Birds are small. I bet you don’t think about it that much. But, have you ever seen a Cardinal out in the yard, and pulled out your camera to take a picture, only to find that Cardinal’s a small red speck in the image? Yep. I wanted to image birds, so here’s what I did.
Because birds are small, I need a long focal length. This means a big lens. For those of you who don’t know, a longer focal length means more magnification, to put it simply. A 50mm focal length is considered close to what the eye sees. A 300mm is a telephoto. A 500mm is a super-telephoto. Anybody using anything beyond 700mm is considered mad. Ok, kidding. Kind of. (Nikkor just announced their 800mm F/5.6 lens, with autofocus, VR, ED and Fluorite elements, and about everything else imaginable)
Alright, so I can’t afford the longest Nikkor (big shock). So, are there other options? Yep. There are cheaper lenses from other manufacturers, most of them ultra long zooms like the Sigma 150-500mm F/5-6.3 lens, similar to the lens I owned. I found these lenses are not as good. They’re slow. Their autofocus is reported to be slow. They aren’t sharp, unless stopped down. For very bright sunlight with fast shutter speed and tripod, they’re OK.
There are commercially made, long focal length mirror lenses like this one. But these are Catadioptric and Cassegrain designs, so usually have a slow F/stop. These also have central obstructions. More on that in a bit.
So, guess what? I’m finding more imaging applications for my 6″, F/5 Newtonian telescope. Imaging with a telescope? Yep. What most people don’t realize is that a telescope and a camera lens do pretty much the same thing. They bring light in the visual spectrum to focus. One is just optimized to take an eyepiece, the other to attach to a camera body (and usually has autofocus). They both can have the same problems, such as chromatic aberration, coma, field curvature, and others. My 6″ F/5 Newtonian telescope is a 750mm F/5 lens with manual focus. My setup at left. My scope rides a huge video tripod with fluid head. And don’t tell me I can’t shoot 750mm in wind, because some of these photographs were taken on a very windy day.
There is one big difference, though. My telescope uses a mirror, instead of a lens. Which means it needs a secondary mirror placed in front of it to change the placement of the
focal point. This secondary mirror casts a shadow on the main mirror (the primary), called a central obstruction. It’s usually not a huge problem, but out of focus highlights are doughnut shaped, unlike a pure lens system with nice, creamy bokeh. But this only happens on the brightest of out-of-foucus highlights.
However, if I time my shoot right, or frame it correctly, I can minimize to eliminate that effect. Take, for example, my favorite bird photograph of mine to date (below left). Most everything out of focus is pretty smooth. Why? Because it was taken on a cloudy day. Overcast skies spread light well and evenly, so there aren’t exceedingly bright highlights. So, if
I limit my shooting to overcast days, I get some good shots. That’s not to say I can’t shoot on bright, sunny days, like the cardinal above.
Coma is also inherently bad in fast Newts. I found F/5 fine, I can’t tell unless I frame the subject on the very edge.
The fact that my D3100 doesn’t focus at infinity in the Newt also means it can focus close. Very close. It’s closest focus is something like 12 feet from the front of the tube (I haven’t measured). This also means it can’t focus on the bush 35 feet away. It’s a small range. All these photographs are taken under a feeder in the tree, so it’s a bit easier. Enjoy the photos, I’m having fun shooting birds.