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AF very slow during Live View Shooting on T3


I purchased a T3 as a gift. When taking pictures using the viewfinder everything seemed to worked fine. When using live view shooting it would take several seconds to auto focus and even then it would only successfully focus about 50% of the time. I was shooting indoors with relatively low light. I also noticed that during live view shooting the flash did not automatically pop up or flash to assist focus as it did while using the viewfinder. The setting was on auto and I was using live mode for focus. Is this normal or are there other settings that could help?


This is completely normal and not a defect.

Your camera actually has two independent focus systems. The normal way to use the camera is by looking through the viewfinder (not the live-view screen). When you do this, light is bounced down into the "phase detect" auto-focus sensors (these live on the floor of your camera). Light is basically passed through a prism which splits it into two halves or phases. You can imagine taking a print, cutting it in two with scissors, and then placing them together, but sliding them along the cut edge to match up the two halves -- "phase detect" works something like that (not exactly) but you get the idea. When the image is out of focus at a focus point, the light will be out of phase. The camera can actually tell WHICH DIRECTION it needs to adjust focus AND it can also tell HOW FAR it needs to adjust. Basically it doesn't guess... it snaps right to the ideal focus very quickly. Phase-detect auto-focus is a wonderful thing.

However... when you use "live view", the mirror in your camera has to swing clear to allow light to hit the sensor (that's how it creates the "live view") and THAT means that it can't use the phase-detect sensors because the mirror that was previously bouncing light into those sensor has swung out of the way.

So the camera has a 2nd focusing system... "contrast detection". Imagine that you are taking a photo of a barcode. You've got a black stripes on a white background. But if you think about the "colors" of a barcode... every point in the image is either "black" or "white" and there is no "gray". But it would only look like that IF the image is focused. If it's out of focus you'd have fuzzy black lines that gradually fade to a white background because the image is blurred. In "contrast detect" the camera is analyzing adjacent pixels while focusing in an attempt to maximize the contrast (if in our barcode example... it wants a black pixel right next to a white pixel when inspecting the edge of the barcode -- there should be no "gray" pixels or it's not yet focused well enough. In truth the camera doesn't know what colors you have... but it still analyzes the image looking for patterns and trying to maximize contrast.

The upside of the contrast detect system is that you can use it in live-view AND it's also generally very accurate IF it has a pattern with good contrast and enough light to tell the difference. The bad news is when it's out of focus it has no idea if it's focusing too close or too far and also doesn't know much it needs to correct focus. So the system has to "hunt" -- basically it analyzes contrast, adjusts focus, re-analyzes to see if it got better or worse... if worse it goes the other way. If better it keeps tweaking to until it can no longer improve (and at some point it will go too far and have to back up.)

This means contrast detect focus is much slower and involves "hunting" for focus. But it does need enough light or it will fail.

You will probably want to get used to using the viewfinder as much as possible to compose and shoot rather than use the live-view screen.
Tim Campbell
5D III, 5D IV, 60Da


Tim's reply is great!


I can only add a little...


Part of your difficulty was the low light situation. All AF systems slow down in lower light, until it's too dim for them to focus at all. You can help them by shining a small flashlight onto the subject until the camera focuses. Or with an accessory flash mounted, many of those can emit a Focus Assist grid. If not wanting to use a large flash, a smaller flash controller module called an ST-E2 can be used to emit the same sort of Focus Assist grid. Those grids are not too intrusive, red light.


In good light, you will find that Live View/Contrast Detection focusing is the most accurate, although it's slow.


Using the viewfinder instead, you have several AF modes to choose among. One Shot is the most accurate of those, but is only usable with stationary subjects. AI Servo is for moving subjects, and is designed to track and constantly update as the subject moves. (AI Focus really isn't a focus mode, per se... it's supposed to detect for you whether or not the subject is moving, then change to using the correct mode).


You can help the viewfinder-based focusing system by manually selecting a single AF point, rather than leaving it up to the camera to choose. Use the center point as much as possible, since it's more sensitive and faster to acquire and/or better at tracking.


It also should be noted that the lens being used makes a difference in AF performance. There are basically three types of focus drive: micro motor, STM (stepper motor) and USM (ultrasonic motor). USM is the fastest and most accurate. STM is the quietest and can be preferable with video (but not all camera models can autofocus while shooting video). Micro motor is the least expensive, generally tends to be slower and noisier, sometimes less accurate.


A lens with a large aperture rating, such as "f2.8" or "f2" or "f1.4" also delivers more light to the sensors, so can help with AF performance.


Much more info about getting the best out of Canon AF systems can be found on YouTube in three approx. half-hour videos starting with this one: It's a few years old now, but still very helpful.


Finally, with the new 70D model Canon has introduced a new "dual pixel" focusing system in Live View (and Video)... This is a significant improvement, much faster! I'm sure we'll see it on more Canon models in the future.


Alan Myers

San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7D(x2), 50D(x3), some other cameras, various lenses & accessories


&quot;A lens with a large aperture rating, such as &quot;f2.8&quot; or &quot;f2&quot; or &quot;f1.4&quot; also delivers more light to the sensors, so can help with AF performance.&quot;<br><br>BTW - we talk about &quot;stops&quot; in photography... ISO and shutter speeds all make sense, but f-stops will seem a bit confusing due to the odd numbers. You'd think they'd be simple numbers... 1, 2, 4, 8, etc. And yet they have an odd sequence of f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, and for some lenses even f/32 (and some lenses even go higher).<br><br>The reason for the oddball numbers is that these are ratios and they are based on the powers of the square root of 2 (which is often rounded to 1.41 or even 1.4). If you have a circle with a radius of, say, 1 inch... and you calculate the area of that circle, then if you increase it's radius to 1.4 (well... really by the square root of 2... 1.4 is simply close but not quite accurate) then that new circle will be EXACTLY double the area. Hence as you increase the &quot;area&quot; of the opening in your lens that a factor of 1.4, then you DOUBLE the amount of light that can pass through that opening.<br><br>If you've got a kit lens... the 18-55mm f/3.5-5.6 the focal ratio is stated as a range which goes from f/3.5 (when the lens is at it's shortest focal length of 18mm) to f/5.6 (when the lens is at it's longest focal length of 55mm). If you were taking a shot using the 55mm focal length, your widest possible aperture is f/5.6. If you had an f/2.8 zoom (e.g. the EF-S 17-55mm f/2.8 IS USM is the closest) then that lens would literally collect FOUR TIMES more light when framing and focusing the same shot. That's because f/2.8 is 2 full stops larger than f/5.6 (smaller numbers indicate larger aperture sizes).<br><br>Being able to collect four times more light is not insignificant... it means you can use faster shutter speeds, or lower ISO settings, etc.<br><br>You may eventually want to invest in an inexpensive &quot;prime&quot; lens (a prime lens is a lens that does not &quot;zoom&quot;). The EF 50mm f/1.8 is very affordable... it's cousin, the EF 50mm f/1.4 has a higher build quality and much better performance (in terms of physical response and the quality of blur in out-of-focus areas) but does cost a bit more. The EF 50mm f/1.8 is the most affordable lens in the Canon lineup and to keep it affordable, the build quality involves more plastic than most lenses and it is not nearly as durable, but the optical quality is actually quite good.<br>
Tim Campbell
5D III, 5D IV, 60Da

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