Ryan E. Walters, Cinematographer

Cinematic Excellence at 24 Frames a Second

Why You Should Be Using ND Filters For Interior Work

Last week I talked about the importance of in camera filtration, and this week I am going to address why you should be using ND for interior work. Typically, ND filters are thought of as exposure tools that are only used for scenes that take place outside. While they may have started out as a tool that was primarily used for exteriors, that is no longer the case. As camera technology continues to advance, the ND filter is playing a bigger role when shooting indoors.

If your background is primarily in shooting with DSLR's, you are probably scratching your head, thinking I'm crazy. And I don't blame you. :) With the rapid rate of change in the camera market, it is hard to keep on top of all of the tech, let alone how it is affected by camera filtration. However, if you follow along with me, all will become clear...

Most all of us have worked with DSLR's in some fashion or another, whether it was on a personal project, or on a paid gig. When shooting with a DSLR, like the 5D MKIII in video mode, it is standard practice to dial the ISO up and down according to the light levels. (Day exterior you'll be around ISO 100, midnight in the city, you might be at ISO 2,500+). Not only will the ISO be determined by light levels, but it will also affect noise levels. A lower ISO will yield a cleaner image than a high ISO.

These practices work great on most DSLR's, as the distribution of their dynamic range remains relatively consistent. So, if we take a hypothetical DSLR camera that has +3 of range in overexposure, and -3 stops of range in underexposure, this distribution is constant at any given ISO. At ISO 100 or ISO 2,500, you will only get +3 over and -3 under. The reason you can see more at ISO 2,500 is due to gain being added inside the camera. And, thanks to technology, we are getting cleaner images and more sensitive cameras every day.

As long as you use a DSLR in video mode, you can safely dial the ISO up and down according to your exposure needs. The trouble comes when you make the switch to a camera that shoots in RAW or in a Log format. If you have a camera that shoots in either of these formats, then you have to change the way you think about ISO. The old way of thinking no longer applies (which is why I prefer the term EI).

(You might also like: The Power Of Camera Filtration)

With a RAW or Log based camera, when you change the EI, you are shifting the point at which mid tone falls in the dynamic range of the camera. Although the graphic below is from the Canon C100, the same applies to other RAW or Log based cameras like the Red & Alexa.

Distribution of Dynamic Range in the Canon C series of cameras.
Canon has declared that the base EI of its C series of cameras is 850. At this EI, there are 5.3 stops of over exposure and 6.7 stops of under exposure. As you move to the right, gain is being added to the signal to increase the camera's sensitivity. And, fortunately, the dynamic range distribution remains unaffected. Now take a look at what happens to the left of EI 850. As the EI decreases, the mid tone moves up, which in effect lowers the highlight latitude and increases the shadow latitude. Each stop that you lower the EI, you trade one stop of highlight detail for one stop of shadow detail.

So what in the world does this have to do with using ND's? Everything. It is this change of the distribution of the dynamic range that requires the use of ND filters. If you take your RAW or Log based camera outside, and you apply the traditional way of thinking about exposure to the camera, you will be in a world of hurt. Outside there will be a lot more light, and so you may want to decrease the EI setting on your camera. But if you take your EI 850, and lower it to EI  200, you have just removed 2 stops of range in the highlights, giving yourself 3.3 stops of overexposure latitude. With only 3.3 stops of highlight range, your whites are going to clip quickly, which will give you that "oh so lovely" digital look.

The only way to avoid that look when you are shooting outside with a RAW or Log camera is by keeping the EI set to the base sensitivity of that camera (for the C series cameras, that is EI 850). Then you have to control your exposure externally through the use of ND filters.

This same thinking applies when you are shooting inside as well, especially when there is a window or door in the frame. If you are lighting your talent and there is a window behind them with sunlight pouring through it, the first temptation is to knock down the EI of the camera. But just like shooting outside, if you do that, you will only be making the problem worse for yourself. The window that was 6-7 stops overexposed to your camera is now going to be 7-8+ stops overexposed depending on how low you set the EI. And this again will give you the "lovely" digital look...

Instead what you should do in this case is to add ND to the camera to bring the window to the exposure level that you want it at, and then add more light to your talent. This will typically be the quickest and easiest way to control your exposure on set. The other alternative is to use ND gel on the windows, or fly a large 8' x 8' or bigger net over the window. Either way, by keeping the EI set to 850 (for the C series cameras) you ensure that you are getting as much over exposure latitude as the camera has to offer.

When shooting with any RAW or Log based cameras, it is crucial that they are shot at their base EI and ND filtration is used to help control the exposure (at least if you want to deliver film-like images that have a good roll off into the highlights). As these cameras get more affordable and accessible to everyone, not only will the thinking about exposure have to change, but investing in a quality set of ND's will become imperative. This is why I ran the Great ND Test of 2013.

Have you been using ND filters for your interior work? If not, why? How have you dealt with controlling the exposure? Do you have any other recommended uses for ND filters?

Until Next Time - Get Out There And Shoot!

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