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When I was a kid my dad had an Olympus OM10, he had a bunch of lenses for it and I remember focusing was done with a special circle in the middle of the viewfinder, where you pointed it at the thing you wanted in focus and made the two half of the circle join up and focus was done.

These days I have an 1100D and my dad has a 400D, we both want to experiment with the lenses from the old Olympus. So I've ordered an adapter ring with an AF Confirm chip.

Because I don't reckon I could get anything in focus without that magic circle in the viewfinder, I'm hoping that AF confirm will help me. My understanding is it allows the camera to "tell me" when I'm in focus.

But how does it tell me? and do I have to enable some settings or be in a special mode for that to work?

  • I also recall that the screen (with the split-prisim circle) could be changed. There was a total blank or gridlines available. I wonder if a manual focus aid can be added to a modern pro-grade DSLR in that manner? – JDługosz Nov 26 '14 at 1:31
  • @jglugosz Yes, you can buy aftermarket focus screens for the major SLR brands. In my experience the focus screen and the manual lenses themselves make in-camera metering unreliable. Very unreliable. If you buy one of these, be prepared to use a handheld light meter. – Kyle Jones Nov 26 '14 at 1:39
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The AF confirmation actually works on a very similar principle to the old focus screen, in that they compare two different optical paths in order to determine exactly how in or out of focus the image is. The exact technique is different, but they accomplish the same end goal. The only difference is a computer is comparing the sides of the circle for you.

In a PDAF (Phase-detect auto focus) system, the camera is looking for the light entering the sensor to have the right relationship to each other. When it isn't (such as when the circles didn't line up), it knows that things are out of focus. Once the phases line up properly, it can confirm that the image is properly focused on whatever is under that focus point.

It normally communicates this by blinking or lighting up the focus point or by beeping. The focus confirmation chip provides a few key details about the lens that allow for this process to be completed.

  • "The AF confirmation actually works on a very similar principal to the old focus screen" Are you sure about that? I don't see how prisms and phase detection sensors have much in common. – JenSCDC Nov 26 '14 at 2:15
  • @AndyBlankertz - both can tell how far it is out of line based on comparing two sides. I didn't say the physics were the same, just the principle of being able to tell how much something is out of focus. – AJ Henderson Nov 26 '14 at 3:26
  • Got it. However, I think an extra sentence or two in your answer clarifying this couldn't hurt. I think it was the word "principal" which threw me off. And finally a nitpick- shouldn't it be "principle"? – JenSCDC Nov 26 '14 at 4:47
  • @AndyBlankertz - probably correct on both counts, I'll fix those. Thanks. – AJ Henderson Nov 26 '14 at 6:12
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With this chip, you will, when you half-press the shutter, get the red dot in the viewfinder and the beeping as confirmation of focus, just as if you used a modern AF lens in manual focus mode.
But, if you long for the split screen focussing aid, there are third-party focussing screen replacements you can install in your camera. (just for a quick example: http://www.focusingscreen.com/index.php?cPath=21_45)

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