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.

[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.

[RobertTheFat]

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@Waddizzlewrote:

"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.

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From page 44 of the June 2016 edition of the 5D Mark III instruction manual:

"In the P/Tv/Av/M/B modes, pressing the <AF-ON> button will execute the same operation as pressing the shutter button halfway."

This is confirmed on pages 70, 201, and 228.

[Waddizzle]

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@RobertTheFatwrote:

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@Waddizzlewrote:

"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.

---

From page 44 of the June 2016 edition of the 5D Mark III instruction manual:

"In the P/Tv/Av/M/B modes, pressing the <AF-ON> button will execute the same operation as pressing the shutter button halfway."

This is confirmed on pages 70, 201, and 228.

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Guess what. That’s not BBF.  If the shutter and the button do the same thing, that’s not BBF.  With BBF, pressing the shutter halfway only activates the metering, but not AF.  Not activating AF when you press the shutter is the whole point.  

I find it useful in One Shot mode, such as focusing on a bird among tree branches.  I can focus, and then press the shutter without the camera refocusing.

[RobertTheFat]

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@Waddizzlewrote:

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@RobertTheFatwrote:

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@Waddizzlewrote:

"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.

---

From page 44 of the June 2016 edition of the 5D Mark III instruction manual:

"In the P/Tv/Av/M/B modes, pressing the <AF-ON> button will execute the same operation as pressing the shutter button halfway."

This is confirmed on pages 70, 201, and 228.

---

Guess what. That’s not BBF.  If the shutter and the button do the same thing, that’s not BBF.  With BBF, pressing the shutter halfway only activates the metering, but not AF.  Not activating AF when you press the shutter is the whole point.  

I find it useful in One Shot mode, such as focusing on a bird among tree branches.  I can focus, and then press the shutter without the camera refocusing.

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Er ... no. If you use a back button (by default, <AF-ON>) to achieve focus, that's back-button focus. Many users define idiosyncratic versions of BBF, using the camera's button assignment capability; and some of those defeat the shutter button's AF capability as you indicate. But they're all forms of BBF. You get to choose how BBF works on your cameras, but you don't get to impose an unnecessarily narrow definition of the term.

[Waddizzle]

Whatever, Bob.  BBF mode means the shutter does not activate AF.  That’s the whole point.  You must another button to AF.

There are lots of articles out there that tell you how to setup BBF on your camera.  All of them tell you to disable the AF on the shutter button.

[ebiggs1]

Bob
Boston, Massachusetts USA

Glad it's you and not me!  

[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!