Friday, October 29, 2010

Depth of Field, Sensor Size, and Rack Focusing.

I’ve had this thought for a while and it’s more of an observation. The effect of pulling focus (I prefer the term “rack focus” because that’s what I was taught to use.), which consists of starting a shot focused on one object, and then shifting focus to another object, has become a gimmick. Or, I suppose is quickly becoming one. For aspiring filmmakers, like me, the majority of training we get with cameras usually falls completely on small-sensored Prosumer (Professional-Consumer) grade cameras.

Panasonic HMC150

The small sensor in these cameras creates a very wide Depth of Field (DoF, the point of focus and the area in front and behind that is also in focus.), which all but eliminates the ability to use the awesome and dramatic rack focus. Getting a rack focus with these cameras usually involves moving the objects well away from each other, and placing the camera a considerable distance away and zooming in. this generally makes it unreasonable to perform, let alone do a good rack focus (and placing a follow focus on the camera just seems silly.)

Zacuto Follow Focus on camera



http://store.zacuto.com/z-focus.html

Here’s a pretty popular follow focus that can be used to very accurately dial in the focus of a shot.

So pretty much, the only way to achieve this effect was the high end professional cameras or film cameras. However, with the arrival of Full frame and crop sensors in DSLR’s (like the sensors used in the Canon 5dII and the 7d), the ability to rack a shot became much easier to accomplish.

Now, two things determine how easily a rack focus can be done: How open the iris is, and how large the sensor is.

With DSLR’s it’s relatively easy to get lenses that stop open to 1.2, or 1.4. Unlike the Prosumer cameras with fixed lenses that generally open to 3, or 4.5 (if you’re lucky.) **Edit: apparently the HMC150b shown above opens to 1.6, so that was a bad example. Haha**

So why does this matter? With the iris open, more light is let in and scatters around the inside of the lens, making the DoF shallow. As you close the iris, less light is let in… actually this is probably the most confusing thing my teachers refused to teach us. Just trust me that a shallower DoF comes from a larger Iris. If you’d really like to understand the science behind it, let me know and I’ll post some links to help explain it.

You can try a simple experiment to see this in action though. First, go outside and find a row of objects; Flowers, or columns, or something that you can view several of, side by side. Then pick your widest f-stop (lowest number) and take a picture. Then, switch to your smallest f-stop (highest number) and take a picture. I’d recommend setting the camera to Aperture Priority (Av on Canon, found on the circular dial on top of the camera) Which of these two pictures gave a more shallow depth of field?

The Dial on the left needs to lineup "Av" with the white hash.

**Note: something to keep in mind. Iris and Aperture are used interchangeably. They are measured in stops (with an “F” which stands for Focal Ratio.). the large the F-stop number (ex. 16, 22) the smaller the iris and the less light that enters the camera. The smaller the F-stop number, the larger the iris, the greater the amount of light that enters. If you stop-up (open iris), you double the light coming in. if you stop down (close iris) you halve the light.

Sensors! A larger sensor allows for more detail to be captured. We’ll keep it simple: Bigger is better when it comes to the relationship of sensor size and achieving a shallow DoF. This is why Prosumer cameras struggle to have the Shallow Depth of Field, that is found in Cinema cameras, their sensor is smaller.

So why did I say rack focusing is a gimmick? Can’t it be used dramatically the way God intended? Yes. But just like how alcohol can bring joy and elation, too much of a good thing is a bad thing. Like I said earlier, a lot of students like myself had a hard time achieving rack focuses. But now we can use this effect whenever.

I see it constantly in my videos and in other peoples. We focus on one flower to the next, or through a series of bottles. There’s no stopping it. Eventually we look for excuses to rack instead of using is subtly to draw the audience’s eye. It’s as if we learned to watercolor and now we think that we are Picasso.

I’m guilty of this, and I could point fingers at others who abuse this gift. But as I’ve been sitting, going through gig after gig of footage from the last five months, I realized that every other shot I have is racked. And as I pound my head against the wall, I know that if someone had brought me this footage, I might have accidentally harmed them for putting me through that. And that’s why racking your focus is in danger of becoming a gimmick (Think lens flares in the new Star Trek, or Transformers films. Or Styles of Color Grading being used in films like Transformers, the matrix and any apocalypse film… but that’s another blog post I’m still working on for later. Haha)

And the moral of the story is? Think before you rack. Modesty is a virtue. Rack with caution. I’d hate for any harm to come to you.

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