This page will chronicle various accessory finders for 35mm, medium-format, and digital cameras.  I will add more as I encounter them.

Cosina plastic viewfinders (15mm, 21mm, 21/25mm, 25mm, 28mm, 35mm, 40mm).  These pop up under a number of names: Cosina, Voigtlander, Ricoh (GV-1), Epson (RD-1 finders), possibly also Konica.  They all feature a plastic unibody (body and foot) and coated optics.  Some focal lengths have framelines, others do not.  These were a good value for the money at $99, but for what some of these command today, you might look elsewhere.

• Clarity: excellent
• Distortion: visible, more so at the wider focal lengths
• Eye relief: adequate for eyeglasses but within bounds
• Frameline accuracy: conservative
• Parallax correction: in finders with framelines, shown by a dotted line (also serves as a field corrector).
• Materials: plastic, multicoated glass
• Size/weight: very small, very light
• Foot lock: none; friction fit (plastic)

Epson RD-1 finders.  See Cosina plastic finders above. These are very specific to the fields of view created by putting typical 35mm focal lengths in front of a DX sensor.

Kobalux 21mm and 28mm metal viewfinders.  These two viewfinders came in black anodized and silver pearl chrome finishes. They accompanied the 21mm and 28mm Kobalux lenses (discontinued 2002).

• Clarity: excellent
• Distortion: acceptable (28mm); moderate (21mm)
• Eye relief: extremely high
• Frameline accuracy: albada type; conservative
• Parallax correction: yes, via dotted line
• Materials: coated glass; anodized or chrome-plated aluminum.
• Size/weight: large but light.
• Foot lock: none; friction fit. Recommend ISO flash shoe.

Kodak Retina 50/90mm Sport Finder.  A bit smaller than the Leica equivalent (ROSOL), the Kodak finder is a nicely made flip-up frame finder that covers 50mm and 90mm focal lengths.  The one thing that is not very convenient about this finder is that to see  the 50mm frame, you need to flip the 90mm mask and 90mm eyepiece restrictor out of the way.  But for what it sells for, it's a solid piece of work.

• Clarity: excellent
• Distortion: none
• Eye relief: poor
• Frameline accuracy: conservative
• Parallax correction: yes, via moving the rear eyepiece
• Materials: chrome-plated brass, spring steel
• Size/weight: very light.  Folds flat onto camera.
• Foot lock: none; friction fit with prongs.

Konica M-Hexanon Dual 21-35mm finder.  This resembles the Cosina plastic finders (see above), but it's larger and a bit less distorted.  Has 21mm and 35mm frames, parallax corrected.

Leica 21mm plastic brightline finder (1200x series). Some have locks, some don't. Most have brightlines. This is really no different from the metal 21mm finder (SBKOO) that Leica has been making since the 1950s.

• Clarity: excellent plus
• Distortion: low (barrel)
• Eye relief: ok
• Frameline accuracy: conservative
• Parallax correction: 12008: markings; 12012: no (reportedly)
• Materials: plastic and coated glass
• Size/weight: very light.  Tiny; about 2/3 the size of the Cosina 21mm viewfinder
• Foot lock: depends on model: 12008 (yes); 12012 (no)

Leica Universal Wideangle Finder M. See separate article.

Mamiya 7 50mm viewfinder. This is the correct viewfinder for a 50mm lens on 6x7 (or 6x9 cropped to an 8x10 print) or a 65mm lens on a 4x5 camera. Includes a handy bubble level visible in the viewfinder, a crosshair denoting the center of the frame, parallax correction corners, and framing marks for the Mamiya 7, and integral diopter correction(!).

• Clarity: excellent
• Distortion: low (pincushion)
• Eye relief: ok
• Frameline accuracy: untested
• Parallax correction: by markings
• Materials: plastic and coated glass
• Size/weight: large but light
• Foot lock: screwdown

Mamiya Super 23 Wideangle Finder (65mm).   One thing that cannot be ignored is that there is a direct relationship between the size of an accessory viewfinder and how low its distortion is.  The Mamiya has this covered; it is a very big finder with very low distortion.  As an added bonus, it also has a cold shoe on its top.  This finder is relatively easy to disassemble to insert a Wratten 90 viewing filter.

• Clarity: excellent
• Distortion: very low
• Eye relief: good
• Frameline accuracy: no actual framelines; conservative blackline framing (6x9 with 6x7 markers)
• Parallax correction: yes, via moving the rear eyepiece
• Materials: glass optics, painted die-cast body, chrome-plated brass shoe and foot; aluminum distance scale, plastic eyepiece frame, plastic retainer for optics.
• Size/weight: big but not particularly heavy.
• Foot lock: none; friction fit with prongs and spring.

Mamiya Super 23 Wideangle finder (50mm).  Like the 65mm counterpart, it is amazingly easy to underestimate the size of this finder, which has a rectangular base with a cylindrical optical unit on top.  The reason it is easy to underestimate the size is that in pictures, it is usually mounted on the top of a Mamiya Super 23 press camera or a Fuji G690, either of which is about double the size of a 35mm rangefinder camera.

• Clarity: excellent
• Distortion: suprisingly low for a lens with an equivalent field of view to a 20mm in 35mm.
• Eye relief: acceptable
• Frameline accuracy: conservative (6x9 with 6x7 markers)
• Parallax correction: yes, via moving a huge dial below the eyepiece.  The whole unit tilts (a lot), which is not suprising given the height from the optical axis.
• Materials: glass optics, painted die-cast body, chrome-plated brass shoe and foot; aluminum distance scale, plastic eyepiece frame, plastic retainer for optics.
• Size/weight: big but not particularly heavy.
• Foot lock: none; friction fit with prongs. Also has lock screw that works on a pin that only the Mamiya press cameras have.

Mamiya Universal Finder (75-100-127mm): same as the 50mm Super 23 viewfinder, with a higher magnification and three sets of reflected framelines visible at the same time.

Ricoh GV-1.  See Cosina plastic finders above.  The GV-1 differs primarily in that it shows dual 21mm/28mm frame equivalents and is offset to accommodate the popup flash on the GR Digital series.

Sandmar Zoom-Vue 35mm-135mm. This large, tubular 35mm-135mm viewfinder uses shrinking masks to delineate what will be in the picture. The rim is striped in white paint, essentially making a variable-frame Albada finder with parallax control via the foot. What's the catch? Well, the finder is not bright to begin with, the front window is completely semisilvered (diminishing the scene brightness), and the framelines are indirectly lit by a plexiglas skylight. The weird thing is that this technological terror was designed for the Argus "Brick" camera. With regard to the "zooming" mechanism, this appears to be the inspiration for the Gaoersi finders you see on Ebay (which, reportedly, make the Zoom-Vue look pretty good).

Tewe 35-180mm Zoom Finder.  This finder, copied both by Nikon and Canon for their rangefinder cameras, uses a combination of changed magnification and apparent frame size to cover the range from 35mm to over 180mm.  There is a parallax corrector on the mounting shoe.  Like all parallax correctors, you will forget to use it.

• Clarity: suprisingly good given the all-around nature of the finder
• Distortion: minimal
• Eye relief: passable
• Frameline accuracy: conservative
• Parallax correction: yes, but not field correction.
• Materials: uncoated glass optics, metal barrel, metal focal length ring, metal frame mask, metal eyepiece.
• Size/weight: reasonable but not lightweight.  Suitable for a compact camera, barely.

Voigtlander Kontur.  This is Voigtlander before the bodysnatcher Cosina got to it.  This is an ingenious (and very small) finder that is available in 2x3 and 6x6 formats (always normal lenses).  It uses a high-powered positive lens to focus your nondominant eye on a brightline mask.  Keep both eyes open, and you can see the frames projected onto your subject.

• Clarity: as good as your eyes
• Distortion: slight distortion in the framelines, none of the subject
• Eye relief: very good
• Frameline accuracy: very conservative
• Parallax correction: yes, but not field correction.
• Materials: acetate mask, plastic body, glass lens, metal shoe
• Size/weight: neglible on both counts

Voigtlander Turnit (Original 35/100mm).  The same person at Voigtlander who figured out that you could use an optical illustion to project framelines into a user's stereo vision also realized that the magnification of a 35mm finder is roughly the reciprocal of a 100mm finder.  The result is the Turnit, which can be rotated 180 degrees to be either a passable 35mm finder or a decent 100mm finder.

• Clarity: very good at 100mm, ok at 35mm.
• Distortion: slight distortion at both settings.
• Eye relief: very good; like the Kontur, it is spectacle friendly (and, inexplicably, the 35mm frame is much clearer when you set your eye back slightly).
• Frameline accuracy: conservative
• Parallax correction: yes, but not field correction.  Rear eyepiece/lock moves up and down.
• Materials: uncoated glass optics, bakelite body, metal shoe and eyepiece/locking mechanism.
• Size/weight: much bigger than it looks in pictures.  This is not all bad; the size of the finder makes the clarity that much better.
• Use for: your Micro 4/3 camera for which you use a 17mm Olympus lens and a 50mm Leica lens.

Zeiss 425.  This unusual finder is a combined 75mm viewfinder and rangefinder unit. The main window shows a 75mm field with a rangefinder spot in the center. There are no framelines or parallax correction marks. Minimum focusing distance (in feet) is just under 5. Although this appears to have been designed for a lens with an f/4 or smaller maximum aperture, testing reveals it to be capable of focusing a 50mm f/1.4 lens on an APS-C camera.