I recently received a few questions from a student on controlling exposures and using ND filters. Here are his questions and my responses – keep in mind these are very general answers and books have been written on these topics. I highly recommend “Understanding Exposure” by Bryan Peterson for anyone who wants to dive into this head to toe. Now for the questions…
Q- How does one avoid exposure blowouts when one has an uneven/ragged skyline? I am thinking in particular of the problem created by sunlight and sky behind treetop leaves, although mountain tops present the same issue sometimes.
A- There are several ways to deal with the problem of dynamic range and the limitations of digital cameras to capture what our eyes can see. Of course, I can’t stress enough how important quality of light is, not only for controlling exposure, but also to the overall success of an image. The direction, quality, intensity, and reflectivity of the light on a particular scene will all determine what you can capture, and what tools and strategies you may need to use. These might include circular polarizers, ND grad filters, multiple exposures and HDR (high dynamic range) photography. The easiest way to determine this is by checking your histogram – if you’re clipping the blacks and the whites, then the scene is too contrasty and you’ll need to use some sort of exposure control.
In your particular case, I would try a soft edge graduated filter since it will make a softer transition from sky to foreground with an uneven skyline and/or trees. These area available in 2 and 3 stop versions. I always carry a set from Singh-Ray, which are high quality and come in many different configurations. If your foreground is properly exposed, but your sky is 2-3 stops over exposed, then a 3 stop grad filter would do a nice job of controlling the highlights while maintaining shadow detail. You can also use grad filters in combination with polarizers, but you will need a filter holder that attaches to the polarizer.
Beyond that, two exposures could be made, one for the sky, and one for the foreground. Lightroom does not offer any way of combining exposures, so you will need to export both images to Photoshop and combine them using masks. If you only have one capture, you can also process the image twice in Lightroom then blend in Photoshop.
Watch this video for an example of this technique
Q- I have been looking at the EXIF data on images on Flickr that I particularly like and am finding that many of my favorite photos are taken at dawn/dusk with exposure times of 60 seconds or more…is this an instance where photographers are using ND filters?
A- Unless you are shooting at night, the only way to have a long exposure of 30 seconds or more is with the use of a ND filters. These are simply filters that limit the amount of the light that reaches your lens. I always carry a 6-stop and 10-stop ND filter for those times when I want that effect.
Seascapes are a good example where long exposures of a minute or more can make water look like fog. They are also great when wanting to blur the movement of a waterfall. Even if it is very bright, an ND filter allows you to use a relatively fast aperture (f/5.6 or f/8) and still use a long shutter speed of a few seconds, giving you the best optical quality from your lens. One thing to be aware of is that with a 10-stop filter on your lens, you will not able to see much at all through the viewfinder, and focusing will be close to impossible. So I always setup my composition and focusing point first, then screw on the filter.
I often use PhotoBuddy, a great iPhone app for photographers. I simply set my exposure without the filter, then once I add the filter, use the app to calculate my new exposure with the filter on. Knowing how to do this w/o a calculator is also a valuable skill.
In the Field
If my exposure without a filter is f/8 @ .5 sec, ISO 800, and I then use a 6 stop filter, I need to add 6 stops to my exposure. With out changing my aperture, new exposure might be f/8 @ 30 sec, ISO 800 OR f/8 @ 4 min, ISO 100. If I wanted a longer exposure while at ISO 800, then I would switch to the 10 stop filter, and add 4 more stops to my shutter speed: f/8 @ 8 min, ISO 800.
Exposures over 30 sec require you to put the camera into ‘bulb” mode and manually open and close the shutter. I use a remote shutter for this, which allows me to lock the shutter, then count off the seconds – just don’t wander off to far!
Please feel free to ask any other questions, and let me know what you think of the video…thanks!