ND Gel, how many stops?

Specs say: Incident Exposure Reading ~ ISO100 1/60 f/1 (Sekonic L-358 @ 1m) which at 800 ISO would be f2.8.

On a sunny day, look at the sunny 16 rule: if your ISO is 200 at f/16, then your shutter speed will be 1/200 seconds. At 800 ISO that would be f16 at around 1/50th (which is not exact same comparison as photometric specs of lights but close enough)

You are going to need 4 stops of ND gel if you want outside to be 1 stop bright then your subject at 1m away from the Fotodiox.

Now all this is just theory and can vary in the real world and especially with your actual setup, so I recommend you have a few rolls of various gel strengths just incase. Also get yourself a light meter, do some of your own measurements and take the guess work out of it completely.
 
In the WYSIWYG era, you can just take any decent scouting camera and dial down the exposure until you like what you see out of the windows, then take note of the adjustment and apply the appropriate amount of gel. I've done it for years with a combination of guesswork (which is a form of know-how) and meter readings but it's so much easier and faster to use the camera.
 
I used to roll with ND 1.2 (no pun intended), but with today's cameras I find ND 0.9 my personal sweet spot.
Of course you can always bring both and audition them in front of camera.
 
The responses above are all good advice.

I'll reinforce what was recommended about having various densities of ND gel on hand. If you can only have one density, go with ND 0.9.

All of this is a function of the nature of the light conditions / time of day when you shoot AND a function of the view through the windows. <<that part of the equation is often forgotten when advice is being given. A window that has a bunch of evergreen bushes filling up the view is going to be a different situation than a window where the view is mostly sky / skyline.

The issue with doing interviews with windows as a background is that interviews can often run a long time. And many times you cannot know when the interview will wrap. ( as opposed to a commercial production or a feature film production where you have a schedule and a better idea of how things will go. ) Sometimes with interviews in tv / corporate you get done when you get done. The interview is all you are shooting that day and the length of the interview is a function of how well the interview goes. I've lit an interview that started at Noon and was supposed to last "an hour or so" but went well past sundown. The worst is when you ND the windows for morning light levels and then sit there captive watching the exposure of the windows go to sh*t because the interview runs long. And the Producer doesn't want to "break" the interview to make any adjustments.

I generally try to avoid having windows in the background of an interview I less I am confident that the interview will be short in length and I have with me a light with some punch to compete with exterior light- a 800w HMI or, better yet, a 1200w HMI.

I am often amazed at Corporate-TV Producers who have been working in video for a long, long time yet they still suggest shooting an interview with a city skyline window-view as a background, wholly unaware of the issues that creates for a DP who has only a limited lighting package available and a "lighting crew" consisting of a Sound OP who still sets C-stands lefty-lucy despite being reminded "righty-tighty" what seems like a thousand times.
 
And all of this can change depending on if it's sunny, cloudy, rainy, etc...

That being said, when we shot a regular show segment in this restaurant about 12 years ago(betacam), we ran with .9(3 stops), but I think the windows already had some tint on them. But back it up a few weeks ago and we were shooting a series of sit downs in an office building and I wanted to shoot wider and use the environment and the windows. A few days were cloudy and I was able to achieve the look and exposure I wanted(somewhat blown, but still with detail) with just my Astra's in soft boxes. The last day the sun was out(actually somewhat in and out) and we put 2-stops on the window and I was keying with my 800 HMI in a 24x36 Chimera.
 
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I should also have mentioned that I'm lighting an interior to a 2.8/4 split at ISO 800 in log mode, so on most modern cameras I'm pushing 14 stops DR, which is why I reduced my 1.2 ND preference to 0.9, generally speaking. The angle of the sun on the b/g also plays into this. Never forget to track the sun when windows are present so you can be prepared when the sun comes over the roof of your building and starts heating up those light colored building in your b/g.
 
A critical factor is what size instruments you are working with, what your key is built out of (are you pushing through diffusion, into a bounce, through a softbox etc). As Bill noted above, you have to start with the stop you want or need to meet with your given setup and work backwards from there. If you have a limited amount of firepower, you are safer going with a heavier diffusion as you can always knock down your interior light. The only risk is if you are shooting into the end of day and you have the possibility of the levels outside dropping down to where you can't open up/boost ISO any more. Then you'd have to take the time to pull the ND off.
 
bill totolo said:
. The angle of the sun on the b/g also plays into this. Never forget to track the sun when windows are present so you can be prepared when the sun comes over the roof of your building and starts heating up those light colored building in your b/g.
Click to expand...

2 handy smartphone apps for this..... LightTrac and Sun Seeker
 
I've had Sun Seeker for YEARS and it used to be dead spot on when I had my iPhone 4S, but with my 6 and 7 it is always off. Not usually enough to make a real difference, but it's still annoying. I don't know if it's changes in the phones or in updates to the app...?

Also, be careful of the environment your trying to use it in. I've been around larger metal structures that throw the compass off and also in the area of military bases(even a few miles away) and it won't work.
 
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