Many landscape photographers use Graduated ND filters to compress the exposure range of the composition to within the dynamic range of their cameras. These filters come in different strengths or stops, and different transition zones, hard or soft. However, the actual transition zone in your captured image is not dictated solely by the filters gradient. There are other variables, some of which can significantly effect the transition, that you need to know about in order to select the right filter during capture. I personally tested four variables to determine how the transition zone on the same filter can vary significantly. These variables are Focus point (near vr far), Aperture, Filter distance to lens, and Focal Length. All tests used a 3-stop hard-edge Hitech Z-size grad-ND filter positioned such that the ‘hard’ transition was in the center. With the exception of the ‘distance to lens’ test all filters were held in place using a Cokin Z-holder. The camera used was a tripod mounted Nikon D300 and Nikon 17-55mm and 70-200mm f/2.8 lenses. The four tests and their results are… Filter Distance to Lens Test 1) Focus Point - Near vs. Far: Variation -Negligible. No noticeable difference was seen when the lens was focused close (foreground) vs. far (background). Testing was performed at f/11 at 17 and 55mm. 2) Aperture: Variation – Noticeable. Smaller apertures have stronger transitions. The test included focal lengths of 17 and 55mm at apertures of f/2.8, 4, 5.6, 8, 11, 16 and 22. Remember, what you see in the viewfinder is not what you get unless you press the DOF Preview button but at small apertures it can be hard to see anything. 3) Distance: Variation – Strong. The closer the filter is to the front element the smoother the transition. In this test I handheld the filter flat against the front of the lens, shot, then moved it out about 2”, and then about 4”. This test was performed at 55mm and at apertures of f/2.8 and f/22. 4) 4) Focal Length: Variation – SIGNIFICANT. The longer the focal length the smoother the transition. Also, the longer the FL the more the 3-stop hard behaves like a 2-stop soft (see histogram image) since the densest part of the filter is not in the composition. This test was performed at f/11 at FL of 17, 24, 35, 55, 70, 105, 150 and 200mm. Focal Lenght Test: 17-200mm Conclusions: The harshest transitions occur at short focal lengths, small apertures and when the filter is placed away from the front of lens. The smoothest transitions occur at long focal lengths, large apertures, and when the filter is placed flush against the lens -which is where you would normally place it when hand holding. The notion that hard-edge grad-ND filters require perfect positioning in a filter holder at long focal lengths is not true and just an old photographers tale. You can, and I have, handhold a hard filter, especially if you feather (jiggle up and down) as you press the shutter because filter density transition at 200mm in the composition is about 10x wider than the same filter transition edge at 17mm. Density Differences/Focal Lenght Since the most significant variable in the density transition is focal length, longer lenses will require a stronger filter and/or harder edge than a wide-angle lens to handle the same break point in the composition. Where a 2 stop soft or hard may do the trick for an ocean horizon or mountain ridge at wide angles, a 3 stop would be needed when you zoom in. This is evidenced by the histogram comparison where two images, same exposure of a flat overcast sky at 17 vs 200mm showed a 1-stop difference. Therefore, care must be taken when selecting the filter and even the brand. Quality brands like Singh-Ray’s 3-stop soft has an unusually large transition compared to other brands often making the transition to 3-stops of density not possible at mid and long focal lengths. Personally, for this strength I prefer and have always used Hitech for just that reason. As a disclaimer, my current set includes five grad-ND filters, all Hitech Z-size, including 1-2-3 hard and 2-3 soft-edge, and hand-hold about 75% of the time. If you would like to comment on this article you can do so in this blog.