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You should already be comfortable with the concept of depth of field and understand how aperture choice impacts the depth of field in an image. If you're not, then go watch the Foundations of Photography: Exposure course or at least the depth of field and aperture bits of it. To shoot shallower depth of field, you need to use a wider aperture. One that has a lower f-stop number. So f/2.8 yields an image with less depth of field than f/11. Now the thing is not all lenses can open their aperture all the way to f/2.8.
For all sorts of complex physics and optical reasons, the lens has to be engineered specifically to manage an aperture of a particular size. This means that every lens has a maximum aperture size to which it can be opened and that size is written on the front of the lens. For example, here is a 50mm lens that has a maximum aperture of f/1.8. I know that because right here it says 1:1.8. Maximum aperture is often expressed as a ratio like this. Now check this out.
This is also a 50 millimeter lens but its maximum aperture is 1.2, as I can see here. Now plainly, the 1.2 lens is much larger than the 1.8 and what you can't see just from watching is that it's also a lot heavier and it's a lot more expensive. We refer to the maximum aperture of the lens as the speed of the lens. So, if someone says, "What speed is that lens?" I would say, "Oh it's a 1.2," and they'd slink off, done in by the status conveyed by this very fast lens I am holding.
With a faster lens, I can use faster shutter speeds because of the ability to open the aperture wider. Now, you can think of a faster lens as being able to gather more light in the same amount of time as a slower lens. It takes a lot of glass to make a faster lens and it has to be very high-quality glass. And that's why a faster lens like this is bigger and more expensive than even just a slightly slower lens like this one. With a fast lens, I can shoot very shallow depth of field and I can also shoot in lower light while keeping my shutter speed fast enough both for hand-held shooting and to stop moving action in my scene.
Now, zoom lenses also have maximum aperture ratings. So for example, this 24 millimeter to 105 millimeter lens has a maximum aperture of f/4, which I can see right here. Other zoom lenses might show an aperture range. For example, this 18 millimeter to 55 millimeter lens, which is a fairly typical kit lens that might be bundled with a camera, shows an aperture range of 3.5 to 5.6. Now, that means at full wide angle, the maximum aperture that you can get with this lens is at 3.5.
At full telephoto, zoomed in all the way, the maximum aperture is at 5.6. Focal lengths in between vary between f/3.5 and f/5.6. Smaller zoom lenses like this often have a variable maximum aperture across their range, because that's the only way to engineer that focal length range in such a small package. Now, here is a 75 millimeter to 300 millimeter lens and here is a 70 millimeter to 200 millimeter lens.
Now this 70 millimeter to 200 millimeter has a shorter zoom range than this 75 to 300, but it's physically larger than the lens with the larger range. Why? Well, the 75mm to 200mm, excuse me, the 70mm to 200mm is faster. The 75mm to 300mm shows a maximum aperture of f/4 to f/5.6. Now, again, that means that at the wide angle, my maximum aperture is f/4. If I zoom all the way into full telephoto, then my maximum aperture can only open to f/5.6. By comparison, this 70mm to 200mm has a maximum aperture of f/2.8.
Sometimes the maximum aperture is also written here on the outside of the lens. Now, that's much faster and this lens has that maximum aperture across its entire zoom range. As I mentioned earlier, fast lenses are expensive and fast zoom lenses especially. Fast zoom lenses like this one that are fast across their entire range, those are especially expensive. For most everyday shooting though a 3.5 to 5.6 maximum aperture is probably fine, but it's still nice to have a fast lens in your arsenal. If you're a Canon or a Nikon shooter on a budget, you should know that both companies make very good, very inexpensive f/1.8, 50 millimeter lenses, like this one.
Now this thing feels cheap. It feels plasticky, but it's got great glass in it and you can get either one of these for under 100 bucks. They are small, they are light. So, it's worth keeping one in your bag just for those times when you need a fast lens because you want to shoot shallow depth of field or you want to shoot in lower light, and your other lenses are all in the 3.5 to 5.6 range. Now, as you'll see in the rest of this chapter, lens speed is not the only factor that controls depth of field.
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