Resolution and Contrast

First of all, I think it is worthwhile to define these two terms, if briefly, since they sometimes are confused with each other. Both refer to the image quality and are influenced by a number of optical design parameters. However, in order to evaluate resolution and brightness, you do not need to know anything about the scope’s construction. All you have to do is look through it and pay attention.
 

Resolution is simply the ability of the scope to distinguish small details within the image.

Contrast is the ability of the scope to faithfully transmit the difference between light and dark (and by extension color fidelity and saturation).

I have heard people say that resolution and contrast go hand in hand. That is not, strictly speaking, correct. They are in a perpetual match of “tug of war”. It is impossible to optimize both of them to be as high as possible. If resolution is fully optimized, contrast suffers, and vice versa. In an image with high resolution, but low contrast, there may be a lot of fine detail, but you might have a hard time distinguishing between them, since they do not stand out much.  Conversely, if an image has high contrast, but low resolution, all the large details will be very distinct (a common term is to say that they “pop” out at you), but small details will simply not be present. While ideally you would want to have an image with both high contrast and high resolution, that is not easy to achieve. For every optical system, the designer has to compromise between resolution and contrast in order to achieve a well-balanced image. Moreover, a riflescope’s contrast and resolution are typically not the same in the center of the image as they are near the edges. The term “sweetspot” usually refers to how large of a spot in the center of the image delivers consistent resolution and contrast (and a few other things). Historically, different companies chose to compromise between resolution and contrast in different ways. For example, between the “alpha” makers, historically, Zeiss used to always put a little more emphasis on resolution, Leica on contrast and Swarovski on sweetspot size. In practical terms, they naturally tried to get everything as well optimized as possible, but they simply had slightly different ides of what the “optimal” compromise is. In recent years, as the optical quality keeps on getting better and cheaper, there is some convergence between the compromises made by top end companies anyway. That is one of the reasons why some people prefer image quality of scopes from particular companies: since we all have different eyes, we are sensitive to different things. Some, for example, are so sensitive to contrast, that a very well resolved image might not look all that good to them due to somewhat muted contrast. Personally, I adjust to contrast differences pretty well, but my eyes favor (somewhat) high resolution and wide sweetspot (hence my affection for Zeiss riflescopes and Swarovski binoculars).

Moreover, the relative importance of contrast and resolution sometimes changes as the light gets low. In bright daylight, when the objects you are looking at are naturally very contrasty, even if the scope diminishes that contrast very well, you are still likely to see details pretty well. However, as the light gets low, if contrast is insufficient, fine details start blurring pretty quickly, and even large details become hard to see. They just do not stand out as much. In order for low light performance to be good, both resolution and contrast have to be up to par (and that is one of the reasons you get what you pay for).

Lastly, I’ll briefly mention MTF. MTF stands for “Modulation Transfer Function”. I see this term misused in all sorts of discussions related to sporting optics. MTF is, in essense, a composite measure that describes how well a particular optical instrument maintains contrast at ever increasing resolution and with respect to spatial variation within the image. Just about every optical design goes through some sort of MTF optimization, and a lot of the potential performance of an optic can be gleaned from its MTF curves. In principle. In practice, since the peculiarities of our eyes are not accounted for, there is no replacement for looking through the riflescope in question. Three years of digging through specs and curves will not replace 20 minutes of messing with the actual product.

If you are interested in more technical details on MTF, resolution and contrast, there are all sorts of wonderful online resources available: Wikipedia, Norman Koren’s website, etc. Just do not expect a whole lot of practical and applicable insight from them, since they are primarily focused on the photography applications, where image acquisition is accomplished via a well-characterized imaging sensor, not your eye. Bottom line is that all optical designers pay attention to MTF and optimize it the best their budgets allow.