DeWijs Macro

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John = Hart=20

de Wijs Dual-Objective Stereo = Macro=20 Lens: Tests

John Hart

Program in Atmospheric and = Oceanic=20 Sciences

University of Colorado

Boulder, CO 80302

hart@tack.colorado.edu

nimbus.colorado.edu/hart/science.htm

April 10, 2006

In this technote we study the performance of the = de=20 Wijs stereoscopic macro lens on a high performance = digital=20 single lens reflex (DSLR) camera (17 Megapixel Canon 1Ds-2). = The=20 lens we tested is one of a series made by deWijs that offer a = range of=20 magnification from approximately 1:3 (magnification 1/3) to = 3.6:1=20 (magnification 3.6). This is based on the width of the = object versus=20 the width of sensor (i.e. magnification 3.6 =3D> 10mm = wide subject=20 on 32mm wide sensor). Our test lens, model A, had the = highest=20 magnification. These lenses are compact stereoscopic imaging = devices=20 that place a left/right pair on a single full 35mm frame. = Our goal=20 is to use this to visualize particle paths in small volumes of = turbulent=20 fluids in three dimensions. However, the test images shown = here are=20 simple easily-photographed common macro subjects.

Lens attached to the DLSR.

Front view, showing the pair of small diameter = lenses at the=20 front of the deWijs. The aluminum frame allows for = positioning and=20 setting the focus without using the viewfinder. Due to the = high=20 f-stop (45, 60, 90, increasing with magnification), the viewfinder = image=20 is quite dim.

Close-up of the twin lens system. Small = diameter=20 lenses allow narrow separation and facilitate an increase in depth = of=20 field. Separations vary between units with different=20 magnification. Here are the specifications and links to the = deWijs=20 drawings.

Model=20 A. Subject size: 10x14 mm. lens sep.: 6.5 mm. aperture = 90.
Model=20 B. Subject size: 21x30 mm. lens sep.: 9,5 mm. aperture = 64.
Model=20 D. Subject size: 37x50 mm. lens sep.: 12 mm. aperture = 45.
Model=20 E. Subject size: 52x73 mm. lens sep.: 14 mm. aperture = 45.
Model=20 F. Subject size: 72x96 mm. lens sep.: 15 mm. aperture=20 45.

Each lens paints a stereo (left/right) image onto = the=20 sensor. The lenses are designed for full-frame (36mm wide by = 24mm=20 high) digital or film cameras. A splitter at the back of the = lens=20 keeps the left and right images separate. The splitter = does=20 not interfere with the mirror system of the camera.

Because the viewfinder is dim, a focus guide is = included=20 with the lens. Its use is illustrated above. Some = calibration=20 will be necessary, because you probably want the center focus = point to be=20 somewhat in front of the vertical bars. There are screws at = the=20 bottom of the lens that allow one to move the guide in and = out.

de=20 Wijs Macro Lenses for stereo photography.

Sketch of the deWijs macro lens. (Image = from the deWijs=20 website)

Lens

Model
A
B
D
E
F

object fields
10x14 mm.
18x24 = mm.
36x48=20 mm.
46x72 mm.
72x96 mm.

K
11
22
40
56
80

L
18
27
50
78
100

M
~7
11
32
38
65

N
~3
4
10
20
30

O
-
10
12
14
15

P
-
128
96
88
83

S
~95*
124
175
242

Technical Data from the deWijs=20 Website (except row A, columns K - S, which were measured by = J.=20 Hart). All data in millimeters (mm).

* Depends on setting of focus guide, whose = distance is=20 variable by about 7mm.

K =3D focus guide width. L =3D focus = guide=20 height. M =3D usable depth of field. S =3D = working=20 distance (lens face to start-focus point). N =3D = distance past S=20 to center of focus (mid DOF).

The lens we had to test (courtesy of Jon Golden at = 3D = Concepts) was=20 model A. In some ways this is the most extreme (highest=20 magnification, lowest depth of field). Above is a ruler (in = 1/32"=20 units) photographed at 45 degrees. The apparent sharpness = spans=20 about 12/32" or so, which converts to about 7mm parallel to = the=20 optical axis.

Download=20 full-size, only slightly-compressed version of original image=20 (warning: ~ 2MByte file).

The camera records parallel pairs directly onto = its=20 sensor. Above is a raw image (reduced in size for the web, = of=20 course). It comes out in cross-eye format. There is a = narrow=20 fuzzy zone (about 5% of the width of one side of the cross-eye = pair),=20 arising from the septum splitter plate at the back of the lens = (see=20 back-of-lens picture above).

Color anaglyph of the raw image. You can see = the fuzzy=20 zone at the right vertical edge. The image directly out of = the=20 camera is well behind the window.

A simple shift puts the subject closer to the = window, but of=20 course this costs some sensor real-estate (i.e. some pixels are=20 lost). For the model A lens, we found that the shift costs = another=20 5% horizontally. Thus, between the required shift and = the=20 fuzzy zone, you can expect to lose about 10% of the image = width.

Here is an image of a resolution test chart (a = crude printed=20 one). Made with a flash, for stability. Note = that these=20 lenses are fixed aperture and fixed focus. Care was taken to = be=20 within the shallow focus zone by illuminating the subject with an = intense=20 model light and using the DSLR's viewfinder and a magnifier to = critical=20 focus.

Actual pixels of the small zone around the highest = resolution block of the test chart. The optics are limited = by=20 diffraction (and perhaps other aberrations). In order to = optimize=20 depth of field, these units have a very small numerical aperture = (roughly=20 1/(2 * f-stop). The chart is degraded somewhat from what you = would=20 see if you looked at it using a high quality microscope objective = having a=20 near unit numerical aperture (shown below). Of course, the=20 microscope view has essentially zero depth of field. An = important=20 question: Is there enough resolution to enable good quality=20 presentation of stereo images using standard printing and = projection=20 methods? The deWijs can be sharpened dramatically (using USM = in=20 Photoshop, for example). In fact this step is probably = necessary=20 when trying to make a quality presentation.

The smallest blocks of the printed resolution chart used above, = photographed using a high quality microscope.

For presentation in the form of stereocards, or = for digital=20 projection, the vertical format (i.e. the so-called "portrait = mode"), may=20 not be preferred. If you must crop the DeWijs frame to get = to a 4=20 wide by 3 tall (4:3) aspect ratio typical of digital projectors, a = fair=20 amount of image information will be lost. Doing this crop = can=20 alternatively be thought of as an increase in the = magnification. In=20 this process, defects in the lens, like it's diffraction errors, = will=20 become more apparent.

EXAMPLE IMAGES:

The images below are single sides of stereo = pairs. The=20 pairs can be viewed in various formats by clicking on each of = the =20 images shown, which activates our stereo image-server. The = images=20 have a maximum display resolution of 1024x768 per side. If = you want=20 to look at the full camera resolution, you can download some=20 slightly-compressed but fully-sized samples. The full-size = pairs are=20 about 4800x1800. All the images below were sharpened using = USM 150%,=20 1.9 pixels. This is fairly strong, but brings out details in = these=20 particular shots.

Download=20 full-size slightly-compressed stereo pair, (a, left, portrait=20 mode). Warning: large ~ 2MB file.

Download=20 full-size slightly-compressed stereo image (b, right, full = frame 4:3=20 cropped mode) Warning: large ~ 2MB file.

Download=20 full-size slightly-compressed stereo pair (warning: large = file).

In a projection shootout of these images, my = conclusion is=20 that the 4:3 cropped images appear just a tad soft. Square = or=20 portrait mode images look OK. Based on this, the latter = modes would=20 also make reasonably good stereo cards (where the projector = resolution=20 converted to 300dpi prints gives stereo-cards about 3 inches wide, = per=20 side),

CONCLUSIONS:

PROS:

Compared with other=20 methods using beam-splitters and such, this system is much = smaller,=20 faster to set up, and easier to acquire and process stereo = pictures!

You get instant stereo images - no inverting = images in=20 software or making beam-splitter corrections (keystone, color = shift,=20 resizing), etc.

The range of available magnifications cover many = common=20 situations (short of high-magnification microscopy).

Shifting the window and cropping to 4:3 (without = the=20 centerline fuzz), results in an image about 2400x1800 in size = PER-SIDE=20 (using a 17Mpix camera). Other full frame cameras can be up = or=20 downsized according to the square root of the megapixel = ratio. For=20 example, a 12 MPix camera would give images about 2016x1512. = At=20 300dpi you can get prints that are 7 x 5, or at 500dpi you get 4 x = 3.

CONS:

Critical focus on Model A was somewhat difficult = because of=20 the shallow depth of field. Careful setup and adjustment of = the=20 focusing frame is necessary.

The splitter at the back of the lens should be=20 protected. Care should be taken not to whack it. Use a = back=20 lens cap when the unit of off-camera.

Cropping necessary to get to 4:3 aspect ratio = costs a lot of=20 pixels (compared with a native 4:3 twin-camera beam-splitter = system, for=20 example). The increased magnification in projection (i.e. = spreading=20 the cropped image across the full width of the screen, vs. = spreading the=20 full raw deWijs image across the full vertical height of the = screen - and=20 accepting the resulting black bars on the edges), led to images = on-screen=20 that were acceptable but which appeared a little soft. =

Summary: Recommended. The fact = that this=20 is a relatively small single-camera macro system is a major=20 positive. The lower magnification units (e.g. models D - F, = say)=20 would appear to be easier to use than model A because they have = more=20 depth-of-field and larger fields of view. With model A you = have to=20 be quite precise in setting it up for a shot.

The main use for these lenses seems to be in quick = set-up=20 situations for subjects having motion. Synchronization = between left=20 and right sides is automatic. Flash works perfectly. = Compared=20 with other systems aperating at similar magnification, these = relatively=20 compact, light, and effective deWijs lenses have many advantages. =

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