When the camera does this WHILE you are recording video, you'll see the camera make several "guesses" before it finally refines focus and this is VERY distracting. The camera has to "hunt" for optimal focus. It doesn't even know if the focus is too far or too near. It experiements with focus and compares the differences in how rapidly contrast changes and it keeps hunting until it finds a point where it's maximized contrast in the spot you asked it to focus on.īut the downside of the contrast system is that when not in focus, it doesn't actually know how far it needs to adjust focus. The camera looks for "edges" in your image and inspects how rapidly constrast changes between adjacent pixels.
This is basically how "contrast detection" works. But if focused sharply you'd just have a black pixel next to a white pixel with no shades of gray. If your image is not focused, then you'll have some black pixels and white pixels, but the images near the edges of each stripe will be shades of gray (due to the blur.) You'd have a black pixels, next to a very dark gray pixel, next to a middle gray pixels, next to a light gray pixel, next to a white pixel (a slow contrast change from black to white). In theory, if your image is focused, every pixel should be either a "white" pixel (background) or a "black" pixel (a stripe on the barcode) with no other tones. Instead of the camera does something called "contrast detection" focus. So there's no way to do "phase detect" auto-focus using those sensors in the floor of the camera. This is why DSLR cameras focus so much faster than point & shoot cameras.īut when you shoot video, that mirror has to swing up and clear of the light so it can reach the imaging sensor. This system is able to tell (based on analysis of two phases of light after passing it through a prism which works as a beam-splitter) if the image is focused at that point and if not focused it knows instantly how far it needs to adjust focus (and in which direction) to nail the focus.
When you are shooting normal still images and using the view-finder (looking through the camera instead of using the LCD screen), there's a mirror in the camera which bounces some light up to the viewfinder (so you can see to compose) and some light is bounced down into special "phase detect" auto-focus sensor on the floor of a camera. The first camera that supported that feature was the EOS 70D. The T1i does not support "continuous auto-focus" while shooting video. but it will only re-focus when you press the button. If you press that button while shooting video it will force the camera to re-focus. On the back of your T1i there is a button (near the upper right corner) with an asterisk (*). There's "auto-focus", and then there's "continuous auto-focus".
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