(Editor’s note: Photofocus author Steve Inglima premieres his Tech Corner column by explaining how to choose the right focal length lens to photograph this month’s eclipse along with a great set of tips on how to do it. Read Steve’s story of his first total eclipse of the sun experience in July of 1991.)
Focal length by the numbers
I’ve been asked what focal length or lengths would be appropriate for imaging the upcoming eclipse. In part, it depends on the camera and format size. Most of us will be shooting on a digital sensor rather than film, and there are at least 3 major sensor sizes commonly used. It all has to do with how big will the sun be on the sensor.
What is the image size on the sensor by focal length? Doing the math:
The focal length of the lens times the distance from the earth to the sun divided by the diameter of the sun equals the size of the sun will be on the sensor.
The sun is about 885,000 miles in diameter. It is close to 93 million miles from earth.
Both of these have to be converted into millimeters first.
Hint: 1 mile equals 1,609,344.01 millimeters.
Whew. This one requires a calculator with tons of places because these are really big numbers.
An easier way
To make the math easier, we’re going to create a divisor for the focal length.
So, the distance from earth divided by the size of the sun is 107. That becomes our factor to figure image size.
We simply divide the focal length by this number to yield the image size on the sensor. I’ve saved you the calculations. Here are the image sizes for common focal lengths.
The focal length of the lens divided by 107 equals the size of the sun on the sensor.
100mm divided by 107 equals the sun being .93mm in diameter on the sensor.
200mm divided by 107 equals the sun being 1.8mm in diameter on the sensor.
400mm divided by 107 equals the sun being 3.74mm in diameter on the sensor.
500mm divided by 107 equals the sun being 4.67mm in diameter on the sensor.
600mm divided by 107 equals the sun being 5.6mm in diameter on the sensor.
800mm divided by 107 equals the sun being 7.5mm in diameter on the sensor.
1000mm divided by 107 equals the sun being 9.3mm in diameter on the sensor.
1200mm divided by 107 equals the sun being 11.2mm in diameter on the sensor.
2000mm divided by 107 equals the sun being 18.7mm in diameter on the sensor.
Thus, trying to achieve a 24mm image size of the sun’s circle to fill a “full frame” 24x36mm camera would require a lens over 2,000mm! Of course, that might not be possible and most probably not necessary. Even if you stack teleconverters, etc, you will want to leave a little room for any wisps of light that exceed the sun’s diameter, which will be the corona (the outer edge of the sun’s light) and the flare of Bailey’s Beads. Also, if you’re using a camera with a “crop sensor”, like a Canon 7D with a 22.2 mm × 14.8 mm sensor, or a Nikon 15.5×23.5mm sensor, unless you were trying NOT to image the most interesting part of an eclipse (which is the periphery of the junction of the sun and moon), the last thing you’d want is a 24mm image of the sun’s circle!
The math predicts that the 500mm PS lens I used on my Bronica SQ-A during the 1991 eclipse would yield a 4.67mm sun circle image on film (rather small on a 55.1mm square image area to fill). This would be, BTW, regardless of the film back that I used on that camera (6×4.5cm, or 35mm back). It’s about the focal length of the lens that magnifies the image. I took a rough measurement of the original transparency (a.k.a. a slide) shown here, and sure enough, it’s around 5mm. So yea math! Here’s a chart showing the relative sizes of the sun per focal length for full frame and the APS-C cropped sensor. Nikon’s sensor is slightly larger and not enough larger to affect the chart. I would recommend a lens, or a combination of lens and teleconverter to achieve a focal length of at least 500mm. Longer is better as the chart clearly shows. If your sensor is over 20 megapixels, you can relax a little in that even at 50% of that image magnification, you’ll have a really usable image size as I did in 1991 with a 5mm image on a 55mm wide piece of film. The image above is cropped.
Here’s a chart showing the relative sizes of the sun per focal length for full frame and the APS-C cropped sensor. Nikon’s sensor is slightly larger and not enough larger to affect the chart. I would recommend a lens, or a combination of lens and teleconverter to achieve a focal length of at least 500mm. Longer is better as the chart clearly shows. If your sensor is over 20 megapixels, you can relax a little in that even at 50% of that image magnification, you’ll have a really usable image size.
Points to consider
- If you’re using a sheet of solar material, you’ll be cutting and mounting this yourself. Carefully mount your solar cut filter onto either a screw-in ring or a compendium shade, which will in turn be directly attached onto your lens. If you want to try and glue/tape the filter onto something which in turns screws into the filter threads, be REALLY sure that this will be secure for the entire time you’re going to need to filter the sun’s rays. You DON’T want this thing falling off while working the camera, and especially if you’re looking through the viewfinder. It’s yet another reason to consider “live-view” operation as you’re only looking at the LCD of the camera, not into the sun directly. If you’re using a ready made screw in Neutral Density filter, the mechanics are done for you.
- Practice attaching and removing this device from your lens well before the eclipse event. You should consider practicing indoors. You will be setting the camera on a tripod to photograph the ecliupse, so mount the camera on the tripod you’ll be using, point the camera/lens assembly upwards at the ceiling, and rehearse removing and attaching the filter assembly by feel. Keep in mind that if you’re lucky to experience totality, it’ll be fairly dark during that time, and you don’t have a lot to time to fumble in the dark. The maximum time of totality of this coming eclipse is 2 minutes, 44 seconds (in a part of Illinois near Carbondale). I and my viewing mates (including Kevin Ames, Eddie Tapp, Judy Host, my fianceé Debbie, other friends and family will see 2 minutes, 37 seconds from my deck in North Carolina!
- You might be interested in photographing the entire sequence, and making a composite image with multiple photos of the progress to appear on one frame. That means you’ll be photographing the sun and the moon’s progress over the entire time of the entire event, from the first visible part of occlusion through totality and back again. Keep in mind that to our point of view, the sun travels it’s own diameter distance in the sky every 2 minutes or so. You will need to plan to set your array to include the entire scene, and time the minimum images to space them in the desired sequence on the image (or face creating a composite in post).
- The reason you’ll need to be practiced at removing and re-attaching the filter is that at the point when the sun is completely obscured (if your location is IN total eclipse), you are essentially photographing at night. You’ll have to take the filter off the lens to image the corona of the sun and Bailey’s Beads. And, you will need to get the thing back on the lens a few seconds after the first appearance of the sun, which will look like a diamond ring…this is really important. The sun will get really bright in a few seconds after totality and the “diamond ring.”
- During totality, the interesting stuff is flaring around the circle of the sun/moon. But there are HUGE variations between the inner and outer corona in terms of exposure. That’s why you must simply bracket like mad. There are perhaps 10-12 stops of variation on exposure requirements on all of the interesting stuff. If you choose an ƒ stop of 8, let’s say, and an ISO of 200 (pretty standard and a likely native ISO to many sensors), you’ll have a variation of anywhere from 2 seconds to 1/8000 of a second to image everything from Bailey’s Beads to the outer corona. The diamond ring might be somewhere around 1/125, but…..it changes every millisecond at that point. Either bracket manually like mad, or set your camera to do so automatically. You might have only a couple of minutes, but the cusp of the beginning and the end of totality are the most enchanting parts…and you will need to bracket quickly.
- While there are motor driven mounts that will track the path of the sun, you will not really needs this…but you will note that the sun will be changing positions as the Earth rotates, so you want to have an easy-to-use tripod and head so as to quickly make the adjustments when needed. The longer your focal length, the more you’ll have to move to keep the sun in frame.
- Also, since the Earth is a moving body, and so is the moon, you are going to want to have easy and free movement when needed with your tripod head to re-compose when your subject moves out of your locked down frame. Consider a solid and well made ball head.
- If the eclipse photography is going to be really important to you, the usual scenarios apply to mission critical photography. Back everything up, have lots of charged batteries, memory cards, and even a spare body and lens if possible. Make sure your sun filter is ready to go. If you’re attaching a piece of cut filter, absolutely insure the mount you’re making is not going to fall off or apart in the middle of sun viewing. Yup this is so important, I’m repeating myself.
- The other thing to consider is likely viewing locations and weather. The ultimate location for longest totality is meaningless if it’s covered in clouds. If the opportunity exists to travel to a more sure bet in terms of weather, you might consider it if practical. I will elect to roll the dice to be on my own deck for convenience, as it’s just too perfect not to celebrate there!
Finally, I would also suggest that one of your friends assume the task of setting up some kind of video capture, as in good measure, reliving this event will feature the sounds of all experiencing it, not just the photos of the corona.
Enjoy the magic!
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