Hair looks thinner under flash lighting. Does this mean my hair is thinning or not? (Photo)

Any good photographer can explain how light can show detail or hide it. Bright direct light will mask cellulite and facial folds and wrinkles while removing the shadows that hide the scalp under hair. Medical photography is generally most accurate with oblique lighting and a fixed macro lens. A good hair restoration surgeon can photograph you hair over time and provide density information. Generally, speaking, letting your primary care physician know that you have possibly noticed thinning hair is a good idea though I suspect your apparent thinning hair is lighting artifact.

All hair looks thinner under flash lighting. You are correct that patients often first become aware of hair thinning when they see themselves in different lighting conditions. However it doesn't necessarily mean hair loss. I always advise individuals who think they are loosing hair to take a photo and then see a hair specialist (in that order). Your photos will be a helpful guide over time as to what might be happening to your hair - if in  fact there is anything. 

I believe what you are seeing is the natural law of physics.  If you turn off the lights, things are dark.  If you turn on the lights you see more detail.  Flash lighting and thinner hair does not mean you are losing hair.