How can I do astrophotography with my 70-300mm lens?

I got a Nikon 70-300mm lens as it was on discount. I want to know if I can do astrophotography with it? I want to maximize the use of the lens. I wanted to get a 34-300 mm lens, but it was expensive.

I own an N90. How can I do astrophotography or tele with my 70-300 mm lens? What are the lens' capabilities and limitations?

Answer

To do astrophotography with a 70-300mm lens using a film camera is a challenge, but is possible. One of the limiting factors is the speed of the film you use. In general you need a very fast speed film to do astrophotography, especially with longer focal lengths such as 300mm. This is because as the Earth rotates on its axis, the stars appear to move across the sky and they do so much faster than most people think. Here are some tips on how to do astrophotography with your N90 and 70-300mm lens.

• Stabilize the camera/lens You will not be able to take photos of any night sky objects, other than the moon, at shutter speeds that will allow you to handhold your camera. A sturdy tripod that won't blow around in the wind is the minimum requirement.


• Use the fastest film you can find Something like Ilford Delta-3200 B&W film. It has an actual sensitivity of around ISO 1250 to ISO 1600, but can be 'push processed' to ISO 3200 and beyond. With an f/4-5.6 lens, 'beyond' is what you are looking at. When you have the film processed, be sure to tell the lab to 'push' the speed to around ISO 6400.


• Use your lens' maximum (widest) aperture This will be the aperture setting with the lowest number. AT 70mm your lens probably has a maximum aperture of f/4 and at 300mm a maximum aperture of f/5.6.


• Focus manually to 'infinity' You may have to do a little bit of preparation before it gets dark. Point your camera at an extremely distant object and focus on it. Make a note of the position on the lens' focus scale. You can also use the moon if it is above the horizon. Don't assume the infinity mark on your lens is absolutely accurate. It is probably close, but focusing on stars is critical. You should be able to verify correct focus in your viewfinder by looking at one of the brightest stars in the sky through your viewfinder. Fine tune if necessary, but if you get too far out of focus the stars will disappear as the light from them is spread out over too large an area.


• Limit your shutter speed If your camera is on a stationary mount, divide 600 by the focal length you are using. That is the longest shutter speed you can use that will show the stars as points on an 8x10 print. Anything longer will cause them to show up as trails. For 70mm, that figures to 8 seconds or so. At 300mm, it goes down to 2 seconds. You can double the shutter speed for a 4x6 print (divide 300 by the focal length). Even with very fast film, you're not going to capture many stars at f/5.6 and 2 seconds. Be sure to point your lens at areas that have several bright stars, such as Orion, Canis Major, or Gemini.


• Consider a tracking mount This will allow you to use longer shutter speeds than covered above and will allow you to capture much dimmer stars in your photos. Many telescopes have mounts that, when lined up with the Earth's axis, will track along the path stars take across the sky. Adding a motor that moves the mount very steadily at the same rate as the Earth rotates allows the telescope to stay pointed at the same spot in the sky's constellations. Many photographers will modify their telescope to allow them to attach their camera and let it 'piggyback' on the telescope. If the mount is made for a smaller, lighter telescope then the mount needs to be adapted to hold the camera instead of the telescope.


• Use the darkest skies you can find Find a spot away from heavy light pollution to take your photos of the night sky. Also, unless the moon itself is your target plan your sessions at times before the moon rises or after it has set. The darker the sky the better your results will be.

Photographing the moon is entirely different than the stars. You can use ISO 200 film, an aperture of f/8, and a shutter speed of around 1/250th second when the moon is more than about 15-20° above the horizon. You'll still need a tripod for good results at that shutter speed and 300mm.