Mechanical shutter vs. electronic shutter

Started Mar 14, 2011 | Discussions thread
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Joseph S Wisniewski Forum Pro • Posts: 34,123
The Nikon D70 lesson...

Itamar Hason wrote:

The opposite is true.

No, it is not.

The rolling shutter effect is a result of the CMOS sensor which doesn't capture whole of the frame at once but doing it gradually from top to bottom.

No, that is not true, either.

The rolling shutter is a result of the sensor not having "snap shutter" circuitry. As Human Target already pointed out, that circuitry literally takes up half of a pixel cell, cutting the light gathering photodiode area in half. This causes pooer low light performance and reduces dynamic range.

It also radically increases the sensor's vulnerability to blooming. The Nikon D70/D50/D40 used a snap shutter CCD, and it was famous for turning sunsets, city lights at night, and Christmas lights into streaks that often crossed the entire image from top to bottom. That camera line pretty much spelled the end of the snap shutter in DSLRs.

The presence or lack of a snap shutter is not a characteristic of either CMOS or CCD. CMOS sensor can be made with or without that shutter circuitry. CCD sensors can also be made with or without it.

The reason CCD sensors often had snap shutters was that they evolved in the early days of video. In those days, sensors were slow, and the circuitry that read sensors was slow, so it was hard to make a sensor keep up with the read rate. If you had a 30 frame/sec video sensor that took just about 1/30 sec to read, you ended up with severe rolling shutter artifacts, movement could cause a tear across the entire image.

But when CCD sensors started being applied to digital still cameras, the snap shutter was often eliminated, to improve low light performance and blooming resistance.

Canon made the first serious, large scale use of CMOS sensors, so that was pretty much "born" as a DSLR sensor, with the snap shutter left out of the equation. There have been specialty CMOS sensors with snap shutters built.

Now, we're even seeing the snap shutter left off video applications. Readout speeds and camera processors have gotten fast enough that sensors can be read at anywhere from 2-10x video rates. So, tearing is down to a small fraction of a screen width, to the point where it's becoming about equal in "audience annoyance" to the "jump" from a snap shutter.

The mechanical shutter enhances this effect when shooting a moving subject (like an helicopter) because the top part of the frame is exposed first and then the bottom (or the other way around...).

Actually, helicopter blades are pretty low speed, 300-500 RPM. That's 5-8 revs/sec, so you won't see curved blades from SLR shutter speeds. You might see it in the tail rotor or an airplane propeller.

But ask any photographer which they'd prefer, better propeller image quality or better low light performance, dynamic range, and blooming resistance...

Propeller photographers are a very specialized niche.

With an electronic shutter you can eliminate this effect by using a fast enough shutter speed. Again - in a 1/1000 sec mechanical shutter speed you get a rolling shutter effect exactly as you get in a 1/200 sec if your flash sync speed is 1/200 sec.

The price you pay for that is too high.

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Rahon Klavanian 1912-2008.

Armenian genocide survivor, amazing cook, scrabble master, and loving grandmother. You will be missed.

Ciao! Joseph

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