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
Basically there're two different camera concepts on the 35mm market:
SLR (single lens reflex camera). This type currently dominates the market
for professional cameras.
Rangefinder (in short, RF). This type can be divided again into
The main difference between these cameras is the way photographic scene
is displayed, accessed and processed in a viewfinder.
“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
P&S cameras (Point and Shoot camera)
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.
The light metering sensors can be positioned at different locations:
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:
behind the semi-transparent mirror
adjacent to the pentaprism
at the base of the lens mount
and many other places, depending on designer’s will
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.
the SLR mirror swings to its upward position (accompanied by a viewfinder
the lens aperture is set to the chosen value
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.
the SLR mirror swings down, back to its normal viewing position
the lens aperture is reset to its maximum value (e.g. f/2.8 for a 60/2.8
film is advancing to the next frame, either manually or by a motor drive
SLR Mirror Lock-Up (MLU)/Pre-fire
Advantages of a SLR mirror that unfortunately come at price.
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.
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.
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:
Using the MLU for handheld shots doesn’t make much sense as viewfinder
blacks out – errors in composition are inevitable.
"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!
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
Ability to use lenses of theoretically unrestricted range of focal lengths
Ease of use various filters
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.
Beam splitter (semitransparent mirror)
Framelines projection/parallax compensation unit
Framelines projection semitransparent mirror
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
(no match of overlay image)
(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
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
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:
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
The shutter is released and film frame is exposed to the light coming through
and… That’s all.
Summarizing, RF design offers several advantages
Unfortunately, RF design has disadvantages as well. We will cover it in
Compactness. The entire system can fit into a small waist bag
Superb for available light shooting
SLR vs. Rangefinder
Direct focusing control
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
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
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
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
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.