Applied use of GND filters

[LandscapeTogger]

I understand the basic principle of using graduated ND filters
i.e. -
-at chosen f number spot meter sky and right down f number and shutter speed
-ignore that for now and spot meter brightest and darkest parts of foreground
-pick a shutter which is in between the brightest and darkesgt part of FG to expose correctly
-take that reading and the sky reading and decide how many stops you need your filter to be

what i need to know is when i have the readings written down i dont know the difference in stops. so say i was shooting a f/16 and to correctly expose the FG i need a shutter speed of 1/2 second and the sky needs a shutter of 1/60th, how do i determine the difference in stops

thank you for listening and i await your replies

regards

LandcapeTogger

 

[chris321]

One stop is double/half the amount of light needed. So if you need 1/2 a second for the darks bits, and 1/60 of a second for the light bits, the difference is about 5 stops

1/60 to 1/30 (one stop)
1/30 to 1/15 (2 stops)
1/15 to 2/15 (3 stops)
2/15 to 4/15 (4 stops)
4/15 to 8/15 (5 stops)

8/15 is 0.53 seconds, so roughly half. Sorry the fractions are a bit rubbish, it's just how it works out sometimes! And welcome to the forums!

Chris

Edit: I think this is right, someone please correct me if I'm wrong!

 

[OneTen]

It's basically counting the number of full steps between 1/60 and 1/2 sec:

1/2 - 1/4 - 1/8 - 1/15 - 1/30 - 1/60

Each step is a stop, therefore 5 stops for your example.

 

[Steve Smith]

The method I use is to meter a mid tone area then meter the brightest part of the sky you want to have detail in.

Generally, the highlights want to be about two stops above the mid tone so if your sky reading is three stops above, use a one stop ND grad. If it's five stops above mid tone, use a three stop ND grad, etc.

The correct exposure to use is the one taken from the mid tone area, not the average of the mid and high readings.



Steve.

 

[Naboo32]

The method I use is to meter a mid tone area then meter the brightest part of the sky you want to have detail in.

Generally, the highlights want to be about two stops above the mid tone so if your sky reading is three stops above, use a one stop ND grad. If it's five stops above mid tone, use a three stop ND grad, etc.

The correct exposure to use is the one taken from the mid tone area, not the average of the mid and high readings.



Steve.

Very informative post .

Can I ask a question, Steve (or anyone else who knows)? Do you recommend metering 2 stops above the mid tone because the Dynamic Range of the sensor/film is around 5 stops and you want to allow 2 stops either side of that, or is it not that simple :shrug:?

Thanks in advance for your reply.

 

[HoppyUK]

There shouldn't be any difficulty in exposure metering normally with a grad, without spot metering. We are shooting digital, not film (I presume) and it is far quicker, easier and more accurate to just fit the filter, meter it as you would any other regular landscape (I use matrix/evaluative 99% of the time) and chimp the result on the LCD and histogram. That will get you close, usually very close, then adjust as necessary. It only takes a moment.

Using a grad like this is no different to metering for a dark thundery sky. There is no need for spot metering for that situation, or when using a polarising filter for example, and there is no reason why the camera shouldn't handle it without problems.

Also bear in mind that the effect of the grad is varied a lot not only by the position in the holder but also by lens aperture, and the best way to preview that is by checking a test shot on the LCD. It is extremely difficult to predict the final effect in any other way as there are a lot of variables and spot metering won't help with them.

At low f/numbers the grad effect is reduced, and increases with higher f/numbers. In practise, this has a very similar appearance to sliding the filter up and down in the filter holder, or using a lighter or darker grad. It's actually a depth of field effect, which also explains why grads on longer lenses can appear almost invisible, and are strong with wide angles.

As a general comment, not directed at anyone here, I get the impression that some people believe certain metering techniques give better results. They do not actually change anything, they just make it a bit quicker or easier to find the right answer - use whichever method works for you. In exposure metering there is only ever one optimum exposure level. It makes no difference is this level is decided upon by spot metering or incident metering, or by using your normal method and chimping the LCD.

 

[Steve Smith]

Very informative post .

Can I ask a question, Steve (or anyone else who knows)? Do you recommend metering 2 stops above the mid tone because the Dynamic Range of the sensor/film is around 5 stops and you want to allow 2 stops either side of that, or is it not that simple.

That's more or less it.

The dynamic range of transparency film gives you about two and a half stops each side of mid grey (negative film gives a bit more). I used two stops in the example because I was refering to a highlight which you want to retain detail in.

You can also meter the darkest area you want detail in and check that it is about two stops below mid. There's not much you can do about it but it's sometimes nice to check.

You may need to experiment a bit with digital as the sensor may saturate a bit earlier. In which case you would need a slightly stronger ND grad or you could reduce the exposure a bit. Digital handles shadow detail quite well so this shouldn't be a problem.


Steve.

 

[Steve Smith]

There shouldn't be any difficulty in exposure metering normally with a grad, without spot metering. We are shooting digital, not film (I presume) and it is far quicker, easier and more accurate to just fit the filter, meter it as you would any other regular landscape (I use matrix/evaluative 99% of the time) and chimp the result on the LCD and histogram. That will get you close, usually very close, then adjust as necessary. It only takes a moment.

If you have chosen the correct strength of ND grad then matrix or evaluative metering should give a good exposure.

The trick is in choosing the right grad in the first place. this is where a spot measurement of a highlight helps as you can see how bright the sky is relative to how bright you actually want it.

And no, I am using film.


Steve.

 

[buckas]

another handy tip is that the reflection of the sky in water is 2 stops darker than the actual exposure of the sky itself

drew

 

[HoppyUK]

If you have chosen the correct strength of ND grad then matrix or evaluative metering should give a good exposure.

The trick is in choosing the right grad in the first place. this is where a spot measurement of a highlight helps as you can see how bright the sky is relative to how bright you actually want it.

And no, I am using film.

Steve.

LOL yes of course. I was thinking of the OP and others probably shooting digital

I agree that with film, it's more difficult. I used to use the lens stop-down button to gauge the effect, then bracket with slides, or with colour neg just over expose a bit.

I never resorted to a spot meter though. Possibly because I haven't got one

 

[Naboo32]

That's more or less it.

The dynamic range of transparency film gives you about two and a half stops each side of mid grey (negative film gives a bit more). I used two stops in the example because I was refering to a highlight which you want to retain detail in.

You can also meter the darkest area you want detail in and check that it is about two stops below mid. There's not much you can do about it but it's sometimes nice to check.

You may need to experiment a bit with digital as the sensor may saturate a bit earlier. In which case you would need a slightly stronger ND grad or you could reduce the exposure a bit. Digital handles shadow detail quite well so this shouldn't be a problem.


Steve.

Thanks for the reply, Steve .

You make a good point at the end, there. I tend to use a medium amount of "Active D-Lighting" on my Nikon DSLR, which is supposed to bring out shadow detail. Combining this fact with the earlier highlight saturation on a digital sensor, probably means that the -2/+2 from mid isn't going to be as reliable on a DSLR , I guess!?

Still, I want to try some landscapes using my 35mm flim cameras too, so it's worth keeping in mind for that .