Extension tube

[Crecybattl]

I'm not familiar with "extension tubes" but read an article that mentioned
using one. B&H site shows some for 35mm but doesn't explain what they're for.
Is this for macro photography?

[James Robinson]

Extension tubes are used for close-up photography. They will allow you
to mount your lens anywhere from a few mm to about 8 cm from its normal
position, depending on how many of the tubes in the set you use. This
allows you to focus on objects closer to the camera, to the point where
you can fill the frame with something as small as a coin.

You can simulate the effect by removing the lens on your camera, and
holding it slightly away from the mount while looking through the
viewfinder.

The advantage of extension tubes is that they are inexpensive, and allow
you to use your standard lens for closeup work. The disadvantages are
that any lens imperfections will be magnified, you lose a substantial
amount of light, and the depth of field will drop to almost nothing.

[Crecybattl]

Thanks. That's the most clear and concise answer I've seen on this board.