Choosing the Right f-stop for Night Shots with Long Exposure
By 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.
The 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.
The 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.
In 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.
A 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,
March 22 2013
By The Image Quality ProfessorCategory: Professor Tips Tags: Capture One Pro 7 Clarity tool f-stop; ND filters Neutral density filter