Choosing the right f-stop for long exposure night shoots

Tip711-Img0 1600 - thisBy using long exposure times for exposing water in movement, its possible to achieve a very nice silky effect. Typically an exposure time of 15-60 sec is enough to turn a moving water surface into a flat silky looking surface.  Depending on the conditions you may need to expose up to a couple of minutes.

To achieve this long exposure time for a night shot, simply by selecting a smaller aperture like f22 and f32, may not be the best solution, as lens diffraction at these apertures have a significant negative influence on the contrast and the sharpness of the final image.

You will achieve a much sharper image, if you use a lower f-stop in the range f8  f11. To ensure the right exposure time, try using high quality Neutral Density (ND) filters instead.

Tip711-Img2A full - thisThe image above shows a zoom into the central part of the image without any image adjustments added.  The same image is shot at both f=8 and f=22. It is obvious using f=22 decrease the sharpness and the local contrast significantly. You can hardly read the Gulf logo.

Sometimes its a good idea to make an aperture sweep to learn more about the behavior of a specific lens. It is necessary to use a very sturdy tripod and manual focus, in order not to be influenced by variations in the autofocus system.

Tip711-Img3 full - thisThe 4 images above are 200% zooms into to central part of an image.  At this zoom level we can see how the lens diffraction effect gradually decreases the sharpness of the image.   At f11 we still have great sharpness, but at f16 an obvious loss of sharpness appears. At f22 we really lose a significant amount of sharpness and local contrast.

Lets take a look at the surface of the water and observe the effect of using long exposure time for the shoot.
Tip711-Img5 full - thisIn this case the 15 sec exposure gives the wanted silky look of the water surface. For the best result, I should use a neutral density filter giving a 15 sec exposure at f8 or f11. The image is shot using a wide-angle lens so there will be no problem with depth of field using f8 or even f5.6.

If I havent got my ND filters with me, and I still want the silky water look from the f22 shot, then I can use the Clarity tool in Capture One 7 to improve on the lost local and also improve quite a bit on the loss of sharpness.

Tip711-Img6 full - thisA 100% zoom into the central part of the image.  The Clarity tool from Capture One 7 has been used in the image to the right.  I use Clarity=40 and Structure=20 for this image.  These settings bring back the loss of local contrast and improve the perceived sharpness of the image.

With the Clarity tool we have been able to dramatically improve the loss of quality due to lens diffraction at f22. A better solution to achieve the 15sec exposure time would have been to use a high quality ND filter and an aperture between f8 and f11.

All the best,

Niels

Photography Travelogues: At the outskirts of Monte Fitz Roy, Argentina

Alpine Stream under Monte Fitz Roy

Above El Chaltern and well before Monte Fitz Roy, deep in Patagonia, there’s a wonderful national park with some very photogenic walking  tracks. Even better, you can hire some of the young mountaineers in the area who will carry your gear from camp to camp, just for a bit of training (and a small amount of cash).

We’d spent a couple of nights at a camp above El Chaltern, waiting and watching the weather as it crossed the ranges in the distance, and now we were walking ten kilometres across to the base of Cerro Torre and a second camp. This stream was at one of our rest stops, but a rest from walking inevitably saw us wandering around with our cameras, looking for things to photograph.

I loved this oddly shaped rock, seemingly a towering mountain on a micro scale, surrounded by a flooding stream. Well, perhaps my imagination was getting away with me, but I thought it looked interesting enough to pull out my camera.

There are two techniques used for capturing this image.

The first is the use of a tilt-shift lens, a Canon TS-E 24mm on an EOS 1Ds Mark II. Canon has since upgraded both. Generally speaking, a tilt-shift lens is used to reduce distortion. When photographing buildings, rather than pointing your lens up to include the top of the building (and creating unwanted converging vertical lines), you shift the lens upwards while keeping the camera back parallel to the building. The result is a technically correct perspective.

However, if you shift the lens and tilt the camera the opposite way, you can distort the edges of the image, effectively stretching the scene. This works well with the distant mountains, stretching them so they look a little higher than they actually appeared through a wide-angle lens.

The photo below shows the straight photo without the lens shift. Note the height of the mountains in the background.

Compare this with the following image that includes a lens shift, and a re-framing of the image as well. The two compositions are very similar, but not identical, but the main difference is that the mountains loom larger and appear more impressive. I like this!

The second technique is the use of a neutral density filter. The ND filter allowed me to lengthen my shutter speed. This exposure is 60 seconds at f8 (it’s a 10x ND filter), during which time the water is recorded as a silvery smear and the clouds have also been beautifully blurred.

Compare the result with the same angle taken at a more conventional 1/250 second (see the previous photo). The clouds are more distinct and the water has much sharper reflections. In comparison, I like the ND filter effect because it takes the photographs one step away from reality.

In the days of film, we had to deal with reciprocity failure and colour shifts because the different layers in the film had different responses to light. I think some digital sensors are similarly affected with colour shifts at very long exposures and this shows up as a colour cast. This is the original exposure from the Canon EOS 1Ds Mark II before editing in Capture One.

The magenta colour cast can be quite enchanting, but not for every photograph you take. Fortunately, it’s an easy matter to correct the colour balance – simply use the White Balance Picker in Capture One and the image’s natural colours immediately appear.

To process this image, I also had to make some strong adjustments to the Brightness and Saturation. I also added in three Local Adjustments, lightening up the middle ground and adding a little sharpening to the rock.

Peter Eastway is a professional photographer and photography magazine editor based in Sydney, Australia. To see more of his photography, visit http://www.petereastway.com/. Peter also offers an online Landscape Photography MasterClass. Details can be found at http://www.betterphotography.com/.

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