20mm f1.7 for Night-time Street Photography

Started Feb 15, 2014 | Discussions thread
s_grins Forum Pro • Posts: 12,003
Re: 20mm f1.7 for Night-time Street Photography

Ontario Gone wrote:

Conor OBrien wrote:

This may sounds like a stupid question: Regarding the smallest square of the single point focus you mentioned, for example if I were to focus on a wall in front of me and used the largest square focus area on the center of the wall and compared it to the exact same shot, except using the smallest square focus are on the center of the wall, would the areas in focus be the same?

I want to know whether it creates a smaller area of focus or is it simply used to pinpoint a specific area to focus on that a larger square couldn't specify?

I have done a few tests with this and here is what it seems to do. A larger square offers more area for the sensor to pick up on contrast, but it does seem to try to lock on to whatever is closest to center. This is good as it will keep it most accurate, if you know it is doing this.

I tested this in two ways. I first shot my brick wall in multiple places from a 45 degree angle and every time it locked to where the center was. No problem with contrast anywhere in the frame, it was consistent in this test. The second test i shot at a pure white wall (no contrast) and it wouldn't lock, then when i kept a light switch just within the edge of the big focus square, it locked on the light switch every time. This tells me it will lock to the center most object it can find contrast on. Very good news. I would imagine this is how the AF squares work no matter the size chosen, but larger squares offer a greater chance to find contrast.

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"Run to the light, Carol Anne. Run as fast as you can!"

Based on my experience, small box makes AF lock faster than larger box. Based on your story, I'd deliberately point my smallest box on the light switch, lock the focus, and recompose. Try this too just to compare.

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