Graduated Filters

Image with Blownout Highlights

As we have mentioned previously in other blogs, digital images have exposure limitations and when you go beyond the range of your camera or film's dynamic range, you will literally have no pixel data and unfortunately an irreparable image.  You can lighten up the dark parts of an image, but you can't do anything with blownout highlights.  Using split grad neutral density filters is an easy fix to this problem of an image having a larger dynamic range than the camera or film's ability to capture.

Highlight Warning System in Photoshop
shows Highlight Problem Area

A graduated neutral density filter, also known as a graduated ND filter, split neutral density filter, or just a graduated filter, is an optical filter that gives the photographer the ability to vary the light transmission that the camera captures.  It is used to tone down an overly bright part of a scene into the dynamic range of the fim or sensor in a digital camera.  It can be used to darken a bright sky so that both the sky and subject can be properly exposed.

Neutral density filters are identified by numbers (0.3, 0.6, 0.9 by Lee and Tiffen) which tell you how many stops of light they reduce the brightness.  You want to choose neutral density grads and NOT grey grads.  Neutral density grads don't cause a color cast on the sky, whereas the cheaper grey grads will produce all sorts of unwanted colors.  The old adage that you get what you pay for, is definitely true with regard to filters.  For consistent results I recommend Lee Filters as the most widely used by professional landscape photographers.  The one I use 90% of the time is a 0.6 ND soft grad.

ND Type   Cokin, Hoya     Lee, Tiffen
1-stop        ND2, ND2X    0.3 ND

2-stop        ND4, ND4X    0.6 ND

3-stop        ND8, ND8X    0.9 ND

The purpose of using a ND grad filter is to control the exposure difference between the sky and the subject.  To determine the strength of filter you need for a particular scene, you need to meter on the subject (landscape) filling up the frame without the filter.  Then repeat this step pointing your camera or meter at the sky, filling the entire frame with sky only.  The difference between these two readings indicates the strength of filter required.  A one stop difference will require a 0.3 ND, a two stop a 0.6 ND and a 3 stop a 0.9 ND.  The soft and hard feature just describes how abruptly the filter changes from darker to clear.

When you capture your image, you use the meter reading you took of the subject, when you set your manual exposure setting for the final image.  This will allow you to capture a scene that has a range of 6-8 stops of light, with a camera that can only properly display 5 stops of light, without losing detail in either the shadows or the highlights.  You can either hold the filter in front of the lens looking through the view finder to make sure you have placed it properly, or you can purchase a filter holder that attaches to the lens.  I personally hold mine, because I am unwilling to buy a filter holder and/or adapters for all the various lenses I use and its another set of equipment I have to carry with me in the field.  If your scene has in excess of 8 stops of light, your only choice then is using HDR methods in photoshop to merge mutliple images shot at different exposures into one composite image.  I personally do not use any of the colored graduated filters, as they create unnatural color tone qualities in the images, in my opinion.



HDR Enhanced Image

 ND Graduated Filters are a quick fix for images that have a bright sky peaking through the clouds and causing your Highlight Warning System to start blinking.  You can use the combination of grad filters and exposure compensation on your camera to meet most needs in the field, especially if you are shooting early morning or late evening type of photography.  During the noon or daytime hours you can easily be dealing with scenes with an excess of 10 stops of light, blownout highlights and/or no detail in the shadows, or both.

Pickup a 0.6 ND soft Lee filter at your friendly photography supplier, and try one out. If you like how it works, you can think about purchasing additional ones both soft and hard as well as the full range of stop coverage, and filter holders for ease of use, if you so desire.   I think you will find it will make a world of difference in your landscape photography images.  For the advanced photoshop users, HDR has come of age, and its many times easier to just take 3-5 images at varying exposure compensation adjustments, and merge them in photoshop than it is to mess with filters in the field.  It used to be a fairly tedious process to do HDR in photoshop, but with the latest version of Photoshop (CS5) its pretty easy if you captured the images properly in the field.

Hope this helped you understand when and how to use ND grad filters on your next outing.  The nice thing about digital cameras you can play with it in the field until you get exactly what you want from your image, as well as advance your knowledge and understanding of how best to use filters and their effect.

Keep in search of The Perfect Tyme for that killer image!