Canon seems to be focusing 6" short???

[Tiggsy]

I have only had my camera a short time and found it very hard to get sharp images when using low DOF (which i do a lot as i shoot indoor sports in low light)

Took some pics of a bird on my trellis today notice the bird was unsharp but the trellis 6 inches closer was perfect. I have checked the focus point i was using and its bang on....could my camera be "wrong"?

7D with 85mm 1.8f


Tiggs

[tangerine_sedge]

Sounds to me like the camera focused on the Trellis. Can you set just the centre focus point and do some more testing?

[matheson]

Download a focus chart and do controlled testing. I had a 24-70L (on a 300D/30D/40D) that back-focused so much it was taking images of my back.

Try here for a chart http://focustestchart.com/chart.html

[Simpo Two]

Can't comment on the 7D specifically but modern DSLRs have a bewildering choice of AF options. Apart from single selectable point AF, mine has group AF, dynamic AF, tracking AF, closest subject AF and probably more I'm not interested in, like trellis AF. So just because you had the bird in the middle doesn't mean that's where the camera will focus. Have you checked settings and/or the manual to see what it's doing?

[Tiggsy]

Its sent for single point focus and my PC software shows me what area the camera was focusing on....and its not sharp there.

Will try the charts - if it does back focus what can you do?

[Simpo Two]

Tiggsy said:

Its sent for single point focus and my PC software shows me what area the camera was focusing on....and its not sharp there.

Will try the charts - if it does back focus what can you do?

If you have a problem it's front-focusing; the sharp trellis was in front of the blurry subject, not behind. But don't panic yet - do some more tests and confirm whether the trellis threw it off somehow.

Mind you an 85mm lens at f1.8 is a steep test for AF; it doesn't always get it right!

[ian in lancs]

Quick check - focus at 45 degrees, 1m ish away on a simple line of black on white text in a magazine with the lens wide open. Take a shot (flash is best with the fastest shutter speed you can sync at) and ensure single focus point set to single, not continuos focus. Zoom in 100% on the camera LCD and the line focussed on should be sharp, obviously and the focus should drop off evenly lines above and below. Do several shots and compare focus points for consistency. If inconsistent there is either lack of contrast and AF is struggling or AF is faulty on either lens or camera. If it is consistent and sharp on the line, fine. If consistently sharp above or below the line then the lens/camera need adjusting. On Nikon's this can be done via the set-up menu. Quite simple to do if time consuming. In addition I auto-imported the pics and looked on a big screen to make it easier.

Focussing at 45 degs makes dof easier to see and it is possible to shoot square on and move the focus point fore and aft until sharp. Either method is fine although you'll be challenged by lens resolution as much as focus point. You could try a stop or two down for sharper resolution. Theres loads on the 'net about focus calibration.

I spent Sunday morning playing about with a couple of Nikon bodies and a few lenses. Most were in single digit adjustment but one was at the max of +20 units.

Good luck it will send you loopy!

[Cabinet Enforcer]

Simpo Two said:

If you have a problem it's front-focusing; the sharp trellis was in front of the blurry subject, not behind. But don't panic yet - do some more tests and confirm whether the trellis threw it off somehow.

Mind you an 85mm lens at f1.8 is a steep test for AF; it doesn't always get it right!

Doesn't a 7D have AF adjustment??

To check if the phase detection AF is 'off', do test shots on a focus chart with phase AF then with live view contrast AF. Contrast AF focuses with the sensor itself so will be correct.

[RobDickinson]

Cabinet Enforcer said:

Doesn't a 7D have AF adjustment??

Yes it does.

How I (and how canon recomedn you MA a lens).


Setup a test target (resolution chart) and put the camera on tripod, use remote release or timed shot.

Shoot wide open and at 50* the focal length (for 85mm this wil be 4m)

Use live view at 10* and manual focus - this will be your best comparative shot.

Now use custom function manual focus adjust and shoot at -20 -15 -10 -5 0 +5 +10 +15 +20.

Compare on the PC screen at 100%

Pick the best one vs the live view shot.

Now shoot 5 each side of that ( so if -10 is good, shoot -14 to -6 in 1 steps).

Pick the best, try out in the real world.

(another hint will be ca, or bokeh fringing usualy purople one side of the focus point, grteen the other).

[Simpo Two]

Note the adjustment required may well be different for each lens. Top Nikons recognise each lens and automatically use whatever correction you set; I guess Canon are the same.

[RobDickinson]

Yep , up to 20 lenses and treats lens + tc as a seprate item.

[Tiggsy]

ok.....did the focus chart thing and it was focusing 20mm to close....adjusted (just for this lens...the 7D has a per lens setting) and now it's spot on......in fact, taking pics of labels on food in the kitchen i cant believe how sharp it is!!!!

Now - i had to adjust by "10" to get it right.....is that enough to indicate why my basketball photos (taken at very high speed settings - so it's not action blur) where not sharp? In otherwords....was being out by "10" enough to put a face out of the narrow DOF when half a court away? - I'm hoping the answer is yes!

[Simpo Two]

Can't say but rather than being a fixed 20mm out, it's more likely a percentage error. So 20mm at arms length could well translate to 6" in basketball, yes. But remember that if you're using big apertures on subjects that can move rapidly towards or away from you, you'll still be pushing the limits. Other things like AF speed and accuracy at focus tracking come into play as well.