Do slightly misaligned viewfinder AF point markers or focus screens affect autofocus accuracy?
Asked 5/12/2014
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On a Canon 1Ds Mark II with an Ec-D grid focusing screen, the illuminated AF point markers in the viewfinder appear shifted slightly to the right relative to the screen grid. The offset is small—about a quarter of the width of one AF-point rectangle. Autofocus performance seems normal, but I’m wondering whether this indicates a problem.
Could this be caused by the focusing screen being seated incorrectly, the AF point illumination/projection being slightly off, or an actual autofocus misalignment? Would a small mismatch like this affect focusing accuracy, and is there anything worth correcting?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
12y ago
2 Answers
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It's worth noting that even the focus point LEDs can be off the mark and may not accurately show where the focus really is. Knowing this, you can see why it'd be hard to say what problems a misalignment of focus screen and point indicators will give.
Most likely, if there is any issue, it'd be that the focus point you're expecting to be in sharp focus is out a little. But of course if the LED is also out you risk pulling out your hair to find the issue. I know my dSLR focus slightly out of alignment with the focus point indicators and I've got used to it.
As to it being corrected, you could always ship it back to the manufacturer and have them look at it ($$$), have the focus screen re-seated/replaced to see if this improves things, or live with it. If it's a minor misalignment I'd be tempted to live with it. If it niggles you a lot then it's probably worth spending some cash on it to get it fixed.
Originally by user27977. Source · Licensed CC BY-SA 4.0
user27977
12y ago
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Probably not. On DSLRs, the AF point boxes you see in the viewfinder are only an approximation of the real autofocus sensor positions and coverage. A slight mismatch between the illuminated markers and the grid on the focusing screen is usually not a sign that the autofocus system itself is off.
A mis-seated or different focusing screen generally does not change phase-detect autofocus accuracy; it can affect what you see in the viewfinder and sometimes metering, but not the AF module’s actual operation. It’s also possible for the viewfinder LED indicators themselves to be a little off.
If your camera is focusing reliably in real use, this small offset is unlikely to matter. At worst, the actual area used for focus may be slightly different from where the red box appears.
If it bothers you, the practical options are to re-seat or replace the focusing screen, or have Canon/service check alignment. But for a minor offset with no observable focusing problem, most photographers would simply live with it.
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