AF test chart

a few weeks back someone posted a link to a website that had an on-screen AF test target that used moire patterns to check for accurate focus, I meant to bookmark it for future reference but must of forgotten, does someone remember the post thread or the page I'm talking about?
Sorry to ask, but I have searched the forums and can't seem to find it!

Thanks!

I think this is the link you are referring to:

http://www.openphotographyforums.com/fo ... php?t=4708

Tom

Do a search on "moire"

There is also a nice tutorial on northlight-images.co.uk

Thanks! thats just what I was looking for.

I see no need to buy any additional tutorials or moire images, the text found on the site below works and I highly recommend all take advantage.

Best,

Chas

yes, just downloading the test targets and following instructions on the web should be enough. Teles are easy to fine tune, wides are a bit trickier.

Unfortunately the target in the link is not ideal for accurate testing.
The background detail is too small for many AF detection lines to read with 100% accuracy, and taking a narrow horizontal or vertical strip (which is what AF reads) results in considerable symmetry of detail. Considerable symmetry of detail is not ideal for reliable AF.
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By way of explanation with a target consisting of 9 symmetrical details eg aaaaaaaaa if the AF attempts to read the 3 central aaa's it may do so with good focus. However if it reads three a's to right or left of centre in error because of the symmetry of detail there is front or back focus due to insufficient readable detail.
If the detail is not symmetrical as in abcdefghij and the AF sensor covers def it splits what it sees into a left and right image and compares the two images. If it sees left and right images as def it knows AF is accurate. If it sees the left image as cde and the right as as efg (or vice versa) it recognises the difference and corrects the focus.
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The longer the focal length and the faster the aperture the more distinct the target detail needs to be for perfect AF.
The reason is depth of focus (what happens at the sensor surface) is the reverse of depth of field except in extreme close focus.
It can be difficult to manually focus an extremely wide slow lens because the out of focus depth of field effect is low.
At the AF detector surface (at the same distance as the sensor) detail is the exact opposite with separation between objects is much greater with wide angles than through the viewfinder. This is why AF can be much more accurate than manual focus with a wide angle using a typical AF camera viewfinder.
With a 300 f2.8 the depth of field separation through the viewfinder is significant, but the image at the sensor can be as indistinct (for distance separation) as a slow wide angle appears through the viewfinder.
This is why AF with a long fast lens is less efficient than with a slower lens.
I first encountered this optical issue on African safari when I discovered the Nikon 80-400 at 300 (it is f5.6 at 300) could read zebra stripes at a greater distance than the 300 f2.8.