How to shoot macro with Canon 17-40L pan/tilt

First of all, 17-40L is not a macro lens, but a close-up at most.

Shooting in the mode set by the camera, such as P mode and macro mode, has operational limitations. Once the light is dimmed, the flash will pop up automatically. You can use aperture priority or shutter priority to shoot, set the aperture size or shutter speed yourself, and the flash will not pop up.

To correct your statement, in fact, even if a polarizer is used, it will not eliminate the reflection caused by the flash. The best angle of polarizer is 45. No matter whether there is a lot of polarized light in the flash, even the light reflected from the front can not be eliminated.

If you focus manually, it is really difficult to focus. The reason is that the focal length is very close and the depth of field is very small in close-range and macro shooting, and the subject or photographer himself will lose focus and blur if he moves a little. It is recommended to use tripod and small aperture to reduce jitter and increase depth of field. When shooting hand-held, ISO can be properly improved to increase the shutter speed, which can also reduce the influence of jitter.