INTRODUCTION

If you own a mirrorless camera then chances are it offers you both mechanical and electronic shutter options. There are additional choices but for the purposes of this post, we'll stick to these two. If your reading this then you've probably encountered banding and the 'rolling shutter' effect.

I've read numerous posts where camera users have had these problems when using electronic shutters, banding, where lines of uneven exposure run through their photos or the 'rolling shutter' effect where moving images are distorted.

Most replies to these posts correctly point to the electronic shutter as the source of the problem, and more often than not switching to the mechanical shutter as the solution.

I've always liked to understand how something works, not just that it does. Knowing why parameters are placed on some functions assists me to achieve the desired results and avoid the unwanted ones.

I believe 'getting' the reason why will help make you a better photographer.

In this post, I will start with how a Mechanical Focal Plane shutter and an Electronic shutter work. I will also explain how incandescent (traditional tungsten globes) and non-incandescent lighting (Fluorescent, LED, Sodium Discharge) react differently to our AC (Alternating Current) power supply.

I will explain how this results in banding with some shutter speeds but not others, what the banding actually is, and how to avoid it.

I will also explain why you get the 'Rolling Shutter' effect, arming you with the choice to use it creatively or avoid it completely.

Note: In this discussion, I will be concentrating on the traditional 35mm frame and its dimensions of 36mm x 24mm. All dimensions are interchangeable to match the sensor size of any mirrorless camera that uses a focal-plane shutter.

MECHANICAL SHUTTERS

Focal Plane Shutter
Cloth Focal Plane Shutter
Vertical Focal Plane Shutter
Vertical Focal Plane Shutter

Traditionally the mechanical shutter in 35mm film cameras was constructed of two cloth blinds that travelled horizontally between the rear element of the lens and the film plane. With speeds of 1/60sec or slower, the first blind would set off when the shutter release was fired then stop at a point that left the film frame exposed to the light exiting the lens. The second blind would then cross the frame, cutting off the light and completing the exposure, illustrated in the image below.

Focal plane shutter 1:60

With speeds of 1/125sec or faster the second blind would begin its travel prior to the first blind reaching its endpoint. The film was exposed by a slit created by the two blinds as they travelled across the frame. The higher the shutter speed the thinner the slit, illustrated in the image below.

Focal plane shutter 1:125

As technology developed the shutter was altered both in its materials, from cloth to titanium and its direction of travel, from horizontal to vertical. As the shutter now had a shorter travel distance. (24mm vertically compared to 36mm horizontally) maximum shutter speeds could be increased.

These changes in maximum shutter speeds included the fastest usable shutter speed for flash photography rising from 1/60sec to 1/125sec.

A quick explanation in regards to flash photography. When using flash the frame needs to be clear of the shutter blinds to accept the flash illumination at the one instant. The fastest shutter speed usable with a horizontal focal plane shutter that allowed a moment between the first blind coming to its finish point, the flash firing and then the second blind commencing its travel was 1/60sec.

With a horizontal focal plane shutter, the first blind had to travel 36mm to reveal the whole frame before the flash could fire. With a vertical focal plane shutter, it only had to travel 24mm. The reduction in the distance travelled meant the sequence was completed in less time allowing the usable shutter speed for flash photography to change from 1/60sec to 1/125sec. (All of the above shutter speed figures may vary slightly depending on the camera brand and its manufacture date but the overall explanation remains consistent.)

Jumping forward to the current crop of mirrorless cameras the mechanical focal plane shutter works basically the same as it did in film cameras. Travelling from top to bottom exposing the camera's sensor as it goes.

ELECTRONIC SHUTTERS

Electronic, rolling or silent shutters are different, they are controlled by the sensor itself simulating the action of the mechanical shutter. Dependent on the shutter speed a row, or rows of pixels are initiated and terminated (turned on and off) as the electronic shutter 'rolls' down the sensor from top to bottom.

The length of time each row is initiated and exposed is determined by the shutter speed. Simulating the size of the slit created by the two blinds of the mechanical shutter. Slower shutter speeds will see more rows of pixels initiated, conversely, with faster shutter speeds the slit, or number of pixel rows initiated is reduced. The images below are created to demonstrate the process, each segment of the grid represents a single pixel.

Slow Shutter Speed
Slow Shutter Speed
Medium Shutter Speed
Medium Shutter Speed
Fast Shutter Speed
Fast Shutter Speed

The difference between the physical slit created by the two blinds used in the mechanical shutter and the simulated slit created by the electronic shutter is the time it takes to complete the exposure cycle. With the mechanical shutter, the exposure cycle equates to the shutter speed.

Using the electronic shutter is different. Each pixel row is exposed correctly, based on the shutter speed set but the exposure sequence, from initiating the top row to the terminating of the last row can take up to 1/10sec and it's this time that gives us the 'Rolling Shutter' effect.

(The actual length of time varies between camera brands and models within each brand. It depends on variables including the physical size of a sensor, it's pixel count and the camera's processing speed)

ROLLING SHUTTER EFFECT

In the images below the cars are travelling left to right at around 60kph. In both images, the cars are sharp, stopped in motion by the 1/2000sec shutter speed but the distortion is clearly visible in the photo on the left.

As the car is travelling at 60kph it's physical position within the frame at the end of it being recorded to the sensor is forward of that when it started.

Electronic Shutter 1/2000 f4 Static Camera
Electronic Shutter 1/2000 f4 Static Camera
Electronic Shutter 1/2000 f4 Left to Right Pan
Electronic Shutter 1/2000 f4 Left to Right Pan
Mechanical Shutter 1/2000 f4 Static Camera
Mechanical Shutter 1/2000 f4 Static Camera
Mechanical Shutter 1/2000 f4 Left to Right Pan
Mechanical Shutter 1/2000 f4 Left to Right Pan

ELECTRONIC SHUTTERS AND BANDING

Depending on where you live AC (Alternating Current) power grids work on either 50 or 60 hertz, or cycles here in Australia we use 50 hertz, the terms are interchangeable. There are 50 cycles per second and each cycle contains two alternations, one positive, one negative, both deliver voltage and voltage equates to light intensity.

AC Power Sine diagram

In the diagram above you can see the current sine wave begins at zero voltage, increasing to the highest positive voltage before dropping through zero voltage and alternating to the highest negative voltage. Imagine it as turning on and off 100 times every second. Each alternation is half a cycle, 50 cycles per second = 100 alternations.

INCANDESCENT, NON-INCANDESCENT LIGHTING AND BANDING

The definition of an incandescent light source is one that works by incandescence, which is the emission of light caused by heating the filament.  non-incandescent lighting (Fluorescent, LED, Sodium Discharge) is everything else. It's important to remember that no matter the style of light illuminating your subject the current is still alternating at 50 cycles, 100 alternations per second.

The difference is in the way each lighting style behaves. Under incandescent lighting, the bulb's filament doesn't cool down enough at the zero points of the voltage cycle for there to be a visible loss of light intensity so there is no banding when using either mechanical or electronic shutters.

When your subject is illuminated by any of the non-incandescent types of lighting the use of a mechanical shutter also results in an image without banding. This is because the whole of the sensor is initiated/on at the time of exposure.

At the same time, it is possible to achieve an image without banding when using an electronic shutter lit by non-incandescent lights, but there are rules we need to follow.

PUTTING IT ALL TOGETHER

When we shoot with an electronic shutter the pixel rows are turning on and off at a consistent rate, based on the shutter speed set. As the shutter rolls down the sensor, each pixel row initiation needs to receive a consistent voltage/illumination, if it doesn't we will get banding.

Consistent voltage/illumination is the key. So the maximum usable shutter speed is one that initiates the pixel rows for a length of time that delivers a consistent level of voltage/illumination to every row. To achieve this it must fit within an area on the voltage sine wave where it will repeatedly receive that consistent voltage/illumination.

That shutter speed is clearly indicated in the diagram below. 1/100sec consistently receives a full positive or negative alternation from zero to maximum voltage and back to zero again.

So here's the rule: Any shutter speed that is divisible evenly into 100 will work as it receives a consistently repeatable voltage. eg 1/50sec as it receives one complete cycle/2 alternations. 1/25sec also works and will receive two complete cycles/4 alternations.

What if we use a shutter speed of less than 1/100sec but not evenly divisible into 100, say 1/80th.

As you can see in the diagram below each time rows of pixels are exposed at 1/80sec they receive the amount of light generated over slightly less than 1.5 alternations.

Sine Wave with Shutter Speeds

In the following simplified diagram, you can clearly see how each block of pixel rows is exposed for the same length of time, 1/80sec but receives a level of voltage over each exposure inconsistent/unequal to the previous or following exposure. Inconsistent voltage/Intensity over varying pixel rows or pixel row blocks equals banding.

80sec

Shot on the Fujifilm GFX-50s the results below are a practical example of the explanations above.

1:50
1:60
1:80
1:100
1:125
1:200sec

BEST PRACTICES

There are pros and cons to each shutter type.

Mechanical Shutter Pros: Use with Flash, less shutter lag, freezes fast-moving subjects Cons: slower maximum shutter speeds, slower maximum continuous frame rate, wear and tear on the shutter mechanism, not silent, possible camera vibration.

Electronic Shutter Pros: Faster shutter speeds, silent operation, no wear or tear, no camera vibration, Cons: Rolling shutter effect, can't use flash, banding.

In regards to using the electronic shutter under non-incandescent lights. It is always best practice to check for banding by shooting a few test frames to confirm all is ok.

CONCLUSION

I hope this post of how both styles of shutter work, how they behave with various forms of lighting and how to solve those issues have informed you as much as the research process has informed me. I have learnt much throughout the research for this post which is as enjoyable as the writing itself.

If you have any questions or comments please leave me a note below.

Cheers, John

 

If you're interested then check out these other posts

Fujifilm's 90mm f2 lens review

Blackrapid Double Breathe harness review

The Upside to having your images stolen

 

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