Experimenting with

Lenses in Stereo

By George Themelis

NSA/ISU - August 2017

Workshop notes will be made available at the Convention web site: http://www.3d-con.com/

DrT’s Blog: http://drt3d.blogspot.com/

• 202 Newsletters • ~1600 pages • Tutorials and other information • Price: $35 (goes to the club)

Stereogram CD

Lens

Camera Body

Sensor

FL = 6.3 - 18.9mm f3.7 - f8.0 wide f4.2 - f9.0 telephoto

$3000-$5000

?

• Camera equipment

• Where to find and how to buy lenses

• Crop factor (using lenses in different cameras)

• Long Focal Length (telestereo)

• Short Focal Length

• Relationship between Focal Length & Stereo Base

• Magnification, Distance, Convergence Ratio

• Maximum Aperture (Using “Fast” Lenses in Stereo)

Summary

Camera Equipment

1. Stereo camera w/auxiliary lenses

2. Single Camera w/interchangeable lenses

3. Twin Cameras w/interchangeable lenses +

variable stereo base

My personal journey:

Film: In addition to Realist and RBT S1, I used RBT stereo cameras with interchangeable lenses. I always supplemented my stereo cameras with a twin synchronized camera system (started with twin Minolta X700 cameras).

Digital: Compact cameras wired by Ekeren. Micro 4/3 system (Panasonic). Currently I am using twin Samsung NX1000, which offers normal stereo base at near spacing, variable stereo base, good synchronization.

Sources for lenses:

Buy new or used (ebay, etc.) Three types of lenses:

1. Camera manufacturer (OEM)

2. Third Party Lenses (Generic)

3. Vintage / Legacy Lenses

Buying Used /Vintage / Legacy Lenses

Older lenses often have issues with: • Fungus • Haze • Element separation • Excessive “dust”

Always inspect a lens using a flashlight

Make sure that you can return the lens. Ask before buying about the optical condition of the lens.

Adapting Vintage / Legacy Lenses Vintage lenses can be adapted to many digital cameras using simple and inexpensive adapters:

These are hollow tubes serving two purposes: 1) Adapt the lens mount to camera mount 2) Adjust for infinity focus

No communication of the lens with the camera: 1) Manual Focus, 2) Manual setting of f-stops

Crop Factor

From: https://photographylife.com/what-is-crop-factor

Digital Format Sensor

(mm x mm) Crop Factor

Full frame eq. of 50mm lens

Full Frame camera 36 x 24 1.0 50 mm

Samsung NX (APS-C) 23.5 x 15.7 1.5 75 mm

Micro 4/3 (Panasonic) 17.3 x 13 2.0 100 mm

Samsung and Panasonic report all lenses without taking into account the crop factor.

Samsung:

20-50mm zoom ~ 30-75mm

50-200mm zoom ~ 75-300mm

45mm lens ~ 70mm

Panasonic: 100-300mm zoom ~ 200-600mm

What Vintage Lenses to Buy?

• Crop factor favors long FL lenses

• There are plenty of excellent lenses sold at very low prices. Example: Fast prime lenses, long lenses, macro lenses.

• Only certain iconic brands or rare lenses or lenses w/cult following, are still expensive.

Minolta 50mm f3.5, my first lens ~$300 in 1988. Under $50 (gently used) today.

Minolta Micro Bellows 25mm lens Expensive in 1988, expensive today

Minolta MD Lenses

How I Store & Carry Pairs of smaller Lenses

• Use generic lens caps

• Glue/tape pair of back caps

• Keep them in pairs

• Protect with OP/TECH foldover pouches

• Use rubber shades

Two Lens Metrics:

⇒ Focal Length Affects field of view

⇒ Aperture Determines the mount of light &

Depth of Field

Example:

Samsung NX 45/1.8 Maker: Samsung, mount: NX Focal Length: 45mm Maximum Aperture: f1.8

In the rest of the presentation we will look into the effect of these two metrics and how these lenses can be used for stereo.

There are other aspects of a lens, for example, close focusing distance. There are specialty lenses, like close-focusing / macro lenses, etc. We will not cover these lenses. We will “focus” on these two metrics.

Focal Length

From: http://www.digital-photography-student.com/wp-content/uploads/2009/11/Focal_Length_Diag.png

Classification of Lenses based on their Focal Length

- Ultra wide lenses: 20mm or shorter

- Wide angle lenses: 20-35mm

- Normal lenses: 40-55mm

- Short telephoto lenses: 60-135mm

- Long telephoto lenses: 150-300mm

- Super telephoto lenses: 300mm or more

Long Focal Length Lenses

Telestereo - Telehyperstereo

Applications:

• Scenics / cityscapes (“compressed perspective”)

• Wildlife (bird photography, zoos)

• Portraits

• Sports & long distance action

• Whenever one cannot get closer to the subject

Example of “Compressed Perspective” http://www.dailymail.co.uk/news/article-3756633/

Spectacular-photograph-blood-red-moon-rising-Manhattan-looks-like-Mars-hovering-Big-Apple.html

Mirror • Long FL & compact size

• They have a fixed aperture

• They have a unique Bokeh

• Lower contrast

• Relatively inexpensive

Most common mirror lens:

500mm f8 ~$50 (Generic)

~$100-$200+ (Name brand)

~$500+ (premium)

Questar 3.5

Focal Length ~ 1300mm

Major Issue when using Long FL Lenses

Alignment

From: https://www.bhphotovideo.com/explora/photography/tips-and-solutions/how-test-your-lens

Vertical Alignment

• If the center of gravity of the lens is in the front of the bar, I control the vertical alignment by adjusting the rail mounting screw.

• Having a grid on the camera

helps align the images.

Adjusting Vertical Alignment

Area of overlap when the lenses

are pointed parallel.

Horizontal Alignment

If the cameras are pointed parallel, there will be partial Image Loss (= M x B)

The lenses must converge!

Another Issue when using Long FL Lenses

Focus • Auto Focus: Might need to adjust the focus area so both

lenses are focused in the same spot. • Manual Focus: Lenses must be focused individually.

Focusing is critical because of very narrow DOF. It helps to enlarge the image area.

Magnification & Convergence

Even though the pictures look similar,

there is a difference in perspective.

Short FL/distances result in exaggerated perspective, long FL/long distances result in a a compressed perspective. It is not the focal length (F) but the distance (I) that causes this effect and it is minimized if the background is blocked.

You can take a close-up picture with longer lenses from a distance or with shorter lenses closer.

Pictures that have:

• The same magnification (F/I)

• The same convergence ratio (B/I)

Look surprisingly similar provided that the background is blocked.

Extreme Cropping

Cropping + Enlarging to fixed size =

increasing the focal length of recording lens

Summary: Long Focal Length Lenses

• Pictures with similar (F/distance, B/distance) look similar, provided that

the background is blocked, so use Long FL Lenses when you cannot get close (Examples: Wildlife, animals, portraits,

sports, long distance action)

• Or, for “perspective compression” (stay far back, increase

FL, increase stereo base)

• Alignment & precise focus are important

• Cropping & Enlarging is equivalent to increasing FL (Effective FL = FL x Crop Ratio x Enlargement)

Short Focal Length Lenses

Wide angle - Fisheye Stereo

Applications:

• Buildings / Architecture / Interiors

• Close-ups, shots behind glass/displays

• Landscapes / Nature / Night sky

• Special Effects (fisheye distortion, space expansion)

Wide Angle/Fisheye w/a Stereo Camera

Two “Distortions” associated with wide angle lenses

Keystone (vertical perspective) “distortion” Can be avoided / minimized if the camera is lifted at the center of the subject. Can be corrected digitally.

Barrel (fisheye) distortion Pronounced in fisheye lenses. Depends on the design of the lens? Can be reduced / removed with digital editing.

Keystone “Vertical Perspective” Correction

Before and After Correction

Drawbacks:

1. Image loss

2. If extreme, it looks unnatural (top looks larger than bottom)

It is better to minimize this effect by keeping camera back parallel to subject when recording, even if you have lift the camera higher.

Barrel Distortion Correction - “Defishing”

Relationship between F and B

What is (or should be) the relationship between:

• Focal Length (F)

• Stereo Base (B) Another way to ask this: As we increase the focal length, should we:

1. Increase the stereo base

2. Don’t change it, or

3. Decrease it

I have explored this topic in Tutorials in Stereogram 12.07-12.10 (March-May 2008)

Both F and B affect the stereoscopic deviation (SD), based on this formula: SD = FB/I or FB (1/Inear-1/Ifar), where Inear and Ifar are the distances of the near and far objects in the picture.

If the goal is to maintain a constant stereoscopic deviation, then the answer to the question depends on what happens to these distances as the focal length changes.

Depending on the situation, all three answers can be valid!

Stereoscopic Deviation

F B / I = F B (1/Inear - 1/Imax)

1. If the distances of the near and far objects do not change as we

increase the focal length, then we need to decrease the stereo

base in order to maintain the same stereoscopic deviation.

Example: Fireworks

2. In many practical situations, as we increase the focal length, we

push the near object back (increase Inear) in a way that the

stereoscopic deviation remains constant. In this case we do not

need to change the stereo base. That’s good news when using a

stereo camera (fixed B).

3. In the case where the subject has limited depth (the background is blocked) the formula above becomes SD = FBt/I2, where t is the thickness of the subject. In this case we need to increase the stereo base as we zoom in and step back (to maintain the same image size or magnification). This is the case when taking portraits or wildlife pictures from a distance. That’s where twin cameras are the best.

Increasing the stereo base with focal length was advocated by H. C. McKay in the 1950s. McKay called this PePax principle, from the words Perspective and Parallax.

According to this principle, if you increase the focal length of the recording lens, you should increase the stereo base proportionally to get a picture that resembles orthostereo.

So what should you do? • Estimate / measure distances of near & far objects. Plug in ste-

reo base calculator.

• Take your chances - experiment and learn Start by not changing anything. Then increase (or decrease) the stereo base and see how the pictures look. If using twin cameras, start with the cameras at the minimum stereo base and then increase their spacing as needed or if needed.

Alternative approach: Do not worry about the focal length but only about the distance to the main subject (I) or the ratio of the stereo base to this distance (B/I). Follow this Table. This approach works best if there is no infinity in the picture (the background is blocked in close-ups/macros).

Type of picture Recommended

B/I

Distant Stereo ~ 1/30

Macro ~ 1/10

Close-Up ~ 1/20

Fast Lenses in Stereo

• Fast lens = Large maximum aperture

• Can use fast shutter speeds and/or low ISO

• Low light action photos are possible / better

• Throws the background out of focus (good, bad?)

• Fast lenses are larger & more expensive

• Conventional stereo photography requires that everything is in focus

• In practice however, parts of the image can be out of focus if they are not distracting, as long as the main subject dominates and it is in sharp focus.

• Fast lenses allow you to use faster shutter speeds and/or lower ISO values, both of which result in sharper pictures.

• Fast lenses used wide open can through the background totally out of focus.

• Drawbacks to using fast lenses include price and weight/size of the lens (if the lenses are large, the cameras cannot be brought sufficiently close together).

• Fast lenses are usually prime lenses. However, because the pictures are sharp, they can be enlarged to large sizes and cropped, which gives some flexibility regarding the effective focal length.

Konica Hexar 35mm Samsung NX 45mm f1.8

Prime or Zoom Lenses?

Zoom Lenses

• Convenience of having a range of focal lengths

• No need to change lenses

• Can match focal lengths using lens markings

Prime Lenses

• Cannot accidentally change focal length

• Faster (larger maximum aperture)

• Smaller In film photography I opted for prime lenses because of the issue of matching the focal lengths precisely. With digital alignment, this is not an issue any more. But I find myself using prime lenses because they are smaller and faster and I don’t have to worry about gross mismatch of focal lengths by accident.

It’s up to you!

Summary • Different lenses allow you to explore different types of stereo

photography.

• In addition to OEM lenses, there are plenty of good inexpensive used lenses that can be used in modern digital cameras with inexpensive adapters.

• If using a camera with a smaller sensor, multiply the focal length x crop factor. Also multiply by enlargement to get the effective focal length.

• Long focal length lenses (telestereo) are used for wildlife, nature, sports, or any situation where one cannot get close to the subject.

• Alignment and focus are important when using long FL lenses

• Short focal length lenses are used for architectural, nature, close-ups of behind glass displays, and special effects.

• Keystone and barrel distortion are common when using ultra wide / fisheye lenses, and can be corrected digitally.

• “Fast” lenses have a use in Stereo. They allow you to use faster shutter speeds / lower ISO. They allow for higher enlargements. Also, they can throw the background out of focus (helps with portraits).