Focus issues?

[Sylon]

In June 2016 I purchased a second hand Canon 650D and a new Sigma 30mm f1.4 ART. Over time i've noticed the focus is often off at apertures larger than 2.0. This is especially the case during night shooting.

Now, this may of course be due to the extremely shallow DOF at this aperture, but still I wonder if something might be off in my configuration.

Is there an easy way to test wether my set-up suffers from misfocus issues (back or front).

And, do you know anyway to fix this issue?

 

[astroNikon]

Set up a test in a room.
Find a fixed object and test focusing on that fixed object.
Then you can use one of many focusing tests sheets you can find on the internet, or use a Ruler and shoot at the midway point with it laying flat and your shot at an angle.

This will give you an idea if the focus is spot on or not.
Best to use with a tripod and remote release to remove you as a potential issue.
This will give you a repeatable process to analyze.

 

[Sylon]

Thanks, I'm going to try that!

 

[table1349]

Jeffrey's Autofocus Test Chart

 

[Derrel]

Does the Canon 650D allow the user to do an AF micro-adjustment? What about the Sigma lens--is it part of their new "Dock" system? The Sigma dock being their fairly new hardware and software setup, in which the lens can be hooked up to their dock, which is connected to a computer and software, and which can adjust the lens's AF performance at different distances.

Achieveing good focus at night can be challenging, on several accounts. One can check focusing by using the viewfinder AF system, or Live View, and in some cases, the distance scale on the lens itself. At night, it can be very difficult to get a good AF target.

 

[Derrel]

No need for any fancy printed charts: just stick a length of silver duct tape along the side of a building. Mark a "O" point with a line through it to make a good focus target, and then make some marks along the tape using a marking pen. Set the camera up at an angle to the wall. Throw the focus to "Out", then aim and let the AF system acquire the target while aiming right at the "O" point. Like this.

Keep in mind--with a short focal length lens, the AF squares are not 100% dead-on accurate; the AF squares are a visual approximation of the actual AF sensor's response area, and the SIZE of the area covered by any AF square depends on how long the lens is, and what angle of view it covers. An AF square seen by a 30mm lens will cover a relatively large area at 10 feet, and even larger an area at 20 feet!

This is a focus micro-adjust test frame, shot with a 135mm lens at f/2.8.

 

[zombiesniper115 14]

Does the Canon 650D allow the user to do an AF micro-adjustment?

To my knowledge there is no focus micro adjustment available on the rebel line of cameras which includes the 650D.

 

[john.margetts]

The 650D certainly does not have focus micro-adjustments.