Friday, May 09, 2008

Darwin Wiggett explores how ND Grads perform with three different lens designs

On April 10 we posted an extensive report from leading outdoor photographer and author Darwin Wiggett following his performance tests of Singh-Ray's 4x6-inch and P-size Graduated Neutral Density filters. In this entry, he checks out a few questions that have been raised about the Canon 17-40mm lens used for the earlier testing:

The web is an amazing source of information on photography, but the veracity of the information is often questionable; and I never really believe anything until I test it for myself. For example, the reason for my previous post was to test the ‘suggestion’ that 4x6-inch grads were not usable or effective on full-frame (35mm) DSLR’s. The person who made this statement was using a Canon 17-40mm lens on a full-frame DSLR. To properly test his assertion I needed to use the same gear that he did. In the end, I found his assertion to be in error.

So why did I feel like more testing was called for? You may recall that one of the surprising results in my previous post was discovering that changing the lens aperture had absolutely no effect on the appearance of the grad line. No matter what focal length I chose on the 17-40, the definition of the grad line remained just as ‘sharp’ at f4 as it was at f22. This seemed totally counter-intuitive! But the results don’t lie -- right? Well, yes and no. In choosing the 17-40 to test grad filter performance I unwittingly chose a lens that behaves differently than many other lenses. According to photographer Tim Parkin, the Canon 17-40-mm lens is a ‘retrofocus’ design and this design is used on a variety of wideangle lenses to reduce the back-focus distance. Tim points out that the aperture ring for this lens is placed far forward -- right behind the front elements -- which may cause ND grad filters to perform a little differently than they would on "normal" lenses. One of the side-effects of retrofocus design is that the aperture has little effect on the size of the grad line (for more details please visit Tim’s blog). Tim suggested that I might want to try other lenses to see if the aperture chosen has an effect on the size of the transition line.

In my previous post, I noted that the further the grad filter was placed from the front of the lens, the more defined the gradient's mid-line became. This was true for the 17-40 and it was true for all three of the lenses I tested for this report. These lenses were Canon’s 24-mm TSE, 45-mm TSE and the 70-200 mm f4L. In all the following tests I used slot #3 on the Cokin P-series holder (the slot furthest from the lens) to hold a Singh-Ray 3-stop hard edge ND grad. I used an overcast grey sky as my test subject. So let's look at some test results.

Assumption #1 – The closer you focus the lens, the less distinct the grad line.

I tested the 24- and 45-mm lens with a Singh-Ray 3-stop hard-edge ND grad with the lenses focused at infinity and at their closest focus distance. The differences were subtle but when the lenses are focused at infinity the grad line was slightly more defined than when used at its closest focus (see photo 1 above).

Conclusion #1 - grad lines get slightly less distinct the closer the lens is focused.

Assumption #2 – The smaller the aperture used, the more defined the grad line.

On the retrofocus Canon 17-40mm lens this assumption was wrong –- aperture settings had no effect on the size of the grad line. But on the Canon 24- and 45-mm TSE prime lenses, aperture did affect the transition sharpness of the grad line. The grad line became more distinct as the lenses were stopped down. F22 gave a much ‘harder’ transition line than did f4 and this difference was more apparent on the 45-mm lens than on the 24mm lens (photo 2 above).

In fact, the longer the focal length of the lens, the more the aperture has an effect on the grad line. On the Canon 70-200-mm f4L lens, for example, the differences between f4 and f22 became greater the longer the focal length became (see photo 3 above).

Based on photos 2 and 3, it's clear that the aperture setting on ‘regularly’ designed normal lenses does affect the sharpness of the filter's grad line.

Conclusion #2 – the more you stop down the lens, the sharper the grad line becomes. Secondarily – the longer the focal length of the lens, the more aperture changes will affect the definition of the grad line. And finally – the longer the focal length of a zoom lens that is used, the less distinct the grad line will be at any given aperture.

What does this all mean? First, I suggest that everyone test their own equipment to see how it performs. Obviously lenses differ, grad filters vary in design, some people use filter holders, some people hand-hold their grad filters etc. Your mileage may vary so test your set-up.

In general, the results from this post and the previous one suggest the following:
• 4x6-inch and P-sized grads give essentially the same results.
• the longer the focal length used, the less sharp the grad line becomes.
• wide angle lenses give the sharpest grad line.
• smaller apertures like f22 result in a sharper grad line (except on some retrofocus lenses).
• the further the grad filter is held from the front of the lens, the sharper the grad line will appear.
• the closer the lens is focused, the less distinct the grad line.

In the field, I now can use different filter slots to harden or soften the edge of my grad filter and I always use the depth-of-field preview button at my shooting aperture to precisely place the grad line in my photo. And finally, I have learned that it is prudent to never make assumptions about anything! I hope this article will spur you on to test your own Graduated ND filters on your lenses. Happy shooting!

By the way... You'll want to check out the June 2008 issue of Popular Photography for Darwin's article called "Paint with Time." It describes how Darwin uses Singh-Ray filters to achieve "painterly" effects by seriously stretching his time exposures. For information on Darwin Wiggett's field photo tours check out his website (his tours book up a year in advance so plan ahead). We also suggest you get Darwin's highly instructional pocket guide "How to Photograph the Canadian Rockies" before it's sold out -- it will not be printed again (the publisher is out of business) and can be purchased from Darwin's website while supplies last.

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