Long exposure is as old as photography itself and it serves both technical and artistic purposes.
Image stacking, on the other hand, is a technique used in digital photography and has the very technical scope of improving the digital signal-to-noise ratio.
What Is A Long Exposure?
Typically, handheld photography is the realm of short exposures. A simple rule of thumb to avoid camera shaking is to use a shutter speed that is the inverse of the focal length of your lens.
With long exposures you are using very slow shutter speeds, ranging anywhere from several seconds to several tens of minutes.
What Is The Purpose Of Taking Long Exposures?
The primary reason for taking long exposures is to properly expose a dark scene.
But long exposures can also be used to introduce artistic effects for crafting more compelling images, like motion blur, intentional camera movement, and light trails.
Long exposures also have the effect of better saturating the colors directly in-camera.
The Exposure Triangle
Shutter speed is one of the three pillars of photography, together with lens aperture and ISO. Together, these settings form the Exposure Triangle and work together to define the exposure for the scene.
If you are stuck in a dark environment and your shutter speed is too slow, to use a faster shutter speed you must compensate for the loss of exposure (light) by using a wider aperture and/or more sensitive film/higher ISO.
What Is Image Stacking?
With image stacking we refer to the idea of averaging a photographic sequence.
Differently than shooting for doing HDR or focus stacking, with image stacking the camera settings and the focus stay the same for the entire image sequence.
What Is The Purpose Of Stacking Photos?
The purpose of doing image stacking is to obtain a single image with a higher signal-to-noise ratio than that of the individual images being stacked.
The result is a cleaner and more detailed image, as details that were “diluted” in the background noise in the single images stand now above it in the stacked image.
The more images you stack, the cleaner your final results are, but the improvement of the signal-to-noise ratio is not linear with the number of images. Instead, it goes as the square root of the number of images you stack.
Can I Stack The Same Image Over And Over To Reduce Its Noise?
No, you cannot.
For stacking to be beneficial, the noise must be random and different across all the images in the sequence. Thermal noise and ISO noise are the types of noise targeted when doing image stacking.
Any “non-random” noise, such as stuck and dead pixels, can’t be reduced with image stacking.
Does Stacking Increase Exposure?
No, image stacking does not increase the exposure.
When you take a photograph, the longer you expose, the longer your sensor records the light from the scene, thus building up a stronger signal.
Stacking, on the contrary, does not create more signal than what is already in the single image: all it does is improve the signal-to-noise ratio by phasing out the random noise in each image.
Long Exposure Vs Image Stacking: Which Is Best?
There is no best, as these techniques serve different purposes. In everyday photography, long exposures are more commonly used.
For night time photography and astrophotography in particular, image stacking is an absolute must to reduce noise while revealing better details in your images.
Unfortunately, the benefits of image stacking are not something you can appreciate directly in the field, as the magic is done during post-processing. Should you get bored while in the field, shoot some backstage long exposures to pass your time.
Image Stacking: Pros And Cons
As with all techniques, image stacking has some pros and cons, but if you are into astrophotography, your best bet is learning this technique, however technical and time consuming it may sound to you.
- You can create some artistic effects
- It is one of the best ways to improve the signal to noise ratio in digital images
- Is the best way of doing astrophotography
- It is quite a technical technique
- Can require dedicated software
- You need to have a tripod
- What you see in camera is not what you get
Long Exposures: Pros And Cons
Long exposures seem like a no-brainer at first, but successful long exposure photography is challenging.
Daylight long exposures require the use of strong neutral density filters, while with high contrast scenes there is the risk of clipping the highlights to pure white and the length of the exposure must be appropriate to (i) properly expose the scene and (ii) create a pleasant effect.
- You can properly expose dark scenes
- You can create artistic effects
- You could clip the highlights and introduce noise;
- Increased chance for vibrations, wind, flares, etc. to ruin your long exposure
- You need to have a tripod
- You may need filters like neutral density (ND) and graduated neutral density (GND) filters
Image Stacking Or Long Exposures For Astrophotography?
With today’s equipment, image stacking is the best way to do astrophotography. And not just deep-sky astrophotography.
Culling your sequence before stacking, thus throwing away the worst images, is a good habit you should develop to ensure you get the best results possible with your data.
Stacking software can automatically reject images not fulfilling particular quality criteria, but it is always better to remove firsthand the very bad images.
After all, garbage in, garbage out.
Star Trails Photography
In star trail photography you want to show the path of the stars across the night sky, like their circular motion around the northern celestial pole, near Polaris.
Typically, a star trail requires exposing the sky for no less than one or two hours, depending on the focal length of your lens: the shorter the focal length, the more time you need to record long enough trails.
While you could do it in a single shot, thermal and digital noise will likely become a problem. A far better strategy is to stack together multiple long exposures a few minutes long.
And with this relatively short exposure time (and low ISO), you will avoid clipping all the stars to pure white, thus retaining their natural star colors.
Starry Landscapes Photography
In starry landscapes photography, the landscape has the same importance as the sky. A majestic view of the Milky Way arching over a dark foreground with no details is not very appealing.
More often than not, the landscape and the sky are photographed separately and then re-composed together with post-processing.
Sometimes, the Moon or light pollution can light the landscape just enough that you could photograph the sky and the foreground together.
In either case, stacking does help reduce the noise, particularly in the dark areas of the landscape and in the sky, while increasing the amount of visible details.
Moon And Planetary Photography
For Moon photography and planetary photography, you need very short exposures of a few tens of a second long or faster.
Because of the high image magnification involved, the atmosphere will create all sorts of optical effects, like refraction, color dispersion, and wobbling: using such short exposure will freeze the image to retain as crisp details as possible.
And you shoot hundreds and thousands of those short exposures, only to choose the top 10-15% to stack. This is called lucky imaging.
For deep-sky astrophotography you will not use a very short focal length: the more you close in on the night sky, the more evident the motion of stars becomes. As a result, stars will trail on your image.
The solution is using a star tracker (or other equatorial mounts) aligned to the celestial pole. This will rotate the camera in sync with the stars, preventing them from trailing in your images.
And even if in a perfect world this would let you take exposures as long as you want, tracking errors will limit you to take exposures from 30s to 10 minutes or more, depending on your gear.
The key is to shoot single exposures that are long enough, take lots of them and stack the best ones to reduce the background noise, thus revealing the faintest details.
Determining the proper exposure for deep-sky can be a very technical task and mostly depends on your gear and the sky background brightness, but also on how faint or contrasted the target you are photographing is.
With a DSLR, a rule of thumb for the exposure is the following:
- Use the widest usable aperture for your lens that gives a good image quality
- Set the ISO to the lowest value in the range where your camera is ISO invariant
- Set your shutter speed so that the histogram peaks at ⅓ from the left edge and that its tail is not clipped on the left side
You can find a more complete discussion in our guide about camera settings for astrophotography.
How To Photograph For Image Stacking?
To shoot for image stacking there is not much you have to do.
- Place your camera on a tripod or on your mount if you are doing planetary or deep-sky astrophotography
- Set your camera with the proper settings for the target you are photographing and the equipment you are using
- Use an intervalometer or a computer to control your camera
- Start taking photographs and remember, the more images you get, the better
That’s all there is in shooting for image stacking. The real magic will be done later at home, with the editing.
Exposure Stacking Software
To some extent, image stacking can be done in Photoshop, but for astrophotography this is better done with specific software.
Stacking software usually allows you to calibrate and align your images before actually stacking them.
To know more about what type of software you should use to stack your images, have a look at our guide to astrophotography stacking software.
Long exposures and image stacking are different techniques that serve different purposes.
If you are doing astrophotography, you will need to combine the needs for collecting enough signal from the sky with long enough exposure and the needs to reduce the captured noise and boost details with image stacking.