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The AF problem on 5D3 (N70-300)

 
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humphrey



Joined: 31 Dec 2012
Posts: 2

PostPosted: Mon Dec 31, 2012 9:54 am    Post subject: The AF problem on 5D3 (N70-300) Reply with quote

Hi, conurus

I just bought the N70-300 from a friend. It was modified on 2009-6-15 (fireware 3.00k, circuit board# 0727084T).

There is some problems of AF performance on the 5D3.
The inaccuracy of AF at 100mm, 140mm, 200mm & 300mm could be corrected by 5D3 focus adjustment. But I could not correct it at 70mm.

I has microadjusted the AF at wide end to +20 and AF at tele end to 5+. 20+ is the upper limit of 5D3 focus adjustment.

But the focus is still 40mm in front of the AF target when the focal length is at 70mm.
Please see the photo which was cropped. The target of this photo is the nose of cat. There is a reference scale beside the cat.

Do you have any solution for the poor AF performance on 5D3? Thanks a lot.

Humphrey



I06A1872_load.jpg
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EXIF Information  Details
Camera make  Canon Camera model  Canon EOS 5D Mark III
ISO  1600 Focal length  71mm
Shutter speed  1/200 seconds Aperture  f/4.0
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conurus
Site Admin


Joined: 15 Aug 2006
Posts: 795
Location: Vancouver, Canada

PostPosted: Tue Jan 01, 2013 12:01 pm    Post subject: Reply with quote

Hi, thank you for your problem report and happy new year.

The previous owner may have put in some adjustment values in the lens. You may want to clear those adjustments first. Instructions tailored to your lens (s/n 12683272) can be found at:
http://conurus.com/index.php/contax/user-manual/micro-adjustment?sn=12683272&firmware=3.00k

In our experience, when we tune for autofocus accuracy, it is the most important to ensure that infinity and medium distances (~10m) are accurate, before attempting to adjust the closest focusing distance (1.5m). So, I wonder if the lens autofocuses accurately at medium distances and at infinity if the lens' adjustments have been cleared and the camera's adjustment is set to 0?

The 70-300 in particular is very prone to a problem where a very big adjustment value leads to grossly inaccurate results, whereas resetting everything to 0 doesn't look all that bad. The reason why this is the case is that Canon's total adjustment range is like -128 to 127 (which can be stored in 8-bit). There is already a very big factory adjustment value and if we pile another big adjustment value on top, 8-bits are not enough to hold that and it will wrap around, leading to a totally wrong result. The remedy is to use adjustment in moderation.

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humphrey



Joined: 31 Dec 2012
Posts: 2

PostPosted: Tue Jan 01, 2013 2:05 pm    Post subject: Reply with quote

Hi, Conurus

Thank for your detailed information.

I will try the infinity and medium distance.

Thank you again and happy new year

Humphrey
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