Exposure

Fred Espenak's How to Photograph a Lunar Eclipse is an excellent source for exposure information. Capturing a lunar eclipse often requires large on the fly adjustments because of unanticipated conditions. I build on Fred's guide with some techniques that I've found make these on-the-fly adjustments easier. Correct exposure is a challenge for any eclipse photographer for several reasons:

• The brightness of the Moon varies tremendously during the course of the eclipse. Exposure can vary 18 stops or more (250,000 times) from the uneclipsed full Moon to the darkest total eclipse.
• The brightness of the Moon varies greatly from eclipse to eclipse, as much as 5 stops during the penumbral phases and 8 stops at maximum totality. The Moon's path through the umbra can pass near the edge or through its darkest center. The light that reaches the Moon during the eclipse is scattered by the earth's atmosphere and this depends strongly on the weather conditions around the edge of the earth.
• Exposure varies very rapidly at times near the U2 and U3 contacts.
• The dynamic range of exposure during partial umbral eclipse (U1 to U2 and U3 to U4) needed to capture both the partially lit crescent and the umbra darkened face can be as much as 8 or 10 stops. Images that capture what your eye sees at these times require multiple exposure high dynamic range techniques.

The Danjon scale is used to rate the darkness of a total lunar eclipse. It ranges from an L value of 0 where the Moon is almost invisible at greatest eclipse to 4 which is a very bright orange. It is a subjective scale. Fred Espenak's guide suggests two stops (4 x) of exposure difference between each step of the scale.

Basic camera settings

The low light, high contrast, and high dynamic range of astrophotography requires differences in settings from daylight photography. These include:

• Turn off all automatic features on your camera. Auto focus, auto exposure, and image stabilization will all work poorly or not at all.
• Shoot RAW images to capture the most dynamic range and allow for exposure correction in post processing.
• Turn on electronic first curtain shutter to minimize vibration. If you use a DSLR, mirror lock up should be used to reduce vibration.
• Do not expose to the right on your histogram. Exposure aids are easily fooled in astrophotography and there is no recovery from over exposure.

Time lapse settings

An intervalometer or time lapse app ensure that your final images are evenly spaced in time. Evenly spaced images allow you to create smooth time-apse videos or time-lapse composite still images. The minimum time interval for shutter triggers should be the total exposure time for your longest exposure sequence during totality plus enough time for you to make any exposure settings or aiming changes, or swap out a low battery. I find that 3 minutes intervals work for me.

Shutter mode settings

You can shoot a single image for each interval. This may be all you want for a simple fixed tripod shot where the Moon will be small in the final image. If you are shooting with a motorized tracker and a longer lens, you may want to shoot a burst of images at each interval. These can be all the same exposure for noise reduction stacking in post processing or an HDR exposure sequence. HDR exposures are required for images near the beginning (U3) and end (U4) of totality to show both the sunlit sliver and the umbra shadowed parts of the Moon correctly exposed.

Lens focal lengths

Your eclipse framing plan will determine the lens focal lengths that you use. It is helpful to keep in mind some field of view limits for lunar eclipses.

The longest lunar eclipse of the 21st century (27-28 July 2018) lasted 3 hours and 55 minutes between U1 and U4 and includes 1 hour and 43 minutes of totality. Capturing a time lapse of the entire eclipse takes about a 60 degree FOV.

Exposure Settings Limits

If you are shooting from a fixed tripod, the rule-of-500 estimates the longest exposure you can set before a camera shows noticeable star trails. This blurring due to the earth's rotation isn't normally a problem shooting a bright Moon, but it may limit your exposure time during a lunar eclipse. The is rule-of-tumb gives:

maximum-exposure-time = 500/full-frame-equiv-focal-length = 500/(focal-length * crop factor)

Crop factors are about 1.5 for APS sensor sized DSLRs and 2 for micro 4/3 cameras. This will give the longest exposure time that you should use during the darkest part of the eclipse with your camera and lens.

If you are shooting with a motorized tracker, keep your exposure times to 30 seconds or less. If you are shooting bursts or multi-image HDR stacks be sure you have enough time for all exposures to complete during totality.

Look up the ISO speed at which your camera gets the greatest signal to noise ration (dynamic range). You want to shoot as close to this speed as possible. Find out if your camera is ISO invariant. A general rule of thumb is that Canon is not, but Sony (above about ISO 400) and Nikon are.

Increasing your ISO beyond this base speed, will only decrease your camera's dynamic range in an ISO invariant camera. If your image is too dark at the base ISO you can simply increase the exposure of your RAW images in post production. This will give you the correct exposure without loosing valuable dynamic range. If your camera is not ISO invariant, additional gain while shooting dark images will decrease noise. If you need to check your focus when the image is too dark, you can temporarily boost the ISO while focusing.

Multiple exposures

I find it very helpful to take advantage of multiple exposures when shooting an eclipse. Multiple exposures can be used in post processing to reduce noise and seeing distortions by lucky image staking and to increase dynamic range by HDR techniques. Many new cameras have shutter modes that make shooting image bursts much easier. If your camera has these, they can be a great help. Burst of 5 or 10 images allow stacking to reducing noise by 2 to 3 times in your final images. HDR exposure sweep modes during the shots near U2 and U3 can be exposure stacked for high dynamic range images.

A sample exposure plan for a total lunar eclipse

You can fully automate your eclipse photography with a tethered imaging application like Xavier Jubier's Lunar Eclipse Maestro, if you have a supported camera and computer. Otherwise use a simple plan with your anticipated exposure and shutter mode changes at the key points in the eclipse. You can get the timing of the key events at Fred Espenak's Eclipsewise or NASA's Lunar Eclipse Page. Rather than keeping track of a just a jumble of aperture and speed settings, I prefer to record the anticipated changes in exposure values. As the eclipse proceeds, I will check the exposure of selected images to adjust for actual conditions. If my plan is in specific settings, the subsequent settings will be wrong. If I plan these as exposure value changes then subsequent planned exposure adjustments are still correct.

You can get times for a specific eclipse from Eclipsewise or a planetarium progam like SkySafari. Estimates for the darkness at peak eclipse are at Calsky.

Times in the table below are relative to first partial contact for a typical eclipse. They must be corrected to the actual times for a specific eclipse.

Time	Event	Exposure Change	Settings	Shutter Mode
-1:00	P1-- No eclipse
-0:20	P1++ Late Penumbral	+1 EV
+0:00	U1 Partial begins	nc
+0:23	30% obscured partial	+1 EV	f/4 1/500 @100	"
+0:34	50% obscured partial	+1 EV	f/4 1/250 @100	check
+0:50	80% obscured partial	+1 EV	f/4 1/125 @100	"
+0:54	90% obscured partial	+1 EV	f/4 1/60 @100	"
+1:02	95% obscured partial	+1 EV	f/4 1/30 @100	"
+1:07	U2 starting totality	+2 EV	f/2.8 1/30 @200	"
+1:10	U2++ early totality	+2	f/2.8 1/8 @200	check
+1:25	Totality	+3 EV	f/2.8 1/2 @400	check
+1:40	Totality	as needed	f/2.8 1/2 @400	check
+1:55	Totality	as needed	f/2.8 1/2 @400	check
+2:08	U3-- late totality	-3 EV	f/2.8 1/8 @200	"
+2:09	U3 ending totality	-2	f/2.8 1/15 @100	"
+2:16	95% obscured partial	-2 EV	f/4 1/30 @100	"
+2:21	90% obscured partial	-1 EV	f/4 1/60 @100	"
+2:28	80% obscured partial	-1 EV	f/4 1/120 @100	check
+2:41	50% obscured partial	-1 EV	f/4 1/250 @100	"
+2:50	30% obscured partial	-1 EV	f/4 1/500 @100	"
+3:17	U4 Partial ends	nc
+3:18	Penumbral	-1 EV	f/4 1/1000 @100	"
+4:15	P4	nc
+4:18	No Eclipse	-1 EV	f/5.6 1/100 @100	"

Here is a more complex plan for the lunar eclipse of January 20-21, 2019 for my f/15 Questar telescope. It takes advantage of HDR burst modes on the camera to capture a 9 stop range of exposure during the rapidly changing conditions at the beginning and end of totality. Maximum exposure in the bursts is limited to 30". Exposure will be corrected in stacked images in post processing. The large shifts in exposure times at those times are because of the 4 stop offset in the minimum or maximum exposure when switching to and from the 9 exposure HDR mode in my Sony a6300 camera.