How to focus test before buying new lens?

[Lonewolf]

I'd decided to buy a Canon 70-200 f/4 L...now I'm having some concerns.

I see that Summerwind 4 and perhaps others are having focus problems with this lens (I've seen it referred to as "back-focus).

Is there a way that I can test the lens while in the camera store (using my camera).

I'd like to get it right before I leave the store...I'm not big on sending equipment that should be good to go back for warranty service.

Thanks - Wayne.

[tim]

To do a proper test you need a tripod, a focus test chart, and a cable release, I don't think you can do a proper test in-store. Without that all you can do is take a few snaps at maximum aperture and see how they look.

[robertwgross]

Why not do a standard ruler test?

---Bob Gross---

[Lonewolf]

Why not do a standard ruler test?

---Bob Gross---

Bob-

Thanks for your response. Unfortunately I'm not familar with the "ruler test." Can you provide a few more details.

Thanks - Wayne.

[scraggles]

If it were me, I'd bring my camera, mini-tripod, and just time release the shots. Put a ruler in your bag too. Esp with store lighting you should get the perfect shots. I also would recommend bringing a laptop if possible, to upload your photos and view them full size before buying anything.

Edit: I would assume the 'ruler' test is the same as the paper test. Pick a point on the ruler, focus on it, and shoot away.

I would HIGHLY recommend bringing the laptop. If you can't, tell the store you'll be back after you test out all the lenses you're looking at. Go home, look at them on your computer. No LCD on any camera is adequate enough to show full detail.

[robertwgross]

http://photography-on-the.net/forum/showthread.php?t=10526

(That is Pekka's version)

Also, some people (like me) use an actual ruler or yardstick in front of the camera lens, leaning back at roughly a 45 degree angle. If you light it well, and then focus on some solid mark, then the captured image should have perfect focus on that solid mark, and it should show the depth of field, roughly 1/3 in front of the solid mark and 2/3 behind it.

If you get some other result, then that is not good. For example, if you focus perfectly on the 10" solid mark, and if the captured image shows perfect at 7" and bad at 10", then you've got a problem. To do this, you want to use the center focus point only.

In some cases, it is the fault of the lens. In other cases, it is the fault of the camera body. In a few cases, it can be a user error.

---Bob Gross---

[cactusclay]

It's very easy to do a test in a store, you don't even need a tripod, if you are using a 20D or an XT. 'The 300D seems to have trouble with up close flash shots, but not the other two. Like Bob said, use a ruler, set in on the counter at an angle and focus on one spot, using your on board flash and see where the focus is. And the LCD screen is more than enough to check for this. Just bring the image up that you just shot, then press the magnify button to zoom in on the spot. I discovered that this technique produces the same results as a tripod, cable release and mirror lock up. Good luck.