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EOS 70D focus issue question

Molybdo42
Enthusiast

Hi,

 

I'm having a hard time using the phase AF on the 70D. The problem is most visible when I try to focus on distant subjects with fast lenses. I need to have targets with black and white contrast, otherwise most pictures are completely out of focus, even in situations where my older Rebel T1i can focus (I did micro focus adjust each lenses).

 

I have several questions :

- First there is a small detail that isn't very clear in the specs regarding the high sensitivity focus point for fast lenses.

In the manual p108, it is said that with lenses between f/1.0 and f/2.8 :

Besides cross-type focussing (vertical and horizontal lines detected simultaneously), the center AF point can also perform high-precision vertical line sensitive AF. The remaining 18 AF points perform cross type focussing as with when maximum lens aperture is f/3.2 - f/5.6

 

And in the datasheet it is said that :

Center AF point is cross-type at f/2.8.

 

That difference is kind of misleading : does it mean that the AF sensor is different from the 7D, who has dual cross type focus (turned 45°) ? Is my camera unable to perform high precision AF on horizontal lines ? Is the 70D's AF sensor completely different from the 7D ?

 

 

 

- Second question : I measured where the sensitive AF areas are located by using my computer screen and moving a black box until I reach the limit where each AF point is able to lock focus (parallelism of the camera and screen was checked with a mirror taped on the LCD screen then removed).

In another test, I displayed a drawing of the 70D AF grid on my computer screen, I aligned it perfectly with the marks visible (with my eyes) through the viewfinder, and took a picture. I than compared it to the AF points displayed in Canon DPP.

 

After otherlaying each pictures in photoshop I obtained this picture :


my 70D af sensitive areas and viewfinder grid misalignment

 

Legend :

- lines in green : the AF grid drawing aligned through viewfinder

- boxes in red : AF boxes displayed in DPP

- transparent rectangles in : red, green, yellow, and blue ; the measured AF sensitive areas

 

 

I compared it to the AF sensitive area from a 7D (image taken from this website : http://www.aerien.ch/articles/31/Canon_7d_Reglages_Photo_Nature_et_Oiseaux.php).

7D AF zones

 

 

There are several thing that look like the 7D, for example the lack of AF between several AF points, the overlapping of several AF points...

I understand that the AF sensitive area always extend beyond what is displayed through the viewfinder.

I also understand that there can be some misalignment of the AF grid visible through the viewfinder with the center of the CMOS, and that this misalignment doesn't affect AF performance (except when framing).

 

But I find it disturbing that some marks are almost completely outside the AF sensitive areas.

 

Is this a "feature" to compensate the lack of precision mode spot AF on the 70D, or is my camera's AF sensor completely misaligned ? If the sensor is misaligned, can this be corrected by sending it to canon or do I have to ask for a refund ?

2 REPLIES 2

bja5150
Apprentice

I just posted a similar focus issue. The issues with my focus is at the wide open settings, which are my favorites. With say my 70-200L lens...those 2.8s will create beautiful blurred backgrounds I want. I get the photo I want, but the focused points are not producing tack sharp shots. Frustrating. I have deteremined it not a lens issue since I see those issue across my other lenses. Man I need to get thos figured otut. Its driving me nuts.
Brent

Canon 70D, Canon 70-200L Series II, Sigma 2.8 24--70 (for now ;)...)

ebiggs1
Legend

My first thought is this isn't a valid experiment or measurement.  There is too much tolerance between the format you are comparing.  

 

Cross type focus points are basically just a spot where the AF is sensitive to both vertical and horizontal lines.  Regular focus points are just sensitive to vertical lines.  If I am following your thoughts.

EB
EOS 1DX and 1D Mk IV and several lenses!
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