How to increase contrast/brightness of product photos?

[WayneF]

That's my problem, I strive for perfection in everything and don't trust such subjective methods like visual judging

OK, but it seems the only purpose of a picture is to look at it. No one will care about the numerical value of anything. And our measurement capabilities are too crude to ever show any precise result. We might debate if we should be using 18% or 12.5% or whatever, but that still has no effect on making the actual scene in front of us conform to that notion. Scenes vary.

I would invite you to look at How Camera Light Meters Work

What matters is how does the picture look? Fix it to look best.

 

[bdroberts]

First, your lighting is flat which gives the look of being lacking contrast. Get a book on commercial lighting and start there. There just is no shadows anywhere. Get the lighting right and you won't been photoshop.