Back in 1938 Sidney Cotton had written a paper on the 'Future Requirement of Photographic Aircraft'. He could see the need for an aircraft, possibly with 2 engines with room for more that 2 cameras, a ceiling of at least 34,000 feet, with a top speed of 450 miles per hour and last but not least, a range of 1500 miles "In principle, the machine should always be faster that the fastest fighters in use" wrote Cotton.
It shows how the Air Ministry attached importance to photo reconnaissance when the first Mosquito off the production line was sent to 1 Photographic Reconnaissance Unit (PRU) at RAF Benson in July 1941. Also the Air Ministry had ordered that the first 20 Mosquitoes were to be photo reconnaissance versions.

The type of cameras installed depended on the type of mission being undertaken. The most widely used fit was a single K17 or K8AB fitted with a 12in lens mounted forward of the aircraft, a split vertical pair of F52 cameras mounted behind the wing and an F24 oblique camera mounted on the port side.
At first, the cameras mounts were made of steel, this was changed to wood as this reduce camera vibration and thus increased picture quality.
Another installation used was a split vertical pair of F52 mounted forward, 2 F52's and 1 F24 behind the wing. The split vertical cameras were installed so each camera was at slightly different angle, this would then give double the photographic coverage. To obtain stereo imagery coverage of a target area, each camera had to produce a run of images with an overlap of 60% between the frames.

The F52 36in focal length camera flying at a height of 35,000 feet would give a lateral coverage of 3 miles.

In the later versions of the Mosquito,  2 forward facing F24 14in cameras were fitted into dummy 50 gallon fuel drop tanks, these aircraft were used for low-level reconnaissance.

There were a number of variants of the photographic reconnaissance Mosquito
PR I, II, IV, VI, IX, XIV, 32 & 34

A total of only 10 PR I's were produced, because this aircraft had proven to so useful, a further 29 Mosquito B.IV bombers were converted to PR I standards and designated PR IV. They had increased fuel loads with tanks being fitted in the bomb-bay, giving them 700 gallons compared to the PR I's  540 gallons. Later some of the original PR Is had this conversion.

The first pressurised cabin version was the PR XVI, there were problems with the Perspex canopy suffered from frosting, this could be cleared with the early versions by opening a side window.

The PR 34 gave the photo reconnaissance Units everything they had every wished for, it was basically a PR XVI with the bomb bay made larger to take more fuel, also 200 gallon drop-tanks could be placed under each wing, in total, 1,256 gallons of fuel could be carried, with a top speed of 425 mph and a ceiling of 43,000 feet. 181 PR 34's were built, it was manly built for service in the Far East and a number were sent for the end of the war in the Pacific in August 1945.