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Canon Rebel XS - doesn't work in AI Focus or ONE SHOT mode

coolscorp
Apprentice

Hello,

 

My camera was working fine until now and it suddenly decided to not work. I have 2 lenses with me, one the standard one 18-55 and other is 75-300. On the 18-55, when I try to take pictures in AF mode, it will keep flickering the flash but will never take a picture. I read some earlier posts and tried to take pictures in P, Tv, AV, M modes by selecting AI-Focus and ONESHOT and it will not take any pictures. In all of these modes, if I select AI SERVO, it does take pictures.

 

I have also tried cleaning my lens but it doesn't help. When I used the 75-300 lens, the camera is able to take pictures in AF with AI-Focus.

 

Can someone please help and let me know if my 18-55 lens has gone bad or can it be fixed in some way.

1 ACCEPTED SOLUTION

If the camera and lens cannot focus even outside on bright day then that's definitely a problem.

 

By any chance do you happen to own any other lenses (or have a friend or family member with a Canon EOS lens you might be able to test on your camera body.)

 

It's more likely that the focus system in the lens has failed rather than the camera body (because all the moving parts associated with the focus system live inside the lens.)  

 

 

Your camera actually has two independent focus systems.  The main system is the "phase detect" autofocus system.  This is the faster system and it uses sensors that live on the floor of the camera (just below the reflex mirror).  This system is used when you use the camera normally by looking through the viewfinder to take the shot.

 

The other system is a "contrast detect" focus system.  This system uses the camera's imaging sensor to assess the focus.  It's not as fast as the phase detect system and it has to hunt for focus (just like a point & shoot camera) but it is accurate.  This system is only used when you put the camera in "Live View" mode (when you are using the LCD screen to compose and take the shot rather than looking through the viewfinder.)

 

If it a problem with the phase-detect focus sensors, you can put the camera into "Live View" mode and take some photos.   If it works in Live View mode but does not work in normal (viewfinder) mode, then the problem is in your camera.  If it doesn't work in either viewfinder or Live View mode then the problem is almost certainly with the lens.

 

Tim Campbell
5D III, 5D IV, 60Da

View solution in original post

7 REPLIES 7

TCampbell
Elite

This tells me your camera is unable to confirm focus.

 

When your camera is using "One Shot" mode, it is ALSO using a feature called "focus priority" (focus priority and one-shot mode go hand in hand).  Focus priority means that the most important priority for shutter release is that the image must first be focused.  If you completely press the shutter button, the camera will insist on focusing BEFORE the shutter opens to take the photo.  Most of the time that's what you want.

 

But there's a counter notion of something called the "decisive moment" -- this is the idea that when shooting an image, there's a moment where everything in the scene is exactly as you want (particularly in "action" photography).  It just wouldn't do to be pressing the shutter button and having the camera take the shot a half second later.  

 

The camera has "AI Servo" mode which is intended for taking photos of subjects whose focus distance is constantly changing -- typical of "action" photography.  When you put the camera into "AI Servo" mode, it uses a feature called "release priority".  That means the camera WILL take the photo when the shutter button is completely pressed and it will do so EVEN if the camera hasn't focused (whereas One Shot mode using focus priority will refuse to take the shot until it can confirm it has focused.)

 

BTW, AI Focus mode is a kind of "pseudo" mode.  Really the camera only has One Shot behavior and AI Servo behavior.  AI Focus is a kind of "smart" mode that tries to auto-detect which of the two modes it should use.  It beings by locking focus on a subject and then monitoring for a very short period (1/2 sec) to see if the focus distance to the subject is changing.  If the distance does NOT change it treats it as a One Shot situation.  If the distance DOES change then it treats it as an AI Servo situation.   The problem with AI Focus mode is that it takes the camera a moment to decide which mode to use -- and when you're trying to capture the "decisive moment", that delay can cost you the shot you really wanted.  

 

So now you know about focus priority and release priority and it should make sense why a camera will take the shot in AI Servo and wont take it in One Shot mode (btw, you'll find it also takes the shot if you set the lens to manual focus mode.)

 

The question is WHY isn't the camera focusing?

 

Usually it's the amount of light which is to blame.  A camera that wont focus in dim lighting situations may work fine when outdoors in plenty of light.   

 

Sometimes the lens' focus system is failing and not responding to the camera (although the camera can report an error message when that happens and you didn't indicate that your camera is throwing an error.)

 

Some lenses naturally collect more light than others.  For example, the EF 50mm f/1.4 lens collects SIXTEEN TIMES more light than the 18-55mm lens (when using the 50mm focal length) using the lowest focal ratio available on both lenses.   That gives that 50mm lens a HUGE advantage when trying to photograph subjects in poor lighting.

 

Tim Campbell
5D III, 5D IV, 60Da

Thanks Tim for your reply and providing clarification. It definitely helps to understand these mods and their purposes, really appreciate it.

 

I took the camera in bright day light and tried to focus in my garden, but still it won't take any pictures using the AF mode. If I set the camera lens as you mentioned to MF, it works fine. 

 

What are my options to get the issue corrected, is the lens to be blamed and not usable? Can the lens be fixed or is the sensor and lens alignment for auto-focusing causing an issue. Can you let me know so that I can take the next steps.

 

Thanks again for your inputs.

 

 

One more thing to add is that when pressing the shutter, it keeps showing "Busy" continuously. Not sure if you need this info or not, but thought would share Tim.

If the camera and lens cannot focus even outside on bright day then that's definitely a problem.

 

By any chance do you happen to own any other lenses (or have a friend or family member with a Canon EOS lens you might be able to test on your camera body.)

 

It's more likely that the focus system in the lens has failed rather than the camera body (because all the moving parts associated with the focus system live inside the lens.)  

 

 

Your camera actually has two independent focus systems.  The main system is the "phase detect" autofocus system.  This is the faster system and it uses sensors that live on the floor of the camera (just below the reflex mirror).  This system is used when you use the camera normally by looking through the viewfinder to take the shot.

 

The other system is a "contrast detect" focus system.  This system uses the camera's imaging sensor to assess the focus.  It's not as fast as the phase detect system and it has to hunt for focus (just like a point & shoot camera) but it is accurate.  This system is only used when you put the camera in "Live View" mode (when you are using the LCD screen to compose and take the shot rather than looking through the viewfinder.)

 

If it a problem with the phase-detect focus sensors, you can put the camera into "Live View" mode and take some photos.   If it works in Live View mode but does not work in normal (viewfinder) mode, then the problem is in your camera.  If it doesn't work in either viewfinder or Live View mode then the problem is almost certainly with the lens.

 

Tim Campbell
5D III, 5D IV, 60Da

Thanks Tim. The camera doesn't work in LiveView or ViewFinder mode, it means the Lens has gone bad. I will show it to a local camera shop just to ensure I didn't mess up anywhere in the steps you gave me and then buy a new lens.

 

Thanks for your time and quick responses. Cheers!!

Hi!  I know your post was from a few years ago but I was wondering what the outcome was for your auto focus issue.  Was it the lens or the camera?  If it was the lens, were they able to fix it or did you have to replace it?  I am having the exact same issue and am trying to go the least expensive route to get my camera working again.

Hi coolscorp and whippy333 - were you able to get your lens fixed or did you have to buy a new one? Thanks!
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