Your brain, frame rates and why you should have a variable ND filter

This is going to be a bit of a long post, but stick with it okay?

Everything* in the visual arts can be traced back to the way the brain
processes the information given to it by the eyes and ears.

Roughly speaking the brain sees and decodes a continuous stream of information coming from the eyes. There is a point at which this decoded data can merge together. This can change given different lighting and motion conditions. Roughly speaking the point at which the brain cannot decode individual pictures and see them as distinct frames is about 25 individual shots per second (this is a massive subject area and would take hours of reading just to start to understand, so go with me on that, okay?).

Because of this, when television was developed it only needed to show a picture 25
times a second** to convince us simple humans that we’re seeing fluid
motion rather than a series of still images. Because of the way early televisions projected these
pictures they could only show half the frame at a time. So if you
slowed down a television picture enough you’ll see it only changes the top half and then bottom half of the frame at a time (progressive) or
alternately refreshes lots of horizontal lines (interlaced). The miss-matching
of these projection techniques is why computer monitors have that
scanning look when filmed. To make 25 full pictures a second the television actually shows 50 part-pictures.

Why is this important to us as DSLR filmmakers? Because these 50 part-
pictures dictate our shutter speeds. The choice of shutter speed should, for maximum
quality of picture, be a multiple of the frames-per-second you’re
shooting. So if you’re shooting 25fps you should have a shutterspeed
of 1/50, 50fps should have a shutterspeed of 1/100.

You should also take into account what you’re intending to do with the footage. If you’re going to slow the shot down in post production you should shoot at a multiple of the native shutter speed for your intended frame rate division. For example if you’re shooting at 50fps and intend to slow the shutter speed down by half to 25fps you should be shooting at double the native shutter speed, so 1/200.

So your shutterspeed is fixed, how are you going to control exposure? There’s two options, one is the aperture and the other is the ISO. One of the reasons for using a DSLR is to get that beautiful depth-of-field and aperture is your key to that,so you can’t really mess with it too much. Thats just ISO left and you can’t go down in ISO only up, which adds noise. What to do? What to do?

Well the best piece of kit to easily control exposure without having to mess with your shutterspeed, aperture or ISO is a variable neutral density filter, something that has been present inside of television cameras for sometime. This piece of glass attaches to the front of your lens like any other filter and rotates just like a polariser. Rotation of a variable ND produces reduction in light entering the lens and getting to the sensor leaving you clear to shoot side open and at your native shutterspeed. Its also infinity variable so when filming continuously from a bright to dark environment you can just rotate the variable ND as you go. They’re not cheap, and you need to make sure the one you buy doesn’t cast a colour across your picture or vignette your shot.

But if you’re serious about using your DSLR as a proper video camera you’ll be wanting this little bit of glass eventually.

*mostly, probably
**different frames per second are possible. These are all processed by
your brain slightly differently, making different cinematic effects.