Photographing The Milky Way

[NedM]

I've been watching a lot of tutorials and videos on how to capture the milky way and I've noticed a couple things that seem odd to me.

I noticed that people use a very wide aperture like f2.8 or f4 and a high ISO like 1600 and 3200.
One exposure setting was 30secs at f4 with an ISO of 3200. (He was shooting directly at the sky)

I don't understand why anyone would want to use a wide aperture?
Wouldn't you want to use a larger aperture to get everything sharp?

Was he using a small aperture to get as much light as he need so he can take the image at 30 seconds and avoid star trailing?

 

[IronMaskDuval]

I've been watching a lot of tutorials and videos on how to capture the milky way and I've noticed a couple things that seem odd to me. I noticed that people use a very wide aperture like f2.8 or f4 and a high ISO like 1600 and 3200. One exposure setting was 30secs at f4 with an ISO of 3200. (He was shooting directly at the sky) I don't understand why anyone would want to use a wide aperture? Wouldn't you want to use a larger aperture to get everything sharp? Was he using a small aperture to get as much light as he need so he can take the image at 30 seconds and avoid star trailing?

I was looking at this today as well, and at 30 seconds, you're going to get star trailing. He shot at 2.8 1250 for 30 secs and it turned out great. I was going to try the same thing tonight but it's raining

 

[NedM]

I've been watching a lot of tutorials and videos on how to capture the milky way and I've noticed a couple things that seem odd to me. I noticed that people use a very wide aperture like f2.8 or f4 and a high ISO like 1600 and 3200. One exposure setting was 30secs at f4 with an ISO of 3200. (He was shooting directly at the sky) I don't understand why anyone would want to use a wide aperture? Wouldn't you want to use a larger aperture to get everything sharp? Was he using a small aperture to get as much light as he need so he can take the image at 30 seconds and avoid star trailing?

I was looking at this today as well, and at 30 seconds, you're going to get star trailing. He shot at 2.8 1250 for 30 secs and it turned out great. I was going to try the same thing tonight but it's raining

But is there a reason why people use a wider aperture? Is it just to get more light into the camera without having to use a longer shutter speed that would induce trailing?

 

[IronMaskDuval]

That's what he said although he mentioned that 30 secs was a bit excessive

 

[NedM]

That's what he said although he mentioned that 30 secs was a bit excessive

Alright then, I'll have to give it a try out in Joshua Tree next month.

 

[bc_steve]

I don't understand
Was he using a small aperture to get as much light as he need so he can take the image at 30 seconds and avoid star trailing?

Yes. Except you mean wide aperture. Basically you need to do whatever you can to get as much light as possible to your sensor. If you stop down or use a low ISO you're not going to capture much but the brightest stars.

 

[shefjr]

I don't understand why anyone would want to use a wide aperture?

Wouldn't you want to use a larger aperture to get everything sharp?

Was he using a small aperture to get as much light as he need so he can take the image at 30 seconds and avoid star trailing?

1. Use a wide aperture to allow as much light as possible to hit your sensor. This also helps to keep your ISO lower which will give you a less noisy image. It also prevents possible hot spots in the image due to the sensor heating up when long exposures are made.

2. At that distance everything will be sharp because of the depth of field. Basically the depth of field is from infinity and beyond.

3. My guess is that is one reason. Again the other reason would be because the image quality is better with a lower ISO. Many people will take short exposures and the photo stack later to get star trails.

Here are a couple great TPF links that will help you out. They will go into great depth do you understand more clearly than my quick reply.

Sw1tchFX
http://www.thephotoforum.com/forum/articles-interest/276016-shooting-night-pictures-stars-stuff.html

Manaheim

http://www.thephotoforum.com/forum/...naheims-ultimate-guide-night-photography.html

 

[shefjr]

I should note that I wasn't clear with my answers. The gist of what I said is correct but, there are some technical inaccuracies. Forgive me for that. I'm not usually up this late but, I have a newborn baby and am helping my wife.

 

[petrochemist]

Getting focus spot on infinity can be difficult for many lenses. Closing down a stop or two can help a lot if the camera can still manage good enough speeds. Other than that you want, a sturdy tripod, the aperture as wide as possible, limit shutter speed to (600/equivalent focal length in s), use the highest ISO that doesn't give noise problems and shoot from a spot with dark skies.

The 600/EFL is an astrophotography guide similar to the handholding 1/EFL limit. Any longer than this and the movement of the stars is enough to make star trails visible on a typical print. It will need modifying if your doing large prints.

 

[pgriz]

If you're shooting the sky, without any foreground objects, then the need for DOF is non-existent, as you will be focusing on infinity. Then the choice of aperture is related to your lens (the sharpness sweetspot), and desire for the maximum amount of light reaching the sensor (which usually means the widest aperture). Focusing almost always is done manually, and when possible, using a tool such as live-view (assuming you have a bright star or planet to focus on). If you don't have anything bright to use as your focusing target, then using the infinity mark on the lens (if your lens has a distance scale), will also work.

Regarding getting pin-point sharp images: the rotation of the Earth is 15 arc-seconds of angle per second of time. A point of light will need to move at least 3 pixels on the sensor for its movement to be apparent. So doing the math for my camera (T1i) I get the following:
Time needed to move 3 pixels at equator (90 degrees from the pole) and at 45 degrees from the pole:
10mm - 15 sec, 30 sec.
18mm - 10 sec, 20 sec.
24mm - 8 sec, 15 sec.
50mm - 4 sec, 8 sec.
80mm - 2.5 sec, 5 sec.
100mm - 2 sec, 4 sec.

This tells me that if I use my UWA lens at 10mm, and I want to get pin-point sharp stars, I need to expose no longer than 15 seconds when pointing the camera at the celestial equator, and 30 seconds if pointing to a point 45 degrees from the pole. If I am willing to have slight trailing, then I can increase the length of time that I use for a single exposure. My maximum lens aperture is f/3.5, and with ISO at about 800, I can get down to magnitude 7 with a 15 second exposure. Now comes the next issue: skyglow due to pollution and reflection of light from the aerosols in the atmosphere. In the city, your limiting magnitude may be as little as 2-3, although it is usually around 4-5. In the country away from major lights, you may get to 5-6. At dark sky locations, it may be as much as 7. This means that while in theory I could reach magnitude 7 with a 15 second exposure, in actual practice, I may be recording skyglow which washes out the faint star-light. So when photographing the sky, the need to find a dark sky location becomes very important.

Magnification tends to reduce the contribution of the skyglow, so using longer focal lengths can somewhat reduce (but not eliminate) the skyglow contribution to the image.