Thom finally resurfaces, with comments sure to be controversial, at least here ...

Started Apr 17, 2014 | Discussions thread
OP photoreddi Veteran Member • Posts: 7,432
Re: Even More Controversial!

fotophool wrote:

Thom's follow up

I'm sure that it will be. I suspect that what he wrote comparing the X-T1 and the E-M1 will be of more interest here and might add to the controversy. He also disproves the false charge written here that he thinks camera manufacturers/engineers are stupid, that they should consult with Thom. Hmm, Thom actually writes :

Back in the film days I was able to talk to Nikon engineers about how the focus system worked. When I did, I learned things about the system that weren’t in any Nikon literature. The F5 “special” custom setting focus mode, which prioritized left-to-right detection order was actually quite useful for some subjects. But you wouldn’t have learned about it from Nikon’s manuals. At least not learned anything useful about it. Give a half dozen of us third-party Nikon documenters a day with the autofocus engineers to ask questions and get answers and I can guarantee you that your camera will focus better the next day ;~).


The thing about most mirrorless systems is that they use contrast detect to do focus or to verify a phase detect focus position. Contrast detection is highly accurate (assuming the right subject is in the autofocus sensor position). Phase detection is very fast and understands subject movement directly. The two together can be used to create both a fast and accurate system. Separately, they each have their weaknesses. The X-T1 and E-M1 I was using for the past two weeks had different weaknesses in that respect. My sense was that the E-M1 was faster to initial focus, less reliable on moving subjects, while the X-T1 was slower to initial focus, more reliable on moving subjects once focus was achieved. Neither is better than the other, they just have different optimal uses.

Hmm, could this be similar to what Tony Northrup discovered in his second video, with respect to AF tracking?


I've also long had suspicions about the accuracy (or as Thom wrote, "precision") of on-sensor PDAF 'pixels', knowing that the precision of DSLR AF sensors vary, depending on whether they're designed to sample light from the f/2.8, f/5.6 or f/8 part of a lens. They're actually little split image rangefinders, and the greater the base distance, the more accurate.

Phase detect on imaging sensor focus systems have one big weakness, though: the math precision needed is getting out of hand. I’ve written before that basic phase detection put on the imaging sensor is variable with sensor size. Why? Because the “depth” between the splitter optics and the detector is very small in the image sensor type of phase detect system, which gives you very small discrimination numbers. So small, that as the DOF decreases due to larger sensor size (CoC increases), you don’t get a lot of precision. At least for the current implementations. This is one reason why the Nikon 1 does so well with phase detect on sensor and the Sony A7 doesn’t.

There are many more interesting points in today's Thom. Maybe these limited quotes will whet some appetites to see the rest, even if some will also find them controversial.

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