Why won't my Canon 400 mm prime lens focus?

[AnneEM]

Hello. I have a Canon 400 mm prime lens on a Canon Rebel camera. I am a wildlife photographer, so my camera has been used in some extreme temperatures. I have noticed lately that my camera does not always want to focus on wildlife. For instance, I was trying to take pictures of owls and while they showed up focused on the viewfinder and the auto focus was going off, they ended up not focused on my actual camera. This has been a problem for 3 months now. Sometimes it will focus, but half of the time it does not. I have never had this issue before. I use the P setting and my 400 mm lens a large majority of the time. I had my camera looked at very briefly at a shop and they found nothing obviously wrong with it. I'm not sure if its a problem with my lens or camera. I want to say it's my lens but I'm not sure because I only use the 400 since I photograph wildlife. Does this problem sound familiar to anyone?

[ebiggs1]

"I do not have that type of post processing software."

Where you put the focus point is unimportant if the entire photo is blurry because of other factors.  Do try what I have suggested.

[jrhoffman75]

Always worked and now it doesn't. Seems like a lens problem.

[ebiggs1]

AnnEM,

The problem just might be using P mode.  Sometimes, maybe most of the time, it will select correct settings.  However, sometimes it won't.  Anything except One Shot and a single focus point can have the same result.  It may select the correct one or it may not.  If you do all the selecting you eliminate that issue.

[ebiggs1]

"Seems like a lens problem."

Perhaps so, but other factors must be resolved first before we draw that conclusion.

[jrhoffman75]

Studying the image of the small bird it doesn't look like anything else hijacked the focus. The bird is the most sharp item in the image.

As suggested, single center point focus should narrow down problem.

[Waddizzle]

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@ebiggs1 wrote:

"I do not have that type of post processing software."

Where you put the focus point is unimportant if the entire photo is blurry because of other factors.  Do try what I have suggested.

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If the focus point is in the center of that frame, in the middle of the wide blue sea, then the entire photo might be blurry. 

"I find it strange because I've had this lens for 4 years now and this is the first I'vve had this issue. I used it extensively in this spring and never had issues then."

If it used to work, but now it doesn't, then something has changed.  What's changed?  Lens Filter?

[diverhank]

As suggested, there is a very good chance that it is the user problems. I'm not saying that it is but to eliminate that, you need to make sure of a few things.

1. You need to have fast shutter speeds. To shoot birds with this lens, never shoot slower than 1/1000 or you will risk soft images that can be mistaken for poor focus.

2. Shoot with P does not guarantee fast shutter so use Tv instead with auto ISO.  You might have had ample light before so the speed was high so the pics looked good... but good light are not always possible.

[AnneEM]

The only thing that may have changed is dust that got in my sensor. I was using my lens a lot in Florida and one day I noticed a lot of spots in my images. After that, I noticed that it was harder to get good focus. I just got my sensor cleaned the other day, but the people at the shop told me that a dirty sensor is likely not the issue. I have no been able to experiment with my camera since I got it cleaned due to weather.

[jrhoffman75]

If you aren't using LiveView the sensor plays no part in focusing. If you got a lot of dust in the mirror chamber it is possible that the AF submirror is coated. I put that low on the list though.

[diverhank]

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@AnneEM wrote:

The only thing that may have changed is dust that got in my sensor. I was using my lens a lot in Florida and one day I noticed a lot of spots in my images. After that, I noticed that it was harder to get good focus. I just got my sensor cleaned the other day, but the people at the shop told me that a dirty sensor is likely not the issue. I have no been able to experiment with my camera since I got it cleaned due to weather.

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The AF mechanism is separate from the sensor so that would not be the factor.

Out of curiosity, what were the Av, shutter speed and ISO values for the two pictures you posted? Frankly I don't think those two looked that bad...they look more to me like motion blur rather than focus blur (due to lack of focus).

Just to rule factors out, even though both of these are highly unlikely.

1. check the menu for AFMA (autofocus micro-adjustment) and make sure it's not enabled...you or someone might have inadvertently enter in a correction value that is not needed.  I think your camera has this function. If not don't worry about it.

2. Check and make sure that you or someone hadn't inadvertently disable the AF function for the shutter button.  One way to quickly verify is to switch the camera mode to the green square.  In that mode, the AF function will be linked to the shutter button.