The Olympus E-1 was the first DSLR designed solely for digital. The sensor format choice was independent of any film legacy and the lens design was specifically focused on the way digital sensors accept light. Leica's S series of 45x30mm medium format DSLRs is the only comparable system we can recall.

It's 21 years since we reviewed the Olympus E-1, arguably the only successful attempt to develop a digital SLR system from first principles, rather than trying to adapt what had come before. As part of our 25th Anniversary celebrations, we look back at the E-1 as one of the most significant cameras of the past 25 years.

In the early days of digital photography the large number of photographers already heavily invested in film lenses put pressure on companies to continue their existing systems, even though 'full-frame' sensors that matched the film format they were designed around were prohibitively expensive for most photographers.

The Four Thirds system was an attempt at a clean slate design, with the intention of developing a new set of lenses designed to suit digital sensors, and built around a sensor chosen because it had a good performance-to-price ratio, not to match the arbitrary dimensions of cinema film stock adapted for photography in the early 1900s. It was a decision that would also allow smaller lenses, particularly at longer focal lengths.

The first open system: the Four Thirds timeline

• Feb 2001: Kodak and Olympus enter patent sharing agreement
• May 2001: Olympus confirms DSLR concept with Type 4/3 sensor
• Sep 2001: Kodak and Olympus announce "Four Thirds" system with Fujifilm participation
• Jun 2003: Olympus reveals E-1 5MP Four Thirds DSLR
• Nov 2003: DPReview reviews the first Four Thirds camera
• Feb 2004: Panasonic, Sanyo and Sigma join Four Thirds

The Four Thirds initiative was started by Kodak and Olympus, with the two companies settling on a 5MP Type 4/3 (17.3 x 13mm) CCD sensor. This sensor size and the use of the 4:3 aspect ratio common in most early digital sensors give the system its name.

They also announced it would be an open standard, with other companies welcome to join, a decision that prompted Fujifilm to announce its interest, followed by Sanyo, Sigma and Panasonic, some seventeen months later. For a while it looked like a critical mass might coalesce, finally bringing about the long dreamt-of common mount, allowing complete interoperability between multiple brands.

In the meantime, although later than the initially suggested Feb 2002 launch date, Olympus developed the E-1, a high-end magnesium alloy DSLR with a 100% viewfinder and that 5MP Four Thirds CCD at its heart. Announced alongside five lenses in mid 2003, the original press release highlights the 'Supersonic Wave Filter' sensor shake system that shook dust off the sensor as solving "a problem that has long been an Achilles heel of ... digital SLRs."

Unfortunately, in his review, Phil highlighted that the Olympus couldn't offer either the speed or the resolution that were typically expected of a camera with the E-1's stated professional ambitions. This was made more stark by its $2199 original price tag (albeit with a 'street price' well below that), at a time when Canon's 6MP EOS 10D would set you back nearer $1500.

It took the more affordable E-300 model for the Four Thirds to really find its audience. By the time the E-330 arrived, two years later, Kodak was out of the picture.

It took 2004's 8MP E-300, with its much more attainable price point (around $1000 with kit lens) for Four Thirds to really find its audience.

By 2006 Panasonic and Leica were alongside Olympus spearheading the system, with each brand releasing variations of technologies co-developed by the two Japanese companies, including "Live MOS" sensors from Panasonic, rather than Kodak. The Olympus E-330 and Panasonic L-1/Leica Digilux 3 were some of the first DSLRs to offer live view, with the E-330 including a more sophisticated/complex implementation. You don't have to squint very hard to see the beginnings of the first mirrorless camera, which Panasonic would introduce just two years later.

Despite a broadly sound initial concept, one challenge of the use of a smaller-than-film sensor in a DSLR was that the viewfinders were often quite small and dark (because the sensor size defines the size of the camera's mirror, which in turn puts a limit on how large or bright you can make an optical viewfinder).

This was resolved with the move to a mirrorless design and the creation of the Micro Four Thirds system, built around the same Type 4/3 sensor format and a lot of technology developed during the Four Thirds era. Perhaps fittingly, the final Four Thirds camera was a direct continuation of the original E-1 line, with the system being officially discontinued 18 months later.

With this in mind, it might seem odd to call a system with a span of just over seven years between the launch of its first camera and its last a success, but I'd focus more on the groundwork it laid. Twenty-one years after the launch of the E-1, the Four Thirds sensor format is still very much with us, and you can trace a direct line from the first all-digital DSLR project to the mirrorless cameras that dominate the ILC market today, even if both Kodak and Olympus have now exited the photography market.