DSLR - Front focus "problem"

[Sm00thSm0k3]

Hi all,

I've had my Canon 550D for about 5 years now and just over a year ago I purchased a used Canon 18-135mm IS lens (non STM) as an upgrade from the stock 18-55mm lens.

Over the last few months I've noticed that when I use the 18-135mm lens the camera/lens tends to "front focus" i.e. let's say I'm taking a wide angle shot like one would typically do for a landscape photo, instead of the lens focusing in the center it will rather auto focus on a random object that is closer to the lens. The result of this is that the object in the foreground is in focus while the rest of image is "soft" or out of focus.

This is quite frustrating because one can't really see/detect this while sampling the images on the camera's LCD but it is very obvious when one looks at the images on a PC monitor.

I don't seem to have this problem with the stock lens so I don't know if the 18-135mm lens is malfunctioning or if the camera's sensor doesn't like the lens?

Where should I start looking to try to establish the root cause? Would it be worth my time and money to send both the camera and lens to Canon to be inspected/serviced?

 

[Progenix Oj101]

The camera will generally try to focus on whatever is nearest. Manually select an AF point which covers the desired subject. Unless I'm tracking something moving at speed, I only over use the middle focus point, and then focus and recompose (put your subject in the middle of the frame so it is underneath the middle focus point, hold the shutter release halfway to lock focus, continue holding the shutter release half way as you recompose your shot and fire away. Back button focusing makes this easier).

 

[fvdbergh]

Where should I start looking to try to establish the root cause? Would it be worth my time and money to send both the camera and lens to Canon to be inspected/serviced?

Judging from your statement that you do not have this problem with other lenses, one can rule out operator error as the most likely cause.

You can try MTF Mapper if you would like to try a controlled experiment to determine if your lens does suffer from front/back focus.

My documentation is rather old, but the user documentation should be a reasonable starting point. The workflow described in section 5.5 is intended for setting AF micro-adjustments, which the 550D does not (unfortunately) have, but the workflow will still help you to determine if the lens is front focusing.

You will have to print out one of the test charts, at least A2 size, but A0 is preferable since it allows you to place the camera at a more realistic distance from the chart. I found that a lens at 35 mm works reasonably well with an A0 chart. If you want to test the lens at 18 mm, you would need a much larger chart; conversely, by choosing a longer focal length, you can probably get away with a smaller chart.

I still like the 45-degree "profile" chart, but you can successfully use the "grid" style chart by using the following workflow:
1. Set up the chart perpendicular to the lens, camera on tripod, etc. Good, even lighting is essential for the AF to work as advertised. I often use indirect sunlight. You probably want a decent shutter speed, about 1/250, even when using a tripod. Use timer trigger delay so that you do not move the camera during the shot.
2. Switch lens to manual focus, set focus so that chart is clearly out of focus in viewfinder.
3. Use the viewfinder AF to focus on one of the blocks in the centre of the chart. Use an AF-On mode so that the camera does not refocus when you depress the shutter (see your manual, button may be labeled AE/AF).
4. Capture a shot, repeat steps 2-3 a few times.
5. Next, try to repeat the process using manual focus, using magnified live view to get focus spot-on in the centre of the chart.

If you find that your manual-live-view-focus shots produce much higher measurements in the centre of the MTF Mapper output, compared to the AF shots, then you know that your lens is not focusing properly.

You can usually send in your lens + body to have them mutually calibrated, but be sure to send in all your lenses in one go. I have never tried to use this service (not a Canon user, maybe they can contribute).