Dave Holland
(K=13074) - Comment Date 1/30/2006
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In the digital era, with a tripod, you can do the same thing as a grad ND filter by taking two shots, one metering on the foreground and one metering on the background sky. Then you combine the two in photoshop and get a similar result. Therefore grad ND filters are not as useful as they were with film cameras. A digital technique also works better when the edge between the bright and dark areas is not horizontal. Sometimes with an ND filter you will have a dark object in the light side, or vice versa. Then you have to fiddle in photoshop later to get it right anyway. I still use my grad ND filter but if I travel light I might leave it behind.
The brand of ND filter holder that most of us use is a Cokin holder, which comes in two sizes based on the lens size of your camera. For my lens set the 'P' type is best (large). P-type may work on smaller filters, but not vice versa. The plastic Cokin holder has mounts for two filters. I followed good advice here and cut off the front mount with a hacksaw, so there was less projection in front of the lens to cause vignetting at wide angles. The Cokin system holds rectangular filters, and the most useful one I find is the 0.6 grad ND, though many use the hard-edge filter for sharply demarcated horizon shots. The system allows you to slide the filter down the lens while you look through the camera, judging how far to move it by eye. That part is easy and obvious when you try it once. Don't buy a round screw-in filter, as it won't allow you to adjust the filter position. The Cokin holder also allows you to spin the filter,for edges that are not exactly horizontal. Overall I find it a good system, and landscape shots are often dramatically better.
You don't need a DOF preview to do this.
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Chris Hunter
(K=25634) - Comment Date 1/30/2006
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I agree that using a ND filter on a digital SLR can be done with good effect, but multiple exposures are a better alternative. However, there is certainly an appeal to capturing the perfect exposure in one shot, and a grad ND will help you to do that.
One consideration is that you will probably need several ND filters, to accomodate the wide variety of lighting you will encounter in the field...where as with multiple digital exposures, you can take as many shots as you need, varying the exposure time to acheive a perfectly blended image, with proper highlights and shadow detail from front to back.
Have a look through my portfolio, as well as Carsten Ranke's and Barry Wakelin's. Carsten uses both techniques effectively, and Barry seems to be a master of using ND grads.
Also attached to this thread see an example of multiple exposures at work - this is three seperate exposures, masked together in PS. Each exposure had approx. 7-9 stops, with a total of 15-20 stops for the entire image...you can see how the shadow detail and the highlights in the sky are pretty much perfect. This wouldn't be possible without either using multis or ND grads.
Cheers,
Chris
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Mt. Hope Bay via Common Fence Point
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Warren Simons
(K=741) - Comment Date 1/31/2006
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Hi Chris, Thanks much for the reply and the example photo. This is very useful information for me. You said "Each exposure had approx. 7-9 stops, with a total of 15-20 stops for the entire image. . ." I'm not sure what this means. Also, do you make these shots using a tripod? The photos in your portfolio are wonderful.
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Warren Simons
(K=741) - Comment Date 1/31/2006
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Hi Dave, Thanks for taking the time to write a reply to my question. It's all very useful information.
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Chris Hunter
(K=25634) - Comment Date 2/1/2006
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Hi Warren, what that means is that a standard digital CMOS sensor or piece of film can hold between 5-9 "stops" of light in one single exposure.
Think of it this way: a piece of film can record a certain amount of contrast before the whites become completely blown out and the shadows total black. This is called exposure latitude.
That amount of light can be described in "stops." Not the standard f/stop that you might be thinking of, but just a measurment of light difference (ie: 1/60th to 1/100th would be one stop. 1/60th to 1/300th would be approx. 6 stops). A stop in this use is a basic measurment of light, and the cameras ability to record it.
So, the more "stops" a medium can record (color negative film has the widest) the greater the contrast that can be captured without losing detail in the shadows or highlights. So, if one digital image can record 5-7 stops, then my composite of 3 digital images has an exposure latitude of 15-20 stops.
I hope this didn't confuse you anymore.
Cheers,
Chris
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Chris Hunter
(K=25634) - Comment Date 2/1/2006
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To your second question, yes - a solid tripod is essential to producing images in the fashion. You can maybe - maybe - get by doing handheld multiple shots, but I wouldn't even try. Also, you normally vary the shutter speed, not the aperature - so you don't affect the DOF in each shot.
Cheers,
Chris
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Warren Simons
(K=741) - Comment Date 2/1/2006
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Hi Chris, Your points on exposure latitude are clear. I'll track down a decent tripod and start experimenting. Thanks again.
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