Constant vs. Variable Aperture Lenses: A look at why some lenses have a fixed maximum aperture and others have a variable one.
Even more about lens apertures...
Nikon's 70-300mm f/4.5-5.6 variable-aperture zoom. This lenses uses a smaller maximum aperture at longer zoom settings.
Nikon's 80-200mm f/2.8 constant-aperture zoom maintains fast f/2.8 aperture throughout zoom range. Expensive but a great lens--one of my favorite all-time Nikkor lenses.
Constant vs. Variable Maximum Apertures
On all single-focal-length (non-zoom) lenses, the maximum aperture remains constant. If you have a 135mm f/2.8 lens on your DSLR, for example, the maximum aperture is always the same. The same is not true of many zoom lenses though--most zoom lenses (especially those in the consumer price range) have a variable maximum aperture that gets smaller (gathers less light) as you extend the focal length of the zoom. You might notice, for example, that your 70-300mm zoom lens has two maximum apertures marked on the lens: f/3.5-f/5.6 is common for a lens in that zoom range.
When you are using the lens at its shortest focal length (70mm) the aperture is indeed f/3.5--a fairly fast lens. But as you zoom out toward 300mm the lens speed decreases and by the time you hit 300mm you're shooting with an f/5.6 lens--a considerably slower lens speed. In practical terms what this means is that as you zoom out your lens the viewfinder gets more dim because the maximum aperture is getting smaller. In between 70 and 300mm you would have a mid-range aperture of roughly f/4. Most people don't notice the difference in viewfinder brightness, especially on bright sunny days; but if you're working indoors a lot in dim lighting, it is something to keep in mind.
There are some zoom lenses (most of them in the professional category) that have a constant maximum aperture. My Nikkor 80-200mm f/2.8 lens, for instance, always has a maximum aperture of f/2.8 regardless of the focal length that's being used. This is a great benefit in terms of viewfinder brightness and, again, it works well in low-light situations, but generally these lenses are heavier, bulkier and cost a truckload more money. If you're young, have strong shoulders and a lot of disposable cash, they're great but living without them is not a major inconvenience.
Speaking of money, here's one last thought on lens speed: Lens speed and lens price are always related. The faster a lens is, the more it costs and that's true for prime lenses and zooms. Before you break the bank and decide that you absolutely need a faster lens, compare the prices. Better yet, let your spouse compare them. Often the difference between an f/3.5-f5.6 variable speed zoom and an f/2.8 constant-aperture zoom, for example, is enough to pay for a week in Hawaii (and I'm not kidding). While I have owned some very expensive lenses in my career and I have loved using them, I'm pretty sure that if I had it to do over again, I'd use the cheaper lenses and watch the sun set more in Maui. Probably.
Caution
If you are using a variable-speed zoom with your DSLR and shooting in the manual mode, one slight problem that you might encounter is that because the aperture is changing, if you were to be shooting at the maximum aperture, you might have to adjust your shutter speed to compensate for the slower lens speed. If you are using a zoom with a variable speed of f/3.5 to f/5.6, for example, and you've set your exposure to, say, 1/250 second at f/3.5 at 70mm and then zoom out to 300mm you will have to slow the shutter speed down because now your lens is set to f/5.6--more than a full stop slower. However, if you have stopped the lens down and are using any other aperture than the maximum aperture, it's not a problem since the decreasing speed won't affect your exposure (see the section on "Auto Diaphragms" that follows).
Great egret photographed at the Merritt Island National Wildlife Refuge with a variable-aperture zoom. Remember that when you are using the longest focal lengths you're also using the smallest maximum aperture (in this case, I was using an f/4.5-5.6 at full zoom, so the maximum aperture was f/5.6).
Automatic Lens Diaphragms
One question that you might well be asking by now is this: Why doesn't
the viewfinder get darker when I have the lens set to a small aperture
that lets in very little light?
The answer is because all modern lenses (though this wasn't the case
until Pentax introduced the concept in 1948) have what is called an
automatic diaphragm built into them. This simply means that no matter
what aperture you have set on the lens, you will always be viewing
through the maximum aperture. The lens only stops down to the "taking"
aperture (the f-stop that you are actually using to expose the shot)
the instant that you press the shutter release button. Once the
exposure is made, voila, you're looking at a bright viewfinder again.