DSLRs: What are they, and how do they work?

From film to DSLRs and now to mirrorless. The technology that powers your life-memories capture system has come a long way. Advancements in DSLRs have really shown their might from the first-ever digital SLR, the Kodak Digital Camera System (DCS) which was released in 1991 to the cutting edge Nikon D850. Moore’s law really does apply to DSLRs too.

But the question arises what is a DSLR actually or to put it appropriately, why are they called that. To understand let’s start with a point and shoot camera. Or let’s back up even further. How does a digital camera work? To make an image first you need a device to convert light signals to equivalent electric signals which can be viewed or stored on a medium. That device is called the camera sensor. It’s the heart of any digital camera including camera phones. But if you just take a sensor and expose it to light you will get a blurry/hazy image. To fix that we have something called lenses. It’s akin to our eyes. The retina acts as our image sensor and our eye lens is eponymous. Coming back to point and shoot’s they have a fixed lens in the front and has a sensor inside the body exactly behind the lens. The light hits the front element of the lens, gets focused by it onto the sensor and a sharp image is displayed on the back LCD screen. Once you press the shutter button the live image gets saved onto a memory card. You can either view the live image on the LCD directly in LiveView mode or peer through the viewfinder which is tucked away mostly on the top left. Directly in front of the viewfinder, you will find the opening to let light through. The issue which comes up here is what you see through the viewfinder is not exactly what is being captured by the sensor since the lens and the viewfinder opening are offset by a few cms.

To solve the above problem of not capturing what you are seeing, scientists developed something called an SLR or Single-lens reflex. As the name suggests the image captured through the lens is the same for the sensor as well as for the viewfinder. DSLR stands for Digital Single Lens Reflex. The difference being the image is captured on a digital sensor. But there is still another problem that needs a solution. If there is only one source for the light to enter (i.e. via the lens) then how does it get “transported” to both the sensor (which is actually in line with the lens behind it) and to the viewfinder which is just above the lens on the backside of the DSLR? I can give you a clue if you want to rack your brains on this. If you turn on LiveView on a DSLR, you will notice the viewfinder turns black/off. Also, if the LiveView is off and you are shooting through the viewfinder you will notice that as you press the shutter button and the camera takes the picture the LiveView again turns off for the duration of the shot. If you didn’t figure it out still dont fret. The light from the lens is just reflected to the top into the viewfinder with some fancy mirror (the mirror is actually translucent to let some light through for the auto focussing mechanism to work) and prism assembly.

When the shutter button is pressed (and the LiveView is off) the mirror lifts up making way for the light to enter the sensor. The shutter curtain lifts/opens for the duration of the shutter speed set for the shot, the image is captured on the sensor, the shutter curtain closes and the mirror flips back to its original position. This happens in less than a second (or the duration of the shutter speed).

There is a great explainer video showing this mechanism by the SloMo Guys.