I was preparing shooting the great total eclipse on 8/21, and learned a lot about shooting with long lenses.
For the eclipse itself, usually there are two types of photos. First is time lapse or multiple exposure using medium focal length, which is able to show the entire process of eclipse from the first contact to the fourth contact (as shown below).
The second type of photos is to use a long lens to shoot the sun disc during totality, during which period you can see the chromosphere, corona, and even the moon surface and stars if you expose for sufficiently long time. But it’s challenging because different parts in the composition needs different exposure value. The corona is still bright and asks for a fast shutter speed. On the other hand, you need long exposure in order to get the moon surface or stars. Therefore, the standard practice for this is bracket exposure, e.g. from 1/1000s to 1s. Generally, the focal length for such a task is at least 400mm (for full frame), possibly better at 800mm.
But then comes a practical problem: it’s often prohibitively expensive to buy such long lenses – the price can easily go above $10,000. Other than shooting for the eclipse, the long lenses are not very useful in the daily life, if you are not a birder or shoot wild animals. It’s not a good deal to buy one just for the eclipse. So I did some research on some more economical ways to shoot far away objects, and had some interesting findings.
First we need to understand who need long lenses. Usually they are two types of people, one is astrophotographers, and the other is birders. Both will have a telescope, but astrophotographers usually use astronomy telescopes, and the birders use spotting scopes. There are methods of hooking up a camera with each kind of the telescopes. It became especially popular to use a spotting scope to replace a heavy-duty telephoto lens among birders since 1990s. And this technique is called digiscoping.
So what are the pros and cons of using a telescope against a telephoto lens? First telescopes usually have relatively large diameter. 130mm is not a very impressive number for telescopes, but it means f/3 for a 400mm lens, which is very high-end in the lens world. Second, telescopes are much cheaper compared with lenses with the same focal length. For example, the 600mm f/4.6 (130mm diameter) telescope mentioned above can be bought for mere $300+, including a tripod and Equatorial mount. But a Canon 600mm f/4 lens asks for $11,000, which is 40x more. (I feel my aperture calculation may have some problems here. Comments welcome.)
But telescopes also have disadvantages. First, compared with lenses, the aperture of telescopes is fixed. Telescopes also don’t have auto focus, which is OK for astrophotography, but not for birders. Second, the telescopes are designed for humans instead of cameras. That’s why it has a small exit pupil. Or in other words, you have to place your eye in a certain small area in order to see anything, otherwise you can only see pure black. Note the diameter of human pupil is only 5mm while the lens diameter is in the order to 50mm. So 1) it’s relatively harder for the camera lens to find a good spot of imaging; 2) there will be severe vignette if a wide angle lens is used. So, usually point and shoots are used for digiscoping, and their zooming feature is used to get rid of the vignette. Or iPhones, which have tiny lenses just like human eyes, are also often used in digiscoping. The third drawback is the portability. Although larger than a regular lens, it’s not too hard to carry a telephoto lens during hiking. But it’s not even imaginable to carry an astronomy telescope while hiking…
As mentioned above, there are two types of telescopes, astronomy telescopes and spotting scopes. What are their differences and which one shall we choose? First, the image from an astronomy telescope is up side down. (precisely, the refractors have up-side-down images, but Newtonians have horizontally flipped images) But the spotting scopes give “usual” images like cameras. Second, astronomy telescopes are often bulky and fragile – they are not designed for moving around anyway, and are more suitable for observation in the backyard. But spotting scopes are designed for the wild, and thus much more portable and durable. To choose one from the two, just ask yourself a simple question, will you spend 80% of the time shooting the moon or the birds? The following will be simple.
For the specific case of myself. I don’t shoot the moon or birds. The lens will be rarely used after the eclipse. So I put price as the first factor when shopping lens. There are some cheap long lens online, but the sample shots are terrible. After some research, I found Leica had some long lenses from 1960s, that are extremely cheap (was $8000 when they were firstly out, but $200 now), and the image quality is OK. One interesting thing you can learn from these historical lenses is, when talking about long lenses, the general impression is they are long but more on the fat side. This is because these modern long lenses are telephoto lenses. They have special telephoto elements inside, which is able to make the length of the lens shorter than the focal length. But back to 1960s, this technique is not especially popular. That’s why the Leica lens is super long, 400mm focal length basically means the lens body is 400mm. There were also no aspheric lens that time. So the purple fringing (chromatic aberration) is actually noticeable. And because it’s so long, the focusing is done by pushing/pulling the lens body instead of rotating. That’s very interesting.
P.S. I didn’t realize the lens can bring so much fun when I first wrote this post. Two months after buying this lens, I think it’s a good deal and very fun to use.