Introduction

Multiple exposure of the setting sun

You may once have seen a photo of several regularly spaced suns that are setting over an ocean or mountain range or so, and maybe you wondered how this is done. The technique is very simple, but you will need a camera that can do multiple exposures. These photos require a lot of patience, and you have to keep concentrated, but the results are usually well worth it.

Ofcourse you can create a similar photo by digital manipulation, but that is cheating! The one-hour dedicated, bone-chilling, teeth-grinding session that created that stunning multi-exposure of the moon at 4 AM in the morning surely is going to give you more of a feeling of accomplishment and satisfaction than sitting at a computer...

... in my opinion.

What you need

With multi-exposure I mean exposing one frame and without advancing the film, exposing another frame over the previous frame.

In general, a camera that can do double exposures can do multiple exposures as well. If your camera cannot do multi-exposures, you cannot try this technique. Even if you can fool your camera into doing double exposures by winding the film advance lever with the film rewind mechanism unlocked, as possible with the older Olympus, Pentor and Praktica cameras, you won't be able to do a multi-exposure because the film may move a little.

Nikon FE camera bodies do multi-exposures fine, and are not too prohibitively expensive, although you need a lens as well. Good Nikon lenses have the reputation of being prohibitively expensive, unless you buy them used.

For a lens, use a short-telephoto lens like 85mm or 135mm. Wider angle lenses make the sun's image on the film quite small, and besides, you will need a longer time for the photo (it doesn't look very good to have the sun start somewhere in the middle of the frame; rather let it drift into the frame from the top). Longer telephoto lenses are an option, but then there will be few images of the sun or moon next to eachother on the frame.

Most importantly, you will need a solar filter if you want to do a nice multi-sun exposure where the sun appears as a well-defined orange ball. I highly recommend getting a visual/photographic filter that is made for the purpose. Baader and Thousand Oaks are two companies that distribute affordable solar filters. You need a filter that is ND5, which has an transmission coefficient of 10^-5 (i.e. 0.001 % of the light gets through). The sun will appear deep yellow-orange through such a filter.

If there are thick cirrus clouds, you might be able to get the sun properly exposed on the frame by using just a fast shutter speed and no filter, but the clouds together with the sun may look strange on the frame later, and the sun may be blurry as well.

If you do multi-exposures of the moon you don't need any filter.

You also need a sturdy tripod and cable release.

The technique

Multiple exposure of the rising moon; since the moon wasn't full, the time interval could be shorter. Note that the atmospheric absorption near the horizon shows quite well.

The technique is quite simple, but requires patience and exact timing. You take a series of exposures on a single frame with the tripod and camera fixed, at regular intervals; Earth rotation makes the sun (or moon) move across the sky. When the sun or moon has set, or when it has moved out of the frame, you make another exposure (without filter) of the background sky.

While you make the sun/moon exposures, the background sky will virtually not expose the film, because of the huge brightness difference (assuming that you will not try the technique with the moon during the daytime). Only when you do multi-exposures of the moon during the night, you might expect a bright star or planet that happens to be near the moon to show up as a chain of points, also.

Exposure times

Most difficult is to determine the proper exposure for the sun or moon to have it properly exposed on the frame. Also important is that you use exactly the same exposure settings for all the exposures, even though the sun and moon may dim appreciably when they are near the horizon. If you were to change the exposure settings, the result would look quite bad!

Below is a chart that lets you determine the proper exposure to within one stop or so. Keep in mind that atmospheric scattering and absorption can make these times drastically different, depending on the atmospheric humidity, your altitude, and dust content of the air. The table below is for clear conditions, with the sun or moon about 10^o high in the sky.

ISO	f-stop
100	2.8	4	5.6	8	11	16	22
200	4	5.6	8	11	16	22	32
400	5.6	8	11	16	22	32	44

Object	Exposure time
Sun (with ND4 filter)	1/1000	1/500	1/250	1/125	1/60	1/30	1/15
Sun (with ND5 filter)	1/125	1/60	1/30	1/15	1/8	1/4	1/2
Crescent moon	1/250	1/125	1/60	1/30	1/15	1/8	1/4
Half moon	1/500	1/250	1/125	1/60	1/30	1/15	1/8
Gibbous moon	1/1000	1/500	1/250	1/125	1/60	1/30	1/15
Full moon	1/2000	1/1000	1/500	1/250	1/125	1/60	1/30

[Exposure times for the sun and moon at about 10^o to 20^o degrees high in the sky, under clear-air conditions. Note that a ND4 filter is about 3 stops brighter than a ND5 filter.]

Determining the proper time interval

The sun and moon move in right ascension by approximately 15^o per hour, which is 15 arc-minutes per minute. Since both the sun and moon appear about 30 arc-minutes in diameter, and both are near the equatorial plane of Earth, they move a distance about equal to their own diameter in about 2 minutes. So if you want a dense chain of suns or moons in your frame, make exposures at about 2m 30s intervals; I recommend to start with an interval of 4 minutes. That will give the multiple sun/moon images separated by about their own diameter from eachother.

Composition

When doing artistic photography such as this, proper composition is essential.

The apparent track of the sun will make an angle with the horizon that is approximately equal to 90^o minus your latitude, if you photograph the sun during the equinoxes. E.g. if you are at latitude 35^oN, the sun or moon will make an angle of about 55^o with the horizon. You need to know this in order to determine the composition properly beforehand; once you start the exposure series you cannot adjust the camera!

The horizon intercept angle will be less if you don't photograph during the equinoxes and may also be different if you photograph the moon. The moon orbit makes a small angle with the ecliptic and so the moon doesn't need to be on the equatorial plane at the time of the equinoxes.

Have the sun or moon go into or out of the horizon slightly off-center to the frame, if the track makes an angle with it; the photo will have more balance.

Tips

• The technique gets very easy if your camera supports interval-shooting. But I am not sure if there are consumer cameras that do interval-shooting and multi-exposures simultaneously.

• When shooting during sunset/moonset, do the multi-exposures first, and then the exposure of the background. During sunrise or moonrise, do it the other way around, if you know exactly where it will rise. This is because you don't want the sun or moon near the frame when you do the longer background exposure, to avoid flares or spurious sky glow; and you probably don't want to wait an hour or so between the multi-exposures and the background exposure for the sun or moon to be far enough out of the way.

  Make sure there are no (sharply defined) clouds in view at any time, or the resulting superimposed solar image will look strange.

• Keep in mind that the camera viewfinder often views only about 90% to 95% of the full frame; you need to do some exposures even when the sun or moon is not visible in your viewfinder.

• Make sure that the tripod doesn't move. One frame will probably have the sun or moon half over the horizon, and it is crucial that the horizon doesn't move, otherwise the cut-off body will appear to hover above or into the horizon.

• You cannot do multi-exposures of the moon during the daytime, since you need a huge brightness difference between the moon and background in order to not expose the background during the multi-exposures.

• For multi-exposures of the moon, the background frame will show star trails. This is unavoidable; the star trails will be shorter if you use a faster film, since the exposure then takes shorter. Expect to expose the background frame several minutes if you use 100 ISO and f/2.8.

• Use higher f-ratios such as f/5.6 or f/8 for the multi-exposure series, if the light level permits, to get sharper detail. The background frame may also show vignetting if you use a wide-angle lens at low aperture numbers.

• Timing is essential! Keep a record of all exact times (writing them as, for example, 07:21:00 and such), so you won't get confused as to whether you forgot an exposure or not. It may seem unnecessary but trust me, it is very easy to get confused with this if you don't keep a log! Also, take all pictures within a few seconds accuracy to the proper time interval, otherwise the sun or moon will be irregularly spaced on the photo.

• Either use a dark filter such as ND4 or ND5 for the sun, or use short exposures and no filter at all (if at all possible). Don't use a high-transmission filter such as a +3 stops filter or so, or the photo will have nasty reflections. Do not get confused by the various filter designations though - some filters are (improperly?) sold as ND4, while they have a transmission coefficient of 1/4 the incoming light, or 2 stops, not 1/10000 (about 13 to 14 stops) for the proper ND4 filter! A true ND5 filter will appear completely dark when you attempt to look through it, save for the sun or another extremely bright object. Most filter designations are something like ND4x though, meaning the filter requires exposures 4 times as long to get the same exposure as without the filter - so watch out for the 'x' in the filter designation - these filters are not useful for solar photography.

• For all photography that involves the sun, take great care with your vision. Do not look directly at the unfiltered sun, not even for a moment, and make sure the filter you use is safe for solar viewing.

• The ultimate multi-exposure of the sun is the analemma, the figure-eight that the sun makes in the sky throughout a year if you photograph it at exactly the same time of day once every week or two weeks. Few people have successfully accomplished this. The figure-eight arises from the fact that the orbit of Earth around the sun is elliptical and the fact that the orbital plane of Earth and the equator are not coplanar.