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03-03-2019, 07:21 AM   #1
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Naive DSLR shutter question

In a standard dslr shutter one curtain follows another to produce an exposure, then returns to the original position before the next. This is why a machnical shutter has a "speed limit". Why?
Can it not do an exposure going in the opposite directions? Seems doing exposures in both directions would increase frame rates.

03-03-2019, 07:31 AM - 1 Like   #2
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There would need to be reversing mechanisms, which would increase the complexity of an already complex system, a great deal. It is amazing that shutter speeds are as fast as they are already! Seeing some pro-level Canikons being shot on tv and the speed is faster than 6-700 shots per minute in bursts. (Compared to an M-60 machine gun, the Canon was faster burst speed-wise!) So A faster burst is possible but does not seem to be a Ricoh/Pentax priority, and even though your idea could logically work, I imagine engineering and manufacturing costs would rule it out.
03-03-2019, 09:15 AM   #3
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Yes, shutter frame rates could be increased at the cost of a more complex shutter mechanism with a bidirectional shutter. And it would make the shutter significantly more expensive.

But the shutter frame rate isn't the true source of the speed limit in a DSLR. Two other subsystems impose a speed limit, too. For a DSLR, a faster frame rate also requires a faster mirror mechanism so that the photographer gets a glimpse of the image between frames. The mirror is a far larger, heavier, and more complex mechanism. Making that mirror move faster is the bigger mechanical design challenge. A faster reflex mirror mechanism also presents a serious power drain problem -- double the frame rate means 4X the peak power drain from the battery.

The second subsystem is the sensor and limitations with how quickly it can read all the pixels. The sensor's speed limit comes from the design of the chip itself and a rather deep trade-off in electrical engineering and physics between read-out speed and IQ. Carefully reading every photoelectron in every pixel takes time especially with large sensor chips.

----

As an aside, a bidirectional shutter would also create some strange quirks in the images. For scenes of fast moving objects, the odd-number frames would look a little different from even-numbered frames because of how the object moved while the shutter was going in different directions.

P.S. Older computer users may remember early dot matrix printers and teletype printers that could only print in one direction. The printhead had to move back to the left edge before starting the next line. Eventually, printer makers put enough smarts inside the printer to print in both directions.
03-03-2019, 09:31 AM   #4
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QuoteOriginally posted by photoptimist Quote
P.S. Older computer users may remember early dot matrix printers and teletype printers that could only print in one direction. The printhead had to move back to the left edge before starting the next line. Eventually, printer makers put enough smarts inside the printer to print in both directions.


Oh yes !! Eventually there were "band" printers, with a print-hammer for every character space on the line and a rapidly revolving metal band with several copies of the alphabet etc. on it that travelled between the print-hammers and the paper at high speed. The appropriate hammer would fire as the required character came past and the whole assembly would print several LINES of 132 characters a second! The noise level was considered dangerous and one had to wear muffs if working on the machine with the lid open!

03-03-2019, 09:31 AM   #5
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QuoteOriginally posted by photoptimist Quote
4X the peak power drain from the battery.
So, doubling speed requires an logarithmic increase in power, similar to audio measurements which I thought were more like exponential increases when doubling the audible signal, with a Log scale for noticeable increases in sound, >3db.
03-03-2019, 10:57 AM   #6
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Thank you all, I did not know about some of the complications associated with the mechanisms. I do think it is quite amazing that these shutters can operate at the speeds they do. Some cameras capable of frame rates of eight plus frames per second is just astonishing. Thank you again for the information and education.
03-03-2019, 11:06 AM - 1 Like   #7
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QuoteOriginally posted by SSGGeezer Quote
So, doubling speed requires an logarithmic increase in power, similar to audio measurements which I thought were more like exponential increases when doubling the audible signal, with a Log scale for noticeable increases in sound, >3db.
Technically speaking, the kinetic energy in a moving system scales quadratically (x^2). Thus, a mirror going 2X the speed required 2^2 = 4X the energy, a mirror going 3X the speed has 3^2 = 9X, 4X speed has 4^2 = 16X, etc.

BTW, a logarithmic increase in power (power increase = log(x)) would imply a 10X increase in speed only takes 100% more power, a 100X increase in speed only takes 200% more power, and a 1000X increase in speed might only take a 300% increase in power.

An exponential increase would have the power required equal to c^x, where c is a constant that depends on the specifics of the system. If c=2, then 2X the speed takes 2^2 = 4X, 3X the speed takes 2^3 = 8X, 4X the speed takes 2^4 = 16X, etc. The relationship between exposure stops and total light, for example, is exponential with c=2. Thus, designing a photoflash system that gives 3 stops more light means using a flash capacitor and flash tube with 8X the power rating.
03-03-2019, 12:02 PM   #8
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QuoteOriginally posted by photoptimist Quote
Technically speaking, the kinetic energy in a moving system scales quadratically (x^2). Thus, a mirror going 2X the speed required 2^2 = 4X the energy, a mirror going 3X the speed has 3^2 = 9X, 4X speed has 4^2 = 16X, etc.

BTW, a logarithmic increase in power (power increase = log(x)) would imply a 10X increase in speed only takes 100% more power, a 100X increase in speed only takes 200% more power, and a 1000X increase in speed might only take a 300% increase in power.

An exponential increase would have the power required equal to c^x, where c is a constant that depends on the specifics of the system. If c=2, then 2X the speed takes 2^2 = 4X, 3X the speed takes 2^3 = 8X, 4X the speed takes 2^4 = 16X, etc. The relationship between exposure stops and total light, for example, is exponential with c=2. Thus, designing a photoflash system that gives 3 stops more light means using a flash capacitor and flash tube with 8X the power rating.
My math skills are weak! Mea Culpa!


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