How to take a photo of defects on a mirror?

Question

Taking a picture of a reflected image of object is depicted in this image. Camera focus is on the image of a piece of paper.

Besides the depth of field (cheap Panasonic DMC-FX8 in makro mode, compact camera) I'm content with the picture.

Defects on the mirror

• The mirror has imperfections like small areas with missing golden mirror film.
• Defects near the rim of the mirror are hard to focus on. Autofocus focuses on the piece of paper instead the mirror surface. I understand that. Possibly manual focus should do the job. Maybe there is a trick to work around?

The Question

Is it possible to take the photo using the Panasonic DMC-FX8? If yes, how to focus on the defect area on the mirror?

Answer 1

Can you do shutter button half-press to focus? That is usually an easy workaround.

Just focus on a simple object with the same exact distance, half-press the shutter button, then recompose (keep the button half-pressed, target the camera on the mirror), then press the button fully.

Usually works with most cameras...

Another way is to put a non-transparent mark close to the mirror defect. Anything with a sharp contrast will do, a piece of paper, ink mark, etc. If it is very close, you will have nice focus on the defect as well.

Answer 2

To find defects on mirror surfaces you need to use coaxial light. I buy them from Advanced Illumination.

You can also make them yourself, buying one way surface mirror at Edmund optics and a light plate (uniform area light).

The best versions are made from cube prisms, though.

Example

Answer 3

In addition to TFuto and AJ Henderson's answers I can say a few things more.

To visualize surface defects you need to illuminate the surface with a very low angle from the surface. You can use single source as in the diagram but the best solution would be to use a ring light.

Answer 4

You can try to use a focal point selection and place it directly over something with relatively high contrast (such as a joint in the mirror). The trick is that the AF looks for whatever has highest contrast in the scene and focuses on that. Your camera is contraast detect auto-focus so any high contrast line should work, however if you had a DSLR with phase detect auto focus it would also help to use either a vertically or horizontally oriented feature because of how PDAF works.

This may still not matter though as what you are trying to do is very tricky for both the photographer and the camera. Manual focus is generally the most surefire way to deal with it.

Alternately, if you are using a tripod, you could put a piece of paper with a line on it over the mirror, focus the camera on the page and then remove the paper before taking the shot.