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05-17-2008, 09:45 AM   #16
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Gheesh, I don't know what kind of arguments I have opened myself up to, but as a mechanical (I admit, not an optical) engineer, I could no longer resist..

I have seen this test chart, and one (of many, many) problems with it (from a scientific point of view) is that you are focusing on the thing in the center (ok) but reading results from the outer edge (not ok) we are talking about a wide angle lens here, this lens has a fairly decent curvature of field, Especially wide open......

I would never expect the area directly adjacent to that focus line to be in focus with this lens (or anything short of a telephoto macro, shot from farther away, which would have a much flatter plane of focus, but even then, well I'll leave it at that for now)...

However, I would expect with this lens, if you filled the frame as the instructions state, and you shoot wide open, as instructed, that if anything at all at the sides were in focus, it would be the area in front of, and not adjacent to, or further back.

05-17-2008, 12:36 PM   #17
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Interesting august. I didn't quite understand that last sentence. Could you explain what you meant in more detail?

I have gone back to just checking focus by pointing the camera at my daughter's face! I should never have wasted any time on this "focus chart!"
05-17-2008, 02:32 PM   #18
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Pentax Poke:

as soon as I find a particular published article I'm looking for, I'll post the link.

It will explain this better than I can, and it is a much more dependable source than I am.

Wide angle lenses, for the most part, and dependent mostly on optical design, have a slightly curved field of view (which is how it can put a larger area of view on a particular sized target (film or sensor size), which results in a slightly curved plane of focus. Sometimes, zooms more than primes, because of how zooms need to be constructed to accommodate varying focal lengths. If you shoot a chart straight-dead-on flat wide open with a lens that is perfectly in focus you will notice the center being sharp, and the edges and corners less so.

If the sensor was a mere dot, it would have an equal distance to the center of the curve as it would the sides. Then there would be no such thing as barrel distortion or pincushioning. But our sensors are rectangles, and placed within an even slightly curved dome, the edges and corners especially will be closer in distance to the sides, which accounts for edge and corner softness and vignetting. And which is why full-frame lenses fare better as well.

As you step back (depending on the lens design) this improves, and if you step down to get a greater depth of field, it improves more. So I would not want you to over compensate for the edges and corners when adjusting your AF, and thus winding up with a center focus that would then be out of focus. I know you don't have access to an optical bench for accurate testing, but if you could shoot a focus target that you shoot straight on with your camera perfectly centered vertically and horizontally, with the camera perfectly perpendicular to the target, so that the target is centered without any tilt or angle. And have the target occupy the center only, then do your -10 to +10 and look at the results on the computer and see. If it starts out not quite fine and keeps getting worse, your focus is beyond -10, if it starts out not so fine and gets better then worse, then where it is best it most likely a good starting point for real world testing outdoors. Remember most likely a wide angle lens wont be tack sharp wide open across the entire frame.

I'll try to find that report and get back to you when I do find it.
05-17-2008, 02:57 PM   #19
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Hi all,

When I had my Pentax service centre do my Z1-P film body to check focus, they put a SPECIAL lens on it and inserted it into their testing machine (all Pentax equipment). It was the only way to adjust/test the cameras properly apparantly.

My 10c

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