Re: Can taking photos of the sun damage your mirrorless?

Shiranai wrote:

Yes, sunlight is quite intense. Consider a magnifying glass projecting an image of the sun onto a piece of paper. You can easily get enough heat to ignite that piece of paper.

At f/4 you get the same light per unit area on the sensor whether the focal length is 100mm or 1,000mm. However, with a 1,000mm lens that image of the sun covers a wider area of the sensor.

But a magnifying glass makes it even hotter if the light gets focussed on a tiny spot. Isn't a wideangle lens doing essentially the same?

Sort of, but the light is moderated by the aperture. If your magnifying glass is 3 inches (73mm) in diameter and 6 inches (152mm) from the paper, then it's essentially a 152mm f/2 lens. If you masked off the edges of the magnifying glass leaving a 1.5 inch (38mm) diameter opening, it would be a 152mm f/4 lens.

So yes, if we took the same amount of sunlight and focused it into a smaller area, it would get hotter, but that's not generally what a wide angle lens does. The wide angle lens generally has a smaller aperture diameter, so it focuses less of the sun's energy onto that smaller area.

The short answer is that the energy density on the sensor is proportional to the f/stop. At the same f/stop, the longer lens projects the same energy density for the image of the sun, over a wider area.

Longer answer:

There are a finite number of photons from the sun reaching the front of the lens. The aperture blocks some of them. The number of photons making it through is proportional to the area of the aperture. Halve the area of the aperture, and you halve the number of photons getting through.

The focal length determines how those photons are distributed. Longer focal lengths spread them out over a wider area, yielding fewer photons per unit area.

The actual number of photons hitting a spot on the sensor is dependent on both the focal length and the aperture diameter.

It turns out that if the ratio of focal length to aperture diameter is the same, we get the same number of photons per unit area.

If we move from a 50mm lens to a 100mm lens we spread the photons out over four times the area on the sensor.

If we also were to increase the aperture diameter from 25mm to 50mm, we let in four times as many photons, which balances out the reduction from the longer focal length..

As it turns out we have a name for the ratio of focal length to aperture diameter. We call it the "f/stop" or "relative aperture."

A 50mm lens with a 25mm aperture diameter is f/2 (the aperture diameter is the focal length "f", divided by 2).

A 100mm lens with a 50mm aperture diameter is also f/2.

If you are comparing a shot of the sun with a 35mm lens at f/4 to a 200mm lens at f/4, we get the same energy density. The image of the sun covers a much larger area of the sensor, but the aperture diameter is larger, and lets in more light to compensate.