Wednesday, 29 October 2014

Electronic shutter readout speed

Many modern system cameras feature the electronic shutter option. This means that you can take pictures silently, without activating a mechanical curtain shutter. Rather, the exposure is started and stopped electronically.

The Nikon 1 series is pretty much built around electronic shutters. The basic J and S series of cameras do not even contain any mechanical shutter at all, you only have the electronic shutter option.

The downside of the electronic shutter is that not all the sensor is read at once. Rather, the sensor photosites exposures are started and stopped row for row, vertically. This process of starting and stopping the exposure, and then collecting the exposure data, can take around 1/10s to 1/100s, depending on the camera model.

The slower, the less useful the electronic shutter is, as you can risk rolling shutter effects, buildings appear to lean if you pan while photographing, for example.

Rolling shutter effects are not exclusive to digital cameras. Rather, they were present also with early film cameras. Here is a famous photo of a racing car taken in 1913 by Jacques Henri Lartigue using a 4x5 Speed Graphic camera:



The curtain shutter moves relatively slowly on this camera 100 years old camera, when compared with modern SLRs, which gives the distortion of the racing car. The distortion is especially visible in the wheels, which appear to be leaning forward. This rolling shutter effect was later copied by cartoonists when they wanted to give the impression of speed.



One way to test the speed of the electronic shutter is to take a photo at a fast shutter speed in artificial light. For about a century or so, people have been using incandescent light bulbs for electronic indoor lightning. Even when used on alternating current (AC), the light is stable. Since the filament is heated, it emits light also when the alternating current is at zero.

However, traditional incandescent light bulbs are now being replaced with the energy saving fluorescent light bulbs. They tend to flicker at 100Hz (in Europe) or at 120Hz (in the US). The lights don't flicker at 50Hz and 60Hz, as you might expect. This is since during each period, the electrical current reaches two peaks, see the illustration below:



So to measure the speed of the electronic shutter, it is sufficient to look at the picture and see how many periods you get. Each period corresponds to 1/100s.

Here are the results using some Nikon 1 and Lumix G cameras:

Nikon 1 J1Nikon 1 V3
ISO 6400, 1/1000s, f/1.8ISO 6400, 1/1000s, f/1.8
Lumix GH3Lumix GH4
ISO 1600, 1/320s, f/1.4ISO 3200, 1/640s, f/1.4

The Panasonic Lumix camera results are the most easy to interpret. With the Lumix GH3, I count ten periods over the exposure, meaning that the electronic shutter readout speed is 1/10s, which is very slow.

The GH4 gives around 3.3 periods, corresponding to a readout speed of 1/30s. This is a significant upgrade over the GH3, but still rather slow.

With the Nikon 1 cameras, it is a bit harder to count the number of periods. But we can find the distance between the periods. Here is the picture from the Nikon 1 V3:


The distance between the two periods is about 2820 pixels, indicated by the orange line of the right. This corresponds to 81% of the vertical image height. This means that the electronic shutter readout speed is 1/100s divided by 81%, or 1/81s. Let's say 1/80s, since there is some uncertainty.

When using the Nikon 1 J1, the distance between the periods is about 85% of the image height, corresponding to a slightly faster readout speed of 1/85s. This makes sense, as the J1 only has 10MP, much less than the 18MP resolution of the Nikon 1 V3.

Safe shutter speeds


If you are photographing indoors, and you want to avoid these horizontal rows, then you can not use too fast shutter speeds. If you are in the US (or other NTSC countries), then the fastest shutter speed you can use is 1/120s. It is also safe to use 1/60s, 1/30s, and slower. If you are in Europe, or other PAL countries, then use 1/100s, 1/50s, 1/25s, or slower.

Conclusion


The Lumix cameras are still quite inferior in terms of the electronic shutter readout speed. This can lead to significant rolling effects. Here is a pair of example images taken using both shutter types with the Lumix GH3, at f=32mm, f/2.8, 1/320s, ISO 800:



Mechanical shutterElectronic shutter

Setting a faster shutter speed would not help. You would still get the skewed car using the electronic shutter. This is because the speed of the sequential sensor readout is independent of the shutter speed.

You can also use this effect creatively. When photographing vibrating items, you can get fun effects. Here, I have photographed a bass player's hand:



The picture was taken at with the Lumix X 45-175mm lens at 175mm f/5.6, 1/800s, ISO 200. In retrospect, I should have set the ISO higher, to achieve a faster shutter speed, and better defined green string, with less motion blur. The camera was held in landscape orientation, which is crucial here.

The Nikon 1 camera have an electronic shutter readout speed which is fast enough that it rarely is a problem. When using the Nikon 1 V3 camera and the CX 70-300mm lens (my review) to photograph birds in flight, the background can become skewed due to the panning. Here is an example image, taken at 300mm, f/5.6, 1/1000s, f/5.6:


As you see, the fence in the background is pictured as skewed, even if it is square in reality. This is because of the rolling shutter effect.

Apart from these panning side effects, I rarely find any negative rolling shutter side effects when using the electronic shutter in the Nikon 1 cameras. The 1/80s readout speed appears to be fast enough. The flash sync speed is still set slightly slower, to 1/60s, probably for some margin when firing off the flash.

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