There are several ways to compensate for poor lighting or dark lighting:

1. adjust the ISO rating upward (This will increase noise in dark areas)

 2. increase the aperture; open the aperture as much as possible – to as much as F. 2.8 or whatever the largest setting your camera allows.  Keep in mind, however, that the best aperture may be f.8 or f.11 or higher - to provide a balance between the need for more light and sufficient Depth of Field.

3. decrease shutter speed; Slow the shutter speed to 1/10th or 1/15th of a second or less.  (Note that since we are unable to hold a camera still at 1/60 of a second or less, it is really necessary to use support).

4. Learn about your camera’s exposure compensation and adjust for brighter + or darker - stops as required.

5. use a flash.   Flash will allow greater DOF, because you can bring the F-stop down (higher number) Problems: when too close to the subject, get blowout; harsh shadows  Lens hood gets in way.  (Diffuser or weaken flash intensity so it matches natural light) ring light also works well. If you take your photo in the morning or late afternoon, when the light is low - assuming your mushroom is in decent condition - use daylight as your main light source  (on right or left). Then use an extension cable with the flash coming from the upper left (or right) and have a white card to reflect light into the shadows on the opposite side.  Use an aperture correct for the flash (say f/11).  The typical shutter speed when using a flash is 1/125 sec, which will result in a very dark background.  Shutter should be low at 1/2 to 1 second so the shadows will fill in and the background will be visible.

6. Try bracketing to give you a better chance of getting a properly exposed photo.  In this setting the camera will take a series of shots from under exposed to overexposed.  You can pick the best of the lot. 

7. A tripod is a necessity!  There’s no getting away from it.