Long exposure with OM computational photography
Long exposure (LE) photographs, particularly of seascapes, rivers or waterfalls, can create magical effects that allow you to express your creativity, and to show elements of the scene that the eye alone cannot discern. However, it is one of the most difficult photography techniques to master, and can be enormously frustrating. The traditional method to achieve long exposure times has been to use physical ND filters to act as “sunglasses”, and I provide exposure tables and techniques here for that method. If you are unfamiliar with EV and LE concepts, I strongly recommend you read that post first.
Most digital cameras provide no specific capabilities for long exposure photography other than a primitive bulb function to allow for very slow shutter speeds. The Nikon Z7 does not even provide a timer on the bulb function, or indeed any indication at all – in this area the camera is as dumb as a pile of bricks.
On the other hand, Olympus and OM System cameras have a superb set of features to assist LE photography.
- Firstly, when used with physical ND filters, Live Time not only provides a timer, but actually displays the image as it is being exposed. This is invaluable, as a wrongly exposed photo can be terminated very quickly instead of waiting for 2 or 4 minutes to find the error.
- Secondly, every OM camera provides computational photography functions like Live ND and High-res shot which not only replace some or all of the physical ND filters, filters but also dramatically reduce the noise compared to a normal long exposure as well as providing extra dynamic range and resolution to the images.
With OM system cameras, you can do what other cameras can not. For example, I took the featured photograph above with no tripod, and no filters, balancing on a rock in the middle of a fast-moving river in central Cambodia. Impossible with my Nikon Z7 full frame camera, or any other vendor’s cameras I am aware of.
How does computational photography work?
The basis for the CP method of long exposure is “exposure stacking”. The principle of this is that the smoothing effect of a single 128-second long exposure is the same as 128 stacked one-second exposures. However, because the one-second exposures are far faster, you have significantly reduced or eliminated the ND filters required to expose correctly. In fact, a 128 shot stacked exposure has an effective ND of 7 stops (2 to the power of 7 = 128).
The huge additional benefit of stacked exposure CP is that a 1 sec exposure has massively less noise than a 128 second one, and the stacking process reduces that further, while the dynamic range is significantly increased. This stacking process is used both in Live ND (from 2 to 128 stacked images) and High Res shot (8 or 12 stacked images), and a high quality Raw file is produced in seconds in both cases.
The doyen of dynamic range (DR) is Bill Claff, who in his excellent site “Photons to Photos” produces the only authoritative source for DR across all camera brands. Here are his measurements of the effect on DR of a Tripod high-res shot (8 images – black line) and LIve ND (64 images – blue line). The DR of both modes exceeds that of the Nikon Z7 full frame camera. The noise of a stacked shot is also far lower for OM Live ND and High-res shots than on the Nikon, at any shutter speed or ISO.
Live ND and high-res by OM camera
All cameras since the EM-1 MkII have had Live ND and High-res shot mode. High-res shot bracketing is the same for all cameras at 8 images (3 stops) for the tripod version, and 12 images (3.5 stops) for hand-held. Live ND varies by camera:
- OM-5: up to 32 images/5 stops
- OM-1 Mk I and OM-3: up to: 64 images/6 stops
- OM-1 Mk II: up to 128 images/7 stops (but see below)
Typical Long exposure photography exposure times
The exposure time used for LE of moving water depends on the speed of movement. This is my usual set of times:
- Waterfalls and sea flowing over rocks to give ‘lines’ of water: 1/5s
- Heavy seas on rocks to give the ‘mist’ effect: 1-2 minutes
- Urban rivers and lakes and flat seas to give a mirror effect: 4 minutes
Live ND filter tables for waterfalls and sea flowing over rocks (1/5s)
The Live ND values on OM cameras are selectable from 1 to 7 stops. For my normal 1/5 sec LE exposure for fast-moving water, this is a very useful range, and in principle could be used to fully replace physical ND filters for this kind of exposure. However, the fastest shutter speed for LND is 1/128s. This limits the fastest LND exposure at 7 stops to 1 second (7 stops is 128 exposures, which at 1/128s each gives 1 second equivalent). The EV values for which LND applies are therefore from 10-12 EV, and LND alone is only usable on overcast days or for scenes fully in the shade.
For waterfalls, which are often in the shade, an EV of 12 is very often the case, but for seascapes, full shade is far less common (except at British seaside resorts).
For bright days by the sea, the 6 and 7 stop LND settings do not provide a true ND capability for 1/5s exposures. As a result, some kind of physical ND filter will be needed to drop two stops to ND32, at which point LND acts like a true ND. In some circumstances a circular polariser may well get you into the range.
For brighter days at EV 14 and above, I am experimenting with a screw-in variable ND filter, to drop luminance by more than 1.5 stops, and then use the LND features to get a super high dynamic range low noise shot. A combination of CP plus ND 4-64 VND is available from K&F and others and this makes a good all-condition option. The filter is screw-in, because I have lost too many magnetic VNDs which are very vulnerable to being flipped off into the foaming briny.
If OM systems increased the maximum shutter speed for LND to 1/512s they could get a genuine 7 stops of ND, usable in sunlit situations up to EV 14. Hopefully, this could be implemented in updated firmware.
Live ND filter tables for urban rivers and lakes and flat seas (1-4mins)
The table below is from my post on physical ND filters but it applies here also. At the longer end of LE exposure times, LND is limited to 60 seconds. This is sufficient to get a misty effect on heavy seas but not enough for urban rivers and lakes.
For 1 minute shots, using the OM-1 Mk II and LND 128, another 6 to 9 stops of ND are still required. Around 8 stops can be generated by the CPL/VND option above, so this seems viable. For seascapes, this single filter could be used for short (1/5s) and long LE (60s) shots all day, without removal. This would be a workflow breakthrough. I am about to test this in Cape town, and will report back.
For 2 and 4 minute shots, I rarely if ever use LND, for two reasons. Firstly, above 8-9 stops, a VND is not usable, and stacked ND filters must be used. In that case, there is no particular LND workflow advantage, although the CP technology gives improved DR and noise.
But the second, and most important reason is that there is an alternate computational photography method, which enables long exposures of up to 12 minutes to be achieved, with improved DR, noise and resolution.
Long exposure photography using High-Res shot mode (HRLE)
While larger numbers of images can be stacked to get higher ND values using LND, the smoothing effect is just as effective with the 8 or 12 images taken for Tripod HR (THR), or Hand-held HR (HHHR) respectively.
Because each exposure can be up to 60 seconds, long exposures of up to 8 minutes (THR) or 12 minutes (HHHR) can be achieved. With HRLE the Dynamic Range (DR) is significantly increased, image noise is massively reduced, and the resolution can be up to 80pmx. The ND effect is still useful at 3 stops.
Of the two HR types, I have found THR produces the best result. Shooting data for this mode is shown below.
THR ND table: 8 frames, 30s for 4m, 15s for 2m
For most conditions, a 10 stop plus 3 stop ND filter should work. I have tested both methods on 4-minute exposures of the River Thames in London and compared them to physical ND long exposures on the 46 mpx Nikon Z7. The resolution, water rendition, DR and noise are all better than with the Nikon.
Movement artifacts
Unlike landscape photography, most LE shots are explicitly designed to have movement (usually water) in the image. I find the OM-1 Mk II and the OM-3 to be excellent at handling moving items like boats coming into the frame, and they blur them exactly as a normal LE shot would do. Earlier cameras like the EM-1 series were much less effective and could produce jagged artifacts. In all LE shots, the water is intended to be smooth. If a faint shape appears, whether for a normal or LND LE shot, I can usually clone stamp it out from the rest of the smooth water. No hassle.
Conclusion
OM system cameras are unique in providing multiple tools specifically for long exposure photography. For the kinds of photography I do, the main benefit of using computational LE methods is for image quality, not to replace actual filters, as in almost all cases real filters are still needed. But the image quality is excellent, and gives nothing away to larger full-frame cameras.
