DK on the a99

Started Dec 27, 2012 | Discussions thread
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copejorg1 Senior Member • Posts: 1,626
Re: Real world experience

Very interesting subthread between you and Nordstjernen.  Both very experienced, both know what you're talking about, but having a strong disagreement about a technical aspect of DSLRs.  Perhaps I can help ...

You may be right in your contention that many (most? -- I sure don't know) DSLRs use one or more electric motors for cocking the shutter and mirror mechanisms against the tension of a mechanical spring, and that the actual raising of the mirror (and presumably release of the shutter) is accomplished using the energy stored in the springs.  This certainly would explain why the cameras in your experience can hold the reflex mirror locked up for extended periods of time without the apparent use of additional electrical energy from the battery.

But that doesn't necessarily mean that the Sony DSLRs work that way.  Indeed, the information available for the A900 seems to indicate that the mirror is raised using motor power alone (although this is not conclusive).  Take a look at the shutter and mirror items on this page of DPR's A900 review ...

I have no reason to dispute Nord's contention that a long exposure drains the battery quickly on his Sony DSLRs, but not on his SLTs.  Here's a little engineering jargon that could explain why:

A DC electric motor not only continues to draw current when it is "stalled" against a load, it actually draws maximum current (and produces maximum torque or linear force) -- for any given input voltage -- under those conditions.  Ironically, the faster the motor spins, the less current it draws (and the less torque or force it can produce) -- again at any given input voltage.

So if we imagine a mechanism design that incorporates a fairly powerful DC motor to accelerate the mirror out of the light path very quickly, it theoretically still could be very energy efficient in normal (non-"long exposure") usage, because:

  1. It goes through the complete up/down cycle very quickly, without pausing for a very long time with the mirror raised (during which the motor presumably is "stalled" while pressing the mirror mechanism against its travel stop), and
  2. The motor is spinning (the mechanism is physically moving) during the majority of the cycle time, reducing its energy consumption during the exposure cycle.

But if we take that very same mechanism, using the same powerful DC motor, and stall it against the mirror's travel stop (forcing the motor to draw its maximum possible electrical current) for the full duration of a long exposure, it's pretty easy to imagine how that could drain the camera's battery (and produce quite a bit of heat in so doing) during that long exposure.

The above is just my own conjecture, concerning the nature of Sony's DSLR mirror-lift mechanisms.  But it does provide a very plausible explanation for Nord's observations.



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