pls help me understand how the GND filters can give us those extra 2 or 3 stops

[ocean76]

I really want to understand how the Gnd filters help us on the field.

to give an example: I am standing in a meadow filled with flowers that is under shadow and there is a mountain ahead which is bathed in great sunlight. So the dynamic range seems big but how big? does it exceed my camera's dr range, this is sth I don't know how to calculate.. (what I can do at that situation is: shoot with evaluative and check the histogram to see if there's clipping on each sides and if yes then continue shooting with + and - ev comp. until there's no clipping)

although I don't know a better way to calculate how many stops the dr of the scene exceeds my camera's dr, I am sure of one thing:

knowing that my canon eos 6d has a dynamic range of 12 EV, let's assume that this scene I want to photograph has a dynamic range of 15 EV, so the difference is 3 stops. And I know that if the ev difference between the lightest and darkest areas of the scene (carefully metered with cw or spot) is bigger than 12 stops (for the 6d) then I should either make bracketing or use ndg filters.. So here it is a must to use the NDG filter..

Now the metering and shooting parts start:

1) I point my lens at the foreground (for the darkest part's metering) and take an average CW meter (or spot meter) of the foreground.This is my base exposure..

2) now I do the same thing for the sky (to take the meter reading of the brightest area of the scene)

3) calculate the stop difference between the foreground and sky (let's assume it is 4 stops here) and then I decide that it is better to use a 3 stop NDG filter ( if I had used a 4 stop one, then the sky and the land would have the same brightness which is sth I should not prefer, right?)

4) I switch to manual and adjust my exposure to my base exposure (foreground's exposure) and the 3 stop ndg is attached to the lens, I start shooting..

If all the thing's I have written above are true, below are the things I can not understand::

1) in this example I have used a 3 stop ndg (for a 4 stop difference between the lightest area and darkest area..)
a) does it mean that I have managed to exceed the dynamic range of the scene by 3 stops by using this 3 stop NDG filter.
b) because the stop difference is 4 stops, does it mean that the dynamic range of this scene is 4 stops bigger than my camera's dr so here it is 12+4 = 16 stops...

it is complicated for me and I'd really like to understand how the use of ndg's help to imrprove the dynamic range..

thx in advance

[NorthernFocus]

Your logic is flawed. The GND isn't intended to bring the EV value of the sky to the same as that of the foreground. It is intended to effectively compress the dynamic range. In your example of a scene with a dynamic range of 15EV, let's assume that the sky is the highest end of the range. By using a 3-stop GND filter you bring down the brightness of the sky and effectively reduce the dynamic range to 12EV. If your camera can handle the 12EV then entire image will now fit within the histogram without having to choose one end or the other to clip.

[Donald]

Nothing to add to Dan's answer.

[ocean76]

Your logic is flawed. The GND isn't intended to bring the EV value of the sky to the same as that of the foreground. It is intended to effectively compress the dynamic range. In your example of a scene with a dynamic range of 15EV, let's assume that the sky is the highest end of the range. By using a 3-stop GND filter you bring down the brightness of the sky and effectively reduce the dynamic range to 12EV. If your camera can handle the 12EV then entire image will now fit within the histogram without having to choose one end or the other to clip.

Thank you Dan for the great explanation, now it is clear for me...

[rpcrowe]

Great explanation Dan. I might add that sometimes, a CPL filter will darken the sky enough to bring it within the dynamic range capabilities of your camera.

The angle of the sun and lens needs to be correct for the CPL to work effectively. Of course, UWA lenses and CPL filters don't get along too well. However, I seldom use UWA lenses when shooting landscapes.

I usually have a CPL on my lens when shooting landscapes...