A few minutes ago I went outside to take a panorama on my lawn with my DSLR, but I accidentally looked at the sun through the viewfinder. Is that dangerous? I just saw a black spot for about 30 seconds and it disappeared, does it depend on the physical aperture of the lens? (It's not the first time it happens.)
(Normally I would not post an answer to a question I've already voted as a duplicate. But the danger of irreversible damage to a user's eyes compels me to write this.)
We've all seen or heard this Henny Youngman joke:
Translated to your situation, it goes something like this:
Patient: "I looked at the sun through my camera's viewfinder and saw a black spot for about 30 seconds and it disappeared. I'm worried about damaging my eye."
Doctor: "Then don't do that."
Let's talk about your eyes. Just because you feel no discomfort is no guarantee you are safe to look at the sun with your naked eye. From a NASA news release about safe solar viewing during an eclipse:
Damage to the eyes comes predominantly from invisible infrared wavelengths. The fact that the Sun appears dark in a filter or that you feel no discomfort does not guarantee that your eyes are safe.
If you really want to read about what happens when you stare at the sun too long, read this article from Sky and Telescope. An excerpt:
When longer wavelengths of visible and near-infrared radiation pass into the eye, they are absorbed by the dark pigment epithelium below the retina. The energy is converted into heat that can literally cook the exposed tissue. Photocoagulation destroys the rods and cones, leaving a permanently blind area in the retina. This thermal damage also occurs during extended exposure to blue and green light.
Both photochemical and thermal retinal injuries occur without the victim's knowledge, as there are no pain receptors in the retina, and the visual effects do not occur for at least several hours after the damage is done.
So you can understand how much thermal energy is contained in the sun's infrared output, let's look at photographic gear damaged by being pointed at the sun:
Lensrentals.com has posted a blog entry in which what happened to some of their rental equipment that were used without proper solar filtering during the recent Great American Eclipse in the United States is shown in photos of the damaged equipment.
You need more than a UV filter, ND filter, or polarizer to protect your eyes and your camera when imaging the sun. You need a solar filter specifically designed for imaging the sun. The danger to your eyes and camera are very real if you are pointing the unprotected or underprotected camera at the sun.
- Most ND filters and polarizers only block visible light.
- The sun emits very high levels of UV and infrared radiation as well as high levels of visible light.
- A filter that only blocks visible light will not mitigate the energy contained in the UV and infrared portion of the sun's light.
- A UV filter will have no effect on infrared radiation at the other end of the visible spectrum from UV light.
- You can literally cook your retinas in a matter of seconds by exposing it to the sun's infrared energy focused through a telephoto lens. Since our retinas have no pain receptors, you won't even realize it until hours later when the effects of all that heat cause scar tissue to form on your retina. The damage to your vision will be permanent and could be as severe as total blindness.
- Your camera is also vulnerable to IR and UV in such concentrated amounts.
- Even in the visible portion of the spectrum, it takes about 15 stops of neutral density to lower the light from the sun to a safe viewing level.
Does it depend on the aperture of the lens?
Most cameras do all metering and focusing with the lens wide open regardless of what aperture setting has been selected. The lens is only stopped down a split second before the shutter opens to expose the scene. Even if you've got f/22 or f/32 selected you're almost certainly looking through the lens' widest aperture (lowest f-number) with a camera that has an optical viewfinder and very probably doing it with a mirrorless camera that uses an electronic viewfinder (or a DSLR in Live View mode). Of course with an electronic viewfinder, your eye won't be exposed to the energy from the sun. In that case your camera's image sensor that is supplying the image to the EVF will, though.
There are some variables to be considered when deciding whether it is safe to point your camera so that the sun is in the frame:
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. An 18-55mm f/3.5-5.6 lens has a maximum entrance pupil diameter of less than 10mm.¹ A 600mm f/4 lens has an entrance pupil diameter of 150mm and collects 225X as much of the sun's energy as the 55mm f/5.6 lens does!
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 for the human retina, which is generally less tolerant of direct exposure to the sun's energy than most modern cameras. With that 600mm f/4 lens mentioned above, your eye can be almost instantly damaged enough to blind it by looking directly at the sun without an adequate solar filter! 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.
¹ Keep in mind that the pupils in your eyes are usually around 2-4mm in diameter in bright light. Even that 55mm f/5.6 lens is letting about nine times as much of the sun's energy into the camera. With an SLR, a portion of that energy is allowed through the semi-reflective part of the mirror that allows light to go to the autofocus sensor array. Some of it is absorbed by the viewscreen at the top of the light box that is viewed through the viewfinder. But most of that energy is concentrated into the viewfinder's small exit pupil.
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.
At very wide angles the danger is much less and taking photos with the sun in the field of view doesn't normally harm the camera or lens. When the sun is very low on the horizon the energy is also reduced as there is much more of Earth's atmosphere to absorb much of that energy between an observer on the ground than when the sun is high in the sky.
More importantly, with wider angles of view it probably doesn't do any harm to the photographer's eyesight. Keep in mind, though, that to the best of my knowledge NO manufacturer of cameras or lenses has ever said anything to the effect of, "It's okay to look at the sun through our camera's viewfinder." Some of us do it on occasion, but we do so at our own risk. If in doubt, use Live View. You can replace a camera. You can't replace retinas cooked by the sun's infrared light!
I've taken plenty of wider angle photos (say between 17-24mm on a FF camera) with the sun high in the sky and in the field of view without any detrimental effect on my lenses, sensor, or vision. When doing so I consciously avoid looking directly at the sun in the viewfinder. The sun will not be clearly visible in such a photo if it is exposed for the terrestrial scene. If the photo is exposed so that the sun will not be overexposed then the rest of the scene will be very dark or even black. There's just too much difference in brightness between the sun and the ground to capture both at the same time unless the sun is very low near the horizon.
For some example photos of this, please see this answer to Do you need a solar filter for a wide-angle camera?
Yes. Light rays from the scene pass through the lens and camera directly to the matte screen where they form a dependent source of light. The scale of this light, and consequently the energy it is channeling, depends on the focal length and your aperture (though many cameras close down aperture only for the shot itself or for "depth preview"). With a tele lens, you can do tremendous damage when aiming it at the sun. Not just to your eyes, but also to your aperture blades (which warp due to the heat) and your sensor. With wider lenses, however, it becomes actually comparatively common to have the sun in the field of view, but it's kept at a size and brightness where it tends to be reasonably harmless.
A UV filter keeps at least the more powerful invisible part of the spectrum out and thus some of the potential damage in check.
If your subject actually is the sun so that you are going to aim a tele lens at it, you need to block most of the light with a filter explicitly specified for sun photography in order to protect both your equipment as your eyes and not just reduce the visible part of the spectrum, but also block the invisible part.
Can it be dangerous to look at the sun through the viewfinder?
Yes. The viewfinder of DSLRs use a technology that allows image updates to occur at the speed of light. Light rays from the scene pass through the lens and camera directly to the photographers' eyes. Any light source that can damage the eyes when viewed directly are also capable of damaging the eyes when viewed through a DSLR.
... does it depend on the aperture of the lens?
Most DSLRs operate in what is known as auto aperture mode. While the photographer frames the scene, the aperture is kept wide open. It is stopped down while the image is being recorded on film, then opened again for framing.
Older DSLRs may operate in stop-down mode. The aperture changes immediately according to the aperture setting. This reduces the light passing through the camera while the photographer is framing the scene. At F22, the amount of light passing through the camera is 1/128 the amount that passes through at F2. However, even with this reduction, viewing the sun through the viewfinder may still be harmful.
I accidentally looked at the sun through the viewfinder... I just saw a black spot for about 30 seconds... It's not the first time it happens.
Consider being more careful in the future.