January 21, 2009
By Jay Holben
There’s no doubt that smaller-chip DV, HDV and HD cameras have put more high-quality, inexpensive cameras into more users’ hands than ever before, but the cheaper, smaller chips come with a few compromises. In last month’s column, I discussed the problems of achieving a wide-angle field of view with a small-chip camera, and how to incorporate lens adapters to compensate. This month, I’ll discuss a different kind of lens adapter, the “film lens” or “35mm” adapters such as the P+S Technik Mini35, Redrock Micro M2 and Letus35 Mini, which are designed to work with small-chip cameras with fixed lenses.
Above: My Canon XL2 fitted with a Letus Ultimate lens adapter and Lens Baby unit.
The primary advantage to using one of these adapters is to create more “film-like” depth of field. With these adapters, it isn’t the 35mm lenses that give you the shallow depth of field; it’s the larger size of the target (secondary ground glass) that creates this result.
Depth of field is an optical phenomenon whereby a range of space within an image is rendered into acceptable focus through a combination of target size, lens focus distance and aperture size.
Notice that I didn’t include focal length in any of the factors above. As I’ll discuss later, everything being equal, focal length does NOT affect depth of field. I’ll let that sink in a bit and address the focal length misnomer a little later.
Why is shallow DOF so coveted? Not only does it provide a more “film-like” image, it provides the visual storyteller with an essential tool for focusing the audience’s attention on a specific portion of the frame. It communicates to the audience that this space is what is important in this scene.
Breaking down the components that define depth of field, I’ll start with the focus point. This is the distance that you set your lens to, generally at the position of your main subject. Technically speaking, there is only one plane in any photographic image this is perfectly in focus. The rest of a particular range of depth of field may fall into “acceptable” focus — that is, the subjects within that defined space are rendered sharp enough to the eye to appear in focus. The further away your focus point is from your focal plane (the camera’s chip), the more depth of field you will have. With the camera focused at 2', you might only have 1-2" of depth of field, but focused at 50', you might have 20' or more of depth of field. Below: The same subject photographed with (from top) an 18mm lens from 1' at f/5.6, a 36mm from 2' at f/5.6 and a 72mm from 4' at f/5.6. Note that the change in field of view limits the background seen, but the depth of field remains consistent.  
Aperture size is one aspect of depth of field that you have the most control over. As you tighten your iris/aperture, the depth of field grows. As you open the iris up, depth of field decreases.
Lens adapters such as the Redrock M2, Letus35 Mini and P+S Technik Mini35 allow the user to incorporate larger-format lenses from still or motion picture cameras and artificially create a larger target size by incorporating a secondary ground glass inside the adapter, upon which the image from the larger lens is projected and then re-photographed by the smaller-chip camera’s fixed lens.
This process of re-photographing the image allows the adapter to take advantage of the larger-format target size, generally that of a 35mm film frame, and therefore adopt the depth of field properties of the larger format. A ground glass is a piece of optical glass that has been ground down on one side to give it a rough texture so that an image projected on it can be seen rather than just pass through the glass. As the camera’s fixed lens is focusing on the glass itself, it is also focused on the texture of the ground glass, which will end up in the final picture. To avoid this, all of the adapters mentioned above incorporate a vibration, oscillation or rotation of the ground glass so that the texture of the glass itself is rendered invisible.
It is, of course, important to note that when you incorporate multiple lens elements, in addition to the camera’s lens, you’re going to lose light transmission through the additional elements. In most cases this is desirable, as you’re looking to shoot with a wide aperture to achieve shallow depth of field, but it is important to note that these devices generally require up to four times the amount of lighting you would need with the small-chip camera alone.
The depth of field is then defined by the iris set on the larger-format lens and the focal distance to the subject. Exposure is controlled with the camera’s iris.
As the larger-format lens projects its image onto the adapter’s ground glass, it is — by the nature of optics — upside down. This means the camera is recording an image that needs to be flipped in post. It also means that the image in your viewfinder is upside down. Some cameras, such as the JVC GY-HD250U, have a built-in image flip so that the display is right-side up. Some users incorporate an external LCD monitor mounted upside down onto the camera to view a corrected image. (The Letus35 incorporates an internal prism in the adapter that corrects this problem.)
Note that as I broke down the various aspects of depth of field, I did not mention focal length in this equation. Although it is optically correct that a longer focal length creates larger circles of confusion and, therefore, less depth of field, where this becomes a misnomer is in the idea that if you want less depth of field, you can achieve the effects with a longer lens. This is, in fact, incorrect. It’s important to understand that I’m comparing apples to apples here. Meaning that for a given composition and subject size in the frame, the focal length of the lens you choose is irrelevant to your depth of field.
As I mentioned above, the focal position plays a significant role in depth of field. This optical factor is what cancels out the larger circles of confusion for longer focal length lenses.
For example, let’s say you’ve lined up a shot of your subject with a 50mm lens and the subject is 5' from the camera. This gives you a depth of field from 4' 10" to 5' 3", equaling a total of 5" of DOF (according to the ASC Manual, 9th edition). You decide you want less depth of field, so you switch to a 100mm lens. However, in order to keep the same composition and framing of your subject, you now need to move the camera further away. Suddenly you’re on a 100mm lens at 10' from your subject and your depth of field is now 9' 10" to 10' 3", or a total of 5" — exactly the same as when you were on a 50mm lens only 5' from your subject. The fact that you moved the camera further away, thereby requiring you to adjust your focal position to a greater distance, you will have the same depth of field as your previous composition with the 50mm lens at 5'.
Now, by moving to a longer lens from further way, you will have compressed the background elements and this can sometimes result in a perceived more shallow depth of field, but, really, what you’re seeing less of is the overall background area and you’re compressing the background closer to the subject, not reducing the actual area of the frame that is rendered in acceptable focus.
All in all, these lens adapters, when correctly utilized, can help to refine the image created by small-chip cameras and present the filmmaker with another powerful tool in their storytelling arsenal.
|