Monday, September 1, 2014

How to photograph the aurora: Part 2

I just saw this page shared with a beginner to help them photograph the aurora. It's typical of the advice given to amateurs, and (in my opinion) typically bad and/or irrelevant. Here's why:

Location is a key factor when preparing to photograph the northern lights. A clear shot of northern and eastern skies is helpful.

Right from the start they're advising you to do something boring. If you go somewhere that's clear and empty all the way to the north so you just see the aurora above a distant horizon, your picture will be boring.

As activity increases it normally starts from the east as Earth rotates into the aurora.

...No. The auroral surges come from the east pre-midnight because that's the direction from which the magnetic field lines are convecting. Post-midnight the auroral surges come from the west. Earth's rotation had nothing to do with this.

there are lots of other [places to photograph the aurora], just look for cars parked along the roads

And you will spend your whole night fighting the headlights and flashlights of all those people you stopped next to. And for what? The aurora looks the same a mile down the road as it does right there.

How about a tripod? If you've got one, dust it off and figure out its use before you're out in the cold, where the plastic becomes brittle and has a tendency to break in extreme temperatures.

Don't get a plastic tripod. They wobble. The whole point of a tripod is not to wobble.

Camera batteries in the cold lose power real quick ... At 40 below zero, a camera battery lasts around 20 minutes.

*Shrug* I've never needed to swap my batteries during a shoot. If I'm going to leave the camera running hands-off for a while I pull a neck gaiter over the camera, with the lens poking out one end, to keep the wind off, and my standard battery will last a couple hours at 40 below. I don't suppose it's bad advice, but it's not that big of a worry.

Keep a Ziploc bag handy for storing your camera

Take the memory card out of the camera before you go back inside, put the camera in some kind of a bag you can close, and don't open it till morning. Your backpack or regular camera bag is fine; it doesn't need to be waterproof. If you forget to take the memory card out before you go inside, just open the bag, take it out, and reclose it. Your camera will be fine.

Make sure your in-camera storage media card is clear of any other photos. Long exposures require a lot of room.

I... what? It takes the same amount of memory to store a pixel of the same value, regardless of how long it took to capture that pixel. I find aurora photos often require less storage room than daylight photos.

Lens filters can also produce what's known as a ghosting reflection between filter and front glass of the lens.

Not unless you're looking at something really bright and localized, like a street light or the moon. The aurora will not do this. The filter will, however, work as an interferometer and leave diffraction rings in your photo. This happens anytime you use the filter, but it's only noticeable when looking at a monochromatic light source like the aurora. So yeah, take your filters off... that's just not the reason why.

Rotating a lens's manual focus clockwise will set it to infinity. A good rule of thumb is to rotate the lens to infinity and then back it off just a hair. 

Which direction you rotate the lens to focus to infinity is absolutely dependent on the lens. Nikon brand lenses usually go the opposite direction of Canon brand lenses, and I use lenses on my camera that go both ways. And regardless, this is not how you should set your focus. Find the brightest far away point source, which will be the Moon if it's up, a distant street light if one's visible, or the brightest star otherwise. Point at it, switch to live view, zoom all the way in (digital/live view screen zoom, not lens zoom), and focus. You can use the chromatic aberration to set the focus: When you pass through focus the fringes change from purple to green.

Set your lens aperture to the lowest number.

Probably, but maybe not. Fast lenses usually have a lot of aberration when used wide open, and it's a lot more visible on point sources like stars than it is in daylight pictures. You're going to have to decide for yourself where you want to set the compromise between speed and sharpness. I usually shoot my f/1.8 lenses at f/2, and if the aurora is bright enough to get away with it I'll go to f/2.8.

Some cameras also have a long exposure noise reduction function. If yours has it, turn it on.

I would never use this. The camera takes a dark frame after each exposure and subtracts it from the light image. You can do this yourself in Photoshop or whatever, without wasting the time to take a new dark frame every single exposure. The aurora isn't going to wait for that.

On DSL cameras, mirror opening and closing creates most vibrations in long exposures. There are other options, though. Most cameras are equipped with a self-timer. Set it to a short time and push the shutter release to minimize camera movement. If "mirror lock up" is an option, use it.

If mirror slap is seriously causing vibrations in your aurora photos, your tripod is so worthless you might as well be balancing the camera on a rock. This is a consideration for long focal length astrophotography, not wide angle aurora work.

If the preview shows up black, do not, repeat, DO NOT, delete. Your camera will capture more than meets the eye. Due to long exposures, cameras record a lot more than the eye can process, and more than likely you won't be able to see it on the camera screen.

This is all kinds of What. A long exposure is not going to be black on your preview screen by virtue of the fact that it's a long exposure: The value of each pixel is integrated over the exposure, and the preview shows the final image, which is the total light in that pixel. If it's black on your preview screen, it's not going to get any less black on your computer. If it's merely dark on the preview, you may be okay depending on how noisy it is. And you should look at the histogram and not rely totally on the preview image: The preview image is a processed .jpg, if you're shooting RAW (and you should be) the histogram is a better idea of what you have and haven't captured. The RAW file will hold low-level stuff that doesn't show up on the preview, but this is because the preview can only show 8 bits while the RAW captures 12 or 14; it has nothing to do with the fact that it's a long exposure.

Once you've found a good exposure setting, avoid looking at the preview.

So how do you know when you need to change the exposure?

Don't breathe on or around the camera.

Okay, I will stop breathing. :/ Just direct your breath away from the optics, okay?

1 comment:

  1. Hilarious! I can't wait until I actually get an opportunity to photograph it.