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Mirrorless DSLRs vs Traditional DSLRs (Part - 1)

Panasonic Lumix DMC-G1 The first compact DSLR in the world
Panasonic Lumix DMC-G1 The first compact DSLR in the world


Olympus and Panasonic first introduced compact DSLR cameras in August 2008 based on the Four Thirds sensor standard. Called Micro Four Thirds, these cameras retained the 18 x 13.5 mm sensor of their traditional DSLR cameras, but dropped the mirror box assembly. 

The Pansasonic offering was called Lumix DMC-G1, while Olympus marketed its product as E-P1.

Panasonic's latest offering is Lumix DMC-G3, while the latest offering from Olympus is E-P2.

Samsung jumped on the compact DSLR bandwagon in 2009 with its NX10 series. In September 2010, Samsung pushed the envelope with the NX100.

Sony unveiled its compact DSLR concept at the PMA 2010 and announced the NEX-5 and NEX-3 in May.

Nikon and Canon have announced plans for compact DSLRs. While Cannon has indicated it will achieve a compact form factor while retaining the mirrorbox assembly, Nikon has been ambivalent, saying its compact DSLR could be mirrorless.

Pentax plans to release a mirrorless camera that it says is distinct from models already on the market.

How do DSLRs become compact?

Compact DSLRs do away with the  mirror box assembly and pentaprism required with the optical view finder (OVF) of traditional DSLRs. (See Fig 1 below.)

Eliminating the mirror box allows the main camera lens to be moved closer to the image sensor, and the the diameter of the main lens to be reduced (See Fig 1 and 2 below), reducing the overall size of the camera dramatically. 

One obvious and important way compact DSLRs differ from traditional DSLRs is - they don't have an OVF. Instead, you compose your photos using an LCD, as in a Point and Shoot (P&S) compact camera, or an Electronic View Finder (EVF). 

In an EVF, the camera view through the main lens is electronically projected from the main sensor onto a miniature display visible through an eyepiece, like that on an optical viewfinder.


A comparision between the capabilities of OVF and EVF is essential to understanding how compact DSLRs differ, or maybe don't differ, from traditional DSLRs.


Optical or Electronic, a view finder is used for 

  1. Composing the photo.
  2. Focusing on the object of interest.
  3. Tracking the object, in case it is moving.
  4. Setting Exposure, Depth of field and focus, Shutter Speed and other camera modes.
Fig 1 Traditional DSLR block diagram with phase shift autofocus
Fig 1 Traditional DSLR block diagram with phase shift autofocus

Optical View Finders (OVF)

An OVF uses a mirror box and pentaprism to reflect the light coming through the lens to small sized optical assembly for you to look through.
When using an OVF you look into it through a single eye, with the camera held close to the body and one of your hands supporting the main lens. 

A supporting hand under the relatively heavy main lens steadies the camera and makes tracking a moving object easier, while leaving your other hand free to operate camera controls and shutter release.

However, the need to hold the camera close to your body can limit its usage; for example, when you want to take a photo over a crowd. It's for that reason that high end SLRs now support both an OVF and EVF.

Autofocus with  OVF

OVF equipped DSLRs typically use a phase detection autofocus system using a special focusing sensor. The  main mirror is slightly translucent in the center allowing some light to pass through it to a secondary mirror, which reflects it to a special focusing sensor. (See Fig 1 above.)

Two separator lenses are placed in front of the focusing sensor to form two images from the incoming beam on the sensor. The distance between the two focusing images is optimum when the main lens is in focus. (Fig. A)

If the main lens is closer to the image sensor than it should be for correct focus the distance between the two images on the focusing sensor is more than the optimum distance, and vice-versa.

The deviation of the image distance from the optimum not only allows the camera electronics to instantaneously determine how much the main lens needs to move in order to achieve focus, but also in which direction it needs to move.

Phase Detection: Near instantaneous and predictive

Phase detection autofocus is near instantaneous. A single analysis of the two images on the autofocus sensor is enough to determine where the main lens needs to be positioned by the lense drive.

It can also be made predictive when photographing a object that is moving away or towards you. 

Fig 2 Compact Mirror-less DSLR with contrast detection autofocusF
Fig 2 Compact Mirror-less DSLR with contrast detection autofocus

Electronic View Finders (EVF)

Instead of mirror box and additional optics, EVFs use the main image sensor to display a 'live view' on either the main LCD or a small LCD monitor visible through the eyepiece

Composing using the live view on the main LCD can be challenging in bright sunlight. However, new live view display, like the AMOLED (Active-Matrix Organic Light Emitting Diode) that comes with the Samsung NX-10 can be viewed in bright sunlight.

EVFs also pose a challenge when tracking a moving object. 

Autofocus with EVF

Lacking the mirror box, EVF equipped DSLRs use a contrast detection autofocus system.

Sony NEX-3
Sony NEX-3

Contrast Detection Auto Focus

Contrast detection works on the logic that image contrast will be maximum when the image is in focus. 

The technique allows the main sensor of the camera to also be used for focusing.

The camera is equipped with electronics to analyse image contrast.

The lens drive moves the lens in one direction. The image is immediately analysed. If contrast has reduced, instead of increasing, the lens drive reverses direction of movement.

Once the correct direction of movement is determined, camera electronics continuously analyze  the image and move the lens to achieve maximum contrast.

More Accurate?

According to Panasonic, contrast detection autofocus is more accurate than Phase detection even when shooting at small F values.

Phase detection AF system is susceptible to mechanical margin of error, whereas contrast detection AF is  controlled by the image sensor so that no mechanical margin of error occurs to achieve precise focusing.

However, it is unlikely this is true for low light conditions when contrast detection is difficult.

Slower, non predictive

Contrast detection auto-focusing requires intensive processing and is iterative. It can be very slow as is evident when using a compact camera. 

When Panasonic introduced its Micro Four Thirds, they improved contrast detection auto-focusing to an extent that made its use in high end DSLRs feasible.

Olympus is so sure of its improvements to contrast detection that it is rumored to  be ready to do abandon the traditional DSLR format completely.

Pansonic's DMC GH-2 is claimed to have a 0.1 sec contrast detection AF time, which is as faster than phase detection AF.  Fast AF is achieved by reducing the detection time for focusing by doubling the drive speed from 60 frames per second to 120 frames per second. 

Detailed Illustration of Phase and Contrast Detection autofocus. 

Detailed illustrations and explanation of phase and contrast detection autofocus maybe viewed at the Nikon site.