Thread: A comparison of HD vs non HD high end scopes

"HD" glass means High Density glass. Wikipedia says this: "The density of soda glass is 2.4 g/cm3 (39 g/cu in) or below, while typical lead crystal has a density of around 3.1 g/cm3 (51 g/in3) and high-lead glass can be over 4.0 g/cm3 (66 g/in3) or even up to 5.9 g/cm3 (97 g/in3)."

Why use higher density glass? you get better refractive index - which leads to probably sharper image and less CA.

But, it is should not provide better light transmission, which seems to be what you were measuring.

What affects amount of light transmitted are:
1. objective size divided by focal length (also known as magnification)
2. number of glass used. each additional glass will cut light transmission by some degree. more more glass means more/better correction and also more complex design to accommodate more adjustment range.
3. quality of coating.

Generally speaking if you have all scopes set at the same focal length, the scope with largest objective length should pass the most amount of light. But design and coating also affects this.