Friday, August 31, 2012

Tip: Time Exposures and Night Photography

This 4 minute exposure (with LED light painting on tree and silo) was taken with long exposure NR.


     
In recent months I have been doing a fair bit of night photography, from late evening-twilight city shots in Vancouver, to aurora borealis (northern lights) under really dark skies in the Fraser Canyon, to an attempt at shooting the Perseid meteor shower east of Abbotsford in the Fraser Valley a few weeks back. In all cases, I was shooting on a solid tripod and was doing exposures ranging from 5 seconds to 10 minutes in duration, at a wide range of ISO settings. In this article, I will give you some pointers for successful nighttime shooting of city shots and star trails...

Long Exposures and Hot Pixels

For night shots, having a good camera and lenses are quite important for the reasons you will see. A cheap P&S probably won't give you results that you'll be happy with since they start getting noisy very quickly with exposures of a second or more. Even many Micro-4/3 cameras will struggle with time exposures and you will definitely want to activate long exposure noise-reduction (NR) if the camera has the option. What is long-exposure NR and why do you need it? If you shoot raw, does it have an effect? Read on...

When you do a time-exposure, even the best cameras will start looking noisy once you start getting into minute-plus exposure times. You will see lots of colour pixels randomly scattered throughout the shot, or sometimes white pixels too, and these are generally called “hot-pixels.” In fact, the temperature of the camera, or specifically the camera's sensor, can indeed have an impact on this sort of noise, so the hotter the temperature, the more hot-pixels you may have. This is why astrophotographers use specialized digital cameras that have their sensors cooled, in extreme cases cooled with liquid nitrogen or sometimes even liquid helium (much colder), for their hours long exposures of the night sky.

Depending on the raw converter you use, some of these specks might get cleaned up automatically but at other times, you might see a flurry of hot-pixels that can really ruin the shot. Some raw converters have specialized noise-reduction algorithms that can often identify and clean up hot-pixels for you and for example, both Phase One's Capture One v6 (on sale and mentioned elsewhere in this newsletter) and the very latest version of Nikon's Capture NX 2 software have specialized NR features useful for astrophotography. Notably, Adobe Camera Raw and Lightroom are actually fairly bad at removing hot-pixels for you these days and my recollection was that they used to be better at this, sometime in past versions.

So if your workflow is tied to Adobe's products, what can you do? Well the best thing is to enable your camera's long-exposure NR feature. Let's say you are doing a 2 minute exposure and have this feature activated. What will happen at the end of the two minutes, when your camera's shutter closes again, is that the camera will make another exposure of exactly the same duration with the shutter closed, capturing a so-called “dark-frame”. A dark-frame should be exactly that, totally black when no light is allowed to hit the sensor, however in practice, a dark-frame may show a fair bit of noise, depending on the duration of the exposure. Luckily, the behaviour of sensors is quite predictable and repeatable so if certain pixels are “hot” in the original exposure, generally those same pixels will also be hot on the dark frame. What the camera then does, is mathematically subtract the dark frame from the original exposure pixel by pixel. Surprisingly enough, this in itself is actually quite fast and doesn't add much processing time, a few seconds at the most. If a dark-frame pixel is black (so has RGB values at zero), well then subtracting those numbers from the original exposure will have no effect, but let's say a dark-frame pixel has a red value of 100, so a bright red hot pixel. Well, the original exposure will then also have a pixel in that exact same position having a red value that is numerically 100 too large, for example at 130, so subtracting that number yields the correct value of 30 for that pixel. There are other subtleties but that is more or less what happens in the camera. If you are Photoshop savvy, you could actually create a dark-frame exposure yourself, using a lens-cap for example, and then layer the images and use the “difference” blending mode, however in practice that technique usually requires additional tweaking and adjustment and never seems to look as good as letting the camera do it for you. So if you have the time to wait for an in-camera dark frame, then by all means turn on long exposure NR and let the camera do it. It will save you trouble later!

Lenses at Night

With a good lens, really bright streetlights will not cause any flare or ghosting.


       
Next, let's talk about lenses and filters. Well actually, when it comes to filters, I would suggest not using any whatsoever for night shots, not even clear protective filters, no matter how high the quality. Even the best filters will bounce back light from a bright streetlight glaring off the lens' front element and with night shots of cityscapes, you will encounter the highest dynamic range of anything you'll ever shoot. If there is an internal reflection of a streetlight that happens to appear in a black part of the frame, the flare will build up and become quite visible, especially if you do any aggressive post processing to try and bring out shadow detail.

When it comes to lenses, a modern lens with the best possible anti-reflection coatings will help, so for Nikon their so-called Nano-Crystal coatings and for Canon, their SWC coatings. These coatings will help keep high contrast in night shots and will resist flare and internal reflection ghost images. Generally I would say that prime lenses (fixed focal length) are preferable to zooms since primes will generally have fewer lens elements and often (but not always) outperform zooms of similar focal lengths when it comes to flare and contrast in extreme situations.

Also, when you are working on a tripod and have the freedom to use long shutter speeds, then stop the lens down in the range of f/8 – f/11 for the best results. For night shots, aberrations and corner softness are often more apparent due to high contrast city lights and buildings at the edge of your frame and you will want to make sure that you are getting the best lens performance possible. Ultimately, it might take a bit of experimentation to see which of your lenses perform best at night and what their aperture sweet spots are. My suggestions are merely good starting points and are certainly not written in stone!

Also, if you are shooting northern lights or meteors, then quite possibly the light gathering ability of your lens and camera are more important than ultimate corner sharpness, so shooting at much wider open apertures to let in more light, and shooting at a much higher ISO will increase your chances of capturing fleeting events like meteors or the flicker and dance of northern lights. Once again, experience with your own gear will tell you what you can get away with as far as shooting your lenses at wide f-stops and your camera at higher ISO settings.

Exposure and Dynamic Range

The foreground greenery was very nearly black before substantial shadow fill in Lightroom v4, and the sky was a lot darker too.
       
When it comes to star trails and aurora, dynamic range is generally not much of an issue, but for nighttime cityscapes it certainly can be, especially if you are trying for a pseudo HDR look, with photos that have both good shadow detail and highlights that are not badly blown out. Also, you will benefit from shooting raw and post-processing your shots with a good raw converter like Lightroom v4 or Phase One's Capture One Pro v6. Lightroom, in particular the latest v4 which is far better than previous versions, has some superb highlight recovery and shadow fill capabilities and if you are shooting with a modern DSLR that shoots cleanly in low light, it is amazing what dynamic range can be wrung from a single, optimally exposed raw file. Some people like using multiple bracketed shots and creating HDR images (high dynamic range) but personally I am not a fan and prefer using a single optimal exposure and post processing that one shot.

This next bit is something you'll also have to experiment with, but I find that in practice, some modern cameras can have tremendous amounts of recoverable shadow detail, far more than you might expect, so exposing to limit highlight blow-out to only the brightest streetlights, or most brightly lit parts of a shot, will then still allow a lot of shadow detail to be recovered, even in parts of a shot that look almost black on the back of the camera. Make sure that you have your camera's blown-highlight warning activated, which will blink alternating black and white in the highlights where all detail has been lost. This will allow you to choose an optimal exposure and limit blow-out to unimportant parts of the frame.

Some images will start getting noisy with substantial shadow recovery, so this is where the new adjustment brushes in Lightroom v4 come in handy. You can actually paint in more noise reduction in shadows if they are too noisy, or if the shadows have a lot of fine texture detail, you can actually brush in less noise-reduction to bring out the detail that may have been lost to overly aggressive noise-reduction. Dynamic range difficulties brings us to the next section...

Best Times to Shoot
        
Shooting at twilight when there is still some ambient light helps to tame a high dynamic range situation.
       
For me, the most attractive city shots are near the end of twilight, when there is still some blue-sky, or sunset colour, but where many city lights have already started to switch on. Alternatively, dawn is a very good time as well, and during certain times of the year, you can get fog or other atmospheric effects that add drama to the shot and don't occur at sunset.

During twilight, you will have a much easier time seeing into the shadows or bringing out the blue sky in shots. The dynamic range will generally be less extreme than in city shots taken when it is completely dark. Also, if you shoot with the sunset (or sunrise) at your back, you may get some beautiful warm glow reflections in building windows that contrast with the cool blue sky.

When it comes to night sky shots, the best times are well after sunset, about two and a half hours after, if you want to capture star trails or faint aurora. It is also good to avoid the moon for sky shots as it is bright enough to mask all but the brightest stars for most parts of the sky. On the other hand, moonlit landscape shots can be eerie and beautiful too, looking very unusual and stark, but with surprisingly vivid colour on long time exposures.

Also, for long time exposures, be sure to bring fully charged batteries since shooting on bulb for long durations will eat a lot of power.

When to use High ISO

Shooting at ISO 6400 with only a 20 second exposure allowed for more or less pinpoint stars and well defined auroral spikes.
        
Whenever possible try to use the lowest ISO that will let you get the shot. This will definitely maximize your image quality and keep in mind that long-exposure NR does nothing to help with random noise you get with shorter, high ISO shots. On the other hand, if you are trying to capture something that is moving quickly and/or is very faint, then boosting the ISO might be your only option. When I was photographing the aurora recently, there were many slowly moving vertical bars of light. They were moving quickly enough that shooting at a low ISO and exposing for several minutes would have rendering the bars totally blurred and invisible, lost in the bright overall glow of the aurora. I ended up boosting my ISO to 6400 and shooting at f/5.6 for only 20 seconds. Not only did this allow me to capture the vertical bars, but with my 24mm f/1.4 wide-angle lens, a 20 second shot still rendered stars as pinpoints and not trails, and I felt this added an immediacy and realism to the shot, despite the added noise. The reason I shot at f/5.6 was to ensure the stars were still pinpoints in the extreme corners and not aberrated blobs, although in hindsight, I could have shot at f/4 at ISO 3200 (maybe even f/3.5) and the stars likely would still have been sharp enough.

Another example of needing a higher ISO was for some shots I took of BC Place, where I had the water of False Creek in the foreground. When I did 8-15 second exposures at ISO 100, the lights being reflected in the water ended up looking quite smeared. I noticed that with shorter exposures, there was some intriguing drawing of the reflected lights going on, with interesting random “squiggle patterns” in the water. These squiggles were blurred out during longer exposures, but by boosting my ISO and getting the exposure down to around 1-2 seconds, it allowed me to capture the reflections in a far more interesting way.

So as far as high ISOs, avoid them unless they are absolutely necessary to capture the scene in the way you envision!

Timer Remotes

Phottix Wireless Timer Remote
       
While a regular cable release that allows you to lock a bulb exposure is all you really need, getting a fancy LCD timer-remote, like the Canon TC-80N3, the Nikon MC-36 or a third party remote from, for example, Phottix or Pixel, will take a lot of tedium from your night photography. The most useful feature these remotes have is the ability to program in a precise bulb-exposure duration. Most cameras limit you to 30 seconds, 60 at the most, and your night shots might often exceed that duration. Being able to program the remote for 2 minutes and 45 seconds, for example, means that you set it, start the exposure and then walk away (or sit down), providing for a much more relaxing evening of shooting!

Another useful feature of these remotes is being able to set a start delay. For example, say you are on a trip and camping in remote area under dark skies. You are tired and want to hit the tent early, however the sky is beautiful, moonless and clear and it would be great to do a time-exposure when it is really dark in a few hours! Well, just set your remote for a two hour start delay, set the bulb exposure for the desired duration and go to sleep. Your camera will start shooting automatically for the exposure you set it for!

Lastly another useful feature of these remotes is the ability to do time-lapse photography, being able to specify an interval and a number of shots. In the hypothetical camping example, you could even set your camera to shoot 10 different 15 minute exposures throughout the night, setting the time interval to 31 minutes. This will allow for the 15 minute exposure, the 15 minute long-exposure NR dark-frame and some extra time for processing and saving to the card. Just make sure you are running with a fully charged battery! Also remember that in cold conditions, battery capacity might be substantially reduced.

Post-Processing “Ugly” Colour
        
The intensely orange and distracting foreground light from the left was toned down with an opposite colour wash in Lightroon 4.

        
One thing I love about nighttime cityscapes is the huge variety of colours from all the different kinds of light sources. Play with your white-balance and green/magenta cast sliders, when post processing your raw to try and find an optimal overall colour balance. However sometimes you might tweak your white balance and most of the image looks great, save for a larger area with an ugly dirty-orange or nauseating-green colour cast. This is where the tools of your raw converter can come in very handy!

Phase One's Capture One is especially adept at allowing you to isolate a specific range of colours and then fine-tune them by changing their hue, saturation and lightness. For example, often times just desaturating the ugly oranges can vastly improve a shot, or tuning the greens towards more of a yellow tone and dialling back their saturation can make a huge difference. I perform these fixes quite often after boosting the overall saturation or vibrance of an image to bring out all the wonderful colour variations in the light sources.

Lightroom's colour adjustments are not quite as sophisticated as those in Capture One, so you might need to take an image into Photoshop after. With Lightroom, I often find that adding a subtle opposite colour wash with the gradient tool or adjustment brush to an area with “ugly” colour can help minimize it, without affecting other similar hues in the image which might not want altered since they look good as they are. For example, an overly yellow area can be corrected by adding a very slight blue-wash using either the gradient tool or adjustment brush in Lightroom.

Final Thoughts
        
Shooting a sharp photo on this bouncy pedestrian bridge meant waiting for a gap in the foot traffic!

       
Anyway, that's more or less it for my nighttime shooting tips. The last thing I want to touch on is stability. Use a good sturdy tripod and make sure it is not too windy or to bouncy where you've set up. Bouncy? Well say you are doing shots of the city from the middle of a bridge, something I've done a lot. You'll be amazed at how much vibration and bounce there is from trucks and buses driving across the bridge, certainly enough to ruin the sharpness of your carefully focused long exposure! Time your shots to coincide with gaps in the traffic or set up closer to a bridge support or near the ends of the bridge for increased stability.

Also, if you are setting a tripod up on spongy ground, be careful if you are standing or walking near it since you could easily cause movement in the tripod as well. Lastly, if you are using a stabilized lens, turn off the VR or IS, even if the owners manual brags the lens' stabilization is “tripod aware!” This tripod-awareness might be fine for shorter durations but for a longer exposure of many seconds or minutes, the stabilizer itself is very likely to cause blurring and image movement, not to mention unnecessary battery drain!

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