DIY Lens Focus Test
|Table of Contents|
|Intro||Who is this for?||Quick test||Comprehensive test|
|Back focus||My Verdict||Where to buy||Alternatives|
For users of DSLR's, have you ever been sure that you (auto) focused on the eye of your subject/model but your shot shows a blurry eye but sharp ear? Try this simple test to see if you need to send your camera and lens(es) to the service center for adjustment.
Actually, there's 2 tests...read on...
Every photographer who needs to check their camera / lens accuracy when it comes to focusing. Some DSLR camera body's have an adjustment that can correct minor focusing issues. Please check your manual. For larger adjustments, the lens and camera will have to be serviced.
The picture here is the first test. Take 3 identical objects spaced evenly apart and staggered (one behind the other).
Set your camera on a tripod (highly recommended), ensure your lens is pretty much at the same level of the objects your aiming at. Now use only your middle focus point and focus on the middle object. Use the biggest aperature your lens can go (use Av mode and select the lowest possible value for you aperature). Take the shot.
View your results. The middle object should be in focus and the front and rear objects should be equally blurry. If not, then you may have an issue depending on the severity of the differences. Which means you should do the next test...
Now you will have to go online and google something like lens focus test chart. Which is a test that includes some measurements on it. Download the free chart and print it out. The one I used is here: Focus Test Chart
Basically, print out the sheet, place it on a flat surface, aim your camera at a 45 degree angle to the paper (this is important). The angle will make the measurements accurate. Which is important because if only a minor adjustment needs to be made, some camera bodies will allow you to do it. On a canon 50D for example, it would be called micro adjustment.
So, same procedure as before, but this time with the markings on the paper you can see by how much your lens is back focusing (focus' more behind what you originally focused on) or front focusing (opposite of back focusing).
I found out all this the hard way, was beating myself up over a lot of not so good photos. This can just happen, I think, because I didn't notice it before I've been shooting with my lenses for almost 2 years before I noticed it. So I suggest doing these tests when you buy your new lens. (actually, you can do the first, "simple", test right in the store and request to test a different unit if the results are not to your satisfaction.
Keep reading to see the results of my tests for the worst of my lenses...
This lens is my Canon EF 50 mm f1.8 II
As you can see (if not, the photos are in my flickr, there's a slideshow at the end of this lens) the text behind where I'm supposed to focus is a lot "sharper" (more in focus) that the line I was focusing on. Yes, I'm sure I focused on the right place. You can use the software that came with your camera to show the focus point(s) used in the photo.
Pretty brutal, eh? Ok, check out the next one...
Just to make sure I didn't mess the first shot up, I took another shot...
See? (if not, a larger version is on my flickr) now it front focuses.
This is the SAME lens !
After seeing these results, I test all my other lenses (4 others) and some were off but acceptable others not so much.
So, I brought all my gear (camera body also) to Canon Service Center to have all my lenses calibrated. I must say they did an awesome job. I sent them in on a Monday morning and got it all back on Thursday afternoon.
Here's the photo of my fixed EF 50 f1/8 II
Just to be sure, I took multiple shots (like about 10) and all came out like this one here...yeah too small, sorry, please refer to my flickr set.
All my gear was out of warranty and each had "electrical adjustments" done to it. I was only charged for the 2 lenses that needed replacement parts, the EF 50 f1/8 II (surprise surprise) and my telezoom (needed some IS part). I was charged around $43 USD for each. I think that's a good price to pay for having all my gear working at optimum performance.
To be even more precise, I would highly recommend the Datacolor SpyderLensCal. There's a link below.