ND8 filter and strobe exposure. Need to check something!

Hi all
So I'm going to be using an ND8 filter with one strobe on its lowest setting (1.5, Elinchrom), at 1/125th.
I'm getting f4.5 on the light meter. I want to go down to f1.8. So once I put the filter on, I'll put my fstop at 1.8 given that
it's roughly 3 stops down, and I'll keep my shutter at 1/125, but what happens to the strobe - do I bump it UP three stops?
I've confused myself with numbers yet again.
Please help
Thanks
Ps. This is in studio, not outdoors so it's not about blocking out sun or anything. Just the crazy DOF.

I am assuming that you cannot turn the strobes down anymore then what they already are? OK, so you are measuring at F4.5 (@ your desired ISO, of course), then adding a 3 stop ND8 will mean that you will need 3 stops more light to hit the film.

SO, you have 2 choices
1) Adjust your lights up so you have 3 stops more power
2) Open the lens up so you have 3 stops more light

So, since you want to shoot @ F1.8 (kind of close enough), you wouldn't touch the lights.

That being said, that if this was in the studio, if I had a digital on hand, I would be using that to preview what you are doing.

Yes, the flash won't go down further than that. I think I've done my calculating right, going from f4.5, to f1.8 will be roughly 3 stops. But just to check that I'm working it out right: say my scene is metering at f11 instead, and put the filter on, I'm going to need to come down to f4. Yeah?
It is in the studio and there is a digital on hand. Definitely will check it first before I sacrifice precious film! I have Pola too.
Would there be much of a difference in the image if you were to pump the flash output up rather than open up the lens?
It just comes down to giving the film the right amount of light - the method shouldn't matter...?

Can you move the flashes further away Holly ?, because Using The Inverse Square Law light reduces in intensity inversely to the square of the distance, so if the studio is big enough to move the flashes twice the distance away from the subject which will reduce the light intensity to a quarter of its brightnes, thus not needing N.D. filters

f/1.8 is 3 stops less than f/5, so bump your flash up 1/3 of a stop (or move it a smidgen closer) so that you get f/5 on the meter (also known as f/4+6 on most flash meters).

So your flash (without filter) measures f/5 at the subject at minimum power? If you put the ND8 on there, you will get enough light for f/1.8 - you can see that for yourself by metering through the filter. If you want to shoot at f/1.8 then just go ahead and do so by leaving the flash on min power and putting the filter on.

You're correct that shutter is irrelevant to all of this except for balancing ambient. However with large apertures like this, you are likely to get some ambient contribution so you might actually need to pay attention to the shutter speed - especially true if you've got natural light coming into your scene at any point. If you put the ND on the camera, you will cut down the ambient very nicely but it's quite hard to focus accurately. The other alternative is to put the ND on the flash - focusing is easy, but obviously it means you have 8x more ambient light coming in.

benjiboy said:

Can you move the flashes further away Holly ?, because Using The Inverse Square Law light reduces in intensity inversely to the square of the distance, so if the studio is big enough to move the flashes twice the distance away from the subject which will reduce the light intensity to a quarter of its brightnes, thus not needing N.D. filters

Click to expand...

Unfortunately the backdrop I need is in of all places, a bathroom! So there's basically zero options for moving the lights back. I have the shot all worked out apart from the DOF I want, so it's just a matter of getting the filter/light settings down.

polyglot said:

f/1.8 is 3 stops less than f/5, so bump your flash up 1/3 of a stop (or move it a smidgen closer) so that you get f/5 on the meter (also known as f/4+6 on most flash meters).

So your flash (without filter) measures f/5 at the subject at minimum power? If you put the ND8 on there, you will get enough light for f/1.8 - you can see that for yourself by metering through the filter. If you want to shoot at f/1.8 then just go ahead and do so by leaving the flash on min power and putting the filter on.

You're correct that shutter is irrelevant to all of this except for balancing ambient. However with large apertures like this, you are likely to get some ambient contribution so you might actually need to pay attention to the shutter speed - especially true if you've got natural light coming into your scene at any point. If you put the ND on the camera, you will cut down the ambient very nicely but it's quite hard to focus accurately. The other alternative is to put the ND on the flash - focusing is easy, but obviously it means you have 8x more ambient light coming in.

Click to expand...

Hiya, forgive me, I'm not following why I'd bump the flash up 1/3 rather than 1/2 a stop..? Flash is metering at f4.4 to be exact, on my trusty Minolta.
Another thing while I think of it - with focusing, can I not just focus, meter, do all that stuff, then pop the filter on and fire the shot? Why am I seeing so many people on various internet forums saying it's hard to focus..surely you just focus first then put the thing on? (I've yet to have the filter in my hot little hands, it's being delivered next week so I am a bit hypothetical at this stage.) Sure it's dark, but don't you just focus without it to solve that issue?

We seem to have some confusion over "f/4.5". It normally means "aperture is focal length divided by 4.5", which is 1/3 of a stop smaller than f/4. 1/3 again is f/5 and then a third again is f/5.6 - the next whole stop. However on a Minolta flash meter if it says "f/4 5" where the 5 is a little digit just to the right of the f/4, that doesn't mean f/4.5. It means "f/4 plus half a stop". Similarly, "f/4 9" means "f/4 plus 0.9 of a stop" (very very close to f/5.6) not f/4.9.

So if your flash meter reads f/4+0.5 and you take 3 stops off, you get f/1.4+0.5 which is about f/1.7 on the aperture dial - halfway between f/1.4 and f/2 on a mechanical aperture ring

With a stationary subject you can certainly put the camera on a tripod, focus and then add the filter - it's what I do when shooting with infrared filters. But it's a real hassle unless you're using a TLR (look through one lens, shoot through the other-with-filter) and it's not feasible for even the most slightly-moving subject or when shooting handheld. Since you're shooting indoors, focusing critically wide open is difficult anyway; doing it handheld with an ND8 on there is crazy-talk.

Edit: if there's no human in the shot, you could just light it with a lightbulb and use a longer exposure; no need for flash. Or you could use a smaller (battery powered) flash. It's really useful to have a couple speedlights on-hand because the good ones go to really-low power, they're easy to move around (no power cord!) and they're cheap. They make good accent & rim-lights if nothing else.