P+S Technik Image Converter (35mm Lens Adapter, DOF adapter)
MINI 35 Compact
“Creating the shallow Depth of Field”
This lens adapter creates the identical depth of field of 35mm film on a video camera with a smaller size of the sensor (Chip)
1. Why do 35mm film and large sensor HD cameras have so little depth of field compare to 16mm or small sensor video cameras?
A B
A. Here we see a hypothetical 25mm lens mounted on a 35mm film camera or an HD camera with a 35mm-sized sensor. The lens casts a circle of light, and the sensor/film gate “samples” the center of the image. The depth of field remains roughly the same for both cameras because the film gate and the sensor are roughly the same size.
B. Here’s the same lens, shooting the same subject at the same distance, but with a Super 16mm film gate or 2/3” sensor. All that’s changed is that we’re now sampling a smaller section of the image projected by the lens. We’ve gone from a wide shot to a close up just by changing film or sensor size, but the depth of field remains the same for a subject that distance from a 25mm lens. And while we’d expect a lot of depth of field for a wide shot, we don’t expect it on a close up--but try to throw the background out of focus on a 25mm lens in Super 16mm or on a 2/3” HD camera. It’s hard to do.
To shoot the same shot on all the cameras we’ve spoken about (35mm, 2/3” and 1/3") we’d be looking at using (approximately) the following lenses to keep the same angle of view as in the top diagram (a wide waist-to-head shot):
35mm film = 25mm lens (a lot of depth of field)
2/3” = 12mm lens (tremendous depth of field)
1/3” = 6mm lens (near infinite depth of field)
All will yield the same size shot, with the camera at the same distance to the subject, but with drastically different depths of field.
This is why you’ll never get much out of focus when shooting with an1/3 inch chip camera.
Depth of field doubles if you halve the focal length of the lens. Depth of field halves if you double the focal length of the lens. Depth of field doubles by stopping down two f-stops. Depth of field halves by opening up two f-stops.
If you want do see how much tighter your shot will be if you doubled the focal length of your current lens, just look at 1/4 of the frame--between the center cross hair and any corner. That’s the frame if you put on a lens that’s twice as long.
If you reduce the size of the sensor or the film gate by a factor of 2 (say between 35mm film and 16mm film), your depth of field will effectively double because you now need a lens that’s 2x as wide to get the same shot that you had before.
If you set up a shot with a 50mm lens, and then set up the same shot on a 25mm lens by moving the camera toward the subject, you’ll have exactly the same depth of field for both shots.
2. What is a 35mm lens adapter?
It is used to achieve shallow depth of field on a video camera whose sensor size is not adequate to achieve it natively. The term 35mm adapter is common, since most designs use a focusing screen (target) the size of a 35mm film frame and interface with lenses designed for 35mm cameras.
It resolves the 35mm image from the lens onto a ground glass located at the position of the traditional film plane. The image is then optically frozen, downscaled and transmitted to the camera’s CCDs. Since this is just a linear squeeze of two-dimensional image, all optical characteristics, like depth of field and angle of view stay the same. The video camera’s only responsibility is to record what is being projected onto the focusing screen (target / ground glass). This means that the video chip is NOT the place of image creation (this is the target), but of image recording.
3. P+S Technik, MINI 35 Compact
Image Plane / Pin for measuring focus
Back Focus Adjustment Ring
Adjustable ND filters
Target Speed Control Wheel
Run Button (Start Adapter)
· Very light sensitive, loss only ½ stop
· Power supply via camera, no extra battery needed
Recommendations
Choose a film-like frame rate and progressive capture (24P, 30P)
Choose a film-like shutter speed (exposure time) (1/48th, 1/60th)
Control your DOF with your F-stop and focal length (as you know)
“Lighting, framing, lens perspective and other important cinematographic techniques: they can optimize the visible result more than all the technical adjustment!”
Light Loss
The inherent light loss comes with attaching the unit to the front of the camera. Using any one of these adapters requires that the scene being shot by the camera be adequately lit.
Target Speed
Background Information
Static (non-moving) adapters suffer greater image degradation from low-light situations because texture on the focusing screen (target) becomes more noticeable. The camcorder used in conjunction with the adapter must focus on the focusing screen inside the adapter that is used as a projection surface. As a result, the camcorder also picks up the pits, dimples and/or specks in the material that give it its translucent properties. The solution to this problem is to shake, rotate or otherwise move the focusing screen (target) so that the texture of the screen is blurred. (The target itself carries a structural pattern to visualize the image created by the front lens. If the target is motionless, the pattern is stamped on the image and appears like some kind of dirt on the lens. Once you turn on the switch on, the target starts oscillating whereby the structural pattern appears random, as in a film grain-like manner.)
What is target?
The target is an element out of glass inside the Image Converter, which is located at the position of the image plane of the MINI 35. Often described as focusing screen or ground glass. It displays the picture, which is projected from the lens.
Adjust the target speed to its optimal speed. This has to be tested before shooting.
a. Connect the camera to a quality monitor
b. Shoot a flat and even lit surface like a grey or colored wall
c. Use a wide angle lens
d. Turn on the Image Converter
e. Stop down on the front lens until you can clearly see possible moving artifacts (this might only happen on a high F-stop)
f. Adjust target speed by turning the speed wheel until the movement of visible artifacts stops (it will be still visible, but no more moving)
g. Open the iris on the lens to your working stop.
h. Now you got the target speed, which gives you the freedom to use almost any lens without the need of readjustment.
In principle, a fast moving target leaves you more freedom than a slow moving target.
Low Speed / Medium / High speedPossible in case of low-resolution camera / Necessary in case of high-resolution camera
Possible in case of long exposure time / Necessary in case of shorter exposure time
Possible in case of telephoto lenses / Necessary in case of wide angle lenses
Possible in case of open aperture
* Very Quiet / * Maybe louder operating noise
Suggestion: JVC HM700 (with the setting: 1080, 24 or 30P, and 1/48th or 1/60th shutter speed) – Medium to High Speed (4-8) depending on the noise. Make sure that you’ll do some test shooting and check your adjustments. How? Use your laptop to see the tested image to make sure of the result.)
When do I have to readjust the target speed?
a. If the camera is changed
b. If the camera frame rate is changed
c. If your operation temperature changes heavily
d. Short focal length lenses are usually more critical than telephoto lenses.
Think about this:
In general, we don’t want to have a visible target glass. However, users use the target speed in a creative way and add grain to the picture intentionally by running the target in very slow speed. You really want to test before shooting and decide your target speed based on the “look” you’re looking for your film.
Back Focus Adjustment
With JVC HM700, It’s really easy to know when you need to adjust a back focus.
How do you turn on focus assist on this camera and what does it do?
: Focus assist button on handle or side of camera, when on B&W is out of focus, color is in focus.
If the back focus is off, then you won’t be able to use this “Focus Assist” function on camera as the camera “sees” everything as “out of focus.” Then, you need to adjust the back focus.
Unlock the back focus adjustment ring on the relay lens and turn it to adjust for best sharpness. Once you do this, you’ll be able to see “color” in “Focus Assist” function.
Also, if the distance engraved on the lens’s focus ring do not match to the measured distances to focused object, that’s another way you know that the back focus is off.
When do I have to readjust the back focus?
In case of changing the camera
In case of serious variation of temperature
Adjustable ND filters
The MINI 35 offers a second iris diaphragm. The DOF does NOT change by using these ND filters. This is only a kind of light valve.
This offers the opportunity to shoot in bright environment with a wide opened front lens aperture (in order to obtain a narrow depth of field) without overexposing the video cameras CCD-chip. But there is no influence on the depth of field.
Do not use the aperture of the front lens at T4 or higher in order to achieve the shallow depth of field and avoid the structural pattern of the target on your image. In order to control exposure, use the lens adapter’s iris (ND filters) and JVC camera’s ND filters.
Image Orientation
Image orientation is an issue. The adapter flips the image upside-down. The actual recorded footage must still be rotated 180 degrees in post-production.
Another option is to use a flip module. A flip module is another addition to 35mm adapter construction set that is made up of a set of two chambers lined with a series of mirrors that literally "flip" the image while shooting. This avoids the burden of needing an external LCD monitor, but at the cost of additional light loss. Many adapters already have a flip module within them, so the filmmakers do not have to worry about flipping the image in post-production.
P+S Technik Image Converter MINI 35 doesn’t have a flip module but you can “flip” the image with JVC HM 700 camera. How?
Do you remember the HM 700 test question #52?
*How do you flip the image being recorded? (In menu, go to “Camera Process” then to “Reverse Picture” and select “Rotate”)
4. Cine JVC HM 700 Package
· P+S Technik, MINI 35 Compact Image Converter is already mount on the camera body
· Comes with two cases
o Case 1: Matt box items (We’ll learn and practice to assemble these items.)
o Case 2: Nikon 35mm still camera lenses
a. Zoom (28-85mm) T3.5
b. Prime 105mm T2.5
c. Prime 85mm T2
d. Prime 50mm T2
e. Prime 35mm T2
f. Prime 28mm T2
g. Prime 18mm T3.5
Focal lengths in different formats
Film format / Wide angle / Normal / Telephoto16 mm / < 16 / 16 - 25 / > 25
35 mm / < 50 / 50 / > 50
5. Test footage screening and Discussion
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