The Firing Line Forums - View Single Post - What is the difference in scopes magnification?

jmr40 · August 8, 2012, 07:49 AM

Don't waste money on a 50mm front objective. The amount of light that gets through to your eye is a factor of several things. The front objective is only one of them. You also have to consider the scopes magnification, and light transmission rating.

Assuming equal quality glass a 50mm scope set on 10X lets in exactly the same amount of light as a 40mm scope set on 8X or a 20mm scope set on 4X. These 2 factors (front objective diameter divided by the scopes magnification) determine the diameter of a ray of light coming through the scope. Imagine a beam of light from a flashlight. You can adjust the beam from a large circle or a narrow circle. All of the above examples will let in a beam of light to your eye 5mm in diameter. That is about as large as the average human eye can dialate. Any more light is wasted. If you are still in your 20's with exceptional vision you might dialate to as much as 7mm.

But even this does not tell you how much light is getting through, only the diameter of the beam of light. The scopes light transmission rating tells you how much light gets through. Many scopes in the $200 and under range only let around 80-85% of the light that enters the front objective through to your eye regardless of how large the beam of light is. The better mid-range scopes are in the 90-95% range. Your really high end scopes are in the 95-98% range.

A decent $200-$300 mid range scope with a light transmission rating of around 90-95% with a 40mm objective will be far better in low light than a 50mm scope with a light transmission rating of only 80-85%. A scope with a 50mm lense, and 90% or greater light transmission is far more expensive than a 40mm scope with the same rating.

A 50mm scope is only an advanage at 10X. At any other power setting it lets in no more useable light than a 40mm scope. It is only an advantage for 2-3 minutes in the morning, and another 2-3 minutes in the evening, and only if you spend big money to get a high end scope with at least a 95% light transmission rating. Otherwise you are wasting money.

August 8, 2012, 07:49 AM	#9
jmr40 Senior Member Join Date: June 15, 2008 Location: Georgia Posts: 10,808	Don't waste money on a 50mm front objective. The amount of light that gets through to your eye is a factor of several things. The front objective is only one of them. You also have to consider the scopes magnification, and light transmission rating. Assuming equal quality glass a 50mm scope set on 10X lets in exactly the same amount of light as a 40mm scope set on 8X or a 20mm scope set on 4X. These 2 factors (front objective diameter divided by the scopes magnification) determine the diameter of a ray of light coming through the scope. Imagine a beam of light from a flashlight. You can adjust the beam from a large circle or a narrow circle. All of the above examples will let in a beam of light to your eye 5mm in diameter. That is about as large as the average human eye can dialate. Any more light is wasted. If you are still in your 20's with exceptional vision you might dialate to as much as 7mm. But even this does not tell you how much light is getting through, only the diameter of the beam of light. The scopes light transmission rating tells you how much light gets through. Many scopes in the $200 and under range only let around 80-85% of the light that enters the front objective through to your eye regardless of how large the beam of light is. The better mid-range scopes are in the 90-95% range. Your really high end scopes are in the 95-98% range. A decent $200-$300 mid range scope with a light transmission rating of around 90-95% with a 40mm objective will be far better in low light than a 50mm scope with a light transmission rating of only 80-85%. A scope with a 50mm lense, and 90% or greater light transmission is far more expensive than a 40mm scope with the same rating. A 50mm scope is only an advanage at 10X. At any other power setting it lets in no more useable light than a 40mm scope. It is only an advantage for 2-3 minutes in the morning, and another 2-3 minutes in the evening, and only if you spend big money to get a high end scope with at least a 95% light transmission rating. Otherwise you are wasting money.