|
|
Introduction
Planning
Camera
Film
Choosing a Lens
Solar Filters
Other Accessories
Calculating Exposures
Composing the Image
Practising
Eclipse Day
Further Information
Introduction
Capturing a successful image of a total solar eclipse is one of the most rewarding achievements. However, most images record less than a second's worth of an event that lasts for around 2 hours. A multiple exposure sequence image is a visually compelling record of the entire eclipse, allowing you to record both the partial phases and totality on the same image. In essence you are photographing 'time'.
Outlined below is some advice and tips on the equipment and techniques specific to multiple exposures sequences, and should therefore be used in conjuntion with more general advice on photographing and viewing an eclipse available from other sources.
Planning
Gather as much specific information on the intended viewing site as possible, including:
- The exact GPS coordinates of your intended viewing site
- Details of surrounding topography (large mountains, trees, rivers...)
- Timing data for key phases of the eclipse (first contact, mid-totality...)
- Altitude of the Sun above horizon and its direction at these times
Most of this information is readily available on the web, however you may need to dig a little deeper to acquire topographic maps of the area. Make sure that if the eclipse occurs near sunrise/sunset, there won't be a large mountain obscuring your view.
Camera
For multiple exposure photography, a camera with the following features will be required:
- Ability to override the automatic film wind
- Fully manual aperture and exposure settings
If you are unable to override the film wind then you cannot create a multiple exposure sequence, unless it is digitally composed afterwards. The multiple exposure button needs to be easily accessible when the camera is mounted correctly (horizontal or vertical) on your tripod. A 35mm SLR camera will be fine, however if you plan to have the image enlarged a medium or large format camera may be better.
Film
Choosing the best film is a matter of personal choice and depends on what you want to do with your photo. For maximum reproduction quality, a fairly slow (ISO 50 or 100) slide film is best due to its fine grain. The drawback is that slide film has a smaller exposure latitude (tolerance of incorrect exposures) than print (negative) film. Using a slow film also increases the length of exposures and hence risk of vibration, however a solid tripod should minimise this. Generally any film suitable for outdoor photography will be fine.
Choosing a Lens
To select a lens the predicted motion of the Sun must first be calculated. The Sun moves 0.5o (it's own diameter) every 2 minutes, or 15o per hour. For an eclipse that lasts around two hours, the Sun will move 30o through the sky.
The exact trajectory of the Sun (it's altitude and azimuth at different times) can be calculated for any location by entering the times for important phases of the eclipse (1st contact, mid-totality etc) into some astronomical software programs. Then select a focal length which gives the required field of view to fit the entire sequence into the frame. As an example, some of the altitude and azimuth coordinates for the 2002 eclipse are listed below:
Event|
| Time |
| Azimuth (degrees)|
| Altitude (degrees) | | | | |
1st contact|
| 18:41|
| 255o|
| 19o | | | | |
Mid-Totality|
| 19:41|
| 248o|
| 7o | | | | |
Sunset|
| 20:21|
| 243o|
| 0o | | | | |
Note: For this eclipse, sunset occured before 4th contact - for other eclipses the coordinates of the Sun at 4th contact would also be needed.
From this data, we see that the Sun traverses an angle of (255o - 243o) = 12o in azimuth (horizontally) and (19o - 0o) = 19o in altitude (vertically) during the eclipse. So a lens with a field of view of at least 12o x 19o is required. Listed below is the field of view for some standard lenses (35 mm format only):
Focal Length |
Field of View |
Diagonal |
Size of Sun |
28mm |
46o x 65o |
79o |
0.25mm |
35mm |
38o x 54o |
66o |
0.3mm |
50mm |
27o x 40o |
48o |
0.45mm |
70mm |
19o x 29o |
35o |
0.6mm |
105mm |
13o x 19o |
23o |
0.95mm |
135mm |
10o x 15o |
18o |
1.2mm |
The field of view, φ, for other focal lengths of for larger format film can be calculated using:
φ = 2 arctan (x/2f)
where f is the focal length in mm and x is the height or width of the film in mm. For 35mm film, x = 24mm for the short axis and x = 36mm for the long axis. For 6cm x 9cm format film, these values would be 60mm and 90mm respectively. The diameter of the Sun on the film can be calculated using:
Diameter of Sun (mm) = Focal length of lens (mm) / 110
This formula works for all lenses and is independant of the film size.
To include the horizon in your shot, work out the greatest altitude that the Sun will reach and use this value for the field of view. For example, if the Sun was at a horizontal altitude of 45o at 1st contact, a 35mm lens (or shorter) would be needed since it has a horizontal field of 54o. This allows some room around the edge of frame and for foreground detail as well. See also Composing the Image below for more details.
Solar Filters
Using the correct solar filters for both your own eyes and for the camera is by far the most important factor in eclipse photography, since an incorrect filter may result in permanent loss of vision for you and irrepairable damage to your camera. There are many sources of information on Solar Filters which you should be read before starting out.
One possible source of confusion however is in the terminology used. Many articles state the requirement for a solar filter of density 4.0. This is a logarithmic density and means the filter reduces incident light by a factor of (104) = 10,000 times. This is not the same as a photographic neutral density filter with a filter factor x4, which only reduces the light by 4 times.
You need to make absolutely sure that you have a filter of logarithmic density 4.0 or greater and not just a normal photographic neutral density filter before pointing your camera at the Sun. If you able to see anything through the filter, it is definately not the correct one. Stacking up photographic neutral density filters is very bad practise since they do not reduce the harmful UV radiation even though it may be comfortable to look at the Sun. Most people in camera stores will never have heard of a logarithmic density 4.0 filter, so you will need to seek advice from a specialised dealer.
Remember that the Solar Filter is only necessary for the partial phases of the eclipse and must be removed for totality.
Other Accessories
In addition a number of other accesories are required. The most important is a solid tripod. An investment in a heavy wooden tripod or a well built metal one is well worth it, since a cheap, flimsy tripod will not damp out vibrations and may lead to a blurred image. If you are going to spend time planning and preparing a mutiple exposure sequence, the last thing you want is to have the image ruined by a lightweight tripod. A heavy tripod will also reduce the chance of an accidental bump moving the whole camera. Another good idea is to hang a large weight under the tripod, such as a bag full of sand or rocks, and/or partly bury the legs into the ground if possible. Remember that even the slightest movement will be obvious as a jump in the Sun's smooth path across the sky.
Other essential items include:
- A cable release
- Torch for the low light of totality
- Spare batteries (to be installed on eclipse day)
- A shade cover for the camera
- A darkroom timer, watch, GPS or laptop
Calculating Exposures
Determining the correct exposures for partial phases of the eclipse is a matter of trial and error. The exposures used depend on how close the Sun is to the horizon, since the intensity of the Sun near sunrise/sunset diminshes with the increased atmospheric path length. Exposure compensation only needs to be made when the Sun is less than about 20o from the horizon. The ICQ journal article, correcting for atmospheric extinction, contains a detailed analysis of this subject.
Start by making a series of trial exposures around the same time of day as the eclipse. Using the same camera, film, lens and filters as you plan to use on eclipse day, take say 12 exposures. You can obtain eclipse exposure guides from various sources, but a good starting point would be to bracket around 1/250s at f/8 for ISO 100 film, (depending on your equipment). Then critically analyse the film to find the best exposure. This process must also be repeated on some cloudy days to give you a feel for how much exposure compensation to make in the event of cloud on eclipse day. Keeping detailed records of all exposures made with notes about the results obtained is a necessity.
If the Sun will be high in the sky during the eclipse all exposures will be roughly the same, but you should still run some tests to make sure. No compensation is neccessary for the eclipse itself, since the Sun has a constant surface brightness and therefore the exposure needed for a partially eclipsed Sun is the same as that of a full Sun. However for the very thin partial phases compensation for solar limb darkening should be made. An extra 1 stop of light for exposures made about 10 mins before and after totality and 2 stops extra for exposures 5 mins before and after will be fine. Values of 1/2 stop and 1 stop respectively are also OK.
To expose the foreground and sky in your image requires either a fairly long totality exposure or another one when the Sun is well out of frame. A long totality exposure will show the Moon's shadow band and your surroundings nicely, but will also create an overexposed corona. A shorter totality exposure and a subsequent exposure does give more control, although care must be taken to ensure the sum of these will not result in an overexposed sky. The sky should ultimately be underexposed by about one or two stops. Looking at other people's images of totality and the settings used will give you a feel for how long the exposure should be.
Composing the Image
In order to create a successful and well balanced multiple exposure sequence, you need to have a pretty good idea of what the final image will look like long before it is taken. Standard rules for composing a photograph apply, however this will be much more difficult since the subject matter evolves over time.
The best method is to plot a scale diagram of the predicted trajectory of the Sun. Altitude is the angular position in degrees above the horizon, while azimuth is the direction or bearing of the Sun. By converting these values to centimeters or inches using an appropriate scale (eg 5o = 1 inch or 2o = 1 cm etc), the Sun's path can be plotted onto graph paper with a line representing the horizon at the bottom. Plotting (say) 5 data points, such as 1st and 4th contacts, mid-totality and then two points in between, will give a pretty good indication of the Sun's motion. For example, if the Sun's altitude was 20o above the horizon at 1st contact, this would be plotted at a point 4 inches above your horizon line (when using a scale of 5o = 1 inch).
Once all points have been plotted, cut a window frame using the same scale to represent the field of view of your lens. For example, a 50mm lens with a field of view of 27o x 40o would require a (27 / 5) x (40 / 5) = 5.4 x 8 inch window (when using a scale of 5o = 1 inch). By placing the window over your plotted diagram, you can see whether the entire eclipse will indeed fit into the field of view of that lens, how much foreground can be included, etc. By making windows for a few different lenses, it is easy to work out which will be the best lens to use and how the Sun will trail through the frame.
One of the most difficult aspects is determining where to position the first image (usually 1st contact) in the camera's viewfinder. One way is to place the Sun at the edge of the viewfinder somewhere between 5 and 20 minutes before 1st contact to allow it to trail into the frame. A viewfinder with a grid pattern or some other markings will make postioning easier, however this technique must be practised well in advance.
Practising
Now that most of the details have been worked out, a number of practise runs should be undertaken. If, however, you are travelling to a another country to view the eclipse, the Sun's trajectory will be different to home. Nonetheless a trial run will quickly show up any problems, such as camera movement while operating the multiple exposure switch, inaccurate exposure etc, while there is still time to rectify them.
Arriving at the intended eclipse location two or three days early will allow plenty of time to find a site for the camera and to check the Sun's motion. Set the camera up and place the Sun in the top corner of the viewfinder at the same time as 1st contact. Then check the Sun's motion and make sure that 4th contact does not move out of the frame and that any trees or building's you wish to include do not obscure the Sun. This process should be repeated twice if possible.
Eclipse Day
When the big day arrives, make absolutely sure that all equipment is functioning properly and that new batteries have been installed in all cameras, light meters, timers etc. Try to set up the camera a couple of hours before 1st contact and run through everything that needs to be done. Remember, in the excitement of totality it will be easy to forget things like removing the solar filter, activating the multiple exposure switch etc, so everything must be programmed into your mind as well as written on some paper next to the camera. If there are other people around, it would be a good idea to cordon off a small area around the tripod to prevent any accidental knocks since the camera must not be moved during the entire eclipse. Good preparation will reduce the chance of anything going wrong.
Finally, don't forget to enjoy the experience since any photos that you take can never adequately capture the feeling or emotion of a total solar eclipse.
Further Information
Here are some links to other eclipse photography sites with information on multiple exposure sequences:
Fred Espenak > MrEclipse.com
Geoff Sims > Geoff's Photography
Bob Yen > Bob Yen's Way Out Photography
|
|
