In this lesson, you’ll learn the difference between a DSLR and a mirrorless camera, two of the most popular options for professional and amateur photographers on the market today.

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DSLR stands for Digital Single Lens Reflex. It’s a digital camera that has a built-in mirror (the reflex part of the acronym) so the image you see bounces up to the viewfinder. When you look through a DSLR viewfinder, eyepiece, on the back of the camera, whatever you see is passed through the lens attached to the camera, which means that you could be looking at exactly what you are going to capture. Light from the scene you are attempting to capture passes through the lens into a reflex mirror (2) that sits at a 45-degree angle inside the camera chamber, which then forwards the light vertically to an optical element called a pentaprism (7). The pentaprism then converts the vertical light to horizontal by redirecting the light through two separate mirrors, right into the viewfinder (8).

When you take a picture, the reflex mirror (2) swings upwards, blocking the vertical pathway and letting the light directly through. Then, the shutter (3) opens up and the light reaches the image sensor (4). The shutter (3) remains open for as long as needed for the image sensor (4) to record the image, then the shutter (3) closes and the reflex mirror (2) drops back to the 45-degree angle to continue redirecting the light into the viewfinder.

Obviously, the process doesn’t stop there. Next, a lot of complicated image processing happens on the camera. The camera processor takes the information from the image sensor, converts it into an appropriate format, then writes it into a memory card. The whole process takes very little time and some professional DSLRs can do this 11+ times in one second!

If you take off your lens on the DSLR, you can see the mirror and the shutter opening when pressing the shutter button. You can see that there is a mirror right there, you don’t actually see the sensor at all, you just see the mirror reflecting through up into the viewfinder.

Although single lens reflex cameras have been available in various shapes and forms since the 19th century with film as the recording medium, the first commercial digital SLR with an image sensor appeared in 1991. Compared to point-and-shoot and phone cameras, DSLR cameras typically use interchangeable lenses.

DSLR camera limitations

Due to the mirror dependency of DSLRs for through the lens (TTL) viewing, they have these limitations:

  1. Size and bulk. The reflex system needs space for both mirror and prism, which means DSLRs will always have a wider camera body and a protruding top. It also means the viewfinder must be fixed in the same spot on every DSLR, in-line with the optical axis and digital sensor. Basically, there is no other place to put it. As a result, most DSLRs have somewhat similar exterior look.
  2. Large size and bulk also translate to more weight. While most entry-level DSLRs have plastic bodies and internal components to make them lighter, the minimum height and depth issue to house the pentamirror translates to lots of wasted space that needs to be covered.
  3. Complex mirror and shutter design. Every actuation requires the mirror to move up and down to let the light pass through directly onto the sensor. This alone creates a number of issues:
    1. Mirror slap. DSLR cameras produce quite a bit of noise thanks to the shutter mechanism coming up and down each time an image is captured. This mirror slap not only results in noise, but also in camera shake. Although manufacturers have been coming up with creative ways to reduce noise by slowing down the mirror movement, it is still quite audible. Camera shake can also become an issue when shooting at long focal lengths and slow shutter speeds. Once again, DSLR manufacturers had to come up with features like “mirror lock-up” and “exposure delay” to allow mirror to be lifted, then exposure taken after a set delay, all to reduce mirror-induced vibrations.
    2. Frame speed limitation. While the modern mirror and shutter mechanisms are very impressive, they are limited by the physical speed at which the mirror flips up and down. For example, when the Nikon D4 fires at 11 frames per second, the mirror literally goes up and down 11 times within each second, with the shutter opening and closing in between! It has to be a perfect synchronization of both the mirror and the shutter in order for it all to work. Now imagine this process at 15-20 times per second, that’s practically physically impossible to achieve.
    3. Expensive to build and support. The mirror mechanism is very complex and consists of dozens of different parts. Because of that, it is expensive to build and provide technical support if anything goes wrong. Disassembling a DSLR and replacing internal components can be very time consuming for a service center.
  4. No live preview via OVF. When looking through an optical viewfinder (OVF), it is impossible to see what the final image is actually going to look like. You have to look at the camera meter (which can be fooled in some situations) or the LCD in live view mode and adjust the exposure accordingly.
  5. Secondary mirror and phase detection You might already know that all DSLR cameras with phase detection autofocus system require a secondary mirror. In short, part of the light that reaches the mirror ends up on the smaller secondary mirror that sits at a different angle than the primary mirror. The purpose of the secondary mirror is to pass the incoming light to phase detection sensors that are located on the bottom of the chamber. The problem with the secondary mirror, is that it has to be positioned at a perfect angle and distance for phase detection to work accurately. If there is even a slight deviation, it will result in missed focus. And even worse, the phase detection sensors and the secondary mirror have to stay perfectly parallel to each other. If they don’t, some autofocus points might be accurate, while others will constantly miss focus.
  6. Phase detection and lens calibration The problem with the traditional DSLR phase detection system not only lies with the secondary mirror alignment issues, but also requires lenses to be properly calibrated. It becomes a two-way game. Precise focus requires perfect angle and distance of the secondary mirror to the phase detection sensors and requires a properly calibrated lens to the body. If you had AF accuracy problems with your lenses in the past, you might have had experience sending your gear to the manufacturer. Very often, support techs will ask the lens in question to be sent together with the camera body. If you wondered why before, now you have the answer, there are basically two places where things could potentially go wrong. If the technician adjusts your lens to their standard camera environment and your camera is slightly off, your issues might get even worse after such tuning. That’s why it is best to calibrate both the camera and the lens to resolve those discrepancies.
  7. Although manufacturers have gotten much more efficient over the years in terms of DSLR production, assembling the mirror mechanism is no easy task. Lots of moving components mean high precision assembly systems, the need for lubrication in areas where metal components rub against each other, etc. In turn, this all results in increased manufacturing costs. And it does not stop there, if anything goes wrong with the mirror mechanism, the manufacturer must repair or even potentially replace it, which is a very labor-intensive task.


A mirrorless camera is just that, it has no mirror mechanism. The full name for these cameras is a Mirrorless Interchangeable Lens Camera (MILC). When you take off the lens, this camera will expose the sensor in its full glory, nothing is blocking it at all. In contrast, a mirrorless camera is much simpler mechanically – light passes through the lens (1) directly onto the image sensor (4) and the optical viewfinder is replaced with an electronic viewfinder (9) that replicates the image sensor. Mirrorless cameras typically have electronic viewfinders (EVF), since light is no longer reflected on to an optical viewfinder of the type, we are all used to on our cameras (OVF). Which means the viewfinder is an electronic display, a tiny little screen inside the eyepiece allowing you to see everything the sensor is actually seeing, including, for most cameras, the exposure, like LCD do too. In normal operation, the mechanical camera shutter (3) stays open and is only typically utilized at the end of exposure. Due to lack of both mirror and pentaprism, the flange distance (which is the distance between the lens mount and the image sensor) on mirrorless cameras can be shortened significantly, as the illustration above shows. Because of this, most mirrorless camera bodies are thinner and lighter compared to DSLRs, but they can have full frame sensors and the interchangeable lenses of DSLR’s.

Mirrorless camera advantages

With the rise of cameras without a mirror, most manufacturers have already realized that traditional DSLR systems are not going to be the driving force of camera sales in the future. It makes sense from the cost standpoint alone, but if we really look at the current innovation, where are we at with DSLRs? With each iteration of DSLRs, it seems like we are getting closer and closer to hit the wall of innovation. Autofocus performance and accuracy have already pretty much hit the wall. Processors are fast enough to crank lots of FPS and 4K video. Just to keep the word out and sales going, camera manufacturers have been resorting to just re-branding the same camera under a new model name. What else is there to add? More in-camera editing options? Specific features for different types of photography? Those are all great bells and whistles, but are they innovations that will truly drive future sales?

Mirrorless cameras open up huge opportunities for innovation in the future and solve many of the problems of traditional DSLRs. Let’s go through each point above and discuss additional benefits of mirrorless cameras:

  1. Smaller size/bulk and lighter weight. Removing the mirror and the pentaprism frees up a lot of space. This means that mirrorless cameras can be designed to be smaller, less bulky and lighter compared to DSLRs. With a shorter flange distance, the physical size of both the camera and the lens is reduced. This is especially true for APS-C size sensors (full-frame is tougher to address). No more wasted space, no need for extra ruggedness to give a feel of a bigger camera. In short, mirrorless cameras can be made noticeably lighter than DSLRs. The rise of smartphones as compact cameras has taught us a very important lesson, convenience, small size and light weight can potentially overpower quality. The point-and-shoot sales are practically dead, because most people find their smartphones to be “good enough” for those snapshot moments. All smartphone manufacturers are currently pressing hard on camera features, because they want people to think that they are not just getting a phone, but also a great camera in a single compact package. And judging from the sales figures so far, it is clearly working, more and more people are embracing smartphones and leaving their older compact cameras behind. Simply put, smaller size and lighter weight in electronics win in today’s economy. We can observe the same trend in many other gadgets, thinner and lighter TVs, tablets instead of laptops, etc. Hence, people will naturally gravitate towards lighter and more compact, especially if quality is not compromised significantly.
  2. No mirror mechanism. No more mirror flipping up and down means a lot of good things:
    1. Less noise. No more mirror slap, just the click of the shutter mechanism is all you hear from the camera.
    2. Less camera-shake. The only physical component in the mirrorless camera that can cause vibrations is the camera shutter. And even then, it is possible to use electronic front curtain shutter (EFCS), or even disable the shutter completely and shoot with electronic shutter to completely get rid of camera shake from the shutter mechanism. On a DSLR, you need to go into the menu and choose the ‘mirror lock-up’ option before you take your shot, and then remember to put it back down afterwards.
    3. Easier to clean. If dust ends up on the sensor, cleaning mirrorless cameras is easier than DSLRs. You do not need a fully charged battery to lock up the mirror in most cameras, the sensor is exposed once you dismount the lens (some cameras with in-body image stabilization should have their stabilization mechanism locked via camera menu to prevent movement and damage). In addition, most mirrorless cameras do not have an opening under the mirror to house a phase detection sensor and other components, so there is very little chance for dust to circulate after the chamber and sensor are fully cleaned.
    4. Cheaper to build and support. Less moving parts translate to lower cost of manufacturing and support for the manufacturer.
    5. Potentially very fast FPS Speed. Having no mirror means that the capture rate (FPS) does not have to be limited by the mirror speed. This means that mirrorless cameras could potentially capture images at much faster frame rates than 10-12 FPS we see today, with much less noise and viewfinder blackout.
  3. Live preview (WYSIWYG). With mirrorless, you can get a live preview of what you are about to capture, basically “what you see is what you get” (WYSIWYG). If you messed up white balance, saturation or contrast, you will see it in live preview, whether in the EVF or the LCD.
  4. No phase detection/secondary mirror alignment issues. Now that many of the modern mirrorless cameras are shipping with hybrid autofocus systems that utilize both phase and contrast detection autofocus, you do not have to worry about the alignment of phase detection and secondary mirror. On several new generation mirrorless cameras, the phase detection sensors are located on the actual sensor, which means that phase detection will never have to be calibrated for distance, since it sits on the same plane as the sensor that captures the image.
  5. Producing mirrorless cameras is potentially cheaper than producing DSLRs. As of today, most mirrorless camera manufacturers charge heavy premiums for their camera systems, because their overall costs are high, and the volume is not there yet. While the actual manufacturing costs are lower than DSLRs, companies have to spend plenty of R&D money on improving particular camera features, autofocus performance and other technologies like EVF. Overtime, mirrorless camera prices will come down.
  6. Electronic viewfinder. It is the biggest strength of mirrorless cameras and the present and future innovation with it. Without a doubt, an EVF has huge advantages over OVF. While the current implementation of EVF might not be as robust and responsive as it should be, it is just a matter of time before manufacturers fix that. Let’s go over some of the key benefits of EVF over OVF:
    1. Information overlay. With OVF, you never get to see more than some basic grids. There is some static information presented in the viewfinder, but it is mostly fixed and cannot be easily changed. With EVF, you can get any information you want displayed right inside the viewfinder, from live exposure data to histograms. Different warnings could be added, such as a warning for a potentially blurry shot.
    2. Live The same live preview on the LCD can be shown inside the EVF.
    3. Image review. Another key feature that you will never get in an OVF is image review. How cool is it to see the image that you have just captured right inside the viewfinder? With OVF, you are forced to look at the LCD screen, which is a big pain in bright conditions. People end up buying specialized loupes just to be able to see their LCD screen in daylight! With EVF, you never have to worry about this, since you can use the viewfinder for reviewing images instead.
    4. Focus A very useful feature that allows one to see what areas of the frame are in focus. Basically, you can nail focus when performing manual focus without having to rely on your eyes. The area that is in focus gets painted with an overlay color of your choice and you can stop exactly where you want it to be. You cannot do this with an OVF in a DSLR.
    5. No more viewfinder coverage issues. With OVF, you typically get something like 95% viewfinder coverage, especially on lower-end DSLR models. This basically means that what you see in the viewfinder is about 5% smaller than what the camera will capture. With EVF, you no longer have this problem, because it will always be 100% viewfinder coverage, since what you see in the EVF is what the sensor will capture.
    6. Much brighter If the light conditions are poor, you cannot really see much through an OVF. Focusing with OVF in low light is also difficult, because you cannot really tell if the subject is in focus until you take the picture. With EVF, brightness levels can be “normalized”, so that you can see everything as if it was daylight. Some noise might be present, but it is still way better than trying to guess when looking through an OVF.
    7. Digital If you have used a live view mode on your DSLR before, you know how helpful zooming in on a subject can be. With most modern DSLRs, you can zoom in to 100% and really nail focus. With mirrorless cameras, this feature can be built right into the viewfinder! So, imagine manually focusing with a lens, then zooming in to 100% right inside the viewfinder before you take a picture. Pretty much every mirrorless camera can do this. It goes without saying that an OVF would never be able to zoom like that.
    8. Face/eye tracking. Now we are moving to the coolest part of the EVF technology. Because the EVF shows what actually happens on the sensor, additional technologies for data analysis can be utilized to do very cool things, like face and even eye tracking! You may have seen face tracking on smartphones and point-and-shoot cameras, but if you take it a step further, you could have the camera automatically focus on the nearest eye of the person that you are photographing. Many camera manufacturers are already doing this very efficiently on their mirrorless cameras.
    9. Potentially unlimited focus As you already know, most DSLR cameras have a limited number of focus points that are distributed mostly around the center of the frame. While it works out in most situations, what do you do if you need to move the focus point to an extreme border of the frame? The only option is to focus and recompose, but that might not be always desirable, since you are also shifting the plane of focus. In addition, anything away from the center focus point is typically inaccurate and could result in “focus hunting”, where the camera struggles with AF acquisition and goes back and forth continuously. With mirrorless cameras and phase detection sensors placed directly on the imaging sensor, this limitation is lifted. Contrast-detection has already been possible anywhere on the imaging sensor, but now most new mirrorless cameras have also added the ability to focus via on-sensor phase detection, with focus points distributed over most of the sensor, all the way to the extreme borders.
    10. Subject tracking and other future data analysis. If things like face and eye tracking are possible with mirrorless cameras, you can only imagine what camera manufacturers will be able to do in the future. Imaging having a complex tracking system that intelligently combines sensor data with autofocus and uses it to track a given object, or subject in the frame, something already possible on many mirrorless systems. Even top of the line DSLR cameras today have challenges with full subject tracking. If you have tried photographing birds in flight with a DSLR, tracking can get challenging, especially when the bird moves out of the focus point area, or when the light conditions are less than ideal. If data is analyzed at pixel-level and there is no real autofocus area to concentrate on, subject tracking can potentially get very advanced and sophisticated with mirrorless cameras.
    11. Eye damage. When looking through a viewfinder, one has to be extremely careful about photographing extremely bright sources of light (such as the sun), especially with long focal length lenses. With EVF, the image is projected through the sensor and there is no harm to your eyes.

Mirrorless camera limitations

We’ve gone over the many advantages of mirrorless cameras over DSLRs. Now let’s talk about some of their current limitations:

  1. EVF Some of the current EVF implementations are not particularly responsive, resulting in considerable lag. While this is certainly a nuisance compared to OVF at the moment, it is a matter of time before this lag is eliminated. The latest EVFs are already much better than what they used to be before. But as EVF technologies evolve, the lag issue will be resolved completely.
  2. Continuous autofocus/subject While contrast detection has already reached very impressive levels on mirrorless cameras, they are still relatively weak in continuous autofocus performance and subject tracking compared to phase detection AF. However, with the rise of hybrid autofocus systems and their continuous development (where both contrast and phase detection are used together), we have already seen mirrorless cameras with incredible continuous autofocus capabilities. Soon enough mirrorless cameras will fully catch up and surpass DSLR cameras in AF performance.
  3. Battery life. Another disadvantage of mirrorless cameras at the moment. Providing power to LCD and EVF continuously takes a toll on battery life, which is why most mirrorless cameras are rated at about 300 shots per battery charge. DSLRs are much more power efficient in comparison, typically in 800+ shot range per charge. While it is not a huge problem for typical camera use, it could be an issue for someone who travels and has very little access to power. Still, that the battery issue is also something that may significantly improve in the future (and we have already seen much better battery life on the third iteration of Sony A7-series cameras). Batteries will be more powerful and power-hungry LCD/EVF screens will be replaced with other more efficient technologies.
  4. Red dot patterns. Due to the very short flange distance, most mirrorless cameras suffer from a red dot pattern issue, which becomes clearly visible when shot with the sun in the frame at small apertures. Basically, light rays bounce back and forth between the sensor and the rear lens element, creating grid patterns of red (and sometimes other colors) in images. Unfortunately, there is no way around this limitation on all mirrorless cameras with a short flange distance.
  5. Strong EVF contrast. Many EVFs designed today have very strong, boosted contrast, similar to what we see on our TVs. As a result, you see a lot of blacks and whites, but very little shades of gray. While one could look at the histogram overlay in EVF, it is still a nuisance. Manufacturers will have to find ways to make EVFs display images more naturally.

As you can see, the list is rather short, and we expect it to get even shorter within the next few years. All of the above issues may be addressable, and they might get better with each iteration of mirrorless cameras.

In summary, DSLRs simply have no way to compete with mirrorless in the future. It simply does not make sense for manufacturers to continue investing into making DSLRs better, when the technology advantage is clearly with mirrorless.

DSLR and mirrorless camera comparison

When compared to mirrorless cameras, DSLRs by design have some inherent limitations. Part of it has to do with the fact that SLR cameras were initially developed for film. When digital evolved, it was treated just like film and was housed in the same mechanical body. Aside from the circuitry required for digital sensor and other electronics, new digital film media and the back LCD, the rest of the components pretty much stayed the same. The same mechanical mirror, the same pentaprism/optical viewfinder, the same phase detection system for autofocus operation.

While new technological advances eventually led to extending of features of these cameras (in-camera editing, HDR, GPS, Wi-Fi, etc.), DSLRs continued to stay bulky for a few reasons. First, the mirror inside DSLR cameras had to be the same in size as the digital sensor, taking up plenty of space. Second, the pentaprism also had to match the size of the mirror, making the top part of DSLRs bulky. Lastly, manufacturers wanted to keep existing lenses compatible with digital cameras, so that the transition from film to digital was not too costly or too limiting for the consumer. This meant that manufacturers also had to keep the flange distance (the distance between the camera mount and the film/sensor plane) the same between the two formats. Although smaller APS-C/DX sensors and lenses seemed like a great way to reduce the size of DSLR systems, the flange distance/compatibility concerns left them fairly large and heavy physically. 35mm eventually came back with modern full-frame digital sensors, so the mirror and pentaprism sizes again went back to what they were in film days. On one hand, keeping the flange distance the same allowed for maximum compatibility when mounting lenses between film, APS-C and full-frame DSLRs, without the need to re-design and re-market lenses for each format. On the other hand, DSLRs simply could not go beyond their minimum size requirements and the presence of the mirror is what continues to make them so much more complex to build and support.

Having or not a mirror is the main physical difference of the DSLR and mirrorless cameras, but this changes a lot of things for both systems but in general terms two main things:

  1. Using a mirror allows the DSLR to have an optical view of the action in real time, which means you see the light that is coming into the lens, but you don’t see the actual exposure that will be captured by your camera based on the camera’s settings. On mirrorless camera, they are able to give you a preview of what your exposure is in the electronic viewfinder (EVF), in the eyepiece as well as the screen on the back. Today, most DSLR will let you preview the exposure on the back of the screen, but you still have to move the mirror to do that, rendering the optical eyepiece useless.
  2. Mirrorless cameras will tend to be smaller since there is no need for the whole reflexive mirror system, which means there is space for less buttons, making most of the controls menu based and digital on most mirrorless cameras. Size affects everything. Because of the smaller camera size, batteries tend to be smaller, mirrorless cameras will chew through batteries quicker because their sensor is constantly on, along with either the back screen or the EVF. Currently the cost of mirrorless cameras with the same quality settings as a DSLR will be a bit pricier but those prices are coming down and get everyday more similar.

Buying into a system

When we look at the sales data from the past few years, things look pretty confusing, if mirrorless is the future, why do DSLRs still dominate the sales charts globally? There are several reasons for this. First, it takes a while to influence the potential buyer with the message “newer and bigger are not always better”. The word “mirrorless” is relatively new and educating people about its advantages is taking time. Second, people generally resist switching systems due to existing investments. If one already owns a bunch of lenses and accessories, they avoid going through the hassle of selling everything and re-acquiring gear. It is an expensive process both in terms of gear expenditures (selling used gear, especially cameras and accessories, generally does not yield much money to reinvest in an equivalent system from another manufacturer) and time to learn and adapt to new tools. And lastly, before making the move, photographers often assess the camera system as a whole and put deep thoughts into what pros and cons they will have to go through when buying into a new system. Some of the mirrorless systems haven’t fully matured yet and they might have a relatively limited selection of lenses. The same goes for specific accessories that might exist for DSLRs, but not yet for mirrorless cameras.

However, things are changing fast. If a couple of years back mirrorless systems had a small selection of lenses, today that list has grown tremendously, covering many photography needs. The biggest holes to fill are still in specialized lenses like tilt/shift and super telephotos, but that will be coming soon, especially once mirrorless catches up in the autofocus department.

Mirrorless vs. DSLR autofocus performance

Speaking of which, if a couple of years back one could laugh at how bad autofocus was on mirrorless cameras, things are changing rapidly today, in favor of mirrorless. For portrait photography, many mirrorless systems have already surpassed DSLRs in AF performance and accuracy, thanks to specific features such as eye tracking. Cameras like the Sony A9 have already demonstrated that mirrorless can even compete with DSLRs for shooting fast action. It is a matter of time before we see very complex AF implementations that DSLRs will not be able to compete with. For example, some cameras are already capable of recording images before and after the shutter is released, to avoid taking pictures of subjects with their eyes closed, and we have already seen cameras taking a picture at the moment the subject smiles. You cannot have such advanced intelligence on DSLRs, not until light continuously reaches some kind of imaging sensor. Tracking subjects gets easier with advanced analysis of the scene and the camera can even potentially predict subject movement and its direction.

Future innovation

If we compare DSLRs to mirrorless cameras in terms of technological advancements, it is clear that DSLRs do not deliver as much innovation anymore. We can perhaps get better resolution, better video features, better AF modules and perhaps more built-in features like Wi-Fi and GPS, but that’s not enough to truly excite the younger generation of photographers. Mirrorless cameras will continue to provide many more features to be excited about, because possibilities are truly endless. A lot can be done with EVFs and autofocus systems alone, thanks to advancements in display technologies and on-sensor data analysis.

But, are we there yet? While mirrorless is definitely advancing fast, there are some real issues that still need to be addressed. Better battery life, more reliable autofocus systems (particularly for shooting fast and unpredictable action), larger buffer, better lens choices (especially super telephoto and tilt-shift lenses) and improved EVFs are all areas of improvement for mirrorless cameras. The gaps are still there, but they are closing fast. Within the next few years, we should see camera manufacturers offer mirrorless options that can truly compete with modern DSLRs in every way.

Continue to lesson 42: What is crop factor?

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