The Four Thirds System is a standard created by Olympus and Eastman Kodak for digital single-lens reflex camera (DSLR) and mirrorless camera design and development.^[1]

The system provides a standard that, with digital cameras and lenses available from multiple manufacturers, allows for the interchange of lenses and bodies from different manufacturers. U.S. patent 6,910,814 seems to cover the standard. Proponents describe it as an open standard, but companies may use it only under a non-disclosure agreement.^[2]

Unlike older single-lens reflex (SLR) systems, Four Thirds was designed from the start to be entirely digital. Many lenses are extensively computerised, to the point that Olympus offers firmware updates for many of them. Lens design has been tailored to the requirements of digital sensors, most notably through telecentric designs. The size of the sensor is significantly smaller than for most DSLRs and this implies that lenses, especially telephoto lenses, can be smaller. For example, a Four Thirds lens with a 300 mm focal length would cover about the same angle of view as a 600 mm focal length lens for the 35 mm film standard, and is correspondingly more compact. Thus, the Four Thirds System has crop factor (focal length multiplier) of about 2, and while this enables longer focal length for greater magnification, it does not necessarily aid the manufacture of wide angle lenses.

The image sensor format, between those of larger SLRs and smaller point-and-shoot compact digital cameras, yields intermediate levels of cost, performance, and convenience.

Sensor size and aspect ratio

The name of the system stems from the size of the image sensor used in the cameras, which is commonly referred to as a 4/3" type or 4/3 type sensor. The common inch-based sizing system is derived from vacuum image-sensing video camera tubes, which are now obsolete. The imaging area of a Four Thirds sensor is equal to that of a video camera tube of 4/3 inch diameter.^[3]

The usual size of the sensor is 18 mm × 13.5 mm (22.5 mm diagonal), with an imaging area of 17.3 mm × 13.0 mm (21.63 mm diagonal).^[3][4] The sensor's area is about 30–40% smaller than APS-C sensors used in most other DSLRs, but still around 9 times larger than the 1/2.5" sensors typically used in compact digital cameras. Incidentally, the imaging area of a Four Thirds sensor is almost identical to that of 110 film.

The emphasis on the 4:3 image aspect ratio sets Four Thirds apart from other DSLR systems, which usually adhere to the 3:2 aspect ratio of the traditional 35mm format. However, the standard only specifies the sensor diagonal, thus Four Thirds cameras using the standard 3:2 aspect ratio would be possible;^[5] notably newer Panasonic Micro Four Thirds models even offer shooting at multiple aspect ratios while maintaining the same image diagonal. For instance, the Panasonic GH1 uses a multi-aspect sensor designed to maximize use of the image circle at 4:3, 3:2, and 16:9; each ratio having a diagonal of 22.5 mm.^[6]

Sensor aspect ratio influences lens design. For example, many lenses designed by Olympus for the Four Thirds System contain internal rectangular baffles or permanently mounted "petal" lens hoods that optimise their operation for the 4:3 aspect ratio.^{[citation needed]}

In an interview John Knaur, a Senior Product Manager at Olympus, stated that "The FourThirds refers to both the size of the imager and the aspect ratio of the sensor".^[7] He also pointed out the similarities between 4:3 and the standard printing size of 8×10 as well as medium format 6×4.5 and 6×7 cameras, thus helping explain Olympus' rationale on choosing 4:3 rather than 3:2.

Advantages, disadvantages and other considerations

Advantages

• 

  The smaller sensor size makes possible smaller and lighter camera bodies and lenses. In particular, the Four-Thirds system allows the development of compact, large aperture lenses. Corresponding lenses become larger, heavier and more expensive when designed for larger sensor formats.
• Telecentric optical path means that light hitting the sensor is traveling closer to perpendicular to the sensor, resulting in brighter corners, and improved off-center resolution, particularly on wide angle lenses.
• Because the flange focal distance is significantly shorter than those on Canon FD, Canon EF, Nikon F and Pentax K, lenses for many other SLR types including the old Olympus OM System must be fitted to Four Thirds cameras with simple mechanical adapter rings. Such mechanical adapter rings typically require manual setting of focus and aperture.^[9]

Disadvantages

• Compared to a larger sensor with equivalent pixel count, a Four Thirds sensor gathers disproportionately less light per pixel. Not only are the individual photosites smaller, but each loses more of its total area to support circuitry and edge shading than a larger photosite would. With less captured light to work with, each photosite requires additional amplification, with associated higher noise as well as reduced dynamic range. A telecentric lens design can mitigate this problem, but the sensor remains more sensitive to the angle of incoming light, and has more pronounced image corner light falloff.
• The resolution of a sensor is often measured as the total sensor pixel count in megapixels, and this is often a primary decision-making factor in choosing a camera. Smaller sensors are tougher to manufacture with the same pixel count as larger sensors, and place a greater demand on optics, since a lens must achieve greater absolute resolving power to produce an adequate picture on a smaller sensor, compared to a larger sensor of the same pixel resolution. A smaller pixel active area reduces the averaging effect and allows a better sampling of high spatial frequencies, mitigating this problem.^[10]
• To get the same angle of view as with a larger sensor, the focal length of the lens used with a Four Thirds sensor needs to be shorter. However, to get the same depth of field and light gathering capability as with a larger sensor, the lens aperture needs to be kept constant. In other words, the focal ratio of the lens must be smaller on the Four Thirds system to give the same depth of field^[11] and image noise. Since it is more difficult to produce faster lenses (lenses with smaller focal ratios), it can be difficult or impossible to find a lens that produces as shallow a depth of field, and gathers as much light, as an equivalent lens on larger formats. For instance, a 35mm "full-frame" DSLR can match the depth of field of a Four Thirds camera by closing down the aperture by two stops; but it may be more difficult or impossible for a Four Thirds System to match the shallow depth of field of a 35mm camera using a fast lens.

Differences

• Most Four Thirds cameras (notably those manufactured by Olympus) use an aspect ratio of 4:3 rather than 3:2; newer models offer cropping to 3:2, but this results in a reduced image diagonal (i.e., the effective crop factor is then 2.08).^[12]

Four Thirds System companies

As of the 2006 Photo Marketing Association Annual Convention and Trade Show, the Four Thirds consortium consisted of the following companies:

• Fuji
• Kodak
• Leica
• Olympus
• Panasonic
• Sanyo
• Sigma

This does not imply a commitment to end user products by each company. Historically, only Leica, Olympus, and Panasonic have produced bodies. Olympus and Leica/Panasonic have made dedicated Four Thirds lenses, and Sigma makes adapted versions of their "DC" lenses for APS-C format DSLRs. Kodak once sold sensors to Olympus for use in their Four Thirds bodies, but the newer Olympus Four Thirds cameras used Panasonic sensors.

Four Thirds System cameras

The majority of Four Thirds System cameras and Four Thirds lenses are made by Olympus. Many Four Thirds cameras use "sensor-shift" in-body image stabilization, making the need for image stabilization technology in its lenses unnecessary. All Four Thirds cameras also incorporate an automatic sensor cleaning device, in which a thin glass filter in front of the sensor vibrates at 30 kHz, causing dust to fall off and adhere to a piece of sticky material below. Olympus' E-system camera bodies are noted for their inclusion of a wide range of firmware-level features and customization, good JPEG engine, and compact size. Because of the smaller format of Four Thirds, the viewfinders tend to be smaller than on comparable cameras.^[13][14]

Manufacture of Four Thirds cameras came to an end after the introduction of the mirrorless Micro Four Thirds format. Discontinued models include:

• Leica Digilux 3
• Olympus E-1
• Olympus E-3
• Olympus E-5
• Olympus E-30
• Olympus E-300
• Olympus E-330
• Olympus E-400
• Olympus E-410
• Olympus E-420
• Olympus E-450
• Olympus E-500
• Olympus E-510
• Olympus E-520
• Olympus E-600
• Olympus E-620
• Panasonic Lumix DMC-L1
• Panasonic Lumix DMC-L10

Four Thirds System lenses

The Four Thirds lens mount is specified to be a bayonet type with a flange focal distance of 38.67 mm.

There were 41 lenses made for the Four Thirds System standard, including two that were modified and re-released in approximately 2009 with improved mechanisms but otherwise identical optics.^[a][15]

Before announcing that it would stop production of Four Thirds lenses in early 2017,^[16] Olympus produced 24 lenses for the Four Thirds System under their "Zuiko Digital" brand. They are divided into three grades — Standard, High Grade and Super High Grade. High Grade lenses have faster maximum apertures, but are significantly more expensive and larger, and the Super High Grade zooms have constant maximum aperture over the full zoom range; all but the Standard grade are weather-sealed. Lenses within each grade cover the range from wide-angle to super telephoto.^[17][18] The Zuiko Digital lenses are well regarded for their consistently good optics.^[19] The following table lists all Zuiko Digital lenses available at the time Olympus stopped Four Thirds production:^[20]

Olympus also made 1.4× and 2× teleconverters and an electronically coupled extension tube.

Sigma has adapted 13 lenses for the Four Thirds System, ranging from 10 mm to 800 mm, including several for which no equivalent exists: the fast primes (30 mm f/1.4 and 50 mm f/1.4) and extreme telephoto (300–800 mm f/5.6). As of 2014 all Sigma lenses for the Four Thirds System have been discontinued.

Leica has designed four lenses for the Four Thirds System: fast and slow normal zooms and a 14–150 mm super-zoom, all with Panasonic's image stabilization system, and an unstabilized f/1.4 25 mm prime. These are manufactured and sold by Panasonic.

An official list of available lenses can be found on Four-Thirds.org web site.^[21]

As for the system itself, it was silently discontinued in favor of the Micro Four Thirds System.

List of Four Thirds System lenses^[22][23]

Name	Mfr.	F.L. (mm)	Ap., Blades[b]	Splash / Dust[c]	OIS	Const.	Angle	Min. focus	Filter (mm)	Dims. (Φ×L)	Wgt.	Notes / Refs.
Fisheye lenses
ZUIKO DIGITAL ED 8mm F3.5 Fisheye	Olympus	8	f/3.5–22, 7(C)	Yes	No	10e/6g	180°	0.135 m (5.31 in)	—	79 mm × 77 mm (3.11 in × 3.03 in)	485 g (17.1 oz)	[24][25]
Ultra wide angle lenses
ZUIKO DIGITAL ED 7-14mm F4.0	Olympus	7–14	f/4–22, 7(C)	Yes	No	18e/12g	114–75°	0.25 m (9.84 in)	—	86.5 mm × 119.5 mm (3.41 in × 4.70 in)	780 g (27.5 oz)	[26][27]
ZUIKO DIGITAL ED 9-18mm F4.0-5.6	Olympus	9–18	f/4~5.6–22, 7(C)	No	No	13e/9g	100–62°	0.25 m (9.84 in)	72	79.5 mm × 73 mm (3.13 in × 2.87 in)	275 g (9.7 oz)	[26][28]
Wide angle lenses
10-20mm F4-5.6 EX DC HSM	Sigma	10–20	f/4~5.6–22, 6	No	No	14e/10g	94.5–56.8°	0.24 m (9.45 in)	77	83.5 mm × 86.4 mm (3.29 in × 3.40 in)	495 g (17.5 oz)	[26][29]
ZUIKO DIGITAL 11-22mm F2.8-3.5	Olympus	11–22	f/2.8~3.5–22, 7	Yes	No	12e/10g	89–53°	0.28 m (11.02 in)	72	75 mm × 92.5 mm (2.95 in × 3.64 in)	485 g (17.1 oz)	[26][30]
Normal lenses
ZUIKO DIGITAL ED 12-60mm F2.8-4.0 SWD	Olympus	12–60	f/2.8~4–22, 7(C)	Yes	No	14e/10g	84–20°	0.25 m (9.84 in)	72	79.5 mm × 98.5 mm (3.13 in × 3.88 in)	575 g (20.3 oz)	[31][32]
ZUIKO DIGITAL ED 14-35mm F2.0 SWD	Olympus	14–35	f/2.0–22, 9(C)	Yes	No	18e/17g	75–34°	0.35 m (13.78 in)	77	86 mm × 123 mm (3.39 in × 4.84 in)	900 g (31.7 oz)	[31][33]
ZUIKO DIGITAL ED 14-42mm F3.5-5.6	Olympus	14–42	f/3.5~5.6–22, 7(C)	No	No	10e/8g	75–29°	0.25 m (9.84 in)	58	65.5 mm × 61 mm (2.58 in × 2.40 in)	190 g (6.7 oz)	[31][34]
ZUIKO DIGITAL ED 14-45mm F3.5-5.6	Olympus	14–45	f/3.5~5.6–22, 7	No	No	12e/10g	75–27°	0.38 m (14.96 in)	58	71 mm × 86.5 mm (2.80 in × 3.41 in)	285 g (10.1 oz)	[35][36]
LEICA D VARIO-ELMARIT 14-50mm F2.8-3.5 ASPH. MEGA O.I.S.	Panasonic	14–50	f/2.8~3.5–22, 7	No	Yes	16e/12g	75–24°	0.29 m (11.42 in)	72	78.1 mm × 97.4 mm (3.07 in × 3.83 in)	490 g (17.3 oz)	[31][37]
LEICA D VARIO-ELMAR 14-50mm F3.8-5.6 ASPH. MEGA O.I.S.	Panasonic		f/3.8~5.6–22, 7	No	Yes	15e/11g			67	78 mm × 84.5 mm (3.07 in × 3.33 in)	435 g (15.3 oz)	[31][38]
ZUIKO DIGITAL 14-54mm F2.8-3.5	Olympus	14–54	f/2.8~3.5–22, 7	Yes	No	15e/11g	75–23°	0.22 m (8.66 in)	67	73.5 mm × 88.5 mm (2.89 in × 3.48 in)	435 g (15.3 oz)	[35][39]
ZUIKO DIGITAL 14-54mm F2.8-3.5 II			f/2.8~3.5–22, 7(C)							74.5 mm × 88.5 mm (2.93 in × 3.48 in)	440 g (15.5 oz)	[31][40]
ZUIKO DIGITAL ED 17.5-45mm F3.5-5.6	Olympus	17.5–45	f/3.5~5.6–22, 7(C)	No	No	7e/7g	63–27°	0.28 m (11.02 in)	52	71 mm × 70 mm (2.80 in × 2.76 in)	210 g (7.4 oz)	Kit lens with E-500[41]
18-50mm F2.8 EX DC MACRO	Sigma	18–50	f/2.8–22, 7	No	No	15e/13g	62–24°	0.2 m (7.87 in)	72	79 mm × 91.1 mm (3.11 in × 3.59 in)	525 g (18.5 oz)	[31][42]
18-50mm F3.5-5.6 DC	Sigma		f/3.5~5.6–22, 7	No	No	8e/8g		0.25 m (9.84 in)	58	67.5 mm × 67.8 mm (2.66 in × 2.67 in)	270 g (9.5 oz)	[31][43]
24mm F1.8 EX DG ASPHERICAL MACRO	Sigma	24	f/1.8–22, 9	No	No	10e/9g	49°	0.18 m (7.09 in)	77	83.6 mm × 87.9 mm (3.29 in × 3.46 in)	520 g (18.3 oz)	[24][44]
LEICA D SUMMILUX 25mm F1.4 ASPH.	Panasonic	25	f/1.4–16, 7(C)	No	No	10e/9g	47°	0.38 m (14.96 in)	72	77.7 mm × 75 mm (3.06 in × 2.95 in)	510 g (18.0 oz)	[24][45]
ZUIKO DIGITAL 25mm F2.8	Olympus	25	f/2.8–22, 7(C)	No	No	5e/4g	47°	0.2 m (7.87 in)	43	64 mm × 23.5 mm (2.52 in × 0.93 in)	96 g (3.4 oz)	[24][46]
30mm F1.4 EX DC HSM	Sigma	30	f/1.4–16, 8	No	No	7e/7g	40°	0.4 m (15.75 in)	62	77.8 mm × 63.9 mm (3.06 in × 2.52 in)	410 g (14.5 oz)	[24][47]
Superzoom lenses
LEICA D VARIO-ELMAR 14-150mm F3.5-5.6 ASPH. MEGA O.I.S.	Panasonic	14–150	f/3.5~5.6–22, 7	No	Yes	15e/11g	75–8.2°	0.5 m (19.69 in)	72	78.5 mm × 90.4 mm (3.09 in × 3.56 in)	535 g (18.9 oz)	[31][48]
ZUIKO DIGITAL ED 18-180mm F3.5-6.3	Olympus	18–180	f/3.5~6.3–22, 7	No	No	15e/13g	62–6.9°	0.45 m (17.72 in)	62	78 mm × 84.5 mm (3.07 in × 3.33 in)	435 g (15.3 oz)	[31][49]
Telephoto lenses
ZUIKO DIGITAL ED 35-100mm F2.0	Olympus	35–100	f/2.0–22, 9(C)	Yes	No	21e/18g	62–6.9°	1.4 m (55.12 in)	77	96.5 mm × 213.5 mm (3.80 in × 8.41 in)	1,650 g (58.2 oz)	[50][51]
ZUIKO DIGITAL 40-150mm F3.5-4.5	Olympus	40–150	f/3.5~4.5–22, 7(C)	No	No	13e/10g	62–6.9°	1.5 m (59.06 in)	58	77 mm × 107 mm (3.03 in × 4.21 in)	425 g (15.0 oz)	[52][53]
ZUIKO DIGITAL ED 40-150mm F4-5.6			f/4.0~5.6–22, 7(C)			12e/9g		1.4 m (55.12 in)		65.5 mm × 72 mm (2.58 in × 2.83 in)	220 g (7.8 oz)	[50][54]
50mm F1.4 EX DG HSM	Sigma	50	f/1.4–16, 9	No	No	8e/6g	62–6.9°	0.45 m (17.72 in)	77	84.5 mm × 73.7 mm (3.33 in × 2.90 in)	530 g (18.7 oz)	[24][55]
ZUIKO DIGITAL ED 50-200mm F2.8-3.5	Olympus	50–200	f/2.8~3.5–22, 9	Yes	No	16e/15g	24–6.2°	1.2 m (47.24 in)	67	83 mm × 157 mm (3.27 in × 6.18 in)	920 g (32.5 oz)	[52][56]
ZUIKO DIGITAL ED 50-200mm F2.8-3.5 SWD			f/2.8~3.5–22, 9(C)							86.5 mm × 157 mm (3.41 in × 6.18 in)	995 g (35.1 oz)	[50][57]
APO 50-500mm F4.0-6.3 EX DG HSM	Sigma	50–500	f/4.0~6.3–22, 9	No	No	20e/16g	24–2.5°	1.0–3.0 m (39.37–118.11 in)	86	95 mm × 223.9 mm (3.74 in × 8.81 in)	1,830 g (64.6 oz)	[52][58]
55-200mm F4.0-5.6 DC	Sigma	55–200	f/4.0~5.6–22, 8	No	No	12e/9g	23–6.2°	1.1 m (43.31 in)	55	71.5 mm × 92.5 mm (2.81 in × 3.64 in)	330 g (11.6 oz)	[52][59]
APO 70-200mm F2.8 II EX DG MACRO HSM	Sigma	70–200	f/2.8–22, 9	No	No	18e/15g	18–6.2°	1.0 m (39.37 in)	77	86.5 mm × 189.8 mm (3.41 in × 7.47 in)	1,385 g (48.9 oz)	[52][60]
ZUIKO DIGITAL ED 70-300mm F4.0-5.6	Olympus	70–300	f/4.0~5.6–22, 9(C)	No	No	14e/10g	18–4.1°	0.96–1.2 m (37.80–47.24 in)	58	80 mm × 127.5 mm (3.15 in × 5.02 in)	615 g (21.7 oz)	[50][61]
ZUIKO DIGITAL ED 90-250mm F2.8	Olympus	90–250	f/2.8–22, 9(C)	Yes	No	17e/12g	14–5.0°	2.5 m (98.43 in)	105	124 mm × 276 mm (4.88 in × 10.87 in)	3,270 g (115.3 oz)	[50][62]
APO 135-400mm F4.5-5.6 DG	Sigma	135–400	f/4.5~5.6–22, 9	No	No	13e/11g	9.2–3.1°	2.0–2.2 m (78.74–86.61 in)	77	83.5 mm × 189 mm (3.29 in × 7.44 in)	1,280 g (45.2 oz)	[52][63]
ZUIKO DIGITAL ED 150mm F2.0	Olympus	150	f/2.0–22, 9(C)	Yes	No	11e/9g	8.2°	1.4 m (55.12 in)	82	100 mm × 150 mm (3.94 in × 5.91 in)	1,465 g (51.7 oz)	[24][64]
ZUIKO DIGITAL ED 300mm F2.8	Olympus	300	f/2.8–22, 9	Yes	No	13e/11g	4.1°	2.4 m (94.49 in)	43(D)	127 mm × 285 mm (5.00 in × 11.22 in)	3,290 g (116.1 oz)	[24][65]
APO 300-800mm F5.6 EX DG HSM	Sigma	300–800	f/5.6–32, 9	No	No	18e/16g	4.1–1.6°	6.0 m (236.22 in)	46(R)	156.5 mm × 549.4 mm (6.16 in × 21.63 in)	5,915 g (208.6 oz)	[52][66]
Macro lenses
ZUIKO DIGITAL 35mm F3.5 Macro	Olympus	35	f/3.5–22, 7(C)	No	No	6e/6g	34°	0.146 m (5.75 in)	52	71 mm × 53 mm (2.80 in × 2.09 in)	165 g (5.8 oz)	[67][68]
ZUIKO DIGITAL ED 50mm F2.0 Macro	Olympus	50	f/2.0–22, 7	Yes	No	11e/10g	24°	0.24 m (9.45 in)	52	71 mm × 61.5 mm (2.80 in × 2.42 in)	300 g (10.6 oz)	[67][69]
MACRO 105mm F2.8 EX DG	Sigma	105	f/2.8–22, 8	No	No	11e/10g	12°	0.31 m (12.20 in)	58	74 mm × 102.9 mm (2.91 in × 4.05 in)	470 g (16.6 oz)	[67][70]
APO MACRO 150mm F2.8 EX DG HSM	Sigma	150	f/2.8–22, 9	No	No	16e/12g	8.2°	0.38 m (14.96 in)	72	79.6 mm × 142.4 mm (3.13 in × 5.61 in)	920 g (32.5 oz)	[67][71]
Teleconverters
ZUIKO DIGITAL 1.4× Teleconverter EC-14	Olympus	1.4×	1.4×	No	No	6e/5g	approx. ÷1.4	×1	—	68 mm × 22 mm (2.68 in × 0.87 in)	170 g (6.0 oz)	[72][73]
ZUIKO DIGITAL 2.0× Teleconverter EC-20	Olympus	2.0×	2.0×	No	No	7e/5g	approx. ÷2	×1	—	68 mm × 41 mm (2.68 in × 1.61 in)	225 g (7.9 oz)	[72][74]

Micro Four Thirds System

In August 2008, Olympus and Panasonic introduced a new format, Micro Four Thirds.

The new system uses the same sensor, but removes the mirror box from the camera design. A live preview is shown on either the camera's main liquid-crystal display or via an electronic viewfinder, as in digital compact cameras. Autofocus may be accomplished via a contrast detection process using the main imager, again similar to digital compact cameras. Some Olympus and Panasonic manufactured camera bodies also feature phase detection auto focus built into the sensor. The goal of the new system was to allow for even smaller cameras, competing directly with higher-end point-and-shoot compact digital cameras and DSLRs. The smaller flange focal distance allows for more compact normal and wide angle lenses. It also facilitates the use, with an adapter, of lenses based on other mounting systems, including many manual focus lenses from the seventies and eighties.

In particular, Four Thirds lenses can be used on Micro Four Thirds bodies with an adapter; however, "all of the functions of the Micro Four Thirds System may not always be available."^[75]