Long Exposure - A Beginner's Guide
What does a successful long exposure image look like? It’s exposure time is generally between 30 seconds to 5 minutes, it’s critically sharp, has naturally saturated colour, exhibits zero shadow noise and the subject or environment is dramatically exaggerated by the fourth dimension - time.
Long exposure is a very unforgiving niche of photography with a somewhat steep learning curve. This is partly because in nearly all cases your exposure times will be beyond your cameras ability to meter. Exposure then must be carefully balanced by you, not the camera.
What your eye sees is somewhere around 1/125th of a second at the focal length equivalent of 50mm. We see life at a certain speed but life can be much faster or slower if we change it.
Long Exposure Kit Bag
For long exposure your choice of camera is significantly less important than you might think. That’s said, having a camera that’s ISO range drop down to ISO50 can be a great help in quickly and easily calculating your correct exposure once ND filters are added.
Choice of lens is critical and this piece of the kit is where the major investment should be. Wide angle lenses enable you to dramatically exaggerate time and motion. On a full frame that would be lenses with a focal length of between 16mm and 24mm. On a cropped sensor that would be 10mm to 16mm
A good lens for long exposure will perform critically sharp, reproduce colors accurately, have a low amount of flaring and will have very little to no chromatic aberration. A bad lens for long exposure will be soft at the corners, colors will look more flat and chromatic aberration only becomes worse with the larger the sensor or print size.
NEUTRAL DENSITY FILTERSAfter spending a considerable amount of money on a great piece of glass, a lens that is critically sharp and reproduces colours accurately don’t negate that by putting a poor piece of glass in front of it! Actually this goes for any quality lens in any genre of photography. IF you are going to use filters ensure that the quality of the filter, as a bare minimum, matches the quality of the lens.
Neutral Density (ND) filters allow photographers to predictably control the environment by slowing down exposure time, as measured in stops.
Whilst different manufacturers refer to the density in different way from a practical point of view number of stops is most useful. ND filters come in different strengths. Most commonly 3-stop, 6-stop, 10-stop and 15-stop.
The 3-stop is undoubtedly the most versatile. When combined with a circular polariser, low ISO and high F numbers you can get dramatic results immediately, especially during sunrise or sunset.
ND 3-Stop - By Graham Clark
A 6-stop nearly doubles or triples the exposure time of what a 3-stop would, with exposure times typically ranging between 2-5 minutes at sunset
ND 6-Stop - By Graham Clark
TRIPODStability and image sharpness are intrinsically linked, and for that reason a tripod is one of the five essential tools. Tripods can vary hugely in price with the criteria being weight, strength, size. What ever you go for…..you will need a tripod.
SHUTTER RELEASE CABLECameras meter up to 30 seconds, and since we’re going way past that a shutter release cable is needed.With many long exposures it is the critically sharp subjects juxtaposed against movement that can create a compelling image. Even the slightest hand movement touching the camera while exposing can significantly soften the image.
How To Obtain A Long Exposure
Basically we want to let less light in to increase the time taken to expose correctly.
Increase the F-stop - reduces the amount of light through the lens.
Decrease the ISO sensitivity which makes the sensor less sensitive to light.
Select Shutter Priority - selecting longest shutter and letting the camera adjust the ISO and F number up or down to compensate.
This to many if not most of those reading this will know all about this but for long exposure there are pro and cons to achieve the desired exposure in camera.
Don't miss out on the sweet spot of your lens.
All lenses have a sweet spot. A point in their f- range where is achieves peak sharpness. Over time and use most photographers will discern where the sweet spot is. There is software that can show you the sweet spot which can be incredibly useful if it’s a new lens to your kit (lens optimisation test). The sweet spot usually sits somewhere in the middle of the f- range. So for example a wide angle lens with a range fo f-4 to f22 will resolve critically sharp some- where around F8 - F16 with details softening the higher or lower you go outside this range. In the example below you can clearly see a distinct drop off at the widest aperture and a less dramatic drop off at the smallest aperture. Selecting either extreme can, when using long exposures result is less than critical sharpness.
Lens Optimisation Test
So by using f-stop adjustment to control the exposure time you are running the risk of missing the sweet spot of the lens and subsequently the risk of a less and critically sharp image.
Time For Some Examples
ZION NATIONAL PARK, UTAH
25s • F22 • ISO 50 •
In this image you’ll notice that the colour tones appear to be well saturated and it looks natural. In order to increase exposure time F22 was selected, with the adverse side effect of lowering sharpness across the entire frame, and you’ll come to this sharpness vs. duration (technical vs. aesthetic) tradeoff quite often.
GREAT BASIN NATIONAL PARK, NEVADA
30s • F22 • ISO 50 • 24mm
This is the first image in the sequence shot at the lowest ISO setting.But even at the lowest ISO and highest F number the camera would only meter to about 30 seconds - it was bright that night! At this point I put on a 6-stop ND(yes, at night!) and pushed the exposure out to about 900 seconds
GREAT BASIN NATIONAL PARK, NEVADA
901s • F8 • ISO 100 • 24mm
With the 6-stop ND the ISO was increased to 100 to have the flexibility to half it at ISO50 if needed. This particular lens performs sharpest between F8 - F11, so started with a conservative F8 and put on the 6-stop ND filter. It took a few shots to get it right. Starting with an exposure of 400s, then 600s, 700s, 800s. 900 seconds... that was it. The result is the moonlit landscape looks sunlit in the distance, and there is even clarity and details under the water. Notice the trees silhouetted in the distance? Two very different results simply by increasing exposure.
Next week - 'Using ISO To Help Calculate Exposure'
- Anamorphic Lenses
- Circular ND Versus Square ND Filters
- Why Use A UV Filter On Your Lenses
- De-mystifying Infra-red Photography