Equipment & Techniques > Beginner's Questions

Light Metering using a Circular ND Filter

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nvw:
Hello,

I use square ND filters for long exposure and I am considering purchasing a circular ND filter. I use my camera's exposure recommendation to calculate what shutter speed should be. With my square filter I just take it off, get the reading then put it back on. I know that if the light is consistent then I'll only need one reading but what if the light is changing? When using a circular filter, if I don't want to unscrew it then it seems like I will need a light meter. Am I correct that I should have a light meter?

Thanks.

stamper:
IMO no. You will lose 2 stops of light when the polarization is at it's peak. If this gives you a slow shutter speed then raise the iso. If you are using a tripod then you shouldn't need to?

SZRitter:

--- Quote from: nvw on March 09, 2016, 09:04:47 am ---Hello,

I use square ND filters for long exposure and I am considering purchasing a circular ND filter. I use my camera's exposure recommendation to calculate what shutter speed should be. With my square filter I just take it off, get the reading then put it back on. I know that if the light is consistent then I'll only need one reading but what if the light is changing? When using a circular filter, if I don't want to unscrew it then it seems like I will need a light meter. Am I correct that I should have a light meter?

Thanks.

--- End quote ---

I prefer an external meter when I use ND filters for that reason. If you don't want to be constantly unscrewing the filter, it's pretty much the only way to go.

Not sure if you have a smartphone, but there are lots of lightmeter apps that seem to give decent results. I run a Sekonic, so I don't have huge experience with the apps.

Another trick I've used is having a second digital camera. I looked at it's meter readings then applied those to the other camera (this was actually used for a TLR with no working meter, but the principle would be the same).

nvw:
Thanks SZRitter, I had a feel that was going to be the answer. I'll look Lumu's app and meter.

Jimmy D Uptain:
You could try this:
Put your camera in manual mode.
Find a scene, set the adjustments to proper exposure (the middle)
Screw on the filter, and note the difference in the camera's meter. Also, if using a DSLR, keep in mind to cover the eyepiece of the camera as this will allow light in and screw with the exposure meter.

The problem with a light meter is that you have to tell it how dark the filter is so that it can compensate. Or you can dial it in using an app.
I don't have a variable ND filter, but if the filter is marked with this information, it shouldn't be a problem.
However if the filter isn't marked, it can be tricky.
Fixed ND filters could be easier to use.

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