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XSi Auto Focus Issue?


Canon XSi Autofocus issue

Since the launch of the Canon XSi, the authors of this site have encountered numerous claims of the XSi have auto focus issues. These claims range from front focus, back focus, lack of focus, to complete focus failure.

Our feeling on this situation is that the camera does not have any auto focus issues, the XSi is similar to every other Digital Rebel we have used to date. 

The problem perceived extends from a few different aspects of the XSi and digital SLR photography in general. The XSi is an entry level digital SLR, therefore alot of entry level dslr users buy the XSi. 

When moving to dslr’s quite a few new optical fundamentals come into play versus compact cameras. Without a basic understanding of aperture, shutter speed, iso, depth of field, and exposure, I can understand why new users would have difficulty with the auto focus system. SLR’s are not more advanced point and shoot cameras, and it may be a case of people who were expecting an upgrade in their image quality without a need to further their techniques.

Depth of field has a significant impact on the focus of an image, especially of the sort of issues I have seen discussed online.  I will adamantly state that I have had no focus issues with my Canon XSi. 

When you focus your camera, the spot it focuses at is called the focal distance. The focal distance is a narrow location (on your image plane), which has depth of field both in front and behind the focal distance point.

The depth of field surrounding your focal point can be expanded or contracted. Subject matter falling within the depth of field, appear as “focused” and are often considered sharp.

When too narrow of a depth of field is used, users can mistake inadequate Depth of field with a focus error. In fact, the user can undertake a few steps to increase the size of their depth of field. The easiest way to increase depth of field, is to move further away from your subject matter.

The second way to increase depth of field is by stopping down your aperture, and using a narrower aperture. Moving from 50mm F1.8 to 50mm 2.8 at a subject distance of 5 feet will expand depth of field from .2 feet to .32 feet.

Finally, if you are using a zoom lens, by reducing your zoom to a wider angle, you will also increase depth of field. When decreasing zoom from 50mm to 40mm at F1.8, depth of field goes from .2 feet to .31 feet.

Where do focus failures come from?
1) Often times we see a lens such as the Canon EF 50 1.8 II used. The photographer decides to take a photo of someone 5 feet away with an aperture of 1.8. 50mm with a F1.8 and 5 feet from subject provides a depth of field of approximately .2 feet. That is not a very large DOF, and a shift in the DOF by recomposing the image at all could result in total loss of subject focus. Less magnification of the subject by moving further away to say 10 feet, provides a DOF of .81 feet.

2) Low contrast and low light cause focus failure.
I use a Canon Speedlite flash on my camera when shooting at night or indoors. Why? not because the camera can’t focus, but it focuses so much more accurately when there is contrast. The camera will use the autofocust assist lamp which is a red LED light emitted from the speedlite. This only works in one shot and AI Focus mode, not ai Servo. When in a dark, low contrast, or low light environment the camera can not detect that white/black contrast very well. As a result focus is hard to achieve and often time ends up with poor focus.

3) Hand held camera shake or subject movement cause focus issues.
To shoot hand held you must shoot with a shutter speed faster than 1/(1.6x focal length) to capture action you must have 1/500 or faster. Without implementing these rules, you will put yourself at risk for camera shake or subject movement.

4) Using image stabilization when using a tripod causes focus issues.
IS on the kit lens does not account for a tripod, using a tripod will cause the IS to degrade the image with long shutter speeds. This may also be the case for very fast shutter speeds as well.

5) Mirror lock and self timer help reduce camera shake
If you use the custom function for mirror lock, and use the self timer, you will be minimizing camera shake substantially. Not only will the vibrations of your hand no longer be present, but the mirror lifting will no longer vibrate the lens either. These are important functions when using the camera on a tripod for landscape shot.

6) Focus point selection can cause auto focus issues.
If you have the camera in an auto mode, all focus points are enabled. This allows the camera to select the focus point with the subject matter closest to camera. It does not mean all areas of the shot are in focus. Use one shot and manually select a focus point when shooting. If attempting to focus and recompose, try to use a focus point closest to the area you will be recomposing. Do not use the auto modes!

7) Lenses have minimum focus distances
It is important to remember that lenses have minimum focus distances. Not all lenses are macro capable lenses. The EF 50 1.8 II provides a minimum focus distance of 1.5 feet. The EF 70-200 f/2.8 L USM has a minimum focus distance of 4.9 feet. Dont forget that the closer you are to a subject, the more they will fill the frame of the view finder…. as a result you will likely have less contrast in your subject matter.

Canon XSi focus issue causes and remedies:
The XSi has an anti aliasing filter which produces softer images.  Every single XSi picture style provided by Canon has low sharpening by default.  Every single XSi JPG should have the sharpening increased either by adjusting the picture style defaults or by sharpening the image in post processing.

If shooting in RAW you will need to sharpen your images no matter what picture style was used.  If shooting JPG, you can use the cameras in camera sharpening settings to achieve better results that appear “more focused”. If using increased sharpening in the camera, the entire scene will be sharpened. Some people prefer to avoid this due to added noise, but the softness of the default settings will need to be addressed.

Focus Point Selection:
When a lot of users begin using the XSi, they shoot in the full auto modes, these full auto modes have all focus points enabled.  This means the camera will focus on that subject matter which is closest to the camera, NOT necessarily whats in the middle of the view finder or what the photographer expected to be focused on.

Lens Test Charts:
A lens test chart sounds like a compelling test. People advise you to print off a test chart, place your camera above it and fire away to show your auto focus failures.

When using a digital SLR, you have to become familiar with the characteristics of an autofocus system. The system, relies on contrast changes to determine whether a focus point is focused or not. That means, if you shoot in a poorly lit environment, the camera will not be able to determine the difference between white and grey very well. The focus system does not see color, it does not see blue or red, it sees white black or grey. If you have a white next to black subject, the camera will be able to figure out when there is little bleed between the two and then it is in focus.

There is also an issue of which focus point is used, what type of image the camera is focused on, what lens is used, how far away the photo was taken from. Other questions I always would ask are: was mirror lockup used? was a tripod used? remote timer used?

There really is not an issue with the XSi autofocus system, instead its a failure to understand how to use the system. Unfortunately, many users feel the need to do these tests – so while we admit they will continue to be performed, we believe that they are not performed under proper conditions and their outcome should not be taken as scientific evidence of a failure of the camera.

Finally, most of the posts I’ve read state that the focus issues were taken at things like 3.5 and 4.0 for the aperture.  This immediately says to me that the depth of field is causing these users to not understand the focus of their lens. 

I have written short guides that are attempting to explain aperture and depth of field in easy to understand language. One can not take a photo at an aperture of F 2.8 and expect everything to be sharp, and until the users realize this they will not be satisfied with their results.

The XSi is a very competent camera, the auto focus system works very well, far faster then any digital rebel I have used previously.  My shots are sharp and in focus, I’m happy with the results and am confident you will be to.  By referencing this guide, I hope that those with auto focus issues, or those concerned about them will find relief and figure out how to use their cameras.

Here is an example of using added sharpening to the picture styles for JPG shot.

Default Landscape picture style

Default Landscape picture style

Modified landscape picture style

Modified landscape picture style