focus test question

[evilenglishman]

I believe something is out of whack with either my 20D or lenses. It "appears" to be front focusing.
The only lens I curently have is a sigma 28mm f1.8 macro and the canon 17-40.

I'm going to do a load of focus testing tomorrow and wanted to clear something up.

If i shoot a printed out document with text on, do i angle it or the camera horizontally or vertically?

[Scottes]

Have a high contrast item (big black X) parallel to the sensor plane. Aim and focus on this.

To one side, angle a ruler/whatever at a 45-degree angle to the sensor plane. Make a noticeable mark that is in the same plan as the big X, and note that as your "focus point."

Do not focus on the ruler itself.


Bob Atkins has a great setup on this.

Edit: Since I went to look for the link and found it, I think his test is flawed since he focuses on a dark line on the ruler. I don't buy it, and like the way I described.

[evilenglishman]

Just to explain, I've been getting constantly bad results with the sigma lens.
Not scientific but some examples (all shutter speeds are over 1/60th):

Full image showing focus area (between the letters R and A):
http://www.electricvenus.com/dump/focus/brandy.jpg

100% crop:
http://www.electricvenus.com/dump/focus/brandyfull.jpg

Full image showing focus area:
http://www.electricvenus.com/dump/focus/water.jpg

100% crop:
http://www.electricvenus.com/dump/focus/waterfull.jpg

Full image showing focus area:
http://www.electricvenus.com/dump/focus/out.jpg

100% crop:
http://www.electricvenus.com/dump/focus/outfull.jpg

Every image i've taken with that lens on is like this, does it need looking at?

At present I think the 17-40 'seems' okay apart from the images being on the soft side, but I will test it more tomorrow.

[Scottes]

Your tests do look like the lens is front focusing.

But to be sure you really need to go scientific. Focus target & ruler as I suggested, tripod mounted highly preferred (or stabilized somehow).

Oh, you do want a wide-open aperture but it seems you know that.

[evilenglishman]

I'll try proper tests tomorrow but will the f4 of the 17-40 be enough?

[Scottes]

Yeah, that is kinda tough. I'd keep it close. At a distance of 1m at f/4 the DoF is about 10cm - quite a bit, really, but it should do. At min focus distance at f/4 you're down to 2cm DoF, but it might be tough to set up that close. I'd definitely use a tripod since you'll be trying to determine sharpness with little variance between the lines.

[Roger_Cavanagh]

You don't need to pfaff around with rulers, angles, etc., etc., which can give misleading results and be complicated to set up corretly.

Check this link for a simple test advised by Canon Germany:

http://www.rogercavanagh.com/helpinfo/23_focuscheck-1.stm

It won't tell whether you have front or back focus, merely that the AF is off, which is all you need to know to send it back for recalibration.

Regards,

[Scottes]

Personally, I'd go with these tests from Pekka's post:
http://photography-on-the.net/forum/showthread.php?t=35041

This was what I was thinking about in the first post.

(I'm a little too much of a perfectionist for the test recommended by Roger.)

[Roger_Cavanagh]

Personally, I'd go with these tests from Pekka's post:
http://photography-on-the.net/forum/showthread.php?t=35041

This was what I was thinking about in the first post.

(I'm a little too much of a perfectionist for the test recommended by Roger.)

Perfectionism is all very well, but it adds no value in this situation. It will make no difference to the service technicians if you tell them "my AF is off" or "my camera is front-focusing by x amount at y distance using z aperture".

This note (http://www.rogercavanagh.com/helpinfo/29_calibration.stm) may also be of interest.

Regards,

[evilenglishman]

right i set up a crappy copy of that test and i think the 17-40 is fine.

http://www.electricvenus.com/dump/focus/1735test.jpg

[eric1]

thanks for those links Roger, and Scott.

i hope you get your focusing issues resolved Evil. my 20 seems to focus fine with my canon lenses.

[Hellashot]

Just double check your autofocus point if you are using AF. Focus probably probably are not often the lense fault and the overall trickiness of modern autofocus and A-DEP routines.