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Rich Harrington: One of the most important numbers when it comes to really getting to know your gear is the f-stop number, and you could typically find this right on the front of the lens. Now, it's going to be expressed as basically a decimal point, and on some prime lenses, it's going to be down to 1.4, 1.2, but on a longer lens, 2.4, 2.8, that's a really good high quality lens. Robbie Carman: It is, yeah. Basically, the way that f-stops work is that the lower that number is, the more light that's let into the camera. It's a little counterintuitive.
You'd think that a bigger number would mean more light, but no, it's actually quite the opposite of that. So if you had a lens that had an f-stop of say 2.8, you are letting in a lot more light than say an f-stop of 5.6. Rich: Yeah, so when we are looking at lenses, this number is important, particularly when shooting in a low light situation or with a shallow depth of field. But where people run into problems is when they're using their cheaper lenses. A lot of the kit lenses, what's going to happen is they are going to express the f-stop in a range. It's going to be say 3.5 to 5.6 and it's going to all depend upon the zoom.
You can really tell this when you look at the construction of the lens. When we've got the kit lens attached here at the T4i, perfectly reasonable lens if you're shooting in bright outdoor light. That's what they really designed for, entry level, this, much heavier-- Robbie: Much more expensive, great. Rich: This lens costs more than that camera. Robbie: Exactly, and this is where the problem of your exposure changing as you are zooming through the range, the focal range comes in. This kit lens that we have on this Canon body is actually variable in its f-stop. It starts on the wide end of things at 3.5, but as you zoom in and get to the far end of the focal range, guess what, it drops to 5.6, and because we are letting in less light, what happens? The image appears to get darker that we are shooting.
Rich: Yeah, let's see that here. So we're pulled all the way out here, and you see that we're on the 3.5 setting. Now as we zoom in, we'll go ahead and take a look there. You see it goes all the way to 5.6. So as we're pulling through that, the image is getting brighter and it's changing based on the zoom, and that's all because of as we zoom in and out, how things are behaving differently with the actual length of the lens. Robbie: Yeah, this is a really important thing, because you might have exposed properly at the short end of things, but then when you zoom into something, uh-oh, everything is dark.
So this is something that you have to consider when you're out there shooting. The thing about the consideration that really plays into most productions is cost. Typically, the zoom lenses that have variable apertures are going to be less expensive, therefore more accessible by a lot of people. However, sooner or later, this variable aperture thing is going to drive you bonkers. Rich: So, you are right, Rob. This is a very big frustration. When we come back, we're going to talk about how to work around this with the existing lens and when stepping up or perhaps renting makes more sense.
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