Why are microscope lenses so small?
September 3, 2011 9:31 PM   Subscribe

Why are microscope lenses small and camera lenses large? Surely having large lenses on microscopes would let you gather more light from the thing you're trying to examine and give you a clearer picture.
posted by Joe in Australia to Science & Nature (9 answers total) 1 user marked this as a favorite
The larger lenses on a camera actually make getting a clear picture harder to do. When a camera lens is fully open, the amount that is in focus is very small. To get a very clear picture from a camera, especially at close distances, you increase the aperture value, which in essence closes the lens significantly.

The main benefit (other than aesthetic) of a large lens is low light capability, but since you don't need that with a microscope, a smaller lens allows for better sharpness.
posted by markblasco at 9:33 PM on September 3, 2011 [1 favorite]

Smaller objectives facilitate the use of immersion oil to get a higher resolution than regular objectives.
posted by Blazecock Pileon at 9:54 PM on September 3, 2011 [1 favorite]

A small lens is also less subject to chromatic distortion.
posted by Chocolate Pickle at 10:09 PM on September 3, 2011 [1 favorite]

Light-gathering power of a microscope objective (or any lens) is governed by its numerical aperture (NA). Modern microscope objectives have numerical apertures essentially at the theoretical limits, so they can't gather any more light than they already do (without doing exotic things like putting your sample in a circular tube and making a lens that surrounds it).

Therefore, the main purpose to making a physically larger lens would be to image a larger field of view. Most high magnification / high NA lenses have small fields of view (maybe 200 micrometers on a side for a 100x / 1.4 NA lens). The field of view is small partly because people don't often need a larger field of view (cells are small) and partly because distortions get harder to correct over larger fields of view. On high-end research microscopes, you get around this problem by putting your sample on a motorized stage and stitching multiple fields of view together.

The depth-of-field issue that markblasco raises is a double-edged sword. Sometimes it's nice to see the whole thickness of your sample in focus at once, but often it's nice to only have a thin layer in focus. That way you can scan through different focal planes to reconstruct a 3D view of your sample and figure out which objects are above and below other ones. Using a high NA objective automatically gets you a thin focal section.

Finally, imaging in low-light is actually critical for high-end biological microscopy. We often use fluorescent stains to image molecules present at only a few hundred copies per cell, and to see these, every bit of light gathering power helps. With a high-end microscope and a few tricks, it's even possible to see single fluorescent molecules, and this is routinely done.

This is what I do for a living, so if you have other questions, just let me know.
posted by pombe at 10:30 PM on September 3, 2011 [18 favorites]

Also, there are some objectives designed with high NA and low magnification, and just as you'd expect, they're bigger than their high-NA, high-magnification counterparts. For example, here and here (Unfortunately I can't find any side by side pictures to illustrate).
posted by pombe at 10:46 PM on September 3, 2011

Oh well. It looks like my idea of using old camera lenses to build a ginormous microscope is foiled by physics. Curse you, natural laws!
posted by Joe in Australia at 11:00 PM on September 3, 2011 [1 favorite]

Sounds like maybe you shouldn't do important research with it---doesn't sound like you shouldn't build it and see what it does, though!
posted by TheRedArmy at 8:14 AM on September 4, 2011

To get a very clear picture from a camera, especially at close distances, you increase the aperture value, which in essence closes the lens significantly.

While most of markblasco's post is correct, this is an absurd simplification. Aperture size vs. clarity is entirely dependent on which lens we're talking about. And few (if any) wide aperture lenses will be "clearer" at f11 than they are at f2.

However, the larger the lens, the more difficult it becomes to make it optically perfect.
posted by coolguymichael at 10:57 AM on September 4, 2011

Draw a cross-section sketch of a large lens close to a tiny object. Now sketch the paths that light rays would need to follow to get from the object through the lens to a focal point on the other side. You'd need hell's own refractive index to achieve those paths near the edge of the lens, yes?

And if you increase the distance from the object to the lens to get a more reasonable set of ray paths, you've just lost your extra light-gathering power and you might as well just shrink the whole lot down conveniently small again.
posted by flabdablet at 7:00 PM on September 4, 2011 [1 favorite]

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