Cinematography: Shutter Speed Experiments

Over the past week I have had my first two lessons in cinematography, with the main subject of these classes being the relationships between exposure, shutter speed and f-stop.

The first exercise we completed in class was to look at the connection between shutter angle or frame rate and exposure.  Our camera was set up to film at 50i which gives interlaces footage.

We filmed a series of clips at different frame rates to experiment with this, and what we found was that filming with a higher frame-rate caused there to be less motion blur and strobing then the footage that was shot using lower frame-rates.  The higher the frame rate the more crisp the image became, and it began to look almost too clean.  The footage also demonstrated the relationship between the frame-rate, aperture and the resulting exposure of the footage.  As we increased the frame-rate the footage became more underexposed.  Something worth mentioning is that filming at higher frame-rates gives an opportunity in post-production to slow down footage without losing quality.   

Wanting to understand more about this I found an article that discussed the affect of shutter speed on video.   There was some interesting information there that talks about high speed shooting etc, but I also found a youtube clip there that demonstrates essentially what we were trying to explore in class (aside from the link to exposure). 



In animation we always rendered for HD delivery, using 25fps Progressive, so I wasn't sure what effect filming at 50i would have on our footage.  Obviously the main difference between interlaced and progressive is to do with the field order, or how the image is delivered on screen, but how filming in either format affects the footage was beyond me.

Since I really didn't know much about this format I decided I had better do some research about the difference between interlaces and progressive.  I started out by reading a number of articles which talked about this topic.

Essentially interlaced footage is delivered in two passes, or fields.  Each field contains half of the horizontal lines which make up the image, with one field containing the odd numbered lines and the other the even numbered.  Each field is displayed alternately, for 1/50th- 1/60th or a second (depending on the frame rate) which gives an illusion of a complete image

Most of the articles I read agree that Interlaced footage shot will deliver a crisper image of fast movement, but some say that very fast movement will cause softening of the image. Interlaced footage can also cause a strobing effect or artifacting when filming things such as striped shirts, plaid jackets or bricks.  Interlaced footage is sometimes considered to give a more news broadcast look to film, and I read that is can also give good results to sports etc.  

Progressive video displays the entire image at the same time, with each frame containing all of the image information.  Because all of the information is shown at once, the images appear smoother and sharper with less artifacting. It takes more bandwidth than interlaced footage but this is steadily being improved due to digital storage and transmission.  (ISF Forum, 2007).

Articles:

"Choosing your direction: Progressive or Interlaced". (2008). http://www.videomaker.com/article/13755-choosing-your-direction-progressive-or-interlaced

"What's the difference between interlaced and progressive video?". (2007). http://www.isfforum.com/FAQs/view/All-About-HDTV/What-s-the-Difference-between-Interlaced-and-Progressive-Video/33.html

"How does shutter speed affect video?". (2013). http://photography.tutsplus.com/articles/quick-tip-how-does-shutter-speed-affect-video--photo-12092