Originally posted by kcmadr Perhaps if you zoom in on a dark subject you may have to expose for longer time? Or if you zoom out and brighter objects are included you may need to expose for less time?
If you look at zoom lenses that are
not constant aperture, you will notice that as you zoom out your maximum aperture gets smaller. I have a 19-35mm lens that is a 3.5-4.5. That means at 19mm the maximum aperture is f3.5 and at 35mm the maximum aperture is f4.5. Given the same amount of brightness, the camera
must use a slower shutter speed at f4.5 than at f3.5.
The actual physical size of the aperture may not have changed at all....but the focal length has. And f-stops are ratios of opening size divided into focal length. If you have a lens that is 100mm and f4, that means the opening is 25mm across. Now imagine it is a zoom that goes out to 200mm focal length...and the size of the opening is the same old 25mm. That means at 200mm it is f8, which is going to require an extra two stops exposure. So if shutter speed were 1/100 on the short end at 100/4 then it is going to have to be 1/25 on the long end at 200/8. You can see how that would be an annoyance.
Now imagine that the lens was designed so that the physical size of the aperture opening changed in relation to the zoom position so that no matter what focal length you had the lens zoomed to the aperture opening (when wide open) remained 1/4 of that size ("f4" in other words). One doesn't need to be a mechanical engineer to figure out that this is going to take more design work and result in a more complicated mechanism inside. And since this alone will push the price of the lens up, it makes no sense to put second rate glass in there. Hence the quality of constant aperture lenses tends to be better than that of variable aperture lenses.
As has been pointed out in an earlier reply, one of the benefits of a constant aperture zoom is if you are doing some flash shooting with a constant lighting output situation (not varied by technology such as wireless P-TTL). For example, I sometimes shoot events where I set up my remotely triggered lights and will have enough light for about an f4 exposure. If I used a variable aperture zoom I might be fine on the short end, as the maximum aperture there is greater than f4. I would just have the camera set af f4 and the lens would stop down that much and shoot. But on the long end of the zoom the maximum might be something like f5.6 and no matter what I do the lens isn't going to magically open up
wider than that. I would be one stop underexposed. That could be corrected for by using a higher ISO and shooting f5.6, but that introduces a lot of hassle into things. With a constant aperture zoom I know I can work the lens at
any position and be just fine. For example, my SMC-A 35-105/3.5 can shoot at f4 at 35mm or 105mm or anywhere in between. Same for my Tamron 28-75/2.8. It can shoot at f4 anywhere from 28mm to 75mm.