RGB color model spaces for more details. The choice of primary colors is related to the physiology of the human eye; good primaries are stimuli that maximize the difference between the responses of the cone cells of the human retina to light of different wavelengths, and that thereby make a large color triangle. The normal three kinds of light-sensitive photoreceptor cells in the human eye (cone cells) respond most to yellow (long wavelength or L), green (medium or M), and violet (short or S) light (peak wavelengths near 570 nm, 540 nm and 440 nm, respectively). The difference in the signals received from
Kodak Retina Ausf. II Retina IIc, which is later mounted on the Type 019 Ausf. II Retina IB and Type 028 Retina IIIC. The second generation Type 034 Retina Reflex S of 1959 shared "S" type lenses with the Type 027 Retina IIIS (below), and offered fully coupled metering. With these the entire lens detached allowing for a wider range of focal lengths, 28mm to 200mm. At this point economical Japanese SLRs were becoming available and the Nikon F of 1959 would set the standard for future camera system. The Reflex III of 1961 and Reflex IV of 1964 brought minor improvements