p.1 #2 · How can you photograph the inside of a lens?
A macro lens and manual focus will let you focus on the level you want to show. For dust, lighting is the real issue. I find the best lighting varies (probably depends on how reflective the dust particles are, their size, location in the lens, etc.)
As a general thought I've found dust close to the front or rear element usually shows up better by reflection - light shined in at a 20 degree angle or so. For deep dust transmission shining through the lens may show it more.
Either way, make sure you have the most diffused light possible - otherwise you'll see more glare and reflections from the light than dust. I've had a buyer claim I carefully positioned a light glare to hide the worst dust area (like I have that much time on my hands).
All that being said, trying to demonstrate dust for a potential buyer is a tough thing. Lit properly and with a little luck your photo will show far more dust than the buyer will probably see just looking at it. It's just as likely they'll think your photo didn't show how bad the dust was.
p.1 #3 · How can you photograph the inside of a lens?
+1 on what Roger said.
I've had brand new out of the box Nikkors that looked awful when a super bright LED flashlight is shined through them, though by eye with normal illumination they look fine.
How much is significant dust is a relative thing. It can be shown to look horrible, though in practice it won't affect performance. Isolated specs are fine, but a coating of fine dust is not good.
p.1 #4 · How can you photograph the inside of a lens?
Mykal, did a potential owner ask specifically, or are you thinking of just doing this as part of a listing? If the latter, are you sure you want to go there? As the above two posts articulate, you could be showing the lens to be worse than it really is, or even have a buyer claim that you hid stuff. Maybe best to sell the lens with the typical descriptions and photos and skip the dust photos?
