Advanced Advice about Green-Flash Photography

For people who are serious about green-flash photography, here's some more technical photographic discussion:

Choice of film and filters:

I like slides, as most publishers can work from slides and they are easy to mail. The problem there is the limited exposure latitude of reversal materials. The advantage is that I have found one film (Ektachrome 64T EPY) that shifts the color balance in the critical crossover between red and green to be closer to the response of an eye that has been looking at a red sunset. If you shoot straight 64T you get an unnatural bluish cast to the ocean, so I add a Wratten 2E filter — any sharp-cut yellow filter that cuts near 420 nm should do — to suppress this. At those short wavelengths there is very little transmission of direct sunlight so you are not attenuating the blue end of the sunset flash appreciably. (It may be that a Wratten 2A might be slightly better; I'm waiting for good weather to try one.)

Filters can cause problems, though, both from reflections and by not having high enough optical quality for use with those long focal lengths. If you aren't happy with EPY + 2E, you could probably leave off the filter and suppress the excess blue in copying — if the flash is green and not blue. Some people don't like the greenish bias this film produces. Color balance is a matter of taste. Remember that nocolor film can reproduce the extremely saturated colors of green flashes faithfully; don't expect perfection.

I have tried several other tungsten films (Ektachrome 160T and Fuji 64T, for example) and they do not have the required balance and are basically useless for green flashes. The daylight films like Kodachrome 25 are OK but you get an awful lot of red and the green comes out pretty underexposed unless you use a color-conversion filter; I've played around with an 80A and also some cyan filters to reduce the excess redness. Many cyan filters are much more effective than you'd expect, because much of the "red" that comes through in the sunset is really near-IR.

This IR content also means that a lens with very good color correction is a good idea. I'm using a Sigma 500-mm lens that claims to be an apochromat; its focus for IR film is the same as for ordinary film. I've verified that this is true; but unfortunately, my doubler is not apochromatic, so I lose some of that advantage. Just one more complication to bear in mind.

The difficulty with using daylight film and no filtration is that the heavily exposed red-sensitive layer tends to suppress the green through inter-layer adjacency effects. Thus the onset of recording green in the flash is delayed a bit. Even with the Ektachrome 64T I have to wait about a second after I first see "green" at sunset to get green on the film, because of retinal bleaching effects: the flash appears green to the eye when it is still yellow to the film.

Color negative materials have much wider dynamic range, which is a blessing because the exposure when the Sun is right down on the horizon is extremely variable, depending on the amount of aerosol in the boundary layer. With a negative material, you can err on the side of overexposure and get a decent print. The drawback is the interlayer adjacency effect, again. Nevertheless, it's possible to get acceptable GF photos with many different materials — especially if you cut down the excess red response with a cyan or daylight-to-tungsten filter.


Because the disappearing piece of the Sun is so small, it's difficult to use even a small spot exposure meter to set the exposure. The sky nearly always comes out underexposed by 2 or 3 stops in good GF pictures, so you could try using that as a guide. With Kodachrome 25 (or EPY 64T and a 2x tele) the exposure settings I have used run from 1/1000 at f/16 in clear weather to 1/500 at f/8 in hazy conditions. Bear in mind that a green flash is actually considerably fainter than the yellow or orange Sun before the longer wavelengths are cut off. An Italian photographer has some exposure guidelines near the bottom of his GF Web page; you might find them a good place to start. You will probably need to experiment a bit to find what works well for you.


The subject of exposure brings up the need for a tripod. There is a rule of thumb in photography that says any exposure longer in seconds than the reciprocal of your focal length in millimeters needs a tripod. So, for an ordinary 50-mm lens, shutter speeds slower than 1/50 second need a tripod. An experienced photographer, especially with something solid to lean on, can often beat this rule; it takes practice and care, and the time to squeeze off a shot slowly — a luxury green flashes do not provide.

Even with steadier hands than mine, there is often camera shake produced by the wind. There is also camera shake caused by the flop of the reflex mirror. (If you have binoculars, I suppose you could lock the mirror up and avoid this major source of jiggle, watching the sunset through binocs instead of through the camera viewfinder.)

So, because of the long focal lengths needed for green-flash photography, tripods are essential. I'm using a 1000-mm focal length, so in principle I could get by without a tripod in the very clearest conditions, if I stick to 1/1000 of a second exposures. As a practical matter, and because of the difficulty of centering the Sun accurately when such long focal lengths are used, I always use a tripod.

This adds to the bulk and weight of the stuff I have to lug around. If you have to walk a long way from where you can park to where you want to observe, it's nice to minimize the weight. Unfortunately, the steadiness of a tripod depends mostly on its weight. A lightweight, flimsy tripod is better than nothing, but the steadier, the better. If you are willing to spend a lot of money, there are fancy tripods these days that are quite rigid and not as heavy as the old metal ones; these are various kinds of carbon-reinforced plastic.

Sometimes people forget that the screws and clamps that connect the camera to the tripod are a potential weak link. There's no point in having the solidest tripod in the world if the camera sits loosely on top of it. Check this pan-head mechanism for flexure and wiggle. Make sure the connection is tight before shooting.

I suggest trying some test exposures of a sharply-defined distant object to determine how long an exposure you can use without getting unacceptable camera shake with your particular setup.

Digital cameras, video, etc.

In the last few years, silver-halide photography has been all but replaced by electronic imaging: digital cameras, camcorders, etc. The proliferation of imaging makes it more likely that some of the rare displays that have never been photographed will eventually be captured.

However, these new media have their share of problems. Please look at the artifact page for a discussion of these misleading phenomena. It's easy to find pictures that look as if they might be a green flash, but are really just mistakes.

Things to look for

The flashes I see here in San Diego often have complex sub-structure, caused either by atmospheric turbulence or by interaction of the boundary layer with the waves on the ocean. Keep an eye out for unusual structure in your flashes. It seems to me that inferior-mirage flashes taken from the Caribbean and from Hawaii seem to have less structure and look more like "textbook" examples than the ones I get here.

There are also some green-flash phenomena that have seen seen by visual observers, but never photographed (or the photographs never published).

Gaps in the existing photographic record

It would be nice to see some sunrise flashes photographed and published. Most people are, like me, too lazy to get up at 4 a.m. and haul out to someplace from which a sunrise is visible. It's hard to aim exactly at the right place to catch the rising Sun (though if you do this day after day, you will know where to expect it). And you have to be fast on the shutter release to get the rising flash before it turns yellow. Finally, it's harder to judge the right exposure when you have only the pre-dawn sky as a guide to transparency.

There are few objective measurements of flash duration. If you used a motorized back that can shoot 3 or 4 frames a second, it would be great to start firing off frames as soon as you see green, and see how many images actually show green when the film is processed. Systematic observations of this kind would be useful scientifically. The camera I use was not bought with this in mind, so I can't do this myself. Maybe you can.

Another challenge is to photograph a green ray display. I have not seen one myself, but they are reported in a small percentage of visual observations. A typical report is a fan-shaped beam of light extending a few solar diameters above the sunset point a second or two after the GF itself disappears; sometimes the beam appears to grow upward from a flash itself. I think these are green crepuscular rays produced by unusually strong green flashes; if so, I'd expect them to be seen most by an observer standing quite close to sea level. Clearly these will require more exposure to record. It may be that they appear green only to a well-bleached retina, and that they will photograph yellow (though EPY might make the difference there).

Another type of ray display is a narrow band of green light that spreads out along the horizon on either side of the Sun. This, too, may be the local haze illuminated by a thin slab of green light close to eye level from a bright flash. Again the reported durations are just a second or so; you will need to be quick.

If you get any good GF pictures, I'd like to see them. But beware of artifacts, especially in the newer media.

Please bear in mind that it's important to get pictures published where they can be seen by a wide readership. I'd like a reprint (if possible), or at least a reference to any publication of your green flashes. My mailing address is:

Dr. Andrew T. Young
Astronomy Dept.
San Diego State University
San Diego, CA 92182-1221 USA

Copyright © 2001 – 2006, 2014 Andrew T. Young

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