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Tips and Tricks: using a Split ND filter
Welcome back for another installment of Tips and Tricks, a Photo Trek exclusive where I discuss a photography related topic. Each month I try to pick out a topic that has recently come up in my email communications. Over the years I have tackled topics as broad as what makes a good picture, and as detailed as how to effectively use a remote shutter release. I strive to teach with every entry and share some of what I have learned in my short tenure as a photographer. This month we will be dealing with a question that was brought up some time ago and deals with the use of an often misunderstood, or even forgotten filter. I am speaking of a split ND filter, or otherwise knows as a graduated neutral density filter.
I have discussed this filter briefly in a previous Tips and Tricks, but have not really given it its fair shake. Much like the polarizing filter, the split ND filter is invaluable in landscape photography and is one of the must haves in my book. It is the use of this filter that allows the photographer to keep a nice and even exposure between the ground and the sky. I'm sure that we all have taken what we thought was going to be a great picture, but it turned out that the sky was featureless, or the ground was so dark we couldn't make out any features. There are also times that neither the sky nor the ground are exposed correctly and the picture turns out to be a complete flop. Through the use of a graduated ND filter, this problem can be rectified to a large extent.
So, what exactly is a graduated ND filter? Quite simply, it is a filter that has a clear half, and a tinted half that adds density without color to your image. The tinted half is much like a regular neutral density filter in that it reduces the amount of light that is able to pass by without adding any color to the recorded image. It is the clear half that provides the key to this filter. By placing the clear half over the ground, and the dark half over the sky, you are reducing the difference in exposure between the two elements. The filters come in different strengths for different applications with the most popular being a 2-stop filter which reduces the light by a full 2 stops where the dark half is. Below you will see an example of how this works.
This is an unedited photograph straight from the camera which was exposed for the ground. You will notice that the sky is completely blown out in the upper left, and the mountains in the distance are a bit overexposed as well.
This is the same composition, with the same camera settings taken mere seconds later after the addition of a 2-stop soft edge gradual ND filter. You will notice that the ground is largely unchanged, but the distant mountains as well as the sky are all much more accurately exposed. This is the better photograph.
While this effect can be duplicated in Photoshop or other similar image editor, it takes time. Also, if the original photograph is badly blown out as in my first example, you won't have the needed information for the software to recreate the detail. You can bracket your exposure and merge several frames into one later during post processing, but do you really want to spend that kind of time in front of the computer when you can use a simple filter and get it right in the field?
There are two main types of split ND filters, the screw on, and the slot-in filter. Both have their positives and their negatives. The more common is the screw on type and I have mentioned these filters many times in previous tutorials. The benefit here is simplicity. When you need the filter, you take it out of the case and screw it on the front of the lens. This is a two piece filter that works a lot like a polarizer with one ring that screws onto the filter mounting threads on the lens, and another ring that houses the actual filter. With this design, you can rotate the filter to keep the dark section where you want it (usually the sky).
As you can see from the bottom right, the filter is divided in half with a gradual transition between clear and tinted. This represents a huge compromise you have to make in composition. As you will recall, it is usually not a good idea to place the horizon in the dead center of the frame, but with this filter, you are limited to just that type of composition. This is a huge drawback to this type of filter, but I was able to make one work for a couple of years, so it is not insurmountable by any means.
There has to be a better option than having the dividing line smack dab in the middle...right? Well, there is, but it requires a bit more effort, as well as expense. This is the slot-in filter. Basically, this is a sheet of glass, resin, or plastic that is treated with the tint on one half and left clear on the other half. The dividing line can either be gradual, or a hard edge. Yes, I know, the dividing line is still in the middle. How is that going to help my composition? Well, remember this is just a sheet, not a screw on filter. There is a holder which mounts to the lens of the camera which holds the filter sheet.
Here you can see the whole contraption attached the end of my 17-40mm f/4L lens. It is most definitely more cumbersome than the screw on filter, but so much more flexible when it comes to compositions. What this starts with is the basic filter mount, which in this case is provided by Lee Filter Systems.
You can see the slots where the filters slide into the holder. They are held in place by friction and are quite secure. The basic filter holder is equipped to hold three slot filters and one screw on filter. You can modify this by removing slots, or adding more. There is a primary ring that is screwed onto the lens as a screw on filter would. The holder is then clamped onto this ring in a way that allows it to be rotated independent of the mounting ring itself. The rings are purchased separately and can be had for a multitude of different filter diameters. They all step up to the same size for the filter holder. This allows you to keep one size holder, and one size filter for all of your lenses...you just need to invest in the different adaptor rings.
The brass pull is the release that allows you to remove the entire filter holder with filters attached. This leaves the mounting ring which has threads for a screw on filter if that is what you need for the current shot. Also visible in this shot is the length to which the slots protrude beyond the lens. This can potentially cause vignetting (dark corners) with extreme wide angle lenses. If you have a problem with vignetting, it is simple to remove the last slot from the housing without any ill effects. I have only seen a problem with my 10-24mm f/3.5-5.6 lens, and only at the widest setting.
With the filters inserted into the housing, you can now control where the division line is in relation to your composition. Since they are held in by friction, you have total control over where the filter is placed, and since the housing rotates, you can alter the effect from the standard horizontal as well.
There are quite a few filter systems out there to choose from. Cokin is probably the most popular with their Z-Pro and X-Pro line. The sell the holders as well as the filters. It is my opinion that the Cokin holder is not of the same quality as the Lee holder, and there have been many negative reviews on the Cokin system when it comes to vignetting on the wide angle lenses. The Lee holder seemed to do the best in this regard, and offered much more flexibility over the Cokin. The filters themselves are another area where it pays to do your research. Cokin uses resin filters which might not be optically perfect (up for debate). Lee uses glass filters and are generally regarded as very good filters. However, for my money, Singh-Ray makes the best filters around, and they are custom made per your order.
I am using Singh-Ray Split ND filters and could not be happier. Lower quality filters have a tendency to introduce a slight color cast to the scene...the Singh-Ray filters do not. They are available in different strengths, as well as hard or soft edge. I have found that most situations can be covered with either a 2-stop or 3-stop filter. For further flexibility, I use both the hard and soft edged versions. The hard edge is good for anything with a straight horizon like the ocean, while the soft edge is better for horizons with varying terrain like a cityscape, or mountains. Because of the filter diameter on my lenses (up to 77mm) I was forced to get a large sized slot in filter set. I have found the 4x6in filters work great with the Lee holder. The extra length allows me to place the dividing line of the filter anywhere within the frame of the picture, for ultimate control over composition.
The actual use of these filters is pretty straightforward once you have them fitted to the lens. Your first step is to compose the picture in your viewfinder the way you would like it to look. Check the difference in exposure with your internal light meter to see how much you would like to reduce the exposure of the sky. After you know this information, you will be able to select the strength of the filter that is needed. You will then rotate (and slide your slot in filter) to place the dividing line on your horizon. You will need to pay particular attention to whether you need a hard or soft edge if you have a choice. Once you are satisfied with the placement of the filter, you will take your meter reading as you would normally. Your exposure should be much more even between the ground and the sky.
The split ND filter is not just used for evening out the exposure, it can also be used creatively to really saturate the sky during a sunrise or sunset by underexposing without penalizing the ground exposure. Two different split filters can be used at once as well to amplify the effect or apply it differently across the scene. Regardless of how you choose to use this filter, keep in mind it takes a bit of getting used to before you become proficient with it. However, it is very much worth getting familiar with because it can really play a large part in the success of your landscape images.
Written by: Greg A. Kiser
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Great information and very informative. And what awesome pictures you take! Great job.
Sincerely,
Lisa
www.attentiontodetailgifts.com -
Not sure if I'm ready for this particular filter yet but I may purchase some screw on ones and experiment with them first. As always, a very informative entry and I thank you for information I never knew I needed but really really do!!
Here's an "apple" for you teacher
Nancy -
Wow Greg, I don't know if I can get all this in my head. But definitely I can see the need for this filter. In the two pictures you showed the difference is remarkable.
Thanks for writing this for us so that we can learn from you and be better photographers.
I want more pictures of Alaska!!!!!!!!! LOL I am a real pain in butt aint I.....
Myke -
Hi Greg,
I was just popping in to check in on you. I haven't been around much again for weeks and definitely haven't been doing much with photography, my camera isn't working properly. I think I've worn it out! I hope things are going ok for you.
Martha :-)
7/9/08 11:47 AM
Lisa