ND filter to reduce softbox light output?

OK, time for another stupid question by me, and I got almost no sleep last night. So be kind. I've seen the type of ND filters that I can attach to the front of my strobe reflectors to reduce the light output by a stop or two. But after doing a little experiment, this question came to mind.

In the experiment, I used the Elinchrom 600RX with Photoflex medium octa with both diffusers and a grid. I positioned the octa where I wanted it as far as distance from the subject. I then set the strobe to its lowest setting. With my meter set to ISO 100, max synch speed for my camera, it looked like f2 was the biggest aperture reading I could get. I couldn't get to, say, f1.2, without reducing the light output somehow even more. I thought about the ND filters I use with reflectors, but there's no way to introduce one in the light path with the octa attached to the light....or is there? If I don't want to move the light farther away from the subject, is there a way to reduce the light output with the octa attached?

sdipirro wrote in post #8391869
OK, time for another stupid question by me, and I got almost no sleep last night. So be kind. I've seen the type of ND filters that I can attach to the front of my strobe reflectors to reduce the light output by a stop or two. But after doing a little experiment, this question came to mind.

In the experiment, I used the Elinchrom 600RX with Photoflex medium octa with both diffusers and a grid. I positioned the octa where I wanted it as far as distance from the subject. I then set the strobe to its lowest setting. With my meter set to ISO 100, max synch speed for my camera, it looked like f2 was the biggest aperture reading I could get. I couldn't get to, say, f1.2, without reducing the light output somehow even more. I thought about the ND filters I use with reflectors, but there's no way to introduce one in the light path with the octa attached to the light....or is there? If I don't want to move the light farther away from the subject, is there a way to reduce the light output with the octa attached?

Let me start by asking what might be a silly question.

What are you shooting where f/2 is not a large enough aperture? If you want to shoot at f/1.2 then I'm assuming you're going to be using either the 50mm or 85mm f/1.2. You really want to be able to shoot with strobes in the studio at f/1.2?

If you really need that extra 1 1/3 stops and you don't want to use an ND filter on the lens (which is really the easier solution and with a good quality filter you won't degrade the image at all) then you use sheets of ND gel or but it on a roll.

At one time I drove myself crazy trying all kinds of DIY's to get ND's inside modifiers but it proved to not only be a pain in the arse but also very difficult to make sure you covered the area in front of the flashtube evenly, and even then it was a nuisance to have to get inside the modifier to make changes or to add or remove the ND.

So, after a lot of searching, I got some help from Hermes (POTN member) and he suggested either using sheets of ND or buying rolls and cutting it to size and adding velcro to attach them to the front of the modifier. It does cost more but it's the best solution and the pieces can be rolled up and stored.

Thanks, Robert. Well, I thought I might have some creative lighting/DOF possibilities if I could shoot at f1.2 in the studio, with the 85L in particular since the IQ is excellent at f1.2. I wondered if I was missing something basic as far as reducing light output from the strobes, but it sounds like it's as big a pain in the arse as I imagined. So an ND filter on the lens does sound like the better approach. Having never used one, how do I meter for such a configuation in the studio?

sdipirro wrote in post #8397818
Thanks, Robert. Well, I thought I might have some creative lighting/DOF possibilities if I could shoot at f1.2 in the studio, with the 85L in particular since the IQ is excellent at f1.2. I wondered if I was missing something basic as far as reducing light output from the strobes, but it sounds like it's as big a pain in the arse as I imagined. So an ND filter on the lens does sound like the better approach. Having never used one, how do I meter for such a configuation in the studio?

For your shooting, what do you see as the benefit of shooting at f/1.2 in the studio? At relatively close range you obviously know the depth of field is so shallow that you'll barely be able to have the facial mask in focus. If you're able to pull back enough to have the facial mask in focus then the ears, shoulders and back of the head won't be sharp. If you're trying to blur backgrounds you can certainly do that at much smaller apertures with an 85mm focal length on a full frame body.

If you choose to get an ND filter then I would suggest getting a 2 stop (.6) ND. Take a minute before you start using it and just meter and shoot a gray card with and without the filter to measure and make sure it's 2 stops. Sometimes it'll be off by a little and that's good to know when making exposure adjustments.

Then you meter a shot as you normally do and open up the lens by the amount of the ND. If the ND filter is 2 stops and you want to shoot at f/1.4 then you meter and adjust the lighting for f/2.8 and then set your camera for f/1.4.

Why not just shoot using the modeling lights at f1.2?

bobbyz wrote in post #8398288
Why not just shoot using the modeling lights at f1.2?

You lose the motion-stopping ability of flash. Also, tungsten modeling lights will produce different color temperatures at different power levels, potentially inducing color temp issues that are hard to fix.

The ND filter on the lens is a very common technique. Accounting for the light loss should be pretty simple. For a 2-stop filter, set your meter at ISO 25 and your camera at ISO 100 and you're good to go.

Using studio flash and then knocking down the aperture with NDs also helps to kill the impact of ambient light which can otherwise be a problem at f/1.2, even at max-sync.

All great points. Thanks. To answer Robert's question, I'm not sure what I might do with this capability. With a portrait, I could have a single eye in focus and blur the rest. Or I could have a prop in focus and subjects all around the prop out of focus. I'm just thinking it might be cool to experiment with this. The ND lens filter is the best solution since it also gives me creative options outdoors, and the same filter will work with the 85L and 50L. I like the idea of metering for ISO 25. That makes it really simple. And I will have to watch the shutter speed and introduction of more ambient light when I don't want it.

I went back and forth between the Hoya 2-stop and 4-stop filters. The 2-stop is just barely enough with a single 600RX at its lowest setting, and I was thinking the 4-stop might give me more flexibility...although a bit more difficult to meter correctly.

At the risk of sounding like an idiot (once again), I'd like to sanity-check what I'm seeing with my preliminary test results. I purchased the Hoya Pro1 ND8 (4-stop) filter. I metered my subject at f8 and 1/125 and took a couple of test shots using the 85L without the filter, set at f8 and 1/125 and ISO 100. The exposure looks exactly right, as you'd expect. Then I put the ND8 filter on the 85L and took shots at:

f1.2 and 1/125 and ISO 100
f1.2 and 1/125 and ISO 50
f1.2 and 1/250 and ISO 50

All of these were overexposed, but the third and last shot was overexposed by exactly 1 stop, according to Photoshop. So if I could have shot at:

f1.2 and 1/250 and ISO 25

it would have been just right. So what is this filter doing for me in terms of light reduction? I just want to see if my expectations were way off. Thanks.

I think you'll find that an ND8 reduces exposure by three stops so if that's what you have, that would probably explain it.