DSLR vs Mirrorless: The Full Story

Mirrorless cameras are now so similar to DSLRs, that in most cases it makes no sense to separate between them. If you’re shopping for your first interchangeable-lens camera, it rarely makes sense anymore to limit your pool to just DSLRs, or just mirrorless cameras. Yet there are differences that should not be ignored.

Before we get to the differences, let’s review some of the undisputed similarities. Both types of cameras are under the umbrella of the “interchangeable-lens camera” (ILC) category, meaning you can choose which lens you want to use. This separates both types from fixed-lens cameras, which are anything from phone cameras, through superzooms, to large-sensor cameras with a normal-range lens. If a fixed-lens camera offers everything you need, that will generally be the preferred choice, as they tend to be lighter and less expensive than a similar outfit with an ILC. But there is an added flexibility that only an ILC can provide, which is why it is still the primary camera for most self-proclaimed photographers.

Another similarity between DSLRs and mirrorless camera is the image sensor, and the imaging pipeline as a whole. There is great overlap, where a DSLR may have the same image sensor as a competing mirrorless camera, or one that performs very similarly. In any event, if there is a difference in the image quality that you can get with two different cameras, it has nothing to do with the camera being a DSLR or a mirrorless camera, as the mirror is not part of the actual image-capturing process.

With that out of the way, we can talk about the real difference. There is one thing that separates between the two camera types, and that is the presence — or absence — of a mirror. The moniker makes it clear: “mirrorless” cameras have no mirror. What is that mirror used for, and is it even needed?

That mirror in a DSLR, positioned in front of the sensor at a 45-degree angle, serves one main purpose: creating an optical viewfinder. It does that by directing the incoming light to a prism, or a set of smaller mirrors, which then redirects it to the photographer’s eye. This gives a through-the-lens (TTL) view of the world.

While the mirror is in this position, the image sensor is completely obscured. When you press the shutter button to take a picture, light from the lens has to reach the sensor, so the mirror flips up and out of the way. Once the exposure is done, the mirror flips back down.

Before digital image sensors revolutionized the camera industry, this design was basically the only way to see “what the lens sees,” and to get an accurate representation of the photo you are about to take. This is no longer the case — our cameras can give us a near-live feed of the image created at the sensor, so we can actually see what we are about to capture.

That is the idea behind mirrorless cameras. Since the mirror is no longer necessary for TTL viewing before taking a picture, we can just make the same camera without the mirror and its associated components.

This one thing differentiates between a DSLR and a mirrorless camera. This is the only principle distinction.

You may have read or heard about DSLRs outperforming mirrorless cameras in some areas, or mirrorless cameras doing some things better. Those are not necessarily linked to the presence or absence of a mirror, but may be affected by it. We’ll tackle the most talked-about points.

Viewfinder

All DSLRs have an optical viewfinder at the top, giving a TTL view of the world, while mirrorless cameras give an electronic view. That can be with the screen on the back, or with an eye-level viewfinder — EVF (electronic viewfinder). The EVF is made up of a small screen, and optics that let you view the screen from a very short distance.

When you look into an EVF, you get the same view as the rear LCD provides, including useful features, like a live histogram and focusing aids. To get the same features on a DSLR, you would typically have to switch to Live View mode (see the “video recording” section below), or review the image after it’s taken. But with a mirrorless camera that has an EVF, you can use those aids without taking your eye off of the viewfinder.

An EVF is not for everyone, as you will see in the “speed” section below, and neither is a DSLR’s optical viewfinder. I highly advise that you try both, and make your own judgment on which is better, because one is not universally better than the other. Do note that all electronic and optical viewfinders are not made equal — some are bigger, brighter and clearer than others.

Size and weight

For most people, the most tempting aspect of a mirrorless camera is its smaller and lighter weight. A DSLR’s design simply has large and heavy parts that a mirrorless design excludes, so it can be built with a smaller chassis. It was the first thing that drew me to a mirrorless system, but we now have to acknowledge that it might not be as big a difference as we thought.

The camera body is only one part of a photographer’s kit. It could even be a very small part in your kit, if you’re one who needs versatility, shooting with multiple high-grade lenses. Lenses of similar specifications, that are built to similar standards for build and optical quality, are generally very close in size and weight.

If you are looking for a small and light camera, you should carry the same approach over to your lens selection. For example, a zoom lens with a constant maximum relative aperture of f/2.8 is likely to have similar dimensions across multiple camera systems — both DSLR and mirrorless — in the same focal-length range. You can compare sizes and weights in spec-sheets, or visually in the Camera Size comparison website (make sure to switch to the “Top with Interchangeable lenses” view, by clicking the rightmost button at the top).

Autofocus

Mirrorless cameras direct light to their image sensor at all times, so they also gather information about the scene from that sensor. In a DSLR, however, the image sensor receives no light at all while composing a shot, so the information the camera needs to gather for automatic operations has to be collected somewhere else. For this reason, DSLRs use a separate sensor for autofocus, and a sensor for metering.

Much has been said about mirrorless cameras’ autofocusing woes, but most of that is outdated and now false. Camera manufacturers have apparently poured a lot of their resources into this subject, because it has been getting better with every generation of cameras. I do not want this article to suffer the same fate, so I will not categorically claim that one is “better” than the other. It is now less of a divide between DSLRs and mirrorless cameras, but more of a variation between models, so you will have to dig in and do some research on the cameras that fit your budget. It can no longer be generalized.

Keep in mind that there are many different use cases and scenarios for autofocus. One system may outperform another in one test, and the result could be flipped entirely in another test. It is important that you analyze reviews and test samples in parallel to your needs. Keeping a sprinter in focus can pose a different challenge than tracking a soccer player’s movements.

Lens selection

Another topic where anything that is written about it is bound to become false just shortly after it is published: the selection of lenses in a camera system.

The first of the major mirrorless systems, Micro Four Thirds, was launched in 2008. A new lens mount was devised, and soon enough, it had to compete with the longstanding behemoths that are the Canon EF and Nikon F lens mounts, dating back to 1987 and 1959, respectively. With other manufacturers joining the fray later on, each creating its own unique and brand new lens mount, lenses that had already gone through many revisions in the older mounts had to be launched for the first time.

Those days are now gone. At the time of this writing, the three established mirrorless systems — Micro Four Thirds, Sony E and Fujifilm X — have been running for 5–10 years, with gaps constantly being filled. Here, too, we have reached a point where there is no use for generalization, and it only makes sense to see if the lenses you need are available and affordable. There are still some gaps that manufacturers will fill in years to come, but I would say it is now safe to dive in as a new photographer who doesn’t know what lenses they might need.

Every discussion about lens selection for a mirrorless camera must also include adapted lenses. Due to the short flange distance mirrorless systems are built around — shorter than most systems had before them — a simple adapter can open a camera to the lens options of another mount. This may open a can of worms, though, especially regarding autofocus. It is generally advisable to stick to the lenses that are “native” to a system — that way you get all advertised functionality. But if a system lacks a lens you need, review your options for adapted lenses.

Video recording

As with image quality, there presence or absence of a mirror has no bearing on the video quality a camera can produce. A DSLR can theoretically output just as fine of a video file as a mirrorless camera can, with all else being equal. Again, review the specific cases you are interested in — do not try to generalize here.

There is, however, one key element here, and that is ergonomics. I always direct prospective camera buyers to go to a store and handle a few cameras, once they’ve nailed their options down to a manageable few. The way a camera handles, the layout of the buttons … those are critical factors that cannot be appreciated without physically holding the camera.

So far we have talked about a DSLR in its normal setting — composing still images with the optical viewfinder. However, video cannot be recorded in this mode, because it basically needs a constant stream of light to the sensor. To counter that, DSLRs have a mode called “Live View,” where the mirror flips up permanently, and a feed is provided to the rear LCD screen.

When a DSLR is used in Live View mode, it effectively becomes a mirrorless camera in operation. You get the same digital feed to compose your shots. Yet the camera now has several components not in use — including the optical viewfinder, which is completely blacked out. The one thing that defined the camera as a DSLR is no longer in use.

This poses a problem with handheld shooting. Normally, you would have the viewfinder up to your eye, which gives you a steady position to hold. But with a DSLR in Live View mode, you’re left to hold it arm’s length, where it is a lot more difficult to stabilize the camera. Mirrorless cameras that have an eye-level electronic viewfinder (EVF) let you record video while using the viewfinder, so you can more easily stabilize the camera. If all of your video shooting is on a tripod, or with some stabilizer/rig, this might be irrelevant to you.

Another point to add is about autofocus. Mirrorless cameras operate the same way for stills and video, but with a DSLR, you also lose the sensor dedicated to autofocus when you switch to Live View, and you’re usually left with a far slower alternative. Some cameras handle better than others, like Canon’s DSLRs that are equipped with Dual Pixel CMOS autofocus, so it is not a universal mirrorless/DSLR divide. If you need to use autofocus while recording video, this is another point to research about the camera you’re considering.

Battery life

While mirrorless cameras always power a display (whether it is the small, but usually high-resolution EVF, or the larger panel on the back), DSLRs rely mostly on the ambient light to provide a view of the scene through the lens. The latter obviously consumes less power, because it only needs to power a display for useful overlays, such as the active autofocus point and the current exposure parameters.

As such, DSLRs tend to have better battery life. This drops significantly if Live View is used extensively.

One piece of advice: Always carry at least one spare battery, no matter how good the battery life on your camera is. If you’re stuck with a digital camera without a charged battery, you might as well walk around with a brick.

Speed

Nothing can ever be as fast as the speed of light, except for light itself. I referred to the digital display / electronic viewfinder as giving a “near-live feed,” because it can’t be truly live. The optical viewfinder of a DSLR simply shows you the light coming through the lens, so you actually see what is happening in front of you, when it happens.

This delay is another matter that camera manufacturers keep improving, and in most cases it’s completely unnoticeable. It is worth paying attention to, though, at least if you want to photograph fast moving subjects. Of course, it would be best to try the EVF yourself, and see if you miss shots because of it not being responsive enough.


To me, it seems like the differences are now more technical than practical. Gaps are constantly shrinking. There are very few cases now where one type is appropriate and the other isn’t. When looking for an interchangeable-lens camera, review your options by their merits, not by their type.