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?

[jimmy637]

Which Rebal are using the 400mm lens on and is your lens the 400mm f5.6?

[AnneEM]

I have the Canon T1i and the 400 mm f5.6 lens is what I use.

[jrhoffman75]

1. Does it only happen with this lens?

2. If you go to LiveView and focus are the pictures sharp?

3. Are you in One Shot or AI Servo focus?

4. Can you post an image; preferably without stripping EXIF data?

[AnneEM]

It only happens with this lens. The pictures are not sharp on Liveview even though my focus is trying to work. I have been shotting in One Shot. The image below is an example. Lately, my images have been appearing even more unfocused than this one.

[Waddizzle]

---

@AnneEM wrote:

It only happens with this lens. The pictures are not sharp on Liveview even though my focus is trying to work. I have been shotting in One Shot. The image below is an example. Lately, my images have been appearing even more unfocused than this one.

---

It almost looks to me that the AF point that locked focus may not have been on the bird, and there's not a lot of contrast around the bird, either.  Does your post processing software display the focus points that acquired focus?

[AnneEM]

No, I do not have that type of post processing software. Here is another picture with what the focus has been doing lately.

[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.

[Waddizzle]

---

@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.

---

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?

[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.