Sunday, December 30, 2007

The Redrock M2 Cine-lens adapter report Part 1 - Background

As previously stated, Nance and I recently purchased the Redrock M2 lens adapter for our Sony Z1 HDV camcorder. I thought I would break up our report on this in to two parts. This first part is focused on a bit of optical explanation. If you understand the reasons for purchasing the Redrock M2, you can probably skip this posting and move on to the product review post.

The idea here being to answer the question:

What is the Redrock M2 and why would you want one?

The simple answer is:

To allow for the use of other lenses on the Z1 and to allow fine control over depth of field.

However, I thought some of my readers here might not have a reasonable understanding of some of the principals involved with depth of field as well as an understanding of how a lens treats an image. I am not going to delve into optical theory in depth here. Rather I will present some of these principals in a cause and effect explanation. If you want more detail on these topics, I encourage you to search the web.

Depth of field.

What exactly is depth of field? Simply stated, it is the region in an image that is in focus. That's it. Nothing more. The depth of the field in focus in an image. So, why do we care about this? In cinema, it is typically used to force the audience to pay attention to something specific in the frame. As a general rule, we have two methods for doing this: color/brightness and depth of field.

Consider the following image that I used in my previous posting:

In this image, Eeyore is in focus, and Tweety, behind Eeyore, is not in focus. Now in the following image, the reverse is true:

Using this effect, we can draw the viewer's eye to a particular item in the frame. This is used to great effect in shots involving people in a conversation. The focus can be shifted from one character to another at will depending on whom the director wants the audience to pay attention to. As you can imagine, there are a wide variety of applications of this effect in the telling of a cinematic story.

Controlling Depth of Field.

We have 2 variables that we can adjust when using depth of field. They are:

  • What is in focus.
  • What is the depth of the area in focus.

To control the first variable, we merely adjust the focus control on the lens. This process shifts elements within the lens to draw the point of focus from the minimum focus of the lens (this is determined by the type of lens in use) all the way out to infinity.

The second variable is controlled by the speed of the lens, the setting of it's aperture and the siize of the film plane. Let's look at this in some detail.

The speed of a lens is expressed using what is known as an F number (Cinema lenses are rated with a T instead of an F - I will explain the difference below). It expresses the amount of light that a given lens will allow through the glass elements of the lens. A lens with a FAST speed rating allows more light through and a SLOW rating allows less. This is important in situations where the amount of light you have to work with is a consideration.

A fast rating is 1.0. There are almost no lenses rated at 1.0. A more typical rating will be 1.4 for a VERY fast lens. Medium speeds are typically 3.0 - 4.5. A slow lens might be rated at 8.0.

Another side effect of this lens speed rating is that it impacts the minimum depth of field. The faster a lens is, the narrower your depth of field can be.

So, how can we control the DEPTH of the area in focus in a shot? We do this with the aperture.

The Aperture.

The aperture, also called the iris, is a mechanical device inside of a lens that controls the amount of light that a lens will allow to pass through it. It consists of a set of overlapping slats of spring steel that create a variable sized hole. Consider the following diagram:

The smaller the hole is, the less light is passed through and the WIDER the depth of field is. Another way to think about this phenomenon is by using your own eyes. Many times, when we are straining to see something we will squint our eyes. This has the effect of sharpening the image that we see. It's the same affect that the aperture has.

Each setting of the aperature has an F number associated with it as you can see. And that directly relates back to the lens speed. When the aperture is all the way open, the F number for the iris setting is the same as the F number for the lens itself.

F number versus T number.

Typically, still camera lenses are rated with this F number (or F-stop) that we have been talking about. But cinema lenses, such as those that might be found on a Panavision or Arri film camera, are rated in T stops. T stops are considered more accurate. As I understand it, F stops are determined by using a formula that is applied against the lens's design formula and T-stops are determined by measuring the actual amount of light transmitted at the back of the lens.

The Film Plane.

This is our final stop on our journey to depth of field nirvana. The size of the film plane in a camera (the place where the lens focuses it's image for exposure or recording) will impact the amount of depth of field that the lens will allow us control over. The larger that this film plane is, the narrower the minimum depth of field will be. When we have a narrow minimum depth of field, we have more flexability in determining what will be in focus for our shot!

In a 35mm film plane, the amount of the depth of field can be a fraction of an inch (with a fast lens). That's great if you have a 35mm film plane. Not so great if you have an HDV video camera whose sensor is 1/3 inch in size. When that is the case, your minimum depth of field with is several feet.

That film look.

A number of things differentiate how something shot on film looks vs. High Definition video. The principal ones are:

  • 24 frames a second in film vs. 30 in video
  • The lattitude of film vs. video
  • Control over a wide range of depth of field

There are others, but these are 3 of the main ones. The first one can be overcome with cameras that provide 24 frame progressive image recording as well as some tricks done in post processing. Lattitude has yet to be overcome. Depth of field can be overcome with devices such as the Redrock M2.

The Redrock M2 simulates a 35mm film plane and allows your video camera to photograph the simulated film plane and as a result gain the advantages of 35mm film lenses and their associated control over depth of field. It also allows you to use other lenses on a camera that has a fixed, non-interchangable lens.

It does this by placing a film lens in front of a translucent material that mimics a 35mm film plane. The video camera then focuses on this translucent material and sees the image captured by the 35mm lens. Once this system is in place, all exposure and focus operations are controlled by the 35mm lens. The lens on the video camera remains at a fixed focus and zoom position.

An inverted image.

When a lens captures an image and projects it onto the film plane in the camera, it inverts the image. The physics regarding optics that govern this effect are far beyond the scope of this article. Suffice it to say that this phenomenon occurs in ALL optical systems where a device (be it a glass lens or a pinhole) project an image onto a plane.

Consider this diagram:

The image on the left is the subject being photographed. Once that image passes through the lens in the middle, the image is inverted and recorded onto the film or video sensor as you can see by the image on the right.

Now you might be asking "Why don't I see that inversion when I look through the viewfinder of my digital SLR?" And the answer to this is that the optics in the SLR invert the image again before it is displayed in the viewfinder (and, for that matter, on the LCD display).

Now, most devices such as the Redrock M2 do not provide those extra optics to invert the image. As a result, the image must be inverted in post. Additionally, when you use your video camera with a device such as the Redrock M2, your view on the LCD and viewfinder is inverted.

Redrock does supply a free Mac software application that lets you feed the firewire output of your camera into a Mac and it will display the live video image and let you invert it on the Mac. Very handy and very nice. Redrock is also about ready to ship an optical inverter that will go between the Video camera and the Redrock M2 that will invert the image on the fly.


OK, that was a lot of background. I hope I was able to clarify some of the typical questions that I get regarding these types of products. If you have more questions, feel free to post a comment and I will do my best to answer. And if I screwed anything up in this post, feel free to post a correction in the comments as well!

Check out part 2 of the review HERE.

1 comment:

Anonymous said...

This is surely a "Must read" article, really learnt a lot. Thank you so much for your hard work and dedication.

I will remember about you when I get my first public award :)