70-200 zooming after focusing?

[alfonso_bresciani1]

I just received a replacement from Canon, the previous 70-200 2.8 had IS

problems and I'm flipping out on this below:

 

When I focus at something and zoom in or out afterwards the subject ain't sharp no more and I have to

refocus.

The manual clearly says the zooming after focusing may effect the focus, but I don't remember this

happening with the older 70-200 2.8!

 

Does yours keep the subject focused if you zoom in or out after you've focused?

[john_schwaller]

See Chuck Westfall's Tech Tips column: http://www.digitaljournalist.org/issue0606/tech-tips.html

 

It is not specific to the 70-200, but the same procedure can be followed.

 

For ease, I have copied the Q/A:

 

Q: I am writing regarding the parfocal property of Canon lenses. I have read on many forums that the EF24-70mm f/2.8L lens is supposed to be parfocal (including a list of parfocal lenses that you or someone posing as you wrote). My 17-40 and 70-200 are both parfocal and I find this property to be extremely useful. My 24-70 is not remotely close to parfocal (which I noticed since purchasing it). I would like to get it fixed, but I called Factory Service in Irvine and they indicated to me that they couldn't find in any documentation that the lens is parfocal. They indicated that if I were to send the lens for repair, they would test it for factory specifications, most likely find nothing wrong with the lens, and mail it back to me. Is there anything I can do short of selling the lens in order to buy a new one?

 

 

A: I included the EF24-70mm f/2.8L USM on a list of EF zoom lenses that in my experience are parfocal. However, there is more to the story with this particular lens. It turns out that while the lens itself is reasonably if not perfectly parfocal, it is very difficult to calibrate for autofocus over the entire zoom range. There's no way to diagnose the issue properly without having the equipment examined by a Canon Factory Service Center technician, but you can do some troubleshooting on your own:

 

 

 

 

1. Use a tripod and a cable release or self-timer.

 

2. Set the camera at ISO 100 and One-Shot AF.

 

3. Use a flat target with plenty of detail. (Do not use an angled chart.) Make sure the camera is parallel to the target.

 

4. Position the camera about 6 to 10 feet from the target.

 

5. Set Custom Function 4-1 so that the AE lock button only operates AF.

 

6. Set Custom Function 12-1 for mirror lock.

 

7. Use the Standard Picture Style setting.

 

8. Use manual exposure or aperture-priority (with exposure compensation if necessary) to get an accurate exposure at f/2.8.

 

9. Manually select the center focusing point and make sure that Custom Function 17 is set to 0.

 

10. Perform the test in lighting conditions that are at least as bright as office lighting.

 

11. Zoom in to 70mm, autofocus and shoot. The self-timer is a good method to reduce potential vibration, since it automatically provides a 2-second mirror lock prior to the exposure when CF12 is on.

 

12. Check your results on a computer at 100 percent magnification. If the photo is sharp, the camera's AF system is functioning properly.

 

 

 

 

If the test photo is not sharp, then send in your camera and lens with a sample image and an explanation of your testing method.

 

 

Assuming the test photo is sharp, then continue testing as follows:

 

 

 

 

13. If necessary, refocus using the AF system at 70mm.

 

14. Zoom the lens to 24mm.

 

15. Take another test shot without refocusing. (IMPORTANT). The camera will not refocus as long as you followed Step 5 above.

 

16. Take yet another test shot after refocusing with the camera's AF system.

 

17. Examine the results at 100 percent magnification.

 

 

 

 

If the test photo taken at step 15 is sharper than the test photo taken at step 16, it is a clear indication that the lens needs to be calibrated for autofocusing by the Canon Factory Service Center.

[lovcom_photo]

Alfonso, this lens and most by any and all makers is NOT parfocal.

 

A parfocal lens allows you to focus, change the zoom, and in the process the focus is not lost.

 

Some lenses are parfocal, but most are not.

 

This should not be a surprise, nor should it be a hinder to your photography. There may exist a Parfocal Canon lens, and if so, I cannot tell you which lens it is.

[pjmeade]

There was a very similar thread to this a couple of weeks ago

 

http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=00Ga4H

 

I don't think pressing the shutter button to refocus is an overly arduous activity.

 

I hope this helps.

 

Pete

[john_schwaller]

I think that Canon difers on the ability of some of their lenses being parfocal. See: http://www.rogercavanagh.com/helpinfo/30_parfocal.stm

 

John