Light Metering using a Circular ND Filter

[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]

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.

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.

[philaitman]

In all honesty, I'd never go back to screw in ND filters.
I do however use an Iphone app for checking exposure if the light changes dramatically and I'm feeling lazy. It's so easy to just unclip a Lee (Or similar) filter holder re-meter, check focus and reclip and shoot.

With unscrewing a circular ND, it's easy to inadvertently move the zoom, focus, Camera (if your tripod is flexi)

But to answer your question yes use an external meter.

[BAB]

As you mention Sekonic it has a built in function that will compensate to the addition of ND or POL filter with a press of a button! That being said either the in camera meter or external meter can if used correctly be a tool to solve the problem of changing light. But in practice when your camera is on a good set of legs positioned perfectly to frame the image and you just awaiting the correct moment it's a real pain to unhook the camera body from the legs or to move it to a point where you should meter the highlight which is usually higher in the frame and then reposition the camera? Especially when a spot meter hung on your neck or attached to your belt in a pouch is within immediate reach? Also I find using the camera meter or a hand held meter still requires some human interpolation to achieve the correct exposure for the desired image and no in camera meter made is as accurate as a hand held 1degree spot meter!

[rambler44]

Here is a video that addresses the issue of using a meter and ND Filters or CLP.  Goto minute 18: 08

Light meter and filters

To avoid screwing on filters, you might be interested in Xume magnets to hold filters in place.

[NancyP]

Am I missing something? My DSLR meters fine whether or not there is a screw-in ND.

[fdisilvestro]

Am I missing something? My DSLR meters fine whether or not there is a screw-in ND.

If you use a very dark ND filter for long exposures (e.g 10 f-stops or more) then you might not be able to meter with your DLSR through the filter

[BobShaw]

Am I missing something? My DSLR meters fine whether or not there is a screw-in ND.

True, but your DSLR light meter in the camera measures reflected light which is NEVER the correct exposure unless the subject is mid grey. The op is trying to calculate the correct exposure which is the incident light on the subject.

If a bride and groom were standing in exactly the same light then pointing the camera at each one individually would give vastly different camera meter readings. The correct exposure is neither of those. It is the reading of the actual light on the subjects which is correct.

In practice though, I point the camera at the highlights and point the camera at the shadows and read each and that is usually close enough. Film was different.