Variable ND Filter question

I understand how to calculate a solid nd stop filter but how do you know how many stops your at with a variable nd filter. Is there a formula or would I have to just watch my light meter in the camera to figure out the stops. The reason I ask is I am wanting to shoot in full sun with manual flash and maintain 1/250 shutter sync . And was hoping to just get a variable ND filter rather than get multiple filters. I will be metering the light with a incident light meter.

For example, say I want to shoot at 1/250- iso 100 I take my incident light meter reading at the subject and get a reading of F5.6 and what I want is F4 which would be one stop more light so I need 1 stop of ND how do you know when you have your variable ND filter set to one stop. Are there marks for reference? I don't have one yet so I'm not sure. So the next question, If I want my fill light to be one stop brighter than my desired F4 I would have to set my flash meter reading to 2 stops brighter to make up for the ND filter correct? or am I off my mark on my thinking?

jlstan wrote in post #17786704
I understand how to calculate a solid nd stop filter but how do you know how many stops your at with a variable nd filter. Is there a formula or would I have to just watch my light meter in the camera to figure out the stops. The reason I ask is I am wanting to shoot in full sun with manual flash and maintain 1/250 shutter sync . And was hoping to just get a variable ND filter rather than get multiple filters. I will be metering the light with a incident light meter.

For example, say I want to shoot at 1/250- iso 100 I take my incident light meter reading at the subject and get a reading of F5.6 and what I want is F4 which would be one stop more light so I need 1 stop of ND how do you know when you have your variable ND filter set to one stop. Are there marks for reference? I don't have one yet so I'm not sure. So the next question, If I want my fill light to be one stop brighter than my desired F4 I would have to set my flash meter reading to 2 stops brighter to make up for the ND filter correct? or am I off my mark on my thinking?

Heya,

So typically what you would do is set your shutter to your sync speed. ISO to whatever you want. And then meter to get aperture with your light meter, with the ND filter set to 0 or off. Whatever aperture your meter tells you based on Shutter & ISO, will tell you how many stops you have to play with, with the ND filter basically, assuming you wanted to keep your aperture wide open. So let's say you are at 1/250s, ISO 100 and you meter with the ND filter at 0 or off, and you get F5.6 as your aperture from the meter, but your lens is F2.8 wide open and you want to shoot at F2.8. In that case, you would add 2 stops from the ND filter to reach the exposure equivalency from F2.8 to F5.6 but without stopping down aperture (the point of the ND filter).

I would not meter with the ND filter in place. It may work ok at lower ND filter values, but at higher values it will be all over and not really good. Meter with the filter set to 0 or close to it. Then simply add ND stops from the filter based on what you meter and based on what aperture you want to really shoot at (such as wide open) and add ND stops instead of aperture stops to meet the exposure of your meter that you want.

Another example. Let's say you're outside with a 100mm F2 lens and you want to shoot at F2 in fairly bright light. Your shutter is at 1/250s and ISO 100. Your ND filter is at 0 or off. You meter your subject. You get a reading of F11 perhaps. But you want to stay at F2. So you know you need to then simply turn your variable ND filter to 5 stops to get that exposure. Now, let's say you want ambient exposure to be 1 stop lower than your subject. You could then just add one more stop, so go to 6 stops on your ND filter. Now you are metered correctly for subject exposure, while dropping ambient exposure 1 stop below subject exposure, while keeping it at F2 wide open. Just as an example of the process.

As for which filter, the Singh Ray variable ND filter is expensive but it's the one to get if you want a high quality filter of this nature.

Very best,

I understand how to apply the stops. What I am unclear about is when you twist the variable ND filter how do you know when you have twisted it to 2 stops for example? Is there a mark for each stop on the filter? I do not have a variable ND filter yet to look at. All I have and use currently is the Lee big stopper To which I know is 10 stops.

If there are no marks on the variable ND filter couldn't a person just meter the subject with the ND filter off to get the aperture value then put the filter on and turn it until the exposure meter in the camera is two stops under exposed for example then you would know when to stop twisting the filter then change the f stop to bring the exposure meter back to zero?

Long story short, Do the Variable ND filters have marks for each stop?

Depends on the brand, but from the posting here over the years I don't believe the markings are more thsn a rough guide. If you want precision try a 3 stop or 4 stop filter to start.

So what your saying is, the best option would be to get filters for each stop rather than a variable filter?

You most likely dont need a filter for each stop. Strart with 3 stops, if thats too much just bump your iso. 4 is enough for all but the brightest of days. I have a 5 stop and honestly its too much. So you can get away with 2 filters but start with a decent quality 3 or 4 stop filter and you'll be surprised how well you can do.

Thank you

One more quick question will I have to compensate for the ND filter when I meter for flash? Using an incident meter at the subject when figuring % flash to ambient. I think I know the answer is no but just wanting to be sure I'm thinking straight.

Yes, you would have to compensate for the filter if using a flashmeter. But one way to do that is adjust the iso setting on the meter by the filter strength to retain the percentage reading.

So if I reduce with a 2 stop ND I would increase my light by 2 stops?

jlstan wrote in post #17787970
So if I reduce with a 2 stop ND I would increase my light by 2 stops?

No. The nd filter is reducing the ambient and flash equally so the flash power stays the same. You just have to remember to account for that reduction when you meter.

"You just have to remember to account for that reduction when you meter" Can you explain my brain is missing something

The balance between flash and ambient is the same whether the filter is on or not. But if you use a handheld flashmeter and it says f8 you must remember that the nd filter is cutting 2 stops so the camera needs f4.

Example... I' m shooting at f2.8 1/250 iso100 with a 2 stop ND and want my flash to be one stop over exposed I would need to set the flash power to F-8 correct?

jlstan wrote in post #17788054
Example... I' m shooting at f2.8 1/250 iso100 with a 2 stop ND and want my flash to be one stop over exposed I would need to set the flash power to F-4 correct?

Im stuck on the idea you want your flash to be overexposed. Do you mean you want the ambient underexposed?