D3100/18-55 backfocus

Started Mar 4, 2012 | Discussions thread
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blue_cheese Senior Member • Posts: 1,850
D3100/18-55 backfocus

I got so irked at all the D7000 focus complaints and charts, and finally said hey what the heck let me try out one of those focus tests.

The reasons I was concerned about focus operation were:

  • When I pixel peep, i find it difficult to believe that the 18-55 is supposedly as sharp as everyone reputes it to be

  • When I use mye 50mm AI-S, the focus indicator lights up but the image is obviously backfocused, if I back off the focus just before the threshold where indicator lights up everything is fine

So rather than a complaint, I am just curious if anyone can provide possible explanations for my observation. I should say that I find the focus accuracy acceptable and other than the quirk of focusing the AI-S lens I dont have any practical problem with the focusing.

  • When focusing the 55-300 on the focus chart there are no apparent issues

  • When focusing the 50mm AI-S there is no issue and it is correctly focused right when the indicator lights up, however in real world shooting I have to back off the focus to when the indicators just shuts off. Why is that? Does the AF system loose accuracy with further away subjects even if they are relatively large across the focus point?

  • the 18-55 shows backfocus between 18mm and 30mm. When using OVF thhere is about 20mm backfocus according to the chart, when using the live view it is dead on. at 35mm to 55mm it is dead on. Would this explain why sometimes i find the shots from the 18-55 to be soft? is it a possible problem wiht the lens or the AF system? I dont have any other wide lenses to compare, but would the phase detect AF have problems at wide angles and why?

Just curious

Nikon D3100 Nikon D7000
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