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Is it normal that I can't manual focus any of my lenses on my 5D MK iii without having a battery inserted? I can focus on an object, and while looking through the view finder, remove the battery and instantly the image will get darker and blurrier.

Is this normal behavior? If so what's going on here?

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    A lot depends on what specific lenses you are using, what shooting mode you are using, and several other custom settings that may or may not be impacting your experience. But the larger question is, "Why would you want to do such a thing?" WIth a digital camera it is impossible to take a photo with no battery or other electrical source connected to the camera.
    – Michael C
    Feb 21 '15 at 2:38
  • Obviously there is no purpose to it but all my questions still stand. Is this expected and why? Google doesn't seem to have easy to find information about this so it's a perfect piece of information to add to PhotoSE Feb 21 '15 at 16:22
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Yes, it's normal behavior. The reason you're having problems is that the 5DMkIII has an LCD overlay in the viewfinder. This overlay is used to give you grid lines you can turn on and off and different AF point displays. Without power, the LCD becomes opaque. This behavior is identical in Nikon cameras with an LCD overlay in the viewfinder, and has long been noted.

From the Canon Learning Center's article on the 1DX's & 5DMkIII's intelligent viewfinder display:

The LCD overlay does require a tiny amount of electrical power to operate. This is obviously no concern when the camera is turned on, but if the battery is removed the transmissive LCD suddenly loses a lot of brightness and contrast. This is perfectly normal and will return to full brightness once a battery is reinstalled in the camera (the camera doesn't have to be turned on; it only requires a functioning battery pack to draw power for proper viewfinder operation).

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  • That explains why it goes darker but doesn't explain blurriness.
    – Ed Avis
    May 13 '20 at 13:03
  • @Ed Avis: Yes it does. Loss of contrast => loss of perceived sharpness.
    – inkista
    May 13 '20 at 22:42
  • That is true, but it doesn't cover the particular case observed with some camera bodies where there is not only a loss of contrast but a kind of blurry "halo" around objects, as if a lens element had been knocked out of place. This is also caused by the LCD overlay being powered down (it disappears when a battery is inserted) but my question is how that could be happening. I can only guess that the liquid crystal doesn't just become more opaque when powered down, but it also changes its optical properties in some other way that causes the blurry halo effect.
    – Ed Avis
    May 14 '20 at 18:11
  • After all, putting a neutral density filter in front of the lens gives a loss of contrast, but it does not normally result in a perception of extreme blurriness in the viewfinder.
    – Ed Avis
    May 14 '20 at 18:12
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In addition to the LCD overlay in the viewfinder, covered well in this answer, some Canon lenses are focus-by-wire. Any of the STM series of lenses will not move focus elements when there is no power being supplied to the lens by the camera. From Bryan Carnathan at The-Digital-Picture:

Additional typical Canon STM AF benefits include internal focusing, non-rotating filter threads (very beneficial for circular polarizer filter use) and FTM (Full Time Manual) focusing. FTM focusing has one caveat – that the shutter release be pressed half-way for FTM to be enabled.

The STM system is an electronic focusing design that needs power from the camera to function, even in manual focus mode. The camera must be powered on and the meter must be live for manual focusing to be enabled.

There are also a few other focus-by-wire lenses in the Canon lineup. The legendary but now discontinued EF 200mmm f/1.8 L was one. The EF 85mm f/1.2 L II, EF 1200mm f/5.6 L, and EF 200mm f/2 L IS are some of Canon's current non-STM lenses with focus-by-wire systems.

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