Repeatedly the question comes up, why are the long focal length Canon lenses white instead of black? The answers run a gamut, from “it’s all marketing” to “if they did not paint them that color the Calcium Fluorite lens elements would not work in the sun”. The answer is probably someplace in between (and is probably much more complex), but likely both have a shred of truth in them.

Oh, by the way, repeatedly people call the long focal length Canon L lenses “white”, they are not. They are an off-white, more along the lines of beige.

Anyone who has ever parked a car in the sunlight probably knows a dark colored car may be hotter when you get back to it. So it stands to reason a dark lens might be hotter in direct sunlight than a light colored one.

So, I decided to see if the light color did really make a difference.

I have access to an Indigo Systems Merlin Mid Infrared Radiometric Camera. This camera works in the 3-5 micron range and can measure the temperature of any point in the image. The lens used in this test was an Amber Engineering 25mm f2.3 3-5 micron lens.

A two point calibration was done with the camera, one at room temp and one 50 deg F above room temp. This allows for accurate temperature measurement by the camera of any point between those two values.

Two lenses were on hand for this test, the Canon 70-200 f4 L and the Vivitar Series One 600mm f8 Solid Cat.

The lenses were placed in a room with an ambient temperature of 72 deg F for a little over an hour. An image was taken and no point in the image varied by more than +/- 2 deg F.

The lenses were then taken outside and placed side-by-side in direct desert sunlight for 10 minutes.

The lenses were brought back inside and images and measurements were taken at 1 minute, 2 minutes, 4 minutes, and 8 minutes.

In these images white is hot, the whiter the hotter. The darker the cooler.