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09-06-2010, 08:52 PM   #1
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Autofocus test

I recently bought a used 77ltd and because I have a short guarantee so if there are any problems then I want to find them before the guarantee runs out. Over the past few months I have got the impression that it misses focus more often than my da70 does and when my da70 does it is my fault. I'm a bad photographer, my judgement is not always correct. So tested this impression by photographing similar scenes with both lenses. The da70 didn't miss once. Attached are the images the 77ltd struggled with.
AV, center point,
1 The centre point is on the silver metal post. I expected it to miss and it did every time


2 The centre point is on the left red and white square. I did not expect a problem but all I got was a flashing green hexagon.

3 The centre point is on the blue area next to the large s. I expected it to struggle because the tones are similar.

The 77 should, because it focuses at a wider aperture of f1.8, be more accurate than the 70 but it is not. The da70 had no problems focusing.

Therefore I tend to think it is a lens problem and not the autofocus mechanism. Unfortunately I live in Japan and because my Japanese language ability is limited I cannot discuss this with anyone here so I rely on the good will of the experienced members on forums like this for advice.

Any advice would be much appreciated.

Thanks

Attached Images
     
09-07-2010, 11:31 AM   #2
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Are you using center focus, or letting the camera choose the focus point? If it's the latter, I doubt it's the lens's fault.

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09-08-2010, 07:34 AM   #3
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Are these the 77 or the 70 pictures? They all seem in focus the way you want it :S
(I'm not sure which is the silver metal post...)
09-08-2010, 10:29 AM   #4
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QuoteOriginally posted by kiwao Quote
The 77 should, because it focuses at a wider aperture of f1.8, be more accurate than the 70 but it is not. The da70 had no problems focusing.
And the converse is that the shallow DOF of f1.8 will magnify a focus error more than f2.4. Did you use the same aperture for both lenses? (Exif was stripped)

Well, testing methods aside, you're using autofocus - does your camera have per-lens AF adjustment? (K20D, K-7 have it in a Custom Function, and for K10D, K-x it is in "debug menus")

Why does this function exist? Cameras and lenses are consumer goods, and there is variability in manufacturing. This variability can mean that even with two copies of the same lens, one lens is tack-sharp and another either back- or front-focuses on a particular camera body due to tiny variations in the manufacturing and assembly (but all within "acceptable quality tolerances").

It is quite possible that your new 77mm needs a slight adjustment to your camera body, while the DA70 did not. If you have a different camera body (or go to a big store and try one of theirs) you may very well see the opposite results! I bought a lens from a forum member here that couldn't give him sharp results, but it is just crispy on my camera even without adjustment!

The 77mm in general should be "as sharp" as the DA70 at comparable apertures, but the "rendering and bokeh" of these lenses is quite different once you delve deeply into the images. Back or front-focus you can adjust for, but that "magic" of certain lens designs cannot be replicated. Then again, if you FA77 isn't consistent in focus across the field (blurry in one corner, etc.) you can't adjust for that... and it isn't magic!


Last edited by panoguy; 09-08-2010 at 10:36 AM.
09-09-2010, 12:55 PM   #5
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QuoteOriginally posted by kiwao Quote
1 The centre point is on the
The sensor isn't actually a point. It's a cross with quite long arms. It's worth taking the time to map out how big it is. You can do that by putting a black dot on a white sheet of paper and trying to auto-focus with the dot at different positions in the viewfinder. If none of the arms of the sensor cover the dot, and cover plain white instead, the camera won't focus. Where the camera will focus, the dot must be covered by the sensor.

I don't think you say what camera you have, and in any case the exact position can vary. For what it's worth, my K-x has viewfinder marks like [[()]], and the centre sensor cross extends the full width of the ().

I mention this because it seems to me that in your first image, the sensor cross will have covered more than the silver box, and the camera may therefore have decided to focus on the fence behind it.
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