Sigma SA Mount

Started 2 months ago | Discussions thread
Jeffry7 Regular Member • Posts: 243
Re: focusing

xpatUSA wrote:

danski0224 wrote:

It is reasonable to assume that the Sigma auto focus sensor array works similar to what Canon has and other DSLR manufacturers.

The sensitivity can vary, which is evident in claimed "low light" center AF point performance (a -ev number is given).

The performance of the center AF point can also vary between f/2.8 and f/4 lenses.

You are correct that I am not quoting Sigma specs, and I do not have them memorized.

However, I have a Canon EOS 620 film camera with 5 (IIRC) focus points and I had a 5dII with a few more.

Either of those cameras will auto focus faster and more reliably than the Sigma DSLR's I have tried, just sticking with center point only. This is a reasonable comparison because the technology age is comparable as is the market segment (not a "professional" body).

I did some research to find out what is meant by "0 EV" as the low limit for AF on my SD15 (see the manual), Turns out that it specifies the amount of light on the AF sensor needed to make it work. A couple of relevant links:

https://photo.stackexchange.com/questions/95438/why-does-the-autofocus-capability-depend-on-the-aperture-rather-than-the-amount

Scroll down for very good answer to the question.

https://www.grantatkinson.com/blog/canon-dlsr-autofocus-in-low-light-compared

Involves your beloved camera maker and actually uses real light levels in lux rather than the less specific "EV".

In my opinion, the sdQ is a big improvement for AF accuracy compared to the SD1M and earlier, given Global Vision lenses. The DSLR will focus faster due to the different methods involved, and I feel limited to the center point only for consistency. The sdQ can focus lock on any point that I choose, which is nice (the dp/DP cameras also have many more AF points available compared to the sister DSLR). And while the sdQ is better, it is not a match to other cameras with live view focusing for speed.

I do not know if the sdQ AF performance changes with different wide open f stop lenses.

I imagine that it does, based on the references above.

Moderators: please pardon this totally off-topic discussion ...

I had thought the sdq and sdqh where contrast based focusing since they are mirrorless. But looking at the specs they are actually hybrids. They have both phase detect and contrast.

https://www.sigma-global.com/en/cameras/sd-series/specifications/

So the phase detect would be unable to work given the explanations XPAT posted, but the contrast should still work given enough light.

This reminded me about something I read for the fp. The l-Mount teleconverter does not restrict the aperture that lenses will work at. I didn't understand it at the time, but it is because the AF in the fp is contrast only. All the lmount cameras are.

Relevant links

https://www.sigma-global.com/en/cameras/fp-series/specifications/

https://www.sigma-global.com/en/lenses/cas/product/accessories/tele-converter/

In the 2nd link look for the bold text "compatible with autofocus"

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