20D high-precision AF sensor

[csnudelman]

I just read on another forum that the 20D high-precision AF sensor only works with lenses of f/2.8 or faster. Is this true? Please explain. Thanks, Craig

[scottbergerphoto]

It is true.
http://web.canon.jp/Imaging/eos20d/index.html
http://www.bobatkins.com/photography/digital/eos20d.html

[robertwgross]

I can accept that a high-precision AF sensor gets better results than the standard one.

My question is how or why?

I can accept that lens aperture information is passed from the lens to the camera. But then what? Why must this be limited to f/2.8 lenses or faster?

Maybe my question is beyond the realm of idle conversation.

---Bob Gross---

[RDKirk]

I can accept that a high-precision AF sensor gets better results than the standard one.

My question is how or why?

I can accept that lens aperture information is passed from the lens to the camera. But then what? Why must this be limited to f/2.8 lenses or faster?

Maybe my question is beyond the realm of idle conversation.

---Bob Gross---

The AF sensors are single-line pixel arrays, either vertical or horizontal, with the center sensor normally being a crossed vertical and horizontal array. At f2.8, the camera turns on a second vertical pixel array in the center sensor area.

The effect of the second vertical array works in conjuction with the narrower depth of focus (at the sensor plane) and the brighter image to increase the focusing accuracy. It needs all those factors to make the difference.

[robertwgross]

Interesting. I'm going to have to visualize that and think about it for a while.

Do we know why f/2.8 is some magic number? Why couldn't f/3.5 or f/4 be made to work this way?

---Bob Gross---

[EricKonieczny]

SO my questions is , what happens if you are using the 70-200 2.8 IS lens, at f11?

Is it still useing the high precision AF sensor, since you are using the lense, or do you have to be at f2.8 or faster to use it?

[smirchfa]

My understanding has been that the high-precision AF sensor simply needs the extra light provided by 2.8 or faster lenses to operate at the highest level - therefore, 2.8 is not really a "magic number," it just happens to be the smallest setting in which enough light enters the camera for the sensor to function properly.
And Eric, any lens with a maximum aperture of 2.8 or larger will work, regardless of what aperture you are using to expose the image. This is because when you are focusing and composing, you are always seeing through the maximum aperture of your lens. The lens doesn't change its aperture until the moment of exposure. At least that's how I understand it - someone feel free to correct me if I'm way off base!

[lancea]

It must be a manufacturer's decision (most likely the Sales Department), as the level of light reaching the AF system is surely much more dependent on the brightness of the scene than the aperture of the lens ...

[lost]

NO your all WRONG, its a conspiracy to sell more L 2.8 lenses!!!!!!!!!!

[tim]

At least that's how I understand it - someone feel free to correct me if I'm way off base!

You're correct :)

[CyberDyneSystems]

And Eric, any lens with a maximum aperture of 2.8 or larger will work, regardless of what aperture you are using to expose the image. This is because when you are focusing and composing, you are always seeing through the maximum aperture of your lens. The lens doesn't change its aperture until the moment of exposure. At least that's how I understand it - someone feel free to correct me if I'm way off base!

That's 100% correct :)

The lens is allways idle at "wide open" on or off the camera.

It takes electricity from the camera to "stop it down" and that only happens when you press either the shutter or the DOF preview button.