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Foundations of Photography: Lenses

Servo focus


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Foundations of Photography: Lenses

with Ben Long

Video: Servo focus

This is the Canon 24-105 EFL. In addition to its great zoom range of 24-105 millimeters, which makes a fantastic walk-around lens, its four stops of image stabilization, its constant f4 aperture across its zoom range, the low dispersion glass, the red stripe around the end of the lens, which lets people know that you're a discerning shooter. in addition to all of that, it makes a great pendulum. Now the problem is if I stick a camera down there and I want to shoot this lens as it swings back-and-forth, I got problems, because I'm not going to be able to focus on it very easily. Here's why? Focus of course is a matter of the distance between the lens and the camera and if I focus right here, it's immediately swung out of focus, and-- stop that! it's immediately swung out of focus and my image will be blurry.

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Foundations of Photography: Lenses
2h 32m Beginner Feb 11, 2011

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Many of the creative options available to a photographer hinge on an in-depth understanding of lenses. In Foundations of Photography: Lenses, Ben Long shows how to choose lenses and take full advantage of their creative options. The course covers fundamental concepts that apply to any camera, such as focal length and camera position, and shows how to evaluate and shop for DSLR lenses. The second half of the course focuses on shooting techniques: controlling autofocus, working with different focal lengths, and managing distortion and flare. The course also examines various filters and contains tips on cleaning and maintaining lenses.

Topics include:
  • Understanding field of view and camera position
  • Depth of field and lens choice
  • How to choose a lens
  • Examining lens features
  • Using specialized lenses such as fisheye and tilt/shift lenses
  • Focusing techniques
  • Using filters
  • Camera maintenance
Subjects:
Photography Cameras + Gear Photography Foundations
Author:
Ben Long

Servo focus

This is the Canon 24-105 EFL. In addition to its great zoom range of 24-105 millimeters, which makes a fantastic walk-around lens, its four stops of image stabilization, its constant f4 aperture across its zoom range, the low dispersion glass, the red stripe around the end of the lens, which lets people know that you're a discerning shooter. in addition to all of that, it makes a great pendulum. Now the problem is if I stick a camera down there and I want to shoot this lens as it swings back-and-forth, I got problems, because I'm not going to be able to focus on it very easily. Here's why? Focus of course is a matter of the distance between the lens and the camera and if I focus right here, it's immediately swung out of focus, and-- stop that! it's immediately swung out of focus and my image will be blurry.

Now I can try to half press the shutter button along the way and keep up with it, but it's really difficult to do when something is moving that quickly. Now, you may think I don't angle my lenses from the ceiling, so what does it matter? In that case it probably doesn't, but you probably do maybe take your kids to the park and put them on a playground swing, shoot your friends surfing, maybe you go on safari and get charged by a lion and you want to leave some nice pictures behind. Anytime you've got moving subject matter, you're going to have this issue of trying to keep up with the focusing process and this is not just about things moving towards you. Even things just moving across the frame like wildlife or fast-moving sports.

Fortunately your camera probably has a feature called servo focus, sometimes called focus tracking, which can track the subject in your image as it moves across the frame or forward and back and keep it in focus. Here's how it works. You put your camera in the servo focus mode, frame your shot, half press the shutter button, the camera will automatically identify the moving thing and make constant adjustments to focus as that thing moves around. To a degree it even works when you are moving the camera. It can separate background motion from the subject. So we went to the park and stuffed Jacob on a swing and I got down on the ground with a wide-angle lens to get a really dramatic angle and you can see here that I'm just shooting away and I'm not having to worry about focus at all.

Once I'd frame my shot the way that I wanted, I half pressed the button and I heard the servo focus mechanism startup. You'll hear your lens making lots of little noises. You'll feel vibration in the lens. I also put my camera into burst mode. In burst mode, as long as I hold the shutter button down the camera will keep knocking out frames. This particular camera can do about 5 frames per second. So when it would swing into frame I'd start bursting. Meanwhile, servo focus is keeping him in focus. Here's what's happening inside the viewfinder. Now this is a simulation that we've greatly slowed down, but you can see as he moves, he comes to a new point, he is out of focus, and he sharpens up.

It goes to the next frame, and the camera sharpens up. That's because it's constantly staying in focus. As he comes back in, he moves in and he sharpens up. This is all happening very, very, very quickly. It's an ideal, very easy way to shoot moving subject matter. There is a caveat though. You're used to your auto focus mechanism, when you half press the shutter button, at some point you see a ligh, light up and you hear that beep that tells you "I've locked focus and now you can take the picture." And servo focus mode is never going to lock focus. That's the point. It keeps refocusing all the time. So when you're done with servo focus mode it's very important that you switch back to your normal focus mode.

If you don't, you'll do something that I've done a lot of times which is I go to shoot something else later and go well, my auto focus isn't working. Oh, my camera must be in manual focus mode and I look at the lens and no, it's in auto focus mode and finally, by the time my subject is left, I remembered, oh, it's in servo focus and I change it back. So just be aware in kind of a troubleshooting mode that when if your camera is not locking focus, it may have either been left in servo focus mode or you may have accidentally put it there. So if you regularly shoot moving subject matter, take a look at your camera's manual to learn about its servo focus mode.

How to activate it and any caveats there maybe about it.

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