Back-Focus Test Setup
This is GOOD focusing with 17-85mm lens
Back-Focusing with 70-300mm lens at 70mm
Back-Focusing with 70-300mm lens at 300mm
Super Clean lens after Canon Repair
I purchased a Canon Rebel XTi DSLR in late/2006 and have been very happy with it ... especially with the upgrade to the 17-85mm IS lens and then addition of the Canon 580EX flash - great combo. The next logical addition was a long lens, and fortunately, a friend offered to loan me his 70-300 DO IS lens. This uses Defractive Optics technology to make a fairly short and lightweight lens with darn good optical quality. I can attest that it will shoot very good pictures ... on the point where the focus is sharp. Unfortunately, this particular lens that I have appears to have a bias toward back-focusing - keep reading as I present my testing methodology, results, and a possible easy fix for Canon that consumers would appreciate! ;-)
My first main use of this lens was shooting pictures of my kids Boogie-Boarding on the beach in Hawaii. Incredible reach, since with the 1.6x multiplier, it's effectively 480mm. So while I got great pictures, they just weren't tack-sharp. I thought maybe some of the Internet nay-sayers who said this lens was soft were right. But when Canon announced their Canon Mark III (Holy Awesome DSLR Batman!), I read about a new feature that allowed software setting of back/front-focus bias on a per-type of lens. Ummmm ... could my specific lens have this type of problem?
So on a sunny day with no breeze, I jury-rigged the setup you can see on the left and took over 200 pictures with various combinations of hand-held, tripod, IS on/off, different focus modes, and even mirror lock-up. The results were fairly consistent regardless. Note that in all pictures, only the center focus point was used and aimed at the "5" in the middle phone - click on the images to see the mega-pixel originals and you'll see I was pretty accurate.
First, take a look at the image from the 17-85 f/4-5.6 EF-S IS USM which was taken at 85mm F5.6 at 1/1000 ISO 100. Darn good IMHO - sharp on the middle phone at the focus point, with slight blur on the forward and back phones. Expected depth-of-field at about 10 feet is plus/minus 5" - also see here. Very repeatable results and high percentage of spot-on shots.
The next picture is from the 70-300DO at 70mm F4.5 at 1/1000 ISO 100. Slightly larger aperature with expected depth of field of plus/minus 4". Remember we are focussing on the middle phone which is quite blurry. Focus is decent on the *back* phone ... and the focus point may even be slightly past that. Almost every shot was this way.
Similar behavior is seen with the same lens at 70-300DO at 300mm F5.6 at 1/3200 ISO 400. I backed up to about 30 feet for this shot and depth of field should drop to plus/minus 3". Once again, the focus point is past the middle phone to the back phone ... and these results were also quite repeatable. Click on the images to see the 10 mega-pixel originals.
This blur/softness at the desired focus point is consistant with what I saw from my Hawaii boogie board pictures and some subsequent snow skiing shots. Sure, the pictures look good, but they just lack that tack-sharpness that Canon is famous for - damn!
I shot some other "real-life" pictures that also show the back-focus problem. As before, I was shooting aperature priority at 5.6, ISO 400, shutter speeds were 1/1000 second or faster (it was a bright sunny day), and one-shot Auto-Focus with center focus point only. Distances were between 25 and 100 feet ... and the results were very, very repeatable over multiple shots of the same subject. I pulled these into Photoshop to add an arrow to the center of the image and text (plus rotated one of them), but no changes otherwise. Click to see full-res image of gravel walkway, tree in grass, wood posts (best example IMHO), and solid wood fence. BTW, those arrows pointing to the center of the image are darn close to what I was aiming at - not shabby for hand-held at (effective) 480mm! ;-)
Two Back Focus Solutions - $$$ or Free!One solution is to send back into Canon for adjustment. Since this lens is over a year old, it is out of warranty. That is going to cost over $100 - double damn! But there is an easy solution that Canon can provide that would make their customers happy. As mentioned, the recently released Mark III has the ability to set a bias for back/front-focus in the camera for specific lens (by family, not serial number). How cool would it be if Canon released a firmware upgrade that enabled this capability across their family of DSLR's!?!
I suspect that the fundamental focusing algorithm is the same across the line of equipment - read an excellent description from the excellent Fred Miranda forums that includes remarks by Chuck Westfall, who is the Director of Media and Customer Relations for Canon and has been doing this stuff for decades and really knows his stuff. So hopefully it would just be a matter of adding that functionality into the code base and providing a firmware upgrade.
Send to Canon for Back Focus RepairMarch 7th, 2007: I filled out an online repair request at Canon.Com where they estimate it will cost $118 to repair. My guess is that a back-focus adjustment is pretty straightforward; heck, I was even slightly tempted to open up the lens myself and look around! ;-) So hopefully that $118 estimate will be accurate. If anyone from Canon happens to be reading this, the Repair Order Number is WB025508 and the lens serial number #89000537.
Canon Back Focus Problem ... IMPROVEDMarch 20th, 2007: Got the 70-300 DO back from Canon with a comment that "it was found that the adjustment of the AF assembly was incorrect causing rear focus." They also said they did other "adjustments, inspection, and cleaning" ... can certainly attest to the later as there was zero futz anywhere on the lens!
Initial testing shows focusing is much better at 300mm, although perhaps still a slight back-bias. There appears to still be a significant back-focus bias at 70mm ... but I want to redo my tests again to confirm that is the case. I also want to show some sports/action shot to see how it does with real-world pictures.
Full-res images (at 300mm) showing before/after back-focus adjustment
April 4th, 2007: Here's a few dozen ultimate frisbee pictures that I shot this weekend. Sports are a challenge and sometimes the focus just completely misses, but overall, I was happy with the results.
As implied above in my March 20th update, the "fixed" 70-300 lens is decent at the full 300mm ... but does appear to have increasing back-focus as it wide-angles ... here are some full-res pictures of the phones at various zooms: 70 - 100 - 210 - 300. Fortunately, depth-of-field increases as you lower the zoom.
And for grins, I did some tape measures shots of the 70-300 lens at various zooms: 70 - 100 - 200 - 300 that also show back-focussing increasing as the camera zooms out. Finally, here's the 17-85 lens on a tape measure: 17 - 35 - 50 - 85. If you look closely at these pictures, you'll see evidence of a very, very slight amount of back-focusing IMHO; but much less than the 70-300 lens.
BTW, I also had the opportunity to use the 70-300 lens on a friend's Canon 5D. After adjusting for the focal length multiplier, the general focus and image quality appeared similar. I also tried his 70-200 F2.8 lens and yes, it did provide a crisper image on both cameras ... but it is considerably larger and pricier lens. I didn't extensively test the focus, but results seem generally consistent with what I saw above.
SUMMARYMy Canon Rebel XTi can shoot some great pictures! With the 17-85 lens, the focus is very close to dead-on at various zooms. However, the 70-300 was very back-focused, so I sent into Canon for repair. The focus was improved considerably, but is still not perfect, especially at wider zooms. However, this camera will mostly be used for long telephoto shots, so despite there still being some slight back focusing there also, it's probably not worth it to send it back in to Canon again.
As mentioned above, I'm hopeful that sometime in the future Canon will port their firmware upgrade to the XTi that provides the same in-camera adjustment of back/front-focus as they recently offered for their Mark III camera - they did provide this capability on the 40D upgrade - the Canon 50D.
Update: A few months later, I also had a focus problem with a Canon 50mm lens. Please note that sometimes stuff gets outa whack; besides these two anomolies, I've been happy with my Canon gear.