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Lesson 36 from: Mastering Your Digital Camera

Chris Weston

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Lesson Info

36. Autofocus

Understanding how a camera's autofocus works helps capture sharp shots. Understand the difference between phase detection and contrast detection.
Next Lesson: Point of Focus


Lesson Info


cameras focus automatically by detecting contrast. If there's no contrast, there's no water focus. Now I contest this theory by pointing my camera to clear blue sky because there's no contrast in that blue the camel going to what it's called hunt mode, with the lens moving backwards and forwards constantly looking for something to focus on but never actually making a decision. If, however, I point, the camera toe white bird flying through the blue sky in the contrast between the bird in the sky will give the cameras something to focus on our cameras. Sees contrast pretty much depends on the type of camera. DS Alaska it One way Marylise cameras do it another. The two systems are called phase detection and contrast. Detection face detection works off the mirror, which is why it can't be used in a mirror. This camera contrast detection works off the sensor, which is why it can't be using a DSLR because in a DSLR the mirror gets in the way. However, modern technology being what it is, most...

cameras today actually use a hybrid system, a combination of the two, and that's because both of them have advantages. Contrast. Detection is the most accurate, but it's relatively slow and only really good for static subjects such as landscapes. Face detection is much faster and is great for tracking moving subjects. But now we've understood the science of auto focus is your job to tell the camera what to focus on, and you do that Using the auto focus sensors, which are those little squares you see etched in the viewfinder of the layout of the grid, varies between types of camera in DSLR cameras or sensors arranged to cover an area in the center of the viewfinder. While in Marylise cameras, they tend to cover the whole image space. The number of sensors also varies. High end cameras tend to have more, which on the whole means quicker, more accurate. Focusing on this is one of the things you're paying for with a higher price tag. The senses themselves also very between cross type and line type. Now, cross type sensors detect contrast in both vertical and horizontal planes, while line type sensors detect contrast in one plane only which, generally speaking, made cross type senses quicker to focus and more accurate. But only some of the senses in high end cameras. Typically those groups right in the center of the folks in grid across type and an entry level cameras. Sometimes the only cross type center is the middle one, and this is something to be aware off. If your subject dictates speedy auto focus to manually set the active F sensor, that is the sense of the camera starts with. Before tracking any movement, you simply move the focus selector until the appropriate sensor is lit. Then when you turn on auto focus, whatever is covered by the sensor is this subject the camera will focus on. And that brings me to my next question. What object will arian the scene? Do you focus on?

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Ratings and Reviews

mark jacobson

What a marvelous course! What a marvelous teacher! When I went to college, my father would always ask me about my professors, more than the courses themselves. He was passionate about learning and although too busy with earning an income to go beyond an undergrad degree, continued to read 50 books a year. I still remember how he'd get almost visibly excited when I'd tell him about some special professor who taught with such enthusiasm and, more than just passion, evident delight and joy in the subject. 'Ah they're the best, son. How wonderful you have such a teacher." Well, he passed away decades ago but if he were still around I'd get a kick out of telling him about Chris Weston, the 'Prof' of this course. He's one of the very special ones: a teacher who's loved and lived his vocation--his avocation--since he was a boy--and still is as excited about it now as he was then. The result: a course that seems to be more a labor of love--of pouring far more energy and thought into the details then one typically finds in these courses--than anything else. Bravo Chris! I'm already on to your next one.


Chris is an amazing instructor who dissects theory giving amazing analogies that bring concepts to life. I have rarely been able to sit through most video course for more than a half-hour but watched this one from beginning to end. A good refresher course if you've been away from the camera for awhile or there are some concepts that still illude you. I highly recommend this course and look forward to watching his others. Thank you for the clarity and great explanations.

Sky Bergman

This was an amazing class. I have looked at a number of basic photography classes. This one was by far the best I have seen. Chris is an exceptional teacher. He breaks things down into digestible information and then inspires you to be creative. Thank you!

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