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Auto-focus and live view

From: Foundations of Photography: Lenses

Video: Auto-focus and live view

One of the things that's really nice about an SLR is it has a single lens. That's the SL part of SLR. What's nice about a single lens is it means that my eye, when it looks through the viewfinder, is looking through the same lens that's exposing the image sensor. The image sensor sits right back here, directly behind the lens. What's nice about that is it means that when I am looking through the viewfinder, I am seeing the exact same composition that the image sensor is seeing. I can also see the effects of any filters that I put on the lens. In general, it's just a better arrangement than having one viewfinder for me and another one for the image sensor.

Auto-focus and live view

One of the things that's really nice about an SLR is it has a single lens. That's the SL part of SLR. What's nice about a single lens is it means that my eye, when it looks through the viewfinder, is looking through the same lens that's exposing the image sensor. The image sensor sits right back here, directly behind the lens. What's nice about that is it means that when I am looking through the viewfinder, I am seeing the exact same composition that the image sensor is seeing. I can also see the effects of any filters that I put on the lens. In general, it's just a better arrangement than having one viewfinder for me and another one for the image sensor.

Now, for all this to work there have to be some extra little mechanics inside of your camera. You may know all this already, but let's take a quick look here. I am going to take the lens off and what you see here is a mirror. Now, I had said before, the image sensor sits right back here and that should make sense to you. It needs to be directly behind the lens so the light gets through to it, and of course there is a shutter in front of the image sensor, but there is also this mirror. And what the mirror does is it takes the light that's coming to the lens and it bounces it straight up. It gets up into here, into something called a pentaprism.

In a less expensive camera that would be a pentamirror, and that light then shoots it straight out back out the viewfinder. Now, the problem is if I open the shutter to expose the sensor, there is this mirror in the way. Fortunately, the mirror can move. I have got my camera in bulb mode, which means as long as I hold the shutter button down, the shutter will stay open and the mirror will do its thing. So you can see here, when I press the shutter button, the mirror flips up out of the way. You didn't see this because it happened so quickly. The shutter opened and that's the image sensor that you are looking at. When I let go of the shutter button, the shutter closes and the mirror comes back down.

That's the reflex or R part of SLR. That mirror moving around thing. So right now I've got light bouncing off the mirror and coming out here to where my eye can see it, and now I've got light going all the way through into the image sensor. It's a very clever arrangement, works very well. However, with digital cameras, people expect to be able to use the LCD screen as a viewfinder. So for that to work we have a problem. And the problem is this: the autofocus sensors in your camera sit up here. They depend on light being bounced up into the pentaprism chamber to be able to see and when the mirror comes up, they go blind because they're not getting any light.

So that makes autofocus very difficult in live view, because in live view the mirror has to stay up all the way. When you put your camera in live view mode, you probably hear a sound like that. That's the mirror going up and the shutter opening and those just stay open because the image sensor is there gathering light and building the 30 frames per second or whatever it needs to build to put a continuous image back here. Because the mirror is up, the autofocus sensors are blind, so you cannot autofocus during live view. So the way most cameras get around this is they have a button so that when you focus or when you press the autofocus button, the mirror temporarily comes back down.

That gets light back to the autofocus sensors. Your camera focuses, then the mirror automatically goes back up, and now the image sensor can get light, so that it can start building its live view. This is why in live view mode, when you hit the focus button, your screen goes blank temporarily. Now, this is a drag if you're trying to follow motion or track something. If you're really needing to be in the moment of taking the photograph, you are going to get interrupted by that focusing process. Because of that some camera vendors build-in an additional autofocus mode when in live view, and that's a contrast detecting autofocus mode.

And what that means is the mirror stays up all time so your live view screen stays on all the time. The image that the image sensor is getting is analyzed for contrast, because where there's more contrast, there's better focus, and those calculations are used to determine how to focus the lens. The problem with that is it's a very slow mechanism, because it's very computationally intensive and the computer in your camera just can't keep up very well with current technology. So that's not the best solution. Another option is you can manually focus your lens when in live view. What's tricky about that is it's hard to judge focus on these little LCD screens. Even though this has a big beautiful LCD screen for telling fine detail, it can still be a little tricky.

So these are just all caveats that you need to be aware of when using live view and trying to autofocus. Now, there are cameras that get around this problem. Some Sony cameras, some Olympus cameras, a few others, solve the autofocus and live view problem by putting extra focus sensors down here or by using a split mirror or a fancy prism or something like that to get light up here to these autofocus sensors, and then they can manage their autofocus. Also be aware that when you're shooting video with your SLR, if it has that capability, the whole autofocus thing is going to interrupt your video feed and that's no good either.

So plainly, this is a complex subject to learn. How it works with your camera, you need to dig into the live view sections of your camera's manual and learn exactly what autofocus mechanisms it has, because you're going to want to use those anytime that you're shooting with your camera in live view mode.

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Foundations of Photography: Lenses

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Ben Long
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