A comparison of shutter speeds and techniques for slo-mo footage

Back in this article I rasied the question of which shutter speed would be best if you intended to turn your 720@50fps footage into slo-mo. I wasn’t 100% sure, and there was some debate in the comments about which techniques produced the best results so I thought I’d do a little test.

I shot the same exciting footage of pouring a glass of water on a 7D set to 720@50fps using 1/100, 1/200 and 1/400. I then converted it to ProRez422HQ and conformed the 1/200 and 1/400 from 50fps to 25fps in CinemaTools. I then stuck it all into FCP7 into two timelines, one 50fps and one 25fps. As there wasn’t a conform to 12.5fs option in CinemaTools to quarter the 1/400 I took the halved 1/400 footage and slowed the speed by half using FCP’s speed tool, leaving blend/scale/ripple all on.  Below are the two results exported as 720 H.264 25fps and 50fps.

50fps timeline

25fps timeline

I having looked at the clips a number of times, zoomed in on the detail, gone through them frame by frame I prefer the 1/200 slowed by CinemaTools to 25fps in the 25fps timeline. If you were going for a really slow shot I’d defiantly suggest the 1/400/CinemaTools/FCPspeed option as although a little soft, its still very much usable.

But that’s my opinion, and as with all these things its down to personal taste. So which do you prefer? Comments are just below. And if you’d like me to re-do the test in anyway just shout and I’ll see what I can do.

A little something about Picture Styles

Rather than recording all the data captured by the sensor like cameras like the RED (a format loosely known as RAW) most DSLR’s when shooting video save data via a filter algorithm (known as a codec), in Canon’s case the widely used H.264 or MPEG-4.

It’s a simple flow of information from sensor through filter to card. This filter algorithm works out what information isn’t needed to be remembered for the video to still look okay. Its much like the way music is stripped of frequencies not audible to the human ear by the MP3 codec. It’s a simple case of space-saving for the card and buffer.

Now in most Canon cameras the filter algorithm is called the ‘picture style’. These are pre-set filters made by Canon, aiming to record the right information to make your video look its best, given you match what you’re shooting with what the picture style is called. So for example, if you’re shooting a landscape Canon think their ‘Landscape’ picture style would save the right mix of pixels to the card to make your video look just right.

Now these might be fine when you’re shooting photos in JPEG (they were originally designed to process photos not shot in RAW), but if you’re shooting video you’ll want to have control over what is saved to the card. Why? Well to properly colour correct your footage and get its looking its best you’ll need the right mix of properly exposed pixels to work with. So its really nice to know that Canon let you design and install on the camera some of your own picture styles. These can be designed in detail in the EOS utility and uploaded to the camera or quickly made in-camera by changing the Sharpness, Contrast, Colour Saturation and Colour Tone. You’ll also want to look at the White Balance Shift to take out any colour casts your lens or sensor may be making.

Now I know what you’re thinking, ‘Nick, what’s the best picture style for video?’ and the answer is it all depends on what you’re shooting, where you’re shooting and what you need to get out of the pictures. General consensus is that flatter and closer to a RAW photo the profile is, the more information is stored and can therefore be played with in post production. If you don’t want to put the time in messing with it in post you can always use a picture style that emulates your favourite film stock or effect. There’s loads of user defined styles out on the Internets so have a Google, load them up onto your camera and see what you like.

Personally I’d rather get a nice flat images so the camera records as much detail as possible in what will eventually be the highlights and shadows. This gives me more latitude to play with things in post and see where I want my image to sit in the grand scheme of detail and exposure. It’ll also give any colour correcting effects the maximum opportunity to keep detail in the picture.

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.

Hello there!

Welcome to ExposureHD. We are about to launch a full site all about our learning of film making with Digital SLR cameras. From the 500D through to the 1D from Canon, and the Nikon D90, we are very exciting about this new dawn of film making with simple, low cost equipment, empowering all manner of people previously with no access to such equipment.

We aren’t historic filmmakers, we are a group of photographers, developers and authors who are taking a new interest to the web. As such you’ll see us post all manner of stuff we’ve tried and tested, things we’ve seen online and in person and much more, hope you enjoy the site!