How can run a simple test to determine if my lens is front focusing?

I was wondering if there is a simple test I can conduct at home without any special equiment to determine if my lens is front focusing? I have had several shots that I thought were spot on when I looked at the shot on the LCD but discovered they were out of focus. I have tried it with both settings on my 60D - One Shot AF and AI Focus AF. I am shooting a staionary object but it appears to be hit and miss. I am using my 85mm 1.8.


Any suggestions would be appreciated.


-Robert

Robert


You could read this review and employ the Ice Cream Box method. You could set up a vertical target square to your tripod and camera, set a tape next to the target at roughly a 45% angle to your set up and at the minimum DOF (at F1.8) see where it hits.


http://www.the-digital-picture.com/Reviews/Datacolor-SpyderLensCal-Review.aspx


But here are a few things to keep in mind. Lighting will affects how accurate your AF is. Diffrence's in lighting can cause diffrent results. The AF system will hit in the general area with some accuracy, that is not to say it nails the shot each time. Each time you achive focus it will either be right on, slightly Front Focused or slightly Back Focused. What you want the lens to have iswhere the average of the pattern hits the right spot the most. In my opinion it takes quit a few shots to determine what the lens really is doing and form an average of where the lens is actually hitting. Use single point one shot AF mode. Keep in mind that the AF area of the point is much larger than the target point you see in the camera. Shooting at an object that is not square to you could pick a point to focus on slightly out of the focal plane you were looking for.


Rick

You

I

Very helpful thread and I

Ok, so...knowing what I've learned to date, starting with a printed test chart taped to a bifold closet door, and currently having used a LensAlign Pro to test many lenses on multiple bodies, here's what I'd cobble together for AF testing if I didn't have the LensAlign Pro (or the useful cheaper commercial alternatives, the LensAlign MkII or the SpyderLensCal). In essence, I'm trying to recreate most of the important features of the commercial devices using household/free items. IMO, one of the important ones is that the focus target is parallel to the sensor plane - the single-page printed charts that you shoot at an angle to the camera do not have that feature.


It would require a cardboard box, a book, a tape measure, a chopstick, and a printout of this focus target. It could be set up on a table, or even the floor if necessary. If you have a good tripod, you would use that instead setting the camera on a book. The camera-to-target distance should be ~25x the focal length of the lens (regardless of sensor size), so for an 85mm lens that's about 7 feet. The focus target gets taped to the box, down low, and chopstick gets stuck in at the edge of the box, vertically centered on the target. The chopstick supports a tape measure at an angle. Note the measurement on the tape measure where it passes next to the target - that's your 'zero point' against which you can judge front or back focusing (when you review your images at 100% on your computer). Pick a book of a thickness that centers the lens at approximately the same height as the center of the focus target.


The setup would look something like this:


[img]/cfs-file.ashx/__key/CommunityServer-Components-UserFiles/00-00-00-35-15/AFtesting.jpg[/img]


With a zoom lens, you'd use the long end. The aperture should be wide open. As Rick stated, you need to take several shots. Personally, I usually take 8 shots - 2 starting with the focus set at infinity, then two more without touching the focus ring, then 2 more starting with the focus set at the MFD, then two more without touching the focus ring.


Hope that helps!


--John

John


Nice graphic and description.


Robert


You might check out lensalign

WoW! Thank