Help: Focusing issues...

[90sShooter]

I have been having some focusing issues lately. Not sure if it is the AF system on my new 7D (doubt that!) or if my copy of my Tamron 28-75mm f/2.8 has a front or back focusing issue. If this is the case, can I do anything to correct this? Or I just have to deal with it? (manual focus?) I tried Manual focusing but it's hard when your trying to catch a baby smiling, trying different compositions, and sometimes it's hard for me to even see if it is TRULY in focus through the view finder. Here are a couple examples of recent pics that I swear the AF locked on her face but when they came out it was clearly focused on other parts of her body, shirt, etc... I realize most of these were taken with a pretty wide Aperture but I was trying to get a nice bocah effect. I stopped it down to around 5 or so and it seems to help a little, but that is just because there is more in focus...

After looking at some of my recent pictures I notice that this is mainly a problem when doing portraits but I don't know if it is because I am using a wider aperture which makes the front or back focus more evident or if it is because I am trying to focus on something off center and I am not using the AF system correctly. To focus I point at her face press halfway down on the shutter, it locks on then I reframe the shot (without effectively changing the distance of the camera from the point of focus (her face)) and then take the picture. I know the 7D has a pretty advance AF system with selectable AF points but I haven't gotten to that point yet. Maybe it is time to read the manual.

Can somebody shed some light on this?

 

[TitusvileSurfer]

Make sure you are in "One Shot" mode as opposed to "AI Servo". Read your manual. I don't know the finer points of the 7D auto focus system, but there seems to be a wide margin of error if it isn't used properly.

 

[90sShooter]

Newbie question but can you explain real quick the difference?

 

[swjim]

AI Servo is used to shoot moving subjects - sports, kids running, etc. In servo mode you will not get the focus confirmation beep.

You really do need to read the manual, the 7D has a very nice focus system and you'll be greatly rewarded with nice photos if you take the time up front to learn what your options are for any given situation.

That said, if there really is an issue with the focus system on your camera, these shots are all front focused. You can also look up the camera's lens micro adjust feature to compensate for minor focus issues by lens but it's best to perform those adjustments using a static object with high contrast under good lighting conditions so you know if adjustments really are necessary.

 

[90sShooter]

Cool thanks! I am breaking out the manual right now!

 

[90sShooter]

Read the manual cover-to-cover. Learned a couple things! But looks like I was in "One Shot" mode so I am going to play around with it a little and do some controlled tests on a tripod and see if it is front focusing. If it is, somebody explain where I would (if possible) adjust for this. Thanks!

 

[swjim]

Page 211 of your manual should tell you how to perform a microadjustment - CF III - 5

 

[90sShooter]

ok cool! I just kind of scanned through the custom functions section...

 

[footballdude2k3]

i am almost positive the 7d has micro adjust, you could try this, take a ruler lay it on the table, change the focus points and see what is actually in focus with the picture, then adjust accordingly

 

[footballdude2k3]

sorry i did not see the other comment, my mistake

 

[90sShooter]

Thanks footballdude, I was JUST about to do that!

 

[TitusvileSurfer]

You have to be so, so, so careful measuring for micro adjustments. A very small error on your part equals a false adjustment. Now your camera, which was doing it's job perfectly well, actually is off now because you got a false positive.

 

[90sShooter]

It's funny, I talked to my local camera shop and the guy said he would NOT make any adjustments... I said well then what do I do? Just live with it? He said he would send it in and pay whatever it costs to fix it, lol. Then later he again mentioned the micro adjustments and this time he made it sound like it was an option. Can I mess anything up? It is reversible right? Do I need to jot down the original setting or can I just undo the changes? Thanks guys!

 

[swjim]

It's easy to reset, you just change the adjustment back to 0 and you're back where you started.

I believe the reason that TS cautioned you is that you could make an incorrect adjustment and cause yourself to miss shots because of making unneeded changes - you would be programming the camera/lens combo to front or back focus because of misdiagnosing the situation in the first place.

 

[swjim]

One more thing... You might want to open the shots in zoombrowser and turn on the option to show which focus point you were using when you took these shots. It could be that you had the camera set to use the bottom focus point and it was locking in exactly where you were telling it to. Just a thought...

 

[90sShooter]

O don't worry, I will make sure I test properly. Also, I am 99% that when I was shooting I was locking on correctly. 99.9%

 

[90sShooter]

Good news... After some controlled testing I believe the lens and camera are accurate. Bad news... I am a bad photographer

Although it looks like it still might be SLIGHTLY front focusing, not enough to justify messing with anything. Of course it could just be my eyes...

I don't know if I did this right but I think I did. It is also hard to tell because the focus points are pretty large so it's hard to tell what it is focusing on in the box. To me it does look like it might be front focusing JUST A TAD... To me it looks like the mearuing tape in front of the focus point is in focus longer than behind the it. Make sense? What do you think?

 

[90sShooter]

The scaled down pictures are hard to tell but I don't know how else to capture the picture with the focus point on it.

 

[IPT]

interesting...I have had simlar issues and always wondered if it was me or the rig. I agree, there seems to be more in focus infront than behind. However, I don;t know if it should be equal fore and aft. Someone here probably will and I am curious to see what they say.

 

[swjim]

That does appear to be front focused. Were you using a tripod?

As far as the ratio of front to back goes, the scale changes based upon focal length - at wider apertures there will be slightly more area in focus behind the focus point and it gets to be closer to a 50/50 ratio at longer focal lengths. You can calculate the DOF for your specific conditions here: http://www.dofmaster.com/dofjs.html/