Hacker News Reader

Comment by etrautmann | Hacker News Reader

I'm (somewhat sure) the DSLR allowed for a separate light path to a separate AF sensor, so now it has to be integrated into the same sensor which has some cost in terms of sensor area and resources devoted to AF vs. regular imaging? Not an expert so please someone correct me.

View on HN

It’s a separate path, the sensor is under the camera (this allowed it to work with film). 2 sensors per autofocus point which it tries at align. It’s actually kind ofvv be amazing it works.

Gordy details, but the image helps see:

https://photographylife.com/how-phase-detection-autofocus-wo...

On camera sensor focusing was slightly different “contrast detection”. My understanding is modern cameras integrate those phase detection sensors on the image sensor itself. It might effect image quality but nobody notices and focus is fast and reliable. It’s tech that works well so generally nobody cares how it works.

https://www.usa.canon.com/learning/training-articles/trainin...

As far as I understand it, there’s no AF “sensor” on mirrorless cameras apart from the image sensor itself. They just adjust the focus until maximum contrast is achieved - but obviously it’s more complex than that.

You don't have to think about the physical image sensors as just an array of sensors with 1:1 correspondence in the final picture. There was always a lot of computation in between - bayer filters with more green pixels, different grid patterns, multiple sensors per pixel, more pixels on the sensor than in the actual photo and so on.

Recent sensors have AF phase-detect sensors built into the image sensor, by splitting one "pixel" into two sensors (and having lots of them; sometimes in Canon sensors more than half of the pixels have this functionality; on some cameras you can extract the raw image including the different data on those pixels for bigger resolution). The difference between phase detect and contrast being that a phase-detect sensor will give exact values on how to adjust the focus, whilst contrast detection needs focus hunting in small adjustments, so it's slower.

Does that affect overall sensitivity?

Probably yes, as it means less actual sensor surface, so it's a tradeoff between better focus and noise. But it's hard to quantify without comparing electrical signals.

Reply on news.ycombinator.com