Optics 101: Light Transmission Explained!

telescope against the light

Regardless of what kind of optic you have — a spotting scope, a rangefinder, a pair of binoculars, or a rifle scope, there may be subtle tweaks and changes here or there, but they all pretty much work the same way.

They all function to magnify an object and allow you to see it in great detail. To do this, they have to have light.

How it Works

To see an object through an optic, it must gather light. Light gathering is not the most accurate term. The scope isn’t collecting light the same you’d do with water. It doesn’t so much gather and keeps the light as it transmits it.

However, light gathering is one of the biggest industry terms. Just a good-to-know: transmitting light and light gathering means the same thing.  So an optic transmits, or gathers light, through its objective lens.

1. Lens & Image


The objective lens is the lens farthest from the eyepiece and is often the largest one. On the magnified optic, the objective lens plays a significant role. It’s where the real magic happens.

Any magnifying optic is made of multiple lenses. The first lens acts like a net, grabbing the light and forcing it through the optic. This primary lens focuses the light it gathers and transmits it to the second lens. That light is the image you are looking at.

Past the first lens, the image looks the same. The second lens is where the image is magnified for the end user to see. So the first lens is where your light is gathered and then focused.

2. Amount of Light

How much light is transmitted through an optic?

If you had two plain lenses, you wouldn’t get a whole lot. The objective lens would also be reflecting light back and away from the optic. This would make you lose light.

You are already losing light simply by focusing it and magnifying it. The more you increase light, the smaller the exit pupil becomes.

The exit pupil is a virtual aperture that emits from a magnified optic. If you take a magnified optic and hold it away from you, look down the lenses and you can see a little circle of light. More on that later.

3. Lens Coating

With modern optics and technology, we’ve found a way to reduce glare and increase the light being transmitted through the optic. This is known as lens coating.

When it comes to light transmission, the lens coating improves it dramatically. The fully multi-coated lens is the keyword to look for when it comes to quality light gathering ability. With the right lens and lens coating, an optic can transmit 95% or more of light.

4. Magnification & Objective Lens

We already talked about exit pupil a bit earlier, and that comes into play when you are talking of light transmission and gathering ability.

Magnification affects the amount of visible light. When magnification increases, the amount of light gathered diminishes. Less light makes it to the eye anyway. Now the amount of light you lose depends on the size of the objective lens.

The reason you lose perceived light is that the exit pupil shrinks as magnification increases. The overall goal is to match the exit pupil to your eye’s pupil. The larger the objective lens, the more light is let into the optics.

Exit Pupil Calculation

Graphic explaining relationship between exit pupil and eye pupil

To find your exit pupil, divide the objective lens size in mm by the magnification.

So if I have a rifle scope that reads 2-7 x 42mm, this means the magnification is a variable magnification. As the magnification changes, the exit pupil changes too. At 2 power, the exit pupil is 21mm and 6mm at 7 power.

This may lead you to think that when the objective lens is bigger, the better it is; so you should buy the biggest one possible. That’s not exactly correct. There are numerous downsides to it. Size, weight, and cost are all negative aspects of a larger objective lens.

In fact, your eyes can only detect a small amount of light. A large objective lens can transmit a lot of light but your eye can only use some of it. A lot of it is wasted, mostly anything over 5mm or so with most people’s eyes.

Get that Light

In general, just make sure there isn’t a big gap between the magnification and the objective lens. Light is an essential aspect when using an optic. The biggest takeaway should be to focus on the objective lens size, magnification, and the lens coating.