Lens markings

[Mr Felix]

Looking at the new lenses which have a better "auto-focus" function than the older ones, I see that the distance markers on the focus ring are missing.

As are the F-STop numbers to help DOF calculations.

With the new digital cameras, what do they use to detect focus?

"Sharp image detection" or do they still send out a "magic beam" like the older ones used to?

I'm somehow going for the former.

[fillum]

I see that the distance markers on the focus ring are missing.
As are the F-STop numbers to help DOF calculations.

I don't know for sure but probably a combination of things. Many modern lenses don't have aperture rings so there is no reason to mark apertures on the lens. Regarding DOF, photographers can now get immediate feedback on DOF on the camera's LCD, and then change it as required. Many cameras also have a DOF-preview button that also allows checking DOF before taking the shot. Another reason would be that zoom lenses are now probably more commonly used than primes and a DOF scale is not practical because DOF varies with focal length. (Although older style "push-pull" zooms often had a DOF scale on them).

Note that there are exceptions to this such as Zeiss ZF lenses for Nikon which still have aperture rings and DOF scales.

With the new digital cameras, what do they use to detect focus?

In Nikons, a sensor (separate to the main image sensor) is used. The image is split into two and the camera determines correct focus by measuring distance between the two images (known as "phase-detection"). For LiveView the camera measures image contrast in the focus area and moves the lens till max contrast is achieved. Cameras (and some flash units) can send out a beam of light but this is to brighten the subject to help focus in low light. I assume that Canons and other makes operate similarly. There's probably plenty of info on the web if you want more details.


Cheers.

[Mr Felix]

Thanks.

It was only a "passing interest".

So Phase detection huh. Wow that sound neat.
Neraly Star-Trek-ish.


I have seen the beam of light in low light. Yeah.

Thanks for the info.