Your camera's AF system is a contrast detect system. It cannot lock focus on a blank white wall because there is no contrast on the wall. But if this wall were to have black vertical stripes, your camera should achieve a locked focus.
When focusing shots on people, aim for their eyes. Typically, with the eyes being white and some other colour surrounded by skin, there is enough contrast for the camera to achieve focus on the eyes. However in too dark situations, your camera's AF just simply cannot lock focus.
To bypass the inability to lock focus in too dark situations, one can:
- Manual focus (this is difficult depending on your setup)
- Use a wider aperture lens (it lets more light in for your AF system--your 50mm f1.8 will perform admirably in dark situations compared to your 18-135mm)
- Use a speedlite (external flash unit) with a focus assist beam (kinda pricey depending on model of speedlite used and the speedlite flash will change to outcome and look of the image... but the flash can be disabled if need be)
Achieving focus aside, shutter speed must be taken into consideration for a sharp image.
The general rule is that your shutter speed should equal 1 / the focal length you are using (this is for 35mm sensor cameras).
Since your camera isn't a 35mm sensor, your general rule will actually be 1 / (the focal length you are using * 1.6)
Example:
Using your 50mm f1.8
Your shutter speed for an acceptably sharp hand held image is 1/80 seconds because
1 / 50*1.6 = 1/80.
That said, your mileage may vary and you might find that you can get away with a 1/60 second or even a 1/50 second exposure (I know I can). Sometimes, you might even need to use a 1/200 second exposure because you've got the jitters from too much coffee or your subject is simply moving too fast.
T3i | Σ 12-24 4.5-5.6 II | C 18-55 IS II | C 50 1.8 II | C 70-200 4L | C 85 1.8 | 600EX-RT | 580EX II | 430EX II
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