Issues and tips in long exposure photography

March 01, 2014  •  Leave a Comment

 

This week I had a lecture at Downstairs photo club here in Copenhagen. In front of about 30 people, I shown my work and I presented my ideas about long exposure technique and black and white photography. One of the parts that people seemed to be very interested about was the slide containing the tips for long exposure. All the issues that I am presenting here are real, and I was facing them during almost 2 years of using this technique ; the solutions are the ones that I am currently using in order to avoid those problems. 
 
• focus is impossible with extreme ND filters on the lens. This one is rather obvious: when you place a 10 stops ND filter on the lens, the amount of light that is hitting the sensor is about 1000 times less than without the filter; this light is not enough for getting proper focus, neither automatic, nor manually. Focus, lock the focus, place the filters, shoot - this is the correct sequence that always works. For locking the focus, I half press the shooter, get the focus and switch to manual focus. Of course I take a test shot as well to check the focus as its really frustrating to wait for a couple of minutes just to get a soft image.
 
• digital noise - often the sensor gets hot during long exposure and some pixels get stuck, hence they appear having wrong colour or even white in the image. First time I was trying my Lee Big Stopper I thought it was defective and even thinking about returning it, but doing some googling I figured out it is quite normal. There are two solutions for this problem: use Long exposure noise reduction setting of your camera (with the disadvantage of doubling the time necessary to take the image) or remove those spots in post processing. Lots of people are recommending post processing and I was doing the same when using Nikon D7000. However after I upgraded to D800, I noticed a lot more pixels than before. The sensor gets hot faster and it is also bigger, so the hot pixels are everywhere across the frame. Dust and scratches filter in Photoshop can do wonders if applied on sky or water surface however, if large areas with details are included in the frame, the filter can misinterpret the small details with scratches, removing them. This is of course unwanted as the image looks much softer.  On such surfaces, the hot pixels really needs to be cloned out manually with the favourite cloning technique and this is damn time consuming. To conclude on this, it is good to have 2 solutions, however both have their disadvantages.  I usually use Long exposure noise reduction, unless I am pressed by the time and I fallback to post processing removal.
 
example of digital noise , zoomed crop from an original raw file 
 
• large F-numbers can reveal dust spots on the sensor. This one is not necessary related to long exposure photography, but visible when using this technique due to high F numbers. Clean your camera more often or remove them in post-processing: these are the choices. In post processing, is better to place a high contrast adjustment layer on top of the layer you are using to do the retouching, hence making the spots easies to discover. 
 
• the calculation of the exposure time can be quite overwhelming. Use a calculation table until you get familiar with the math behind. Just googling "long exposure calculation table" you can easily find couple of such tables and also many filter vendors are providing them on paper together with the product. There are applications for both Android or iPhone  that are also doing the calculation. 
 
• some light can leak though the viewfinder producing some strange artefacts on the image. The best practice solution is to cover the viewfinder during the exposure. I was hit by this more heavily after I upgraded to D800. Before, I was almost never covering the viewfinder, and unless a heavy sun was shining right into the back of my camera, no artefact was present on the image.  Now, I definitely need to cover the viewfinder (and NIkon were kind enough to provide a simple plastic slide that covers the viewfinder for D800) in all light conditions, otherwise the image is ruined.
 
light leaking through the viewfinder
 
• vignetting is often present when using one or more screw-in ND filters on a wide lens. Some ultra wide lenses, at their wide end, can support only one very thin filter without vignetting. If stacking up more or in case they are not so thin, you can get darker or totally black corners. Lots of people are cropping their images square so they don't have this issue (or some are doing this to solve the issue). I prefer 3:2 crop size of my images and I do not like to loose a lot of pixels so I is use square filters and wide angle adapters. I am using Lee system with Lee and Hitech filters and I am very happy with this setup.
 
• colour cast is almost always present for ND filters. Some produce bluish tints, other reddish cast. This is quite normal due to optical reasons. If you are interested to show your long exposure work in colour, calibrate the white balance in the camera upfront or remove the cast in post processing. For B&W, this is not an issue at all.
 
with and without long exposure example, show the blue colour cast produced by Lee filters
 
• when shooting straight up to the sky, sometimes the zoom creeps due to the
weight of the filters. It happened recently , while in Berlin, using Nikon 24-70 lens. I initially thought is something wrong with the lens, but it doesn't creep without the filters on. My solution was to keep a finger on the lens to stop the creep. Another solution that I read about but never tried is to use a rubber bracelet( like the one that you can get when visiting Heineken museum in Amsterdam) between the zoom ring and the lens barrel, applying enough pressure to keep the zoom ring still. I will test this next time when shooting up to the sky
zoom creep example

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