Testing focus

[BorisMD]

Hi all,

I've had my Canon 24-70 2.8L for about a week now, and am suspicious that the focus may be off. It seems that when I aim the center focus dot at my son's eye, his forehead is in the sharpest focus.

Do you have any suggestions on how to test this more accurately? If it is truly off, do I contact the reseller for an exchange?

Thanks,

Boris

 

[jann1033]

goolge focus tests and you should be able to find charts you can use..you put your camera on a tripod and follow the directions...sorry i though i had the url but i don't

found the url http://photo.net/learn/focustest/

 

[MarkBarbieri]

Just be warned that in most cases I've seen where people are shooting at wide apertures for the first time, their equipment's "focus problems" turn out to be technique problems. When dealing with very narrow DOF, you've got to be careful. Simple little things like "focus and recompose" often introduce more than an acceptable level of error.

Like jan1033 so sagely suggested, do some very controlled tests. I would strongly recommend that they be done on a tripod with either timed release or a remote shutter release as well.

You may truly have a lens or camera that is slightly front or back focusing. In most cases, however, it's the operator and not the equipment.

 

[mabas9395]

I'll just echo what Mark said. With a shallow depth of field, if your son moved even an inch between the time you locked focus and released the shutter you could see his eyes move out of the focus plane and his forhead move into it.

Shallow DOF is hard to master, but I love it when it turns out right.

 

[jann1033]

yeah mark and mabas are to polite to say so but that is what i found when i tested my 50mm...user error

 

[boBQuincy]

One problem is the actual focus area is much larger than what is indicated in the viewfinder. This makes it very difficult to accurately do the "45 degree ruler" test for focus.

One way would be to set everything up on a tripod with the camera aimed at a featureless background. Place a contrasty object (such as a piece of paper with a line on it) at a pre-marked point, focus on the object. Without changing the focus remove the object and replace it with the 45 degree ruler, then take the photo.

If it all works out the result should show any front or back focus. Try this at least three times, maybe even with the focus adjusting from closeup 3 times, then from infinity 3 times.
After this, if your head hurts, go out and take photos and don't think about it again. I tested my 70-200, which is not perfect, but it is always closer than I can ever get with manual tweaking.

 

[BorisMD]

Yeah, I'm very inclined to believe in user error. Especially when I'm the user .

I'll do some testing and see if all is well.

BTW, I used this lens at my son's choir concert last night, and am not real happy with the results. Again, I think it was my lack of skills that caused the result. I thought using the lens at 2.8 was great. I could keep the ISO at 400 and still have a decent shutter speed. But, while I didn't have any motion artifact, I had exposure issues. I think that the risers were lit, but above and below was very dark. That fooled the meter (and me) into too long of a shutter speed, and the highlights were blown. More practice is necessary.

Back to the drawing board...

Boris

 

[JR6ooo4]

also a longer distance from the camera to the subject will increase the area in focus.
When I use my 70-300 at f4.5 (it's lowest) and shoot across a 100 foot stage I get a considerable amount in focus.

Mikeeee

 

[mabas9395]

Boris,
Do you have some sample photos you can post. The folks around here are pretty good at nit picking every single thing you fouled up on. I mean, offering constructive advice.

 

[BorisMD]

Sure. I'll get some on my smugmug and post a link.

BTW--for some reason I'm not getting email notification of new posts to this thread, yet I am subscribed and have instant email notification activated. Any ideas on why that's happening?

Thanks,

Boris

 

[JR6ooo4]

Now that you mention it, mine seem way low also.

The boards seem a little glitchy for me lately. It might be some server work/tweaking.

I hope....

Mikeeee

 

[BorisMD]

Okay, here's the link

http://borismd.smugmug.com/gallery/4425311_T84AL#260013945

I uploaded full size images and EXIF data.

Thanks for your critical help.

Boris