Backfocus on my 7D?

[spurssy]

I've done a simple test to check if my 7D has backfocus prob. The results... satisfactory.

Only on 1 case where there seems to be focusing problem, but that was when I focused very closely, although from the viewfinder the focus seems good.

Testing procedure as follows:
- AF set to center lock.
- Aperture Priority at f2.8
- Mount on a tripod, used 2 sec self timer.
- Lens used: 28-75mm (D)

Here are the results:


Horizontal Focal target: line 35
Distance from Lens filter to Focal pt: 56 cm
Good result, line 35 is sharp, line 34 & 36 slightly (equally) blurred
----


Horizontal Focal target: line 15
Distance from Lens filter to Focal pt: 31 cm
Good result, line 15 is sharp, line 14 is not sharp but better than line 16
Note that minimum focal length of this lens is about 30cm (not very sure how is this measured) so this should be minimum focal distance to achieve good AF.
---


Horizontal Focal target: line 2
Distance from Lens filter to Focal pt: 18 cm
Poor result, line 2 is NOT sharp, line 1 is sharp instead
My guess is below minimum focal distance, the AF doesn't work properly.. funny thing is; when view through the VF after AF, line 2 is the sharpest. Seems that in this case, the sensor doesn't capture what was seen on the VF.
---

That's it... any comments?

[Drudkh]

Originally Posted by spurssy Seems that in this case, the sensor doesn't capture what was seen on the VF.

how can this be? i can't think of any reason cos you're using a tripod.

[Agetan]

Originally Posted by spurssy I've done a simple test to check if my 7D has backfocus prob. The results... satisfactory. Only on 1 case where there seems to be focusing problem, but that was when I focused very closely, although from the viewfinder the focus seems good. Testing procedure as follows: - AF set to center lock. - Aperture Priority at f2.8 - Mount on a tripod, used 2 sec self timer. - Lens used: 28-75mm (D) Here are the results: Horizontal Focal target: line 35 Distance from Lens filter to Focal pt: 56 cm Good result, line 35 is sharp, line 34 & 36 slightly (equally) blurred ---- Horizontal Focal target: line 15 Distance from Lens filter to Focal pt: 31 cm Good result, line 15 is sharp, line 14 is not sharp but better than line 16 Note that minimum focal length of this lens is about 30cm (not very sure how is this measured) so this should be minimum focal distance to achieve good AF. --- Horizontal Focal target: line 2 Distance from Lens filter to Focal pt: 18 cm Poor result, line 2 is NOT sharp, line 1 is sharp instead My guess is below minimum focal distance, the AF doesn't work properly.. funny thing is; when view through the VF after AF, line 2 is the sharpest. Seems that in this case, the sensor doesn't capture what was seen on the VF. --- That's it... any comments?

If you notice the top of the 7D, you should see a little circle with line across it, that is approximately where the sensor is, to measure the focus distance you need to measure from that point to the subject.

Try that... I think sometime, because it is extremely close, you need to measure the fpcus distance correctly... and becareful how you doing it.

that's my thought...

[spurssy]

can someone confirm how is minimum focal distance calculated?

[spurssy]

Originally Posted by Drudkh how can this be? i can't think of any reason cos you're using a tripod.

It sure did... maybe you could try it with a similiar config... focusing on something as near as possible.

[Drudkh]

Originally Posted by spurssy It sure did... maybe you could try it with a similiar config... focusing on something as near as possible.

i have that lens, i'll try out tonight.

[Zerstorer]

Originally Posted by spurssy can someone confirm how is minimum focal distance calculated?

Subject to focal plane. That's what the focal plane indicator on most cameras are for. Basically, Agetan has already provided the answer.

If you were under the minimum focal distance, you would not be able to focus at all.

[Drudkh]

here's my test at minimum focusing distance

KM 28-75 F/2.8 (D) @ 28mm, ISO 400.
100% crop

full resize


100% crop

full resize


the above were taken handheld using my elbow to support on the table. no sign of front/backfocusing.

[TME]

Originally Posted by Zerstorer Subject to focal plane. That's what the focal plane indicator on most cameras are for. Basically, Agetan has already provided the answer. If you were under the minimum focal distance, you would not be able to focus at all.

Yes I thought so too.... but it's quite evident that the subject is not under the minumum focal length since "line 1" is in focus... it's not backfocussing either.... Spurssy, is this reproducible?

[spurssy]

Originally Posted by TME Spurssy, is this reproducible?

Ok, here's the good news, no matter how I try to reproduce (handheld though), I can't get back the same result where the focused point wasn't sharp.

So I thought I made a mistake and remembered the wrong focused point.

Now, here's the bad news... I went back to take a look at the uncropped photo and attempt to map out the exact centre, guess what, I didn't make a mistake. It was as what I've described previously. Take a look at the following photo, uncropped, but at 16.7%. Take note of the center point as determined through PS CS.



Really weird!!!

EDIT: One thing I forgot to add, all the photos were taken at the telephoto end, not sure if this would affect anything.

[TME]

Well I won't worry too much.... might just be a random error....