AF falls off at longer focal lengths?

[stevet1]

I'm using a Canon 55-250mm STM lens. I notived that the autofocus seems to fall off at the highest focal length (250).

I had to manually adjust the focus to fine tune it. The lens seemed to work just fine at around the 135mm length.

Is this normal in any lens?

Steve Thomas

[Ray-uk]

Can you explain what you mean by "autofocus falls off", do you mean it is struggling to get focus or it finds focus but is not sharp.

Also it would be useful to know what camera you are using, what mode you are using ( i.e. auto, P, AV, TV or manual) and what focus points you are using.

[stevet1]

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@Ray-uk wrote:

Can you explain what you mean by "autofocus falls off", do you mean it is struggling to get focus or it finds focus but is not sharp.

Also it would be useful to know what camera you are using, what mode you are using ( i.e. auto, P, AV, TV or manual) and what focus points you are using.

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Ray,

The latter. It finds the focus, but it's off just a touch.

I'm using it on a newly-purchased T8i and normally use it in AV mode with a single focus point.

I've had a color polarizing fileter on it, and I think I'll try taking that off for a while to see if that chnages anything.

Steve Thomas

[Waddizzle]

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@stevet1 wrote:

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@Ray-uk wrote:

Can you explain what you mean by "autofocus falls off", do you mean it is struggling to get focus or it finds focus but is not sharp.

Also it would be useful to know what camera you are using, what mode you are using ( i.e. auto, P, AV, TV or manual) and what focus points you are using.

---

Ray,

The latter. It finds the focus, but it's off just a touch.

I'm using it on a newly-purchased T8i and normally use it in AV mode with a single focus point.

I've had a color polarizing fileter on it, and I think I'll try taking that off for a while to see if that chnages anything.

Steve Thomas

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Yes, definitely remove the filter.  It is really not as necessary with today's digital software.  

What are you shooting that leads to this conclusion?  Personally, I suspect its' an astute obseration.

[jrhoffman75]

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@Waddizzle wrote:

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@stevet1 wrote:

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@Ray-uk wrote:

Can you explain what you mean by "autofocus falls off", do you mean it is struggling to get focus or it finds focus but is not sharp.

Also it would be useful to know what camera you are using, what mode you are using ( i.e. auto, P, AV, TV or manual) and what focus points you are using.

---

Ray,

The latter. It finds the focus, but it's off just a touch.

I'm using it on a newly-purchased T8i and normally use it in AV mode with a single focus point.

I've had a color polarizing fileter on it, and I think I'll try taking that off for a while to see if that chnages anything.

Steve Thomas

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Yes, definitely remove the filter.  It is really not as necessary with today's digital software.  

What are you shooting that leads to this conclusion?  Personally, I suspect its' an astute obseration.

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"Yes, definitely remove the filter.  It is really not as necessary with today's digital software. "

I don't concur with this - a polarizer is really the one effect you can't properly achieve in post. If you have surface glare on water that is obscuring what's below no amount of post processing can add back that information.

[Waddizzle]

Duplicate post

https://community.usa.canon.com/t5/Lenses/AF-seems-to-fall-off-athe-highest-focal-length/m-p/329269/... 

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@jrhoffman75 wrote:

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@Waddizzle wrote:

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@stevet1 wrote:

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@Ray-uk wrote:

Can you explain what you mean by "autofocus falls off", do you mean it is struggling to get focus or it finds focus but is not sharp.

Also it would be useful to know what camera you are using, what mode you are using ( i.e. auto, P, AV, TV or manual) and what focus points you are using.

---

Ray,

The latter. It finds the focus, but it's off just a touch.

I'm using it on a newly-purchased T8i and normally use it in AV mode with a single focus point.

I've had a color polarizing fileter on it, and I think I'll try taking that off for a while to see if that chnages anything.

Steve Thomas

---

Yes, definitely remove the filter.  It is really not as necessary with today's digital software.  

What are you shooting that leads to this conclusion?  Personally, I suspect its' an astute obseration.

---

"Yes, definitely remove the filter.  It is really not as necessary with today's digital software. "

I don't concur with this - a polarizer is really the one effect you can't properly achieve in post. If you have surface glare on water that is obscuring what's below no amount of post processing can add back that information.

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I agree.  A CPL can reveal what is below the surface of water.  I think water scenario is a relatively rare shot, though.  Most people use them to compensate for glare in the the sky, than to see through the surface of water.

[Ray-uk]

Although some long lenses don't like some filters it doesn't mean the filter is faulty. The biggest problem you are likely to have is that a polarizing filter can lose between 1.5 and 2.5 stops of light, as your lens is f5.6 at the long end then this will effectively be reduced to f9 to f13 and that can make the AF struggle unless the lighting is very good.

[jrhoffman75]

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@Ray-uk wrote:

Although some long lenses don't like some filters it doesn't mean the filter is faulty. The biggest problem you are likely to have is that a polarizing filter can lose between 1.5 and 2.5 stops of light, as your lens is f5.6 at the long end then this will effectively be reduced to f9 to f13 and that can make the AF struggle unless the lighting is very good.

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That was my initial thought but OP does state that focus is achieved. I wonder if the light loss is driving up ISO and thus noise giving the appearance of out of focus due to loss of detail?

[stevet1]

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@Ray-uk wrote:

Although some long lenses don't like some filters it doesn't mean the filter is faulty. The biggest problem you are likely to have is that a polarizing filter can lose between 1.5 and 2.5 stops of light, as your lens is f5.6 at the long end then this will effectively be reduced to f9 to f13 and that can make the AF struggle unless the lighting is very good.

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No, the filter is good.

I think I agree with you about the struggling

af in low-light situations. The camera manual warns you several times that focus can be hard to achieve in dim light - thta's what made me think of the polarizing filter in the first place - especially at long focal lengths. The day I noticed it, I was shooting in sort of hazy conditions.

I've kept the polarizer on up to now more as a lens protector than a reducing glare problem. I think I'll pull it off for a while and see if it makes a difference.

Steve Thomas

[jrhoffman75]

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@stevet1 wrote:

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@Ray-uk wrote:

Although some long lenses don't like some filters it doesn't mean the filter is faulty. The biggest problem you are likely to have is that a polarizing filter can lose between 1.5 and 2.5 stops of light, as your lens is f5.6 at the long end then this will effectively be reduced to f9 to f13 and that can make the AF struggle unless the lighting is very good.

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No, the filter is good.

I think I agree with you about the struggling

af in low-light situations. The camera manual warns you several times that focus can be hard to achieve in dim light - thta's what made me think of the polarizing filter in the first place - especially at long focal lengths. The day I noticed it, I was shooting in sort of hazy conditions.

I've kept the polarizer on up to now more as a lens protector than a reducing glare problem. I think I'll pull it off for a while and see if it makes a difference.

Steve Thomas

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Hi Steve.

To clarify - are you saying that focus is harder to achieve or are you finding that focus is not sharp when the camera states focus is achieved (green dot )?