SLR vs Rangefinder
Technology - Technology

PREFACE

35mm cameras slowly evolved over the years transforming into a complex optical-electronic device. Even today's cheap point and shot cameras contain electronic automatic exposure and automatic focus circuitry, which clearly indicates a general trend in 35-mm cameras design: provision of maximum comfort in picture taking. Still, cameras use different technical principles and this page in intended for newcomers as an introductory course to the modern "camerology".

Basically there're two different camera concepts on the 35mm market:

  1. SLR (single lens reflex camera). This type currently dominates the market for professional cameras.
  2. Rangefinder (in short, RF). This type can be divided again into
    • “classical” cameras with coincident rangefinder (Konica RF, Leica M6, Bessa R and many other RF cameras made by Canon, Contax, Leica, Nikon, Voigtlander long time ago)
    • Rangefinder cameras with electronic rangefinder (Contax G1/G2, Konica Hexar AF)
    • P&S cameras (Point and Shoot camera)
The main difference between these cameras is the way photographic scene is displayed, accessed and processed in a viewfinder.

SLR - Single Lens Reflex

The fundamental principal of SLR type camera is the TTL (Through-The-Lens) mechanism. TTL means that the scene is viewed, focused and metered directly through the lens as shown in Fig B. Incoming light is reflected by a “reflex“ mirror towards pentaprism (all-glass, or a roof-mirror type as a smaller and cheaper solution). Then prism re-directs light to the viewfinder. As result, viewfinder shows a scene through the "eye" of the lens. The amount of transmitted light is roughly dependent on the maximal aperture of the lens so the faster the lens the brighter is the viewfinder. Quality of pentaprism also matters and all-glass types usually provide brighter images.
 
figure A: 
figure B: 

The light metering sensors can be positioned at different locations:

  1. behind the semi-transparent mirror
  2. adjacent to the pentaprism
  3. at the base of the lens mount
  4. and many other places, depending on designer’s will
Usually, there is at least one silicon photo-diode. In modern cameras this analog data is processed by a CPU, which calculates appropriate shutter-speed/aperture combinations that in turn could be adjusted by a user, or just indicated as under/overexposure in manual operation mode. After depressing the shutter release button following operations are initiated:
  1. the SLR mirror swings to its upward position (accompanied by a viewfinder black-out)
  2. the lens aperture is set to the chosen value
  3. the shutter is released and film frame is exposed to the light coming through lens. Most modern SLR’s use multi-blade vertically travelling shutters that have two curtains. Initially, the 1st curtain covers the whole frame. After the shutter release button is depressed (and the SLR mirror takes its upper position) the curtain moves upwards exposing the film frame to incoming light. The 2nd curtain follows 1st after a certain amount time that is dependent on the chosen shutter speed. The movement of the 2nd curtain blocks the incoming light again (see figure A) completing exposure of the film frame.
  4. the SLR mirror swings down, back to its normal viewing position
  5. the lens aperture is reset to its maximum value (e.g. f/2.8 for a 60/2.8 lens)
  6. film is advancing to the next frame, either manually or by a motor drive
This is a generic exposure sequence for practically any modern 35 mm SLR camera. Earlier SLR’s did not have so-called “instant-return” mirror, commonly used today. Instead, before and after each shot their mirrors had to be moved manually with a lever located outside the camera body. Interestingly enough, modern medium format SLR cameras have this feature as well and for the good reason. Their mirror is so large and heavy instant-return mechanism would generate enormous amount of vibrations that would be imminently transferred to the film plane, causing a problem that should be taken a special care of.

SLR Mirror Lock-Up (MLU)/Pre-fire

Advantages of a SLR mirror that unfortunately come at price.
  • The distance between lens and film is increased due to the necessary space for the mirror. Thus, a special approach to the lens design is required, known as “retrofocus design”. It works fine for long lenses, but has a profound negative impact on optical quality of shorter lenses.
  • Mirror causes vibrations by moving to its upward position or to be more precise: the vibrations are caused by the impulse produced by stopping the mirror. The power of the impulse is dependent on the camera model.
There is little can be done about (A), whereas the effects of (B) could be substantially reduced. The diagram below illustrates the problem of mirror induced vibrations.

These vibrations are wave-shaped, which means finally nothing else than periodical up- and downward movement of the camera. This movement can cause a certain blur effect which can result in a deterioration of image quality (see picture below). This problem is not relevant at all shutter speeds reciprocal to lens’s focal range (for example 1/200 for 200 mm lens) but it can and will have an effect between, lets say, 1/60s and 1/4s. So what can be done about it? First, modern SLR cameras offer improved mirror-dumping mechanisms. Second, many mid and upper class camera models provide a feature called "mirror lock-up" (MLU). The idea about MLU is that the mirror is activated long before the shutter mechanism is released. Therefore the mirror vibrations have time to be eliminated or at least damped to an uncritical level. There a two types MLUs:

  • "mirror pre-fire" - this is an automated version which is coupled to the camera timer. The camera activates the mirror and waits a few seconds before releasing the shutter.
  • "mirror lock-up" - the user can manually switch the mirror to its upward position and has to release the shutter. Usually MLU is only used with a camera mounted to a tripod. In this case tripping shutter with the conventional shutter release button is meaningless, because this operation would result in much heavier vibrations than any mirror could produce! Therefore, a remote shutter release is a necessity here!
Using the MLU for handheld shots doesn’t make much sense as viewfinder blacks out – errors in composition are inevitable.

Summarizing, there are some benefits of the SLR design:

  • Ease of composition
  • The ability to control the accuracy of focus (manually or by means of AF system)
  • Ability to use lenses of theoretically unrestricted range of focal lengths
  • Ease of use various filters

Rangefinder cameras

The history of rangefinder cameras (RF) goes back to mid-20s of the last century when Oscar Barnack presented Leica A, the camera that started the era of 35-mm photography. Many things that are common for us today, like focal plane shutter or film cartridge, first appeared in Leica cameras. Even now, almost 75 years later, RF cameras find their place in bags of many professional and amateur photographers for reasons we will discuss further in text.

Where Is My Mirror?

Rangefinder cameras differ from the SLR dramatically as they don’t employ through-the-lens (TTL) viewing and focusing, although most modern RF cameras do have TTL metering. Instead, focusing in RF are realized via a rangefinder mechanism that can be either coincident  (“classic” rangefinders) or electronic. Rangefinder is a device that determines a distance to object using the principle of triangulation, the geometric technique known to people for hundreds of years.

Regardless of the nature of signal (optical, radio), the accuracy of such device depends on the effective baselength, which in case of RF cameras is derived from physical distance between beam splitter and rangefinder mirror/pentaprism (see diagram below) multiplied by the magnification of the viewfinder. The larger the effective baselength, the more accurate the rangefinder is.
 

  1. Beam splitter (semitransparent mirror)
  2. Light-gathering window
  3. Framelines projection/parallax compensation unit
  4. Framelines projection semitransparent mirror
  5. Rotating mirror/pentaprism
  6. Viewfinder
  7. Viewfinder frame
  8. Static Image
  9. Secondary Image

This diagram demonstrates the generic design of the optical coincident-type rangefinder mechanism used in many classic RF cameras over the years.

How does it work?

Beam splitter (A) and rotating mirror/pentaprism (E) form two images in the viewfinder – static (H) (through the beam-splitting mirror) and secondary (I) (through the rotating mirror). The lens is linked with the (E) via moving cam at the lens’s base, therefore while rotating the focusing barrel one sees the secondary image moving across the viewfinder. When static and secondary images match the focus is achieved. Several RF cameras (e. g. Leica M6, Konica Hexar RF and Bessa R) also allow split-image focusing using the distinctive edges of static and secondary images, which greatly improves focusing accuracy. How accurate this system is? It’s remarkably precise as long as it operates within specified focal range. For example, Leica M6 with its effective baselength of 40.16-58.86 mm (depending on model) is designed to work with lenses no longer than 135 mm. The longer the RF lens is, the less accurate focusing gets. In contrary, normal and wide lenses focus on RF camera with astounding accuracy.
 
Focus control for optical coincident-type rangefinders
out-of-focus 
(no match of overlay image)

in-focus
(overlay image matches)

Electronic rangefinder uses slightly different principle. There are no coincidental images in a viewfinder, instead camera projects the light beam (infrared or visible) towards the object, times the light that is reflected back and then determines the distance of the object from the camera lens. Consequently, the lens is focused according to the distance supplied by the camera. Electronic rangefinders are widely used in point and shoot cameras and some modern RF cameras (e. g. Contax G1/G2 and Konica Hexar AF).

Viewing and composing with RF cameras is usually done through the combined viewfinder/rangefinder. Besides the focusing patch, RF viewfinder also shows framelines that delineate area that corresponds to field of view covered by lens. Framelines are generated either optically (see C and D in Figure above) or electronically, with small LCD projector. RF cameras with interchangeable lenses have several sets of framelines that are brought up either automatically (for cameras with bayonet mounts), or manually (cameras with screw mount). Some RF cameras, like Contax G1/G2 emulate TTL viewing with their “tunnel” type viewfinders.

Non-TTL viewing creates an obvious problem with close-up focusing. When the RF camera is being focused at object located at approximately 2 meters away, the parallax between optical axis of lens and viewfinder increases, which could lead to compositional errors invisible for user until he/she gets a final print or slide.
 

As result, practically any modern RF camera with optical rangefinder has so-called “automatic parallax compensation” unit that automatically shifts focusing frames towards the lens - to cope with close-up focusing errors.
 

RF cameras also can (and sometimes must) use external viewfinders. Non-TTL viewfinder can’t show 100% of field-of-view for wide and super-wide lenses (<28 mm). Because of this wide RF lenses are designed to work with the accessory viewfinders inserted into a hot-shoe on the camera’s top. User composes the image using the external viewfinder and focuses using the built-in one. Some RF cameras (e.g. Bessa-T) have to use external finders all the time. Awkward ain’t it?

So… Why Rangefinder?

The answer is simple - absence of moving mirror gives RF camera substantial advantages over SLR. Take a look at the operational sequence in RF camera after the shutter release button has been depressed:
  1. The shutter is released and film frame is exposed to the light coming through lens.
  2. and… That’s all.
Additional steps may include automatic aperture operation and film advance by motor drive. Compare this to analogous sequence in SLR cameras you may imagine why RF and SLR are such different animals. Here is more; “look ma – no mirror” approach leads to the whole spectra of unique abilities characteristic to the RF system only. Generally, RF cameras produce vibrations magnitudes smaller compared to those of SLR’s, as there is no mass transfer related to the mirror movement. In practice it translates into exceptional ability to shoot at shutter speeds 2-3 f/stops below limits set by the rule of focal length reciprocity commonly used with SLR cameras without fear to get blurry images. For example, shutter speeds ranged between 1/8-1/15 for 50 mm lens are not unusual. Another hallmark of RF system is its optical performance. Since there is no minimal distance between rear lens element and shutter, the RF lenses are constructed without bulky retrofocus design, resulting is significant advantages in optical quality of wide angle – normal lenses at full-medium apertures. Meaning of this? You can shoot with f/2.0 and results will be barely distinctguishable from those taken at f/5.6 – a dream for street and available light photographer. For comparison, almost any SLR lens in 28-80 mm range must be stopped down to f/8.0-11.00 to obtain maximum optical quality

Summarizing, RF design offers several advantages

  • Compactness. The entire system can fit into a small waist bag
  • Quietness
  • Superb for available light shooting
  • Superb optics
Unfortunately, RF design has disadvantages as well. We will cover it in following section.

SLR vs. Rangefinder

SLR Rangefinder
Pros
  • Direct focusing control 
  • Precise framing
  • Depth-of-field control (if available)
  • Unlimited lens/filter options 
  • Ability to use tilt/shift, macro and long lenses
  • efficient flare check in contra-light
  • Compact
  • Quiet and practically vibration-free
  • Very bright, aperture independent viewfinder 
  • Superb wide-angle and normal lenses
  • Maximum optical quality at f/4-5.6, while excellent at maximum apertures
  • Short shutter lag
Cons
  • Large and heavy
  • Vibrations restrict hand-hold photography
  • Retrofocus design plagues wide-angle lenses
  • Maximum optical quality at f/8-11 while often mediocre at maximum apertures for 28-80 lenses.
  • Often considerable shutter lag 
  • Dark viewfinder with f/5.6 and slower lenses
  • Telephoto lenses are limited to 135 mm or shorter (coincident rangefinder cameras)
  • Awkward macro-photography (if possible at all)
  • Possible parallax errors at close-up focusing
  • Rudimentary depth-of-field control 
  • Focus control is indirect
  • polarizers cannot be used (without major obstacles)
  • potential mismatch between lens flare vs  rangefinder

Conclusions

Looking at this table it is getting pretty obvious that two systems do not oppose, but rather supplement each other. SLR system performs its best with tele- and macro- lenses, suggesting that it’s ideal for sport, action, formal portraits, wild-life and macro photography. The ability to control depth-of-filed precisely also indicate that SLR is excellent choice for portraiture. RF has a definite edge in street, low-light, people and documentary photography. Due to compactness RF system is also ideal a companion for a traveler. Thence, the main conclusion would be “pick the camera that works best for your needs a have fun”. If you discovered that you like both systems - oh well, that’s between you and your wallet.

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