EF 40mm f/2.8 issue

[pitchers]

I use the EF 40mm f/2.8 Canon "pancake" lens on a 6D and have an issue. The lens will appear to "focus" on whatever I select in the focus point in the viewfinder but when I press the shutter, it seems to "prefer" whatever is closest in the viewfinder and focuses on that. I've had the lens for a while but just had the oppportunity to use it for a solid week in a very photogenic environment and this drove me nuts!

The exposure also seems very erratic -- many photos are a stop or two underexposed -- with a very sharp cutoff, omitting about a third of the histogram in Lightroom.  The photos that are properly exposed and focused (about half) are lovely but it's so erratic.  It appears to be a communication issue between the lens and the camera.  I've removed and re-seated the lens a few times but no change. For comparison, my 24-105mm f/4 focuses and exposes perfectly, so this seems to be a lens-specific phenomenon.

Any thoughts...?

[Waddizzle]

What shooting mode [on the mode dial] are you using?

What focusing mode [One shot or Servo] are you using? Which AF point(s) are you using?

What metering mode are you using?

What is your right thumb doing when you take photos?  I ask this question because I used to get the occasional bad exposure, and could not explain what was what happening.  In fact, i still cannot explain it, but I do have suspicions.  When I began playing around with BBF [Back Button Focus] the frequency of my mystery bad exposures seemed to go away.  My suspicion has been that my thumb was wandering to close to AE lock button, and randomly pressing it.  Using BBR gave my thumb something to do.  So, it stopped accidentally pressing AE Lock.

[RobertTheFat]

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@Waddizzlewrote:

What shooting mode [on the mode dial] are you using?

What focusing mode [One shot or Servo] are you using? Which AF point(s) are you using?

What metering mode are you using?

What is your right thumb doing when you take photos?  I ask this question because I used to get the occasional bad exposure, and could not explain what was what happening.  In fact, i still cannot explain it, but I do have suspicions.  When I began playing around with BBF [Back Button Focus] the frequency of my mystery bad exposures seemed to go away.  My suspicion has been that my thumb was wandering to close to AE lock button, and randomly pressing it.  Using BBR gave my thumb something to do.  So, it stopped accidentally pressing AE Lock.

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Even BBF itself could be part of the problem. On the cameras with which I'm familiar, BBF is not disabled by default; but it is (by default) subject to being overridden by shutter-button focus. You may think you've gotten the subject in focus via the back button; but when you press the shutter botton, the camera may have a different idea.

[ebiggs1]

"The exposure also seems very erratic -- many photos are a stop or two underexposed ..."

Whenever things don't go as expected, you should always start by resetting the camera.  Menu, Tools, clear all settings.  Start from square one.  Under good lighting conditions and the mode set to P, try it.  If all is well you know there is nothing wrong with the camera/lens combo.  You must do this first. Otherwise you are just guessing. And, so are Bob and me.

[ebiggs1]

Oh, and BTW, if you call Canon support they will make you do this too.

[pitchers]

Thanks.  My point is that it is only this lens. If I sit in my chair and shoot through an open interior door (with the door frame still in the field of view), I can focus on a point in the next room but the resultant photo is focused for the doorframe.  If, while sitting in the same chair, I swap on my 24-105 zoom, set it at 40mm(ish), take the exact same photo, I get nice crisp distant focus as intended.

But to answer the questions, I use the center focus point and the half-shutter-depress for focusing.  I'm set to spot-meter but it doesn't matter which meter mode I use. It happens if I'm in program [P] mode, or aperture- or shutter-priority.  I don't use the back focus but I use the back exposure-lock button. And I'm pretty much always on "one-shot."

I will need the lens for an upcoming trip and I'm trying to decide of it's worth buying another copy.  It was a joy to have a light, unobtrusive lens, and there's enough rseolution on the 6D that I can crop to compose in post.

Thanks for the help!

[TTMartin]

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@pitcherswrote:

But to answer the questions, I use the center focus point and the half-shutter-depress for focusing.  I'm set to spot-meter but it doesn't matter which meter mode I use. It happens if I'm in program [P] mode, or aperture- or shutter-priority.  I don't use the back focus but I use the back exposure-lock button. And I'm pretty much always on "one-shot."

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In my opinion the only time you should use 'Spot Metering' is when using full manual M and Manual ISO. Otherwise, you will get eratic exposure like you described. 

Use Evlauative Metering. Evaluative Metering is weighted to the focus point in use.

'All EOS models (to date) automatically lock exposure when you’re using Evaluative metering and One-Shot AF mode. Press the shutter button half-way down, and the exposure settings will be locked in-place with no further effort on the photographer’s part. If you keep partial pressure on the shutter button, you’ll see as you move the camera side-to-side that the shutter speed/aperture numbers don’t change. Pull your finger off the button, and the camera immediately begins to update exposure settings as the camera is moved.'

Using Auto Exposure (AE) Lock

[pitchers]

These have all been good photography tips. But does anyone have insight into this particular issue?  It is only with this lens.

"Thanks.  My point is that it is only this lens. If I sit in my chair and shoot through an open interior door (with the door frame still in the field of view), I can focus on a point in the next room but the resultant photo is focused for the doorframe.  If, while sitting in the same chair, I swap on my 24-105 zoom, set it at 40mm(ish), take the exact same photo, I get nice crisp distant focus as intended."

So it's not related to the mode, or any lens-independent user-adjustable camera setting. And no, the lens is not set to MF.

Oh well, was worth a shot.  Thanks, all!

[Waddizzle]

I agree that you should use Evaluative Metering for just about all handheld shooting.  

Are you using any lens filters on your lenses?  You suspect that you probably have a bad lens, though. 

[Waddizzle]

"Even BBF itself could be part of the problem. On the cameras with which I'm familiar, BBF is not disabled by default; but it is (by default) subject to being overridden by shutter-button focus. You may think you've gotten the subject in focus via the back button; but when you press the shutter botton, the camera may have a different idea."

I have yet to set it enabled, by default, on a camera body.  Reseting custom controls disables it..  The metering system is enabled by default, but not metering and focusing.