E-M5 Rattlesnaking: Sources, Solutions, Ramifications

Started May 14, 2012 | Discussions thread
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Timur Born
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E-M5 Rattlesnaking: Sources, Solutions, Ramifications
May 14, 2012

Hello everyone!

In this thread I would like to gather all information about what is called "Rattlesnaking" or "aperture dance" in regard to the Olympus OM-D E-M5.

For my intent "Rattlesnaking" means that the camera quickly and mostly audibly changes the aperture opening when pointing the lens towards differently bright areas. Towards brighter targets the aperture is closed, towards darker targets it is opened. While other cameras do that on purpose as means of "analog" exposure compensation for their sensor/screen (Fujifilm X10) the E-M5 mostly is able to work without (by using digital exposure compensation).

My findings so far:

I encountered three different types of "Rattlesnaking" in that they have different sources and different solution to overcome/workaround. Two of them are reproducible and may be intended behavior by Olympus, the third may even be one of them, but its solution was different. The first two I describe here happen with both an Olmypus 45/1.8 and the the 12-50 kit lens, the only difference is that the kit lens' aperture is a lot smaller and a lot quieter.

One idiosyncrasy of "Rattlesnaking" mode on the E-M5 is that when you dial in negative exposure compensation it will close the aperture, but dialing it back up will not open the aperture again until you point towards a differently lit target, half-press the shutter or use the DOF Preview function.

Drawbacks of "Rattlesnaking"

The main drawback of "Rattlesnaking" is audible noise with lenses that feature a fast (wide open) aperture to begin with. The movement/noise is permanently present while you aim around with the camera. AEL can help against bright targets, but when you lock AE on bright targets you will get an aperture dance when focusing (shutter half-press) on dark targets. That is because the E-M5 always uses the widest available aperture for focusing.

The latter also is the reason for possible additional shutter lag. For focusing the shutter is opened all the way up, then it goes back to its last position that was previously chosen by "Rattlesnaking" mode. And if you combine that with DOF Preview it has to do even twice as many steps.

Possible benefits of Rattlesnaking

As strange as it may seem, there are benefits coming with "Rattlesnaking" mode! Because the aperture closes down in brighter light the depth of field (DOF) increases and thus any semi-automatic tracking modes likes AF-C, Face Detection and Tracking perform better .

There may also be some benefits for live view on screen/EVF, because without "Rattlesnaking" the camera uses digital exposure gain for both the screen and focusing which are limited to only so many levels as are possible within 8 bit processing (256 gradations of gray). Using "analog" exposure via closing down the aperture allows both the screen and focusing system to make use of more gradations. In practice this likely only affects the visuals on screen, because focusing pushes contrast even higher than the +2 you can setup for JPG images and thus kills gradations anyway.

1) Flickering light (conditions) turns the E-M5 into "Rattlesnaking" mode

When you point the camera directly towards a flickering light source - like a 50 Hz incandescent light bulb - it will switch to "Rattlesnaking" mode after a few seconds. The result of this is that the aperture will close down and will thus decrease the visible flickering on the screen.

I was not able to trigger this with power-saving bulbs, but my 30" monitor flickers quite a lot and thus also makes the E-M5 turn to "Rattlesnaking" mode. The same likely applies for older fluorescent lighting that flickers at the rate of the power-source (50/60 Hz).

At least twice I was able to trigger "Rattlesnaking" mode by quickly waving my hand in front of the lens while pointing towards a daylight window. So keep an eye on quickly changing light conditions.

The camera will stay in "Rattlesnaking" mode until you turn it off . Neither letting it go to sleep nor changing modes any anything seems to help.

2) Movie mode will always use "Rattlesnaking", but with a twist

When the camera is in Movie mode and setup to use P or S exposure modes it turns to "Rattlesnaking" immediately, because Movie mode's automatic exposure compensation then has to keep changing the aperture quickly based on light conditions. For video recording itself there is no problem with audible noise because during recording the E-M5 only changes the aperture slowly and quietly_!

Of course the same possible advantages of "Rattlesnaking" I listed before should also apply for video recording, especially the bigger DOF when shooting in brighter light conditions. On the other hand those who want a shallow DOF and make extensive use of the AEL button may need to use A or M mode in order to keep the aperture (and thus DOF) fixed. Yes the latter means that (only) in Movie mode using A and M will turn off "Rattlesnaking" completely.

3) Ominous other possible causes and ramifications of "Rattlesnaking"

At least once I had the situation with the 45/1.8 where turning the camera off and on again did not turn off "Rattlesnaking". What helped then was to remove the battery !

Unfortunately by that time I did not know enough about possible causes and solutions, so I cannot tell how it even came to that situation. But I will keep an eye on that.

Anyone finding other sources and solutions, please feel invited to discuss them here as a central place for "Rattlesnaking" on the E-M5.

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