Collapsible Lens

The Summitar 50 f/2 is the predecessor of the Summicron 50mm f/2. Compared with its predecessor, this lens uses new glass and a new design, which brings its optical level to a higher level. At the same time, it also inherits the compact structure and light weight of its predecessor lens. However, what makes this lens unique is that its glass is radioactive.

Radioactive Yellow Glass

The main source of radioactivity in lenses is the use of thorium oxide as the glass component in the lens elements. Thorium oxide glass provides a high refractive index and low dispersion, allowing lens designers to minimize chromatic aberration and reduce curvature. However, the presence of thorium in the glass can cause the lens elements to yellow over time.

Is Radiation?

If used appropriately, Leica Summicron 5cm lenses containing thorium pose negligible risks to human health. I have 5 of these lenses and measured their radiation levels one by one. The radiation in the early stage is greater than that of the star, and the amount of radiation in the later stage is smaller. Still, prolonged exposure to this radiation can damage the eyes and can lead to cataracts and other vision problems. Its radiation mainly comes from within ten centimeters directly in front of the lens. The radiation value behind the lens is negligible.

Radioactive yellow glass serial number:
NR 920×××
NR 921×××
NR 922×××
NR 993×××
NR 994×××
NR 995×××
No. 1009×××
Nr102××××
No. 104××××

Clarity

When stopped down to f/2, the lens delivers excellent subject separation, background blur, and low-light performance without sacrificing too much sharpness. However, if you stop the aperture down to f/4, it’s sharp and captures all the detail you could ask for. It surpasses its predecessor Summitar by a lot.