Why the A99 is slower than the A77 :-)

Started Sep 18, 2012 | Discussions thread
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JohnFrim Contributing Member • Posts: 572
Re: Are you kidding right?

Thanks for the reply. I agree a detailed discussion would take far too much space, and that is why I started at the higher level of understanding the concept.

Summarizing my point and ignoring a bunch of other factors for the moment, the same shutter traverse speed can only contribute to a higher fps when comparing FF to APS-C cameras because the sensor size, and therefore the required traverse distance, is smaller on the APS-C. And this shorter traverse distance is important not only for how far the shutter travels in the expose phase, but also in how far the shutter has to be moved during reset. In other words, shorter operating distance with the same speed of the mechanicals will allow the shutter to complete one operating cycle in less time, and it can then be tripped sooner for the next frame.

But using a FF camera in crop mode and running the same shutter traverse speed cannot give a higher fps. The required sensor area exposure time is certainly shorter by the ratio of the vertical frame height when in crop mode, but if the shutter still travels the FF distance it will not result in a higher equivalent fps.

That was the statement I was objecting to.

The other basis of my saying your theory is weak is that your math was based on correct values regarding cycle times for 6 vs 10 fps, but you translated that into shutter traverse speeds of mm/s. If you look back at the video links you posted it is really obvious that shutter traverse duration during the expose phase is a very very small fraction of the entire shutter cycle. It just does not make sense to use these shutter traverse speeds as the basis for comparison, especially when your theory is based on a mm/s traverse speed that represents movement in one direction over the entire cycle period.

As I have stated, higher fps requires shorter total shutter cycle times. I would bet that to keep things simple the shutter mechanism is designed to operate at some maximum rate like 10 fps and it is slowed down for 6 fps. And slowed down means a delay before tripping rather than a change in actual cycle time between being tripped and being re-cocked for the next firing.

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