# Canon 400mm f/5.6 focussing issues



## Sabaki (Nov 3, 2013)

Hey everybody 

I'm looking for a little advise to hopefully allay my focussing issues with my Canon 400mm f/5.6.

I was out birding two days in a row where my lens would just not focus on anything in flight. I know on two occasions, my centre focus point (Canon 500D) was slap bang on a bird but the lens would not focus lock.

I experimented by focussing on reeds about 4-5 meters away then focussing on some buildings in the distance, it locked nearly 100% of the time with a very minor lag. The only time it did not focus on the building was when I tried focussing on a white wall, bathed in direct sunlight causing noticeable glare.

A more experienced photographer advised that its not my lens, its due to the cloudless sky being completely monotone blue. He mentioned the sharp sunlight is not giving the lens anything to lock onto.

You guys agree with his assessment? And birder with a 400 f/5.6 also experienced a similar result with her lens.

I had a USM motor of a Canon 100mm macro pack up for no reason and I'm worried this lens may be experiencing the same issue.

Thanks in advance for any advise/experiences everybody


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## BeenThere (Nov 3, 2013)

If you have autofocus mode set to AI Servo (not one shot), and have the center focus point on the bird, it should lock focus and track pretty easily. If not, could be a lens problem. Does the lens/camera lock on a stationary object OK? The clear sky advice doesn't sound right to me. As long as you have contrast between the bird and the sky, you should get focus lock.


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## AlanF (Nov 3, 2013)

What were your settings for focus etc? Without telling us, it is difficult to guess.


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## Sabaki (Nov 3, 2013)

Hey Alan & BeenThere

I shoot AI Servo, Spot Metering, Continuous Shooting, either at f/5.6 in AV mode or in manual with variable shutter speed depending on available light.

Stationary subjects lock in a cinch but when I point the camera skyward, the auto focus struggles. First time in a year I experience this.

Two samples of stationary birds


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## Sebring5 (Nov 6, 2013)

Sabaki said:


> Hey Alan & BeenThere
> 
> I shoot AI Servo, Spot Metering, Continuous Shooting, either at f/5.6 in AV mode or in manual with variable shutter speed depending on available light.
> 
> ...


 Try evaluative metering.


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## J.R. (Nov 6, 2013)

Sebring5 said:


> Sabaki said:
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> > Hey Alan & BeenThere
> ...



And what benefit would that give? I'm not too sure that PDAF has anything to do with metering.


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## Sebring5 (Nov 6, 2013)

J.R. said:


> Sebring5 said:
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> > Sabaki said:
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 Focus is based on the contrast in the picture. When you use spot metering, you limit the contrast and lose or limit the focus. You'll see this when photographing birds against the sky especially when the sky and the bird are close in color.


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## mackguyver (Nov 6, 2013)

Having used one for many years, I think the 400 f/5.6 has one of the best USM motors out there and if it's not working well, it might be your setting for "Lens drive when AF impossible." If it's set to OFF, it can be a lot of trouble in the situation you describe. Also, have you tried using the focus limiter? If it's set to distant, you shouldn't be having any issues locking up focus on birds in flight unless there's very little contrast, i.e (small in frame) white bird in a dim white sky. Pre-focusing can help in those situations.

Also, you said you were using the center point, but if you manually selecting an outside point, that can really drag it down, especially with a 500D.


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## J.R. (Nov 6, 2013)

Sebring5 said:


> J.R. said:
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> > Sebring5 said:
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Do you have any source for this? AFAIK, the metering sensor and the AF sensor do not communicate with each other for PDAF.


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## neuroanatomist (Nov 6, 2013)

Sebring5 said:


> J.R. said:
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> > Sebring5 said:
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Sorry, but...WTF???

Yes, focus is based on contrast...in the scene (PDAF depends on contrast to generate the phase difference). Metering affects exposure which affects the contrast _in the captured image only_. Metering information is used to determine aperture, shutter speed, and/or ISO, depending on your settings. Metering has *no effect* on autofocus (at least, not in the user's T1i/500D; metering data does contribute to face recognition/tracking in the 1D X, and allows compensation for light source, e.g. fluorescent, with dual-layer iFCL metering - but even then, metering type - spot vs. evaluative - doesn't matter).

Light coming through the lens hits the reflex mirror where most of it is directed upward to the viewfinder and metering sensor, and some light passes through the reflex mirror to the secondary mirror, where it's reflected downward to the AF sensor. So, the light hitting the AF sensor never reaches the shutter or the imaging sensor - that means settings for shutter speed and ISO are irrelevant. AF is performed with the lens aperture wide open (it's stopped down to the selected aperture, if necessary, just before the shutter opens), so the selected aperture also has no effect on AF.

So...what effect do you think spot vs. evaluative metering is having on AF? How would that work in fully manual mode (M, user-selected ISO), where metering does nothing except move the little arrow up or down on the light meter display?


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## J.R. (Nov 6, 2013)

Thanks Neuro. 

Just to add to my earlier post, the PDAF works like this


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