Wait a minute - since you used a male-to-male coupler, wouldn't that necessarily dictate that the reproduction ratio will be zero?
Oh, sorry, just couldn't resist!

It's not a very good shot at all, but just think about the scale:
Isn't that just incredible even to consider?!

While I don't pretend to be able to truly photograph reality at the
micrometer scale, still, it approachs much closer to that point
than I'd ever dreamt I could.

You won't have to go much further to find something else fascinating: The effects of the wave nature of light itself. Here is a photo of a small portion of a memory IC. This full D200 frame corresponds to an area on the chip that is about 0.8mm x 0.55mm (please pardon the dust - it wouldn't blow off).

The photo isn't exactly tack sharp. Why is that? Let's take a closer look at the features in this 100% res crop:

What on earth are those horrible distracting rings around all of the small highlights? And why can't we see any detail smaller than about 10 pixels?

At the magnification used, about 28x, one pixel corresponds to about 0.22 microns. Let's zoom in on one of the brighter ring structures and take some measurements:

The square box I've put there shows a 6x6 micron area, which is about the size of a D200 photosite, to give you an idea of the scale! The circle-and-rings pattern we see here is actually the result of light diffraction through the aperture of the lens that I used. The lens was set to f/2.8, and since the dominant wavelength is about 600nm, theory says the size of one cycle or line-pair at the diffraction limit should be 4 microns (it corresponds to the diameter of the first dark ring). Sure enough, this agrees with our observation above.

This diffraction effect will make any object in this photo that is about 2 microns or smaller, appear to be several microns in size. You can't see detail smaller than that, because the finite wavelength of light can't transmit that much information through the lens aperture. Replacing the lens with a better one won't reveal any more detail, if it's still used at the f/2.8 aperture.

If we stop the lens down, our ability to see detail will become even worse. The size of the central disc increases in direct proportion to the focal ratio, so by the time we reach f/16, the disc will be about two D200 photosites wide. This is why, when taking photos with any high quality lens, you will start to see diffraction softening of your D200 images starting around f/16 (or even f/11 if you're a very careful observer).
Isn't physics fun?

  • Marianne

P.S.: Two atta-boys will be officiallly awarded to anyone who can guess which Nikkor I took the pictures with!

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