HD cameras require much more accuracy than 35mm film motion picture cameras. I am referring specifically to those cameras with small chips, such as the 2/3 inch chips used in most of the HD cameras today. Some exceptions are the Arriflex D-21, the Dalsa, the Red and the Panavision Genesis cameras. All of these cameras use a large sensor about the same size as a frame in a motion picture camera which is 24mm X 18mm or slightly larger depending on the camera. As the sensor becomes larger, the lens does not have as wide an angle and the back focus problem is not as great. The focus with the 2/3 inch cameras is very critical, as the image sensor is so small that lenses with a much wider angle of view are required. To see the same field of view as a 25mm lens on a 35mm film camera, the 2/3 inch video camera would require a 10mm lens. This is not even a wide angle view. On a 35mm camera, the field of view does not start to get wide until you reach 16mm or wider. In the 2/3 inch video camera world, a 6.4mm lens would be necessary to see the same field of view as a 16mm focal length lens on the 35mm motion picture film camera.

For the HD cameras there are Zeiss lenses with fixed focal length as wide as 3.9mm and Fujinon and Canon fixed focal length lenses as wide as 5mm. As lenses reach wider angles their back focus becomes very critical. As the lens iris opening grows larger, the back focus and other lens issues are even more crucial. The HD camera lenses are very fast and are often used at very large apertures to reduce the deep depth of field as much as possible. Many of the zoom lenses for the video cameras are also very wide angle and very fast.

What some of you may not be aware of is where a number of focus issues arise. Inside the camera there is an aluminum frame with a special prism attached to it. This prism divides the light coming from the lens into three colors: red, green and blue. The red, green and blue CCDs are attached to this prism at the three points where the light leaves the prism; red at one point, green at another point, and blue at another. In front of this prism cut filters are located. In front of these cut filters are two filters which are mounted into a turret that can be rotated from the camera’s exterior by the camera person. There are always two of these filters in front of the cut filters, at all times.

The aluminum frame mentioned in the previous paragraph presents a problem as aluminum expands and contracts continually as its temperature changes. Aluminum is used in these cameras because it is very lightweight. Remember these cameras’ ancestry is ENG (Electronic News Gathering) and with their use in documentary type work every ounce counts! The problem is that expansion and contraction of the frame affects the back focus to a great degree. Clairmont Camera’s solution to this problem was to create a frame made of a very specialized stainless steel that will neither expand nor contract within a temperature range of minus 100F to over 400F (anything above or below this temperature range will affect this metal). The use of this specific stainless steel has totally resolved the issue of flange depth changing due to fluctuation from the heat of the camera’s electronics as well as the ambient temperature. This very hard steel will neither bend nor flex. This is evident where the lens seats, as the surface is very hard and will not get burrs on it as the original aluminum frame lens-seating surface would. These burrs would affect the back focus and lens tracking.

We also found that the filters in the filter wheel are not always the same thickness or the same refraction index. If these filters do not match after you set the back focus and then change the filter, your back focus will need to be adjusted again. The solution at Clairmont Camera was to take ten cameras apart at the same time, measure all the filters and put into each camera filters of exactly the same thickness and refraction.

The prism block with its CCDs is installed on the new stainless steel frame. At Clairmont Camera, the block is positioned such that the back focus is the same on all of our cameras. The prism block has to be located so the center pixel of the CCDs is located at the optical center of the lens mount. This is done with a complex mechanical, optical and electronic process at Clairmont Camera.

In Clairmont Camera’s opinion the following problem is a silly mistake. The image plane mark on the camera is not at the effective image plane. This is true with all the 2/3 inch electronic cameras, as the mark is incorrectly placed at 46mm; the true mark should be at 65.05mm. This is an error of 19.05mm (3/4” of an inch). The Zeiss lens focus scales come from the factory engraved for this true distance of 65.05mm. At Clairmont Camera all brands of lenses’ focus scales are set up for this true distance. When you purchase a set of Zeiss Digiprimes, they provide a label which is to be placed over the camera’s original image plane mark to correct the true mark. The problem was created when the camera manufacturers set the image plane mark in a location that would be correct only if the filters and the prism were not located between the lens and the CCDs. I can only surmise that this may have had something to do with the old Black and White single tube cameras that have no prism. Apparently, this old standard was used and never changed.

The next issue is with some of the lenses themselves. Both Canon and Fujinon use a few fluorite elements in their lenses. Fluorite allows a very flare free image, however the fluorite elements change optical characteristics as the temperature shifts. Unfortunately, this will affect the front focus of the lens which in turn alters the accuracy of the focus scales as the temperature changes. As a result, many of the telephoto lenses for still cameras have a moveable witness mark. The manufacturers’ suggestion is to look at infinity with the lens and focus until everything is sharp. Next align the lens witness mark with the infinity mark on the lens and then the entire focus will be accurate until the temperature changes. You could also measure off a set distance from the correct image plane mark on the camera (if your camera has a correctly positioned mark) e.g. 5 feet and focus until sharp and then set your witness mark to align with the lens’ 5 foot mark. All the marks will then be accurate. Zoom lenses for the SD and HD cameras do not have a movable witness mark installed by the manufacturer. At Clairmont Camera we have made movable marks for our HD zoom lenses to resolve this situation.

To set and check your back focus, I suggest using either the Century Precision back focus checker or the Zeiss Sharp Max. These collimators only take a moment to use and provide 100% accuracy. Place the collimator in front of the lens and set the lens at somewhere near infinity (not critical at this step), and wide angle with the iris wide open. Adjust the back focus until the collimators’ image in the camera’s finder or on a monitor is sharp. Next set the lens to telephoto and focus the lens by turning the lens’ focus until the collimator image is sharp. Now look at the witness mark. If your lens is from Clairmont, move the witness mark to align with the infinity mark. If the lens does not have a moveable mark, make a new mark with a piece of white camera tape and mark it with a ball point pen. You will need to do this a few times during the day as the temperature of the lens changes. Once you do this, all the marks on the focus scale will be correct if your camera has all the corrections noted previously. Even if your camera does not have all the corrections noted, you will still be better off than you were before.

Zeiss does not use fluorite elements so this is not an issue with their lenses. I do not know if Panavision uses fluorite elements in their HD lenses.

A common complaint is that 2/3 inch HD cameras have too much depth of field. First camera assistant Andrew McDowall explained to me that he would focus the lens short to have the background out of focus similar to what you would get with film. For example, let’s take an object you want sharp that is 5 feet away, but you also want the background to fall off like it would be with 35mm film. For a certain field of view that requires a 25mm lens on a 35mm film camera you would use a 10mm on a 2/3 inch HD camera. With the lens set at T2.8 and 5 feet, the 35mm film camera’s 25mm lens would have a depth of field of 4 feet 3 inch to 6 feet. Using the 10mm lens on the 2/3 inch HD camera with the focus set at 2 feet 9inch and lens opening at the same T2.8, the background would be soft at about the same 6 feet as the 35mm camera 25mm lens. The depth of field in the background would look the same as a film camera. Items in the foreground would be in good focus beginning at about 2feet and would look good in most shots.

I hope that this advice has given you a better understanding of focus problems that you may have had previously with video cameras and will prevent you from having any future focus problems. I have seen projects projected on a 40 foot screen and have been shocked at how sharp a 2/3 inch CCD camera can be!