Tip 5: Graduated Neutral Density Filters

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Many of my landscape images have been captured while using Graduated Neutral Density filters or "ND Grads" (we also call them "Uber filters" over at POTD.com.au, that's because they are generally "uber expensive" lol)

ND Grads work in exactly the same way as regular neutral density filters, in that they reduce the amount of light entering the lens (see earlier ND Filter blog post), but they are as the name suggests "graduated".

This graduation from no ND effect to full ND effect, allows us to reduce the mount of light in one portion of the image, while the remainder is not effected.

The graduation or soft edge of the neutral density allows the photographer to blend the ND effect into the image and achieve a more natural appearance. This is not that easy though and careful selection/use of hard and/or soft graduations and subsequent positioning of these within your image is vitally important for good results.

I will expand on position techniques in a later blog, but before I do this we need to look at a few of the uber filter options available to you.

The Screw-on Filter:

Just like a regular UV filter (many of you will be using these for lens protection), the screw-on ND Grad is circular in shape and screws into your lens’ filter thread. They come in a variety of densities (how much they reduce the light) and edges (how soft or hard the transition is from clear to ND)… unfortunately though, there is no way to adjust the positioning of these filters within the frame.

It screws into the filter thread remember, so where the graduation is positioned is where it stays and there is no way of moving the filter up or down. You can rotate the filter however and this is handy to match off level features within your image, but not having the ability to move the filter up and down is a serious drawback.

For example, if your sunset horizon is 1/3 of the way down your image, you want to position your ND Filter 1/3 of the way down your image to reduce the bright sky and allow a more even exposure between the bright sky and darker foreground. If your Screw-on ND Grad has the ND ½ way on your filter (which they do) you have no option but to recompose your image so that the horizon is now ½ way down your image to match the filter.

This seriously affects compositional choices and for this reason alone, I would NOT recommend the purchase of “screw-on” ND Grads.

The Square Slide-in Filter:

This filter seems more practical, because no longer is the filter physically tied to the filter thread. Instead, a holder is placed on the lens and this holder provides the photographer with a slot, or slots, to slide one or more ND Grad filters into.

Sounds like a winner yes? ….sadly No. :-( The square shape of these filters does not provide the photographer with enough movement up and down, before the filter edge can be seen within the image, especially with the wider Field Of View seen in portrait orientation. These filters are only marginally better than the screw-on filter.

You can of course go to filter that is a bigger square and get greater movement, but these filters are more expensive and I am not sure they offer a great range of graduated effects.

The Rectangular Slide-in Filter:

Now we’re talking!! A rectangular filter is the obvious answer, as this provides us with extra space top and bottom and we can use this space to allow positioning of the graduation where we need it, without the filter edge creeping into our shot.

Like all of the filters mentioned so far, these come in a variety of graduations, intensities and sizes. The photographer will generally have a selection of these in the kit, as each image is different and will require a different filter, or combination of filters to achieve the desired effect.

Filter Width:

OK, so we are using rectangular filters to allow better positioning…. But what size?

This will depend on the focal length of your widest lens. Let’s imagine that we are taking a photo with the camera in landscape orientation (horizontal) and that we are using an ND grad to hold back the sky exposure. The filter will be positioned with it’s longest side vertical (it has to be because of the grad) and this leaves us with the filter’s short width to cover our camera’s Field Of View.

So the width of the filter needs to be great enough, so that the edge of the filter and in fact the filter holder itself, is not seen in the side of our images.

Unfortunately I have no magic formula for you here, but from experience I can tell you that if shooting at 16mm on a 35mm film camera or a full frame digital such as the Canon 1Ds or 5D, you will need to use the 100mm filter/holder system. This will generally allow 2 filters to be stacked, but a third will see the filter holder.

This holds true for focal lengths of 10mm when used on 1.5/1.6x cameras such as Nikon DSLRs or Canon D30/D60, 10/20/30D DSLRs etc.

Many people shooting with the 1.5/1.6x cameras never venture past the kit lens that came with their camera (generally around 18mm), so if this is you, you may like to consider the 84mm wide filter systems.

Regardless of the system that you choose, I would highly recommend a test run with your camera at local supplier prior to purchase. Take a few pictures at your widest focal length and check the image on your LCD for signs that you are seeing (or not) the filter edge/holder.

Remember, you MUST take a picture to test this, as it is only the high level professional bodies that offer 100% viewfinder coverage of what your sensor sees. Many camera viewfinders can be as low as 95% of what the sensor sees and this could easily hide a filter’s edge intrusion.

Note: Some manufacturers offer special “wide” adapters for their holders to help cope with the wide Field Of Views. These include a holder that is limited to a single filter and/or a special mount ring that sits back over your lens when screwed on, rather than out in front… ask you supplier about these wide solutions for the system you are considering.

OK, so now I know what size to get, but I am confused about stops and edges??

Let’s think about what you shoot then….

If you photograph landscapes with distant horizons that are basically straight and have no foreground elements that break into the sky area, then a “hard” edge filter would be useful. If you wish to simply darken the sky slightly, a 1-Stop filter would be useful. (Nice on those overcast days where cloud detail would otherwise be lost)

A 2-Stop filter would provide a greater darkening effect and a 3-Stop filter would be of benefit when the there is a large difference between sky and foreground eg at sunset.

Remember, you can stack the filters as well, so if you need more neutral density and really hold back the sky then stack your 2 & 3 Stop for 5-Stops!! (5 stops will most likely no longer look natural, but some great effects can be achieved)

My personal recommendation would be to have at least a 2 and a 3 Stop hard in your kit for “straight” horizon shots.

If you have image elements that break into the sky area (eg a building), the hard filters become less useful, as it is impossible to hold back the sky without effecting the building also. Here the “soft” edged filter is required, as the more gradual darkening that it provides is more appealing over foreground elements.

Again, good positioning is vital for good results, but there comes a point when one must consider that using any filter at all, may be detrimental to the image and perhaps a brighter sky is a better option if it maintains detail in other image elements. (If this is case, exposure bracketing and merging of these in post processing may be the answer, but we will talk more on this in a later blog.)

My personal recommendation would be to have at least a 2 and a 3 Stop soft in your kit for “broken” horizon shots.

Brands:

There are a number of brands available and the old adage "You get what you pay for!" seems to apply. I personally use "Lee Filters" as I find them to be excellent quality, neutral in colour (yes some of the cheaper brands are not exactly neutral) and quite scratch resistant.

"Singh-Ray" are also extremely good and offer some quite unique graduations and they can even make to order.

Another popular brand is "Cokin" ....I have used these in the past and my experience with them was not great, as I found they promoted a slightly magenta cast. Having said that, I will admit to only using the entry level filters from Cokin and cannot report one way or another on their "Pro" series filters, so by all means check them out.

There are other manufacturers like B&W & HiTech and I would suggest that you Google "neutral density filters" and do a little research to find what is right for you.

I hope this helps you understand Graduated ND a little more. I will be back soon with another blog on positioning these filters for best results.

Cheers

Rusty
AustralianLight - Landscape Photography


Photography help for beginners - Film & Digital Camera Techniques - Post Processing - Photography tips and tricks - Graduated Neutral Density Filters