Whether a film camera or a digital camera, a single lens reflex camera has a hump on the top of the body.
For the rest of this article I will look at digital cameras, so I will use the abbreviation for a digital single lens reflex camera, which is dSLR.
The hump on the top of a dSLR houses the mirror box. It operates like a tiny periscope, and its job is to direct light that comes in through the lens into the viewfinder, using a mirror and a prism.
When you press the shutter to take a shot, the mirror has to get out of the way while the light hits the sensor. Then it has to close to cover the sensor again.
If you shoot at 1/4000th of a second it has to move up and down in 1/4000th of second. That’s a lot to ask of a mechanical device.
When you think that some cameras can shoot at shutter speeds of 1/8000th of a second, it’s amazing that a mechanical device can do it at all.
Not only that but the prism housing has to do it without shaking the camera. So it has to be damped, which adds more complexity to the mechanism.
When you look through the viewfinder on a dSLR you are looking directly at the subject, via that prism and mirror.
Enter Mirrorless Cameras
Once manufacturers could make good quality electric viewfinders (EVFs), they could do away with the mirror box.
Some people don’t like the idea of looking at an eyepiece that is basically a tiny TV, rather than looking directly at the subject. But people have got used to it, and phones probably helped people get used to it.
And some high-end mirrorless cameras don’t have an EVF at all. Instead, you compose in the screen on the back of the camera.
And even when there is an EVF, some people prefer to compose on the screen at the back of the camera.
Autofocus
Mirrorless cameras and dSLRs autofocus differently. dSLRs cannot use the main sensor that records the image when a photo is taken because the mirror box is in the way.
So they use a separate sensor located at the bottom of the lens throat of the camera body and a small amount of the light from the mirror box is directed to this separate sensor and that initiates autofocus.
It works well enough but not as well as if the main sensor was used.
Mirrorless cameras don’t need a subsidiary sensor. Instead, the camera focuses using the whole of the main sensor.
Consequently, we now have accurate real-time eye tracking autofocus that ‘sticks like glue’ to its subject.
Speed
Because there’s no mirror box to flip up and back, cameras can shoot faster.
Cameras that can shoot at 15 frames per second with a mechanical shutter are quite common and some go much higher.
For example, the Nikon D500 dSLR was designed for speed and it could shoot at 10 frames per second. Now we have some mirrorless cameras that can shoot at 30 frames per second or more.
This is not video output; these are individual photos taken at those high numbers of frames per second.
The incredible autofocus that can ‘stick’ on a moving subject, coupled with high frame rates means that photographers can now capture fast moving subjects with pin sharpness in a way that was thought impossible a few years ago.
Just think of the shots you see nowadays in newspapers and magazines of olympic gymnasts or footballers or basketball players caught in mid-air with pin sharpness.
In-Body Image Stabilisation
Another advantage of mirrorless cameras is in-body image stabilisation (IBIS) which allows slow shutter speeds without camera shake.
Of course a slow shutter speed doesn’t help if the subject is moving fast, but for many subjects slower shutter speeds that avoid camera shake is a positive benefit.
Pentax is the only dSLR manufacturer that makes bodies with image stabilisation. But any dSLR still has to get over the problem of mirror slap from the camera body from the mirror and prism housing flipping out of the way and then back when a shot is taken.
No matter how well the mechanism is damped, it is still a tremor that simply cannot match the silent vibration-free shots taken with a mirrorless camera.
Now, even some small mirrorless models have IBIS. I have a Ricoh GRIII It is tiny and it has IBIS, which means I can hand-hold shots at 1/10th of a second and get sharp images.
I shot the photo of Jaffa Street in Jerusalem with this little camera. Not that I shot it at a slow shutter speed. In fact I shot at 1/1600th of a second at F4.5 and ISO 160.
Why Might You Want A dSLR
What you gain with a dSLR is an optical viewfinder because you are looking directly at the subject.
And with no EVF there is no drain on the battery. On a single battery charge, most dSLRs run for three or four times as long as the equivalent mirrorless camera.
Weight
It would be easy to say that mirrorless cameras weigh less than the equivalent dSLRs. It is true, but not by as big a margin as you might expect from removing the mirror box.
Weight hasn’t reduced as much as you might think because manufacturers put more features in their mirrorless cameras in a bid to outdo the competition in what is a shrinking market.
I stopped taking my Nikon D700 around with me because it weighed 1095g. My Canon R6 weighs 680g – a 415g difference, nearly a pound weight saved. But if Canon had decided not to put video capability in the R6 then it could have been even lighter. And who doesn’t like lighter?
I am not singling out Canon for doing this. They all do it.
