The Second Curtain

Where does a curtain come into play in your camera? Think about it.

What acts like a curtain to your sensor? 

Correct, its the Shutter.  Now, how does the curtain/shutter work? 

Lets together come up with a design for the shutter shall we?

Lets assume that the shutter is a curtain that initially sits on top of the sensor.

When I press my shutter release button (say the shutter speed is 1 sec), the curtain moves up and exposes the sensor to light. As below…

It then comes back and closes the sensor completely at the stroke of 1 sec. Here is a pictorial depiction of the same.

curtain 1

All ok?

Really?

Think for a bit and I’m sure you will realize that with this design, the pixels on the bottom will be exposed to light for a greater duration than the pixels on the top.

So how does one overcome this basic issue of exposing each pixel for the exact same time as is set by the photographer.

The simplest of solutions is to have the shutter curtain made up of two curtains instead of one. By default the position of the shutter assembly could be as follows

curtain 2

When the shutter release button is pressed, the first curtain starts moving up. Before the first curtain starts its journey back, the second curtain starts covering the pixels . This ensures that none of the pixels will be getting more or less light. All pixels get the same amount of light thus helping the camera click correctly.

At what time does the second curtain start moving depends entirely on the shutter speed set. For any shutter speed that is faster than 1/200 or 1/250, the second curtain starts after the first curtain has completed the first step of its journey, meaning, after the first curtain exposes all the pixels  of the sensor. 

For any speed that is faster than 1/200 or 1/250, the second curtain starts moving before the first curtain has completely exposed the sensor, thus forming a window/band of pixels that are exposed instead of the entire sensor simultaneously. 

This speed barrier of 1/200 or 1/250 is the fastest shutter speed at which your entire sensor is exposed to light at the same time. This speed is called the ‘Shutter Sync Speed‘ and we will cover why you need to know it in the next blog.

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This entry was posted in Advanced Photography Tips.

One Comment

  1. Vishal Gokhale September 28, 2016 at 8:29 pm #

    The shutter indeed moves top to bottom at least in 7D

    Here’s a demonstration by the famous “The Slo Mo Guys”
    https://youtu.be/CmjeCchGRQo

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