Backfocus Issue, Yes or No?
an article by Chris Hurd

For the past couple of months, nothing has confused the Watchdog more than the current bantering on usenet and mailing lists about the possible existence of a backfocus problem on the XL1. The majority of complaints seem to concern a supposed loss of focus when changing the focal length of (zooming) the lens. Some folks are claiming their images are too soft at the widest setting of the lens.

At first, I thought this might be occuring for a couple of reasons. Either users were seeing the reported diffraction effect at small f/stops, or perhaps people weren't being careful to get critical focus on their subjects... that is, zooming all the way in, focussing, and zooming back out to the desired focal length. Using this method of critical focus, the image should stay crisp no matter what focal length you're shooting at.

But instead, it seems that for more than a few shooters, there is a real focus problem which is not resulting from the diffraction effect or a lack of critical focus. For other shooters, however, the XL1's 16x lens performs flawlessly. What is going on? I have no clear answer... yet.

Read these six testimonials and see if you get confused like I did...

David H. Dennis: "The problem I'm encountering comes up because the infinity is at an extremely long distance. Even at my widest angle setting for the XL1, focus is still critical when the subject is over 100 feet from the camera - that is, if I set my focus to infinity in hopes that it will catch most distant subjects, they will actually be blurred. I found it hard to believe that infinity was that far away for a wide angle shot, but that's how it seems to work."

Frank Prinzel: "When I zoom in on my subject, focus manually and then zoom out, the subject is out of focus. This is unacceptable for professional applications."

Peter Engberg: "I've spent more than 1 month with the XL1 on a documentary shoot in the Himalayas. The camera was among the first 600 units shipped to Denmark, and had a backfocus problem..."

Dan Heimbach: "I purchased an XL1 and used it for five days and returned it for a full refund. When compared side by side with my Sony VX-700 it was immediately clear that the Canon did not focus sharply at wide angle or when the aperature was wide open. The Canon pictures were just plain fuzzy. When the Canon was stopped down to f/8, it sharpened up. I believe that this is a backfocus problem with the lens. Perhaps if the lens mount on the camera body is re-shimmed so that the subjective lens element is either closer or farther away from the CCD', it would bring the camera into focus. This is a real problem. I have been a professional photographer, cinematographer and videographer for fifteen years. The Canon XL1 is the first camera I've ever had to return. I bought a VX-1000 to replace it."

Doug Lauber: "I've had my XL1 for about 3 weeks. I'm happy with the focus, including the wide position. I did some time in photography school. I know when an image is soft or sharp. I pre-focus manually, and use the AF button. In most situations, I'll zoom all the way in on the selected object, and then focus, then zoom out to the wide angle position. This works fine for me. I do not use the auto focus much, and I do not shoot randomly moving objects, like soccer players, etc. I shoot actors on a set."

Daniel G. Trout: "I've had my XL1 for 3 days now and have put it through several manual focusing tests in the field. In bright light, low light, ND filter on and off, and almost every other light/subject situation I anticipate running into, and I've had no problem with it losing focus. I am very pleased with mine..."

Norm Boe: "Having read so much about XL1 problems, I have been checking out my two week old XL1 carefully. As far as the notorious "back focus" problem, if I understand it, I have no sign of it. I focus at full 16x zoom then go to full wide angle at any speed and then back to full zoom hundreds of times and the focus remains perfect every time. Additionally, the autofocus seems very good and I can never improve on the autofocus by going to manual focus. In very low light it pays to go to manual focus, focus via the autofocus button and then release to prevent hunting, but in decent light autofocus seems to do it all."

In case you're wondering just what exactly "back focus" is, an excellent description was posted recently to DV-L which does a very good job of defining it. Many thanks to Per Sverre Wold-Hansen, of Fredrikstad, Norway for the following explanation:

Back focus is the only "operator" calibration adjustment for most pro lenses. It sets the actual (point, plane) distance from the last lens element to the pick-up device (CCD's, tubes, film). The XL1 has no adjustment as most key lens elements are under servo (not mechanical) control. The servos ALWAYS do the fine detail focusing using the back plane/focus.

That's not to say then lens can't be out of calibration, but mechanical lenses have an adjustment to let the operator calibrate the focal plane. This is different than just making the image in focus by using the front of the lens.

"Backfocus" is just an expression of the fact that when changing the focal length (zooming) the focal-plane has to be kept in one place, and the right place - i.e. on the chip(s). That should be catered for in the lens design, and normally is.

If it is just slightly off, it means that the focal plane is in front or in the back of the sensor.

If a lens is just "out of" backfocus it is only a matter of moving the lens in place. On an adjustable (professional) lens, you zoom all the way out/back, aim at an "infinity" motif, adjust the lens to "infinity", and start moving the lens back/forth (backfocusing) for max focus... preferably with a large aperture. When in focus, lock the backfocus adjustment down. The lens now keeps focus at any distance during regular zooming.

So, if backfocus is badly adjusted, why might it look okay when zoomed in (telephoto) and get out of focus when pulling back - Even though the focus plane stays in one place?

When considering the normal depth of field for a lens, a wide-angle lens has the most, compared to a telephoto one. On the same token it is kind of opposite on the back of the lenses. The wide-angle is the most critical in terms of placement with very little tolerance. That's why you adjust the backfocus of a zoom lens at minimum focal length.

One experience of how critical wide-angles are was the use of a 10mm wide lens on a 16mm camera. The center of the picture was always getting slightly in-and-out of focus in each scene. The edges were always sharp. This was because the film was not completely flattened in the filmgate and was "bulging" somewhat while moving. Normal/Tele lenses on that same camera were okay.

If a lens is poorly designed the focus-plane may move while zooming. There is no cure for that. It is almost inconceivable that a Canon lens would suffer that problem! You may, however, experence this when putting an adapter in front of the lens.

A different side to "bad" focus is "dirt on the back lens element". I've experienced that on a film projector. It wasn't that noticeable when on wide, but then the image on screen would get noticeably "softer" when zooming in. After years of use a thin oil film with dirt had formed. Is the cleanliness of the back of the lens more critical (and forgotten) than the front?

Perry Mitchell adds:

Professional camera systems (film and video) allow adjustment of backfocus to cope with inevitable tolerances of manufacture, and also for variations of temperature. Are Canon implying that they manufacture consumer products to a higher tolerance and therefore there is no need for an adjustment on the XL1? To be fair, the pro systems have to cope with different manufacturers' products coming together with only a spec. to ensure a fit, and it is wholly possible that this does generate higher tolerances.

On a related matter, pro lenses often focus through infinity; the correct focus point is a little back from the end stop. A lot of cameramen like it this way since they like to rock around to find the best focus position. Don't know if this has any relevance to the XL1.

Martin Hodge sums up:

The camera stops at infinity when turning the ring counter-clockwise and near focus when turning it clockwise. The ring continues to turn in either direction, but the camera DOES stop at each extreme. It does not jump back to near from infinity, or infinity from near if I keep turning the ring.

The most positive information about this whole issue comes from Bill Domb:

"Canon people I talked with today say that they have been evaluating XL1 focusing issues. They did mention that zooming under autofocus was creating a transitory out-of-focus condition when zooming too fast. Slower zooms, they felt, did not get out of focus."

As soon as I can find out anything concrete, I'll post it here.

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Thrown together by Chris Hurd