Exit Pupil and Low Light performance

Determining low light performance of a riflescope is not nearly as simple as some make it out to be. On the other hand, it is not rocket science either. However, there is no substitute to field trial. If you want to be certain how good (or not so good) your scope is in low light, you have to try it out with your own eyes.
Most of the problems in predicting how well a a particular scope performs in light starved conditions naturally stem from differences in individual eyesight. We all see things slightly differently in good light and in poor light these differences often become even more pronounced.
The most obvious (and easy to observe) difference is in the size of your eye pupil. In good light, a human eye pupil contracts considerably, since there is plenty of light available. The amount of light that gets into your eye is directly proportional to the area of your eye pupil, so if during the day, your eye pupil has a typical diameter of ~2mm, its area is ~3.14mm2. As the sun starts setting and there is less light available, a few things happen. First of all, your eye adjusts to the decrease in ambient light and your eye pupil dilates in response. If it increases up to 4mm in diameter, its area becomes ~12.56mm2, or four times larger than it was during the day. So far so good, except the speed and magnitude of eye pupil dilation varies from person to person and is also effected by a number of other factors like dietary habits, age, time of day, etc. For a typical healthy human being under the age of 30, maximum eye pupil diameter is ~7mm when it gets really dark, but I have run into people with 4mm and 9mm maximum eye pupil dilation, so it varies greatly. On top of that, simple things, like how much coffee you have had on that day make a difference.
Aside from the eye pupil changes, the way we process visual information also changes depending on how much light is available. There are two types of light sensitive cells in the eye: rods and cones. Generally, depending on the amount of light available, human eye operates on one of three different vision regimes: scotopic (ultra low light where only the rods collect information), mesopic (low light when both rods and cones collect information) and photopic (good light when the visual information is collected primarily by cones). Even in rather low light hunting, human eye seldom gets into the scotopic mode of operation, usually there is enough illumination for mesopic vision. As a general guideline, as the light gets lower, you know when mesopic vision starts when the colors start changing. When color disappears entirely, you are in the scotopic region.
On top of the other complications, rods and cones are not evenly distributed in the retina. There is a region in the retina that is directly behind the iris called fovea. It is approximately 1mm in diameter and is responsible for most information collected by the eye. The problem is that there are a lot of cones in the fovea, but not a lot of rods. Hence, as the light gets lower and cones stop collecting information, you need to rely on light falling outside the fovea, which is greatly helped by larger exit pupil in your riflescope.
The exit pupil is a function of scope magnification and objective lens. To calculate its diameter, divide the objective lens diameter by the magnification. For example, a 6×42 riflescope has a 7mm exit pupil. A variable magnification 3-9×40 riflescope has ~4.4mm exit pupil at 9x and it gets progressively larger as you lower magnification. In practice, with a 3-9×40 riflescope, you end up with the best low light performance at somewhere between 4x and 7x magnification, depending on how large your eye pupil gets and how low the light is. To know for sure, it is worthwhile to experiment.
Aside from the exit pupil diameter, high image quality really helps. Human brain utilizes the images from both eyes to extract detail, but there is no binocular vision in riflescopes. However, keeping the off-eye slightly open helps. Also, images with higher contrast allow you to maintain some elements of color vision longer. That also helps your brain extract detail out of the picture.
There are a few other things to consider as well. For one, in very low light you actually see a little better with the “corner of your eye”, so to speak. It is also easier to see moving details than stationary ones. In that case, it helps to have the exit pupil of the scope a little larger than your eye pupil. That way your eye can move a little without blacking out the image. Another factor is the change in the F/# of your eye as the pupil dilates. That causes the perceived depth of field of your eye to be quite a bit shallower in low light. Hence, eye distance behind the scope’s eyepiece becomes more critical.  Moreover, in daylight, when cones are responsible for light collection, your eye is most sensitive to green-yellow part of the spectrum.  In low light, as the rods in your eye starts to collect the bulk of the information, eye sensitivity shifts into the green-blue.  That implies that some scopes with anti-reflective coatings well optimized for daylight will not perform all that well after sunset.  On the other hand, a scope that looks decent, but not exceptional during the day, might look better than you thought at night.  Once again, there is no substitute for experimentation.
When all is said and done, keep in mind that for truly good low light performance, you need both high image quality and large exit pupil. If in doubt, err on the side of top notch glass. If the scope delivers just a bit extra light and higher image fidelity to your eye, a moderate exit pupil works surprisingly well in failing light.