Graduated ND Filters - which should I use?

[Dr007]

Hello friends,

I've read this tutorial about graduated ND Filters and I don't know in which situations I used a 2 stop ND filter or a 4 stop filter

This can be estimated by first pointing your camera at the darker half of the scene, taking an exposure reading, and then pointing your camera at the lighter half and taking a second exposure reading. The difference between these two exposures is the maximum strength that you'll need to use, although you'll likely want something weaker for a more realistic-looking photo.

Why should I used a 2 stop GND when my exposure is between 1/100 and 1/25 ? I don't understand the arithmetic. if my exposure is between 1/2000 and 1/30 which filter should I use? 4 stop GND?

Thank you vor your help!

[Donald]

Hello and welcome to CiC. I hope you stick around a join in discussions and ask questions on a long-term basis.

You've maybe seen that most of us tend to use our real names on here. It makes communication more friendly and CiC is that sort of site. Did you know you can go to Edit Profile and enter your proper name under 'Real Name'. Then it will appear underneath your Username in all your posts.

As to your question.

Excuse me if I'm telling you something you already know. Let's take shutter speed as an example.

From 1/30th to 1/60th is one stop. To 1/125 is another stop and so on .... 1/250th; 1/500th etc. Similarly, from 1/30th to 1/15th is one stop the other way.

So, if your meter reading from the bottom/darker part of a frame is 1/30th at whatever aperture you've chosen, and the reading from the brighter (sky) part of the frame is 1/125th, then that's a 2-stop difference and you need a 2-stop GND.

And as to your other question, the difference between 1/2000th and 1/30th is 6 stops: 1/2000th; 1/1000th, 1/500th; 1/250th; 1/125th; 1/60th; 1/30th.

What you need to memorise are the full-stop figures for shutter speed (as above) and aperture; e.g. f4; f5.6; f.8; f16 etc etc.

Remember they work together. So 1/125th @ f16 is the same exposure as 1/60th @ f8. You're halving one (shutter speed) but doubling the other (aperture). So you end up at the same place, apart from the depth of field you'll have.

[Hans]

Hi Dr007,
It sounds like you might find the following tutorials helpful:
Understanding Camera Exposure
Understanding Camera Metering
Understanding Dynamic Range

Regarding choosing ND grad filters, the actual shutter speeds you are using are essentially irrelevant. It is the number of "stops" between them that is important. This is how I would meter the scene and choose a filter (hope I'm not telling you how to suck eggs so to speak):

It's early in the morning and I want to take a nice seascape with rocks in the forground, waves breaking over the rocks and some nice stormy cloud formations in the sky forming above a bit of headland jutting into the frame, the sun is yet to rise over my right shoulder(so is not in my frame for this one) and that nice pink glow is being reflected onto the clouds and water. After choosing my angles and basic composition, I'd put the camera in manual and set the aperture to whatever I wanted to use. For this image I would want most or all of the elements of the image to render essentially sharp, I'd set my aperture to say f11 (or f16) to give a greater DOF (depth of field).

Now I'd set my meter to CWA (centre weighted average) or "spot" metering mode. I'd point my camera up toward the brighter sky (or brightest area of the scene) and adjust the shutter speed dial until it registered as "correct" exposure (you can take a note of this shutterspeed if you wish). I'd then point the camera toward the darker foreground (rocks) and adjust the shutter again until the camera told me it was at the "correct" exposure (AND WHILE DOING SO I'D COUNT THE CLICKS ON MY SHUTTER SPEED DIAL - 3 clicks on my camera equals a 1 stop adjustment - as most cameras default setting is 1/3 of a stop increments, yours is probably the same).

In this case, let's assume I counted 9 clicks. So at f11 lets say the first shutterspeed reading of the brighter area of the scene was 1/125sec, this would mean that the shutterspeed for correct exposure for the rocks would be 1/15sec. Because 9 clicks of the shutterspeed dial equals 3 stops, I know the foreground is 3 stops darker than the cloudy sky, and 1/15sec is 3 stops slower than 1/125. It needs to be slower, not faster, to allow more light in because the rocks are Darker than the sky. Basically, this "difference in stops" is the only info you need to determine the strength of the ND grad filter, the actual speeds are irrelevant.

Now I know both the "correct" exposure for the foreground and the strength of the ND grad filter I need to use...it is 3 stops (or 0.9 depending on the filter). As the tutorial says, I might be better off using a 2 stop ND grad instead of a 3 stop for a more natural look (a weaker one), but it is my pic and my art so I'll do whatever I feel best reflects my interpretation and what results I am envisioning for this particular shot.

Now I can put my camera on the tripod and get to choosing where to focus (using hyperfocal distance scale) and taking some test shots and adjusting form there. That is why I always write "correct" in inverted commas as the meter is only a suggestion and I might vary a little depending on how it looks in the LCD and how the histogram is looking in the RGB channels (I also know that the sky might well become more than 3 stops brighter as the morning sunrise progresses so I am also prepared to whip out my 1 stop ND grad or my 2 stop ND grad to combine them to make a 4 stop or 5 stop ND grad as soon as the highlights threaten to blow out on me).

Now when I take the shot I find that the sky and foreground both retain lots of detail in both the highlights and the shaddows. Great. But then I think to myself, that water is still not soft enough (burry enough). I might use a 1 second shutterspeed to really soften the water crashing over the rocks (incidentally, 1 sec is 4 stops brighter than 1/15sec). But if I just open up the shutter speed to 1 sec I'd have to compensate by stopping down my aperture to f45 (which I can't do because it only goes to f22 and I don't want to because the image will become soft due to defraction). So what can I do? I can put on a 4 stop ND filter (non-graduated), with my 3 stop ND grad, to stop 4 stops of light coming in which now compensates for the slower shutterspeed without having to adjust my aperture (I don't need to reduce light coming in with the aperture coz the ND filter is doing it for me). So now I can shoot at f11 and 1 sec for "correct" exposure and detail througout the image.

Tip: When pulling the grad down to the horizon it is a good idea to go a little lower than the horizon (sometimes quite a bit lower) as this is usually the brightest part of the image, or you can use a "reverse grad" which is darker in the middle of the filter and grads out to lighter toward one end (kind of like using a normal ND grad up-side-down - which you can do but you need to feather (move it up and down) it while the exposure is taking place as the edge or end of the filter is very hard so will show up as a distinct line if you don't).

Hope this very basic workflow (from a rank amateur) was a little helpful

[Colin Southern]

Just be aware that the theory doesn't always fit with the practice. In theory one might need "x" stops of attenuation - but - remembering the rules of local contrast - if one balances the shot according to the "theory" one may well end up with an image that just doesn't look right because the eye EXPECTS things like skies to be a certain amout brighter than foregrounds.

In reality, I find a 3-Stop GND right for most things.

[koolkat]

Hi, some really good information. Thanks Hans for that story on the beach, made sense to me and something to help in my selection of filters. I could have used that information and filter setup the other day as I was shooting a waterfall with the rising sun just behind it. Slowing down the shutter speed for the soft water effect of course made for to much early morning sunrise. Got some nice shots but not what I'll get maybe next time.
Thanks,