The larger Aperture of the 42's will still show more detail and appear brighter (even in daylight) than a 32 under certain circumstances. Here is an article from "Better View Desired" that explains why:
Objective size: The objective size is the diameter of the objective (light-gathering) lens in millimeters. It is the 35 in 7x35, the 42 in 8x42, etc. The larger the objective lens, the more detail it is capable of delivering to your eye.
Physics dictate that when light passes through a lens, normally distinct points in the object are unavoidably blended together in the image. The process is called diffraction. It is caused by those light rays that pass the edge of an object being diffracted (or bent out of their true course) by that edge, while the path of the light rays passing through the center of the lens is unaffected. When the light rays from the entire lens are recombined in the image, the edge-diffracted rays form small faint halos (or diffraction rings) around the distinct points formed by the undiffracted center rays. This reduces the sharpness of those individual points. When you pass light through a round hole (and a lens is essentially a round hole for the purpose of this discussion), the combined diffraction effects of the continuous edge of the hole amplify each other until you begin to lose the finest details. The smaller the lens, the further apart the points in the object have to be to remain separate in the image. The points that are closer together blur together. You might say that larger lenses simply do less damage to the image than smaller ones, allowing more detail to get through.
Then too, the larger the objective, the more light it is capable of delivering to your eye. The eye is an electro-chemical system that responds to the particular form of energy we call light. The level of energy entering the eye affects our ability to distinguish detail, especially color detail, in complex ways. In general, the more energy the better. Larger objectives collect and deliver more energy to the eye.
More detail, more light . . . when it comes to objectives, bigger is undoubtedly better. Still, high quality objectives as small as 23mm can provide a surprisingly satisfying image of the bird in the field, especially in full daylight and at reasonably close distances. For general field use, objectives in the 30-35mm range will often show you all there is to see, except for the extreme distance and darkness conditions noted below. In fact, at a typical birding distance of 40-60 feet, almost any binoculars will look good and provide a surprisingly satisfying image (at least until you compare them directly to something optically better). However, for binoculars that perform well in any condition, that deliver all there is to see, all the time, you need 40-50mm objectives.
There are three conditions that will clearly show the superiority of larger objectives.
First. whenever the distance gets out over 150 feet, especially on birds of sparrow size, larger objectives will pull out detail that simply is not there in smaller glasses. It is not just a matter of resolving power, though raw resolution has some effect. You will also see more color at those distances. The larger objectives gather enough light to excite the color receptors in our eyes, while smaller objectives leave us seeing some indeterminate shade of gray. Obviously when you combine long distances with low light levels, it only compounds the color problem. To me the lack of color detail in smaller glasses is more limiting, and more obvious, than the lack of raw resolution.
It should be said that the differences in resolution and color detail are there, even at close distances, if you compare binoculars directly in the field on the same birds. However, the limitations of the smaller objective don’t become obvious, or seriously affect your birding experience, until the distances become more extreme.
Second, when looking into deep shadow, especially when portions of the view are brightly lighted, large objectives will penetrate where smaller objectives fail. Again, it is a matter of the amount of energy the larger objectives capture, but this time we are talking about the number of rays (or the width of the wave front) that the objectives intercept from any given point in the object and focus back into the corresponding point in the image. Think of it like this: shadowed points are still reflecting a certain amount of light. The energy radiates outward so that at any given distance the energy from that point could be thought of as being spread over the surface of a hemisphere with the reflecting point at its center. Larger objectives intersect a bigger area of that sphere than smaller objectives do. That greater amount of energy their larger area captures is then focused back into an image of the reflecting point.
Since the larger objective captures more energy, the point appears brighter and we, in effect, see deeper into the shadow. This is why, by the way, larger objectives can appear brighter in all situations, including full daylight, than smaller ones, even though in bright light it should be the contracted diameter of the pupil of our eye that is the limiting factor, and not the binoculars at all. Given a high contrast image, we interpret the full daylight view as brighter through the larger glasses because we see more detail in the shadows, not because there is any more light in the highlights.
The last condition is, surprisingly, the least obvious in actual use. I say surprisingly because it is the only one commonly cited anywhere as a distinct advantage of larger objectives. I am talking, of course, about low light situations: dawn, dusk, twilight, heavy cloud cover, and deep forest. As long as the light is fairly uniform, at moderate distances, a good 23mm objective will perform just about as well as a 50mm objective in dawn, dusk, and overcast . . . right down to darker than you really want to be birding anyway. There is certainly some advantage to a larger objective, but it comes to a matter of minutes or yards. A 50mm glass is useful about 5 to 10 minutes earlier in dawn and later into twilight than a 23mm glass . . . or, to put it another way, will reach 10 to 20 yards further in those same situations. You have to have a certain amount of light to work with for any size objective to work at all. In deep forest, however, the problem is similar to the deep shadow situation above, and larger objectives will give you more evident detail in the shadowed, darker portions of the view.
At the powers common in birding binoculars (7 to 10x) and given the fact that they are generally hand held, you do not gain significant detail or color by going larger than 50mm.
Theory, by the way, would suggest that perfect 8x30 binoculars should be able to deliver all the detail and color the eye can use. In fact, in actual field tests, only two 30mm class binoculars I know of even come close. You can always see a difference by going to a larger objective of similar quality. Is it possible that we need the extra objective size primarily, or perhaps only, to overcome the current limits of real-world manufacturing?
Dennis
