At which focal lengths is it ‘safe’ to have the sun in frame?

Question

I've seen both beautiful wide angle pictures that include the sun as well as camera damage from pointing long lenses at the sun.

At which focal lengths is it safe to have the sun in frame? assuming no filters and that the photographer is quickly framing and taking the shot rather than staring at the sun.

If there are other variables that affect this, an ideal answer indicates how they all affect the equation.

Of special interest is how dangerous is using kit lenses like the 18-55 f4-5.6 or 55-200 f4.5-5.6. The use case also involves letting a kid handle the camera under supervision. So if a one-second exposure of looking through the viewfinder can cause damage with a lens then I will have to be even more diligent.

Answer 1

The variables:

• Height of the sun above the horizon. The closer the sun is to the horizon, the more atmosphere it has passed through before you see it. The atmosphere reflects, absorbs, and dissipates this energy so that less of it reaches a viewing location on the ground.

• The optical density of the atmosphere. Clear dry air allows much more of the sun's energy to reach the surface than air thick with water vapor and other particulates in it. On heavily overcast days the position of the sun can not be seen at all from the surface. Most days at most places are somewhere in between these two extremes.

• Lens focal length/magnifying power/maximum aperture. The size of a lens' entrance pupil (effective aperture) determines how much of the sun's energy is collected by the lens. The larger the entrance pupil, the more energy is collected when all of the other variables are constant.
• The length of time the lens collects the sun's energy. The longer a lens is pointed at the sun the more energy it collects. Heat that is collected faster than it can be dissipated raises the internal temperature of the camera. If the components get too hot, they can be damaged.

The same is true of the human retina, which is generally less tolerant of direct exposure to the sun's energy than most modern cameras. Heat that is pumped into an eye faster than the body can dissipate it can "cook" the rods and cones in the retina, leading to permanently impaired visual function or even total blindness. Since the retina has no pain receptors, you won't even feel it as the infrared energy from the sun heats your retinal tissue. The effect of the damage from that heat can take up to several hours, so by the time you realize you have a problem it is far too late to do anything about it.

Due to the large number of variables and the wide range of variability for each of them there is no single focal length that one can say definitively is the line of demarcation between 'safe' and 'unsafe.' On a totally overcast day it might be possible to point an 800mm f/5.6 lens directly at the sun for an extended time with no ill effects. On a bright, cloudless day in a high desert, it might not be safe to point a very wide angle lens at the sun for much more than a second or two. Maybe even less.

I've never seen an official statement by a camera manufacturer that says it is safe to point a camera to the sun at all without proper filtration that absorbs not only about 15 stops of visible light, but has the same attenuation for the sun's infrared and UV output. On the other hand, they all pretty much universally warn against looking directly at the sun in the viewfinder at any time with any lens.

Answer 2

Wicked kids scorch insects and light fires with a burning glass. This is just a simple magnifier capable of focusing and concentrating sunlight forming a hot tiny image of the sun. You should know, the camera lens is simply a more sophisticated, more corrected variety of the common magnifying glass. Actually if you dismount a camera lens and you have the predisposition, you can go about using it to burn and start fires.

Now your question revolves around the fact that telephoto lenses magnify thus they must perform this task giving superior results. Actually two optical factors intertwine making a burning glass. These intertwine in the camera and effect image brightness.

1. The larger the working diameter (aperture opening) the greater the potential to project a bright, hot image of the sun. This is because a lens acts much like a funnel, the greater its working diameter, the more light energy it can gather and bring to a focus.
2. The longer the focal length, the more the magnification. The greater the magnification the larger the projected image will be. Magnification spreads the image over a larger area. The result is a dimmer less energetic image.
3. The fact that focal length and aperture are intertwined forces us to use a ratio to express the relative brightness of a lens. This is especially useful when we compare one lens to another. To express the light transmission ability of a lens, we divide the focal length by the diameter of the aperture. The result is called a focal ratio (f-number for short).

The f-number is the great leveler of the lens. If we set a lens to f/8, it will deliver the same brightness (heat in this case) as any other lens functioning at f/8, regardless --- regardless. In other words, set any lens to the same f-number, regardless of the other stuff, you get the same image brightness (same heat delivered), they are all equitable in this regard.