Dot Physics

When to not use a camera flash

Check out this picture.

i-d25de6302d6793f2f58dbfca54a1187d-2010-01-12_flash_2.jpg

These camera flashes are just awesome (well, they look awesome). They are not a very useful thing for pictures in this case. Here is the problem. Well, first here is what the flash on a camera is supposed to do. Suppose there is a camera and a ball in a dark room. In order to get light to reflect off the ball and be detected by the camera, you need light. The flash is that light. You can think of it like a flashlight that is only on for a moment. It doesn’t need to be on for long since it doesn’t take very much time to get an image.

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Notice that I drew red arrows for the light reflecting off the red ball to represent red light. Ok, back to the problem with a flash in a football stadium. The thing I want to focus on is how much light from the flash hits the object of interest. How do you measure the intensity of light? One way is to look at the intensity in Watts/meter2. As an example, suppose I have a 100 Watt light bulb that produces 20 Watts of light (I am just making up stuff here). If this light radiates equally in all directions, then at 1 meter away I can make this diagram:

i-493f9c937df8564f5cb23b09c9a93d93-2010-01-13_untitled_3.jpg

So, 20 watts over the area of a sphere with a radius of 1 meter would give an intensity of:

i-a76f3a9ae21eaba4b1d3a0d96a69b126-2010-01-13_la_te_xi_t_1_2.jpg

The problem is that camera flash does not radiate the same in all directions. However, if it is a spherical distribution the intensity will still decrease as 1/r2. For the above case, let me assume that the flash is designed to have an intensity I3 to work at 3 meters (I just made this up – I don’t know the optimal flash distance). How bright would this flash appear at the distance of the skydiver in the stadium? Assuming that light is spherically distributed:

i-3625ba749ace4b7a8a6051d6addd0e4d-2010-01-13_la_te_xi_t_1_3.jpg

What if I want to use the flash for something that is 50 meters away? The intensity at the subject of interest would be just 0.0036 times the intensity at 3 meters. This is probably not bright enough to make the image turn out brighter. Don’t forget that this light reflecting off the subject is also mostly spherically distributed. This makes the problem even worse.

So, why are these flashes on then? Well, the camera sees something dark and just assumes that you need the flash. Plus, it looks pretty. Here is a short video clip so you can see how the flashes change.

Other stuff

Check out all the red lights. I am pretty sure that those are light from video cameras (or phones that do video). Isn’t it standard that in video record-mode there is a red light? If you watch the video, you might notice that you can only see the red lights when my camera is on the opposite side of the subject (the skydiver).

Comments

  1. #1 Paul Browne
    January 13, 2010

    Is the longer time it will take for the light to reach and return from a distant object also a problem, so the light from the flash reaches the camera after the photo has been taken (or part way through)?

    The main problem (apart from running down batteries) with not turning off your flash when photographing distant objects in low light is that the reflected light from closer objects can wash out the distant object that you are trying to photograph.

    Unfortunately my current compact camera defaults to flash-on in low light, which is annoying since most of the time I don’t want the flash to come on…I guess they reckon that most photos taken with compacts in low light are taken indoors but it’s annoying having to reset it every time I turn the thing on.

  2. #2 syliach
    January 13, 2010

    The red lights are probably “focus assist” lights. Many point and shoot cameras(and some SLRs) use a red or orange light to give the camera enough contrast to focus on.

  3. #3 Alan B
    January 13, 2010

    I think syliach is right about the focus assist lights – if you look closely you can see red lights lasting for several seconds, then a flash in the same location, then no light.

    Another problem, at least on my camera, is that the auto-exposure settings correspond to the amount of light the camera expects to receive with a flash, so the exposure is too short when the flash is on.

  4. #4 Hugh
    January 13, 2010

    @Paul Browne: A camera’s shutter is open for something in the region of 1/100 second. Light travels 3,000 km in that time, so unless the object you’re photographing is more than 1500 km away, no.

  5. #5 Rhett Allain
    January 13, 2010

    @syliach,

    I don’t think the red lights are focusing lights. If you step through the video frame by frame, you don’t see a red light changing into a flash. Also, if they are focus lights, a lot of people sure are taking their time to get focus.

  6. #6 Moopheus
    January 13, 2010

    “The intensity at the subject of interest would be just 0.0036 times the intensity at 3 meters.”

    Or to put it in photgrapher’s language, that’s a difference of about 8 stops. So, indeed, pretty useless for most P&S type cameras, which have low-powered flashes and limited speed and aperture ranges. A pro-grade speedlight can be pumped up to reach a good distance, though in a well-lit stadium it hardly seems necessary. The brightest flash in my personal use is the flash gun for my Speed Graphic, that uses magnesium bulbs (I have a stash). Wide open with a fast film, I might be able to get a useful range of about 70-80 meters with that.

  7. #7 Dave X
    January 13, 2010

    Some video cameras have an night-shot/IR light which might appear red on other cameras. Try looking at your IR remote with your camera and you’ll likely see it flash red.

  8. #8 guthrie
    January 13, 2010

    Rhett – I say many of the red lights are focus lights. My camera will produce them even when the flash is switched off, as I often have it. And a lot of them are LED’s these days, of quite a bright sort. So it looks like many people are cluless and don’t switch the flash off, others may have an idea and switch it off but their cameras, trying to focus on a stadium, end up flashing the red light a bit. Then they take the photo and there is no flash.

    Some of the red lights might be other things, I don’t know what, torches?

  9. #9 NJ
    January 13, 2010

    True story.

    While flying out of Chicago O’Hare one night, the plane made a turn by downtown, showing the city lit up with the the sodium vapor streetlights. Nearby passenger grabbed 35mm camera with flash, aimed it out the window and fired away. The flash illuminated the plane’s interior.

    I wish I could have been there when they got the film back…

  10. #10 TGAP Dad
    January 13, 2010

    Having made my living as a photographer in the distant past, and having taken several thousand rolls of film (back when they had that!), many of them in performance venues, I can assure you that a flash is, in nearly all cases, useless for these shots. The critical detail in photography is how much light is hitting your subject. The missing link here is being able to fill the frame with your subject, either via position in the venue, or a sufficiently long telephoto lens. If you can make an educated guess as to the illumination level of your subject, you need only to (manually) set the camera for the appropriate exposure level (EV), then just frame, focus and shoot as much as you want. No flash necessary.

  11. #11 Kevin W. Parker
    January 13, 2010

    I was at a country music concert once sitting at least 50 feet away from the stage when a woman down the row from me started trying to take Polaroid flash photos of the performer. She simply couldn’t understand why the only things visible in the photo were the backs of the heads of the people in the row in front of her. I think she took half-a-dozen photos before she finally gave up.

  12. #12 Steve
    January 13, 2010

    Just another illustration of the sorry state of American education, where “Social Studies” get more time than the real sciences. My favorite anecdote is the lady who screamed “The brakes don’t work!” as her car slid sideways into an intersection after hitting a patch of oil. The brakes were working fine; she had them locked up tight. It would have been funny if I hadn’t been in the car.

  13. #13 Lassi Hippeläinen
    January 14, 2010

    But those flashes are an essential part of the party athmosphere! A good producer makes sure there are flashes all around, for example by mounting fixed bulbs all over the place. If you watch carefully any man ballet program (e.g. professional wrestling), you’ll notice that the brightest flashes come from places where there are no people.

  14. #14 Ian Kemmish
    January 14, 2010

    A few years ago I was at an exhibition of ancient Chinese silk paintings at the British Museum. Unsurprisingly, there were signs up asking people only to use cameras without flash. Nonetheless, people were still marching round at a brisk pace, taking flash photos of every exhibit without even looking at them. Being a busybody, I pointed the signs out to one of them. She looked at me, looked down at the camera with a puzzled look, shrugged, and walked off.

    The question is whether this is a matter of stupidity, or of lousy user interface design? (On my current camera, you pop the flash up out of the camera body to use it, but on most I assume that it’s relegated to a menu selection of a multipurpose button.)

  15. #15 Moopheus
    January 14, 2010

    “The question is whether this is a matter of stupidity, or of lousy user interface design?’”

    Yes.

  16. #16 The Ridger
    January 18, 2010

    My father’s camera (can’t remember the make … Nikon, maybe) gets very obnoxious if you don’t turn on the flash when it thinks you need it. It pops up a very distracting flashing icon on the LCD display. He doesn’t notice it ’cause he uses the view-finder, but others might.

  17. #17 HD
    February 4, 2010

    The question is whether this is a matter of stupidity, or of lousy user interface design?

    A couple of years ago I was at a conference in Sydney. The big dinner was on a boat and as we wined and dined, we got a cruise of the Sydney Harbor at night. I went up on top to catch the sights, and witnessed nearly all of my physicists colleagues, each and everyone of whom work with optics and lasers every single day (except the theorists, but they don’t count) flashing away with their point and shoots. I pointed out to a few of them that their flashes were unlikely to do much for the Opera House (and was also ruining any night vision they might have had), but it didn’t do much good.

    Ph.D physicists, every single one.