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"Dandelion" and other ways of facilitating the process of manual focusing






Manual focusing on a frosted glass has been used since the beginnings of photography. It's a tedious process. There have been numerous attempts to facilitate it with an additional magnifier folding out in the shaft of medium-format cameras or a magnifier on a viewfinder in reflex cameras.





Pentacon magnifier for additional x2.7 zoom of the central part of an image in a viewfinder, installed on Canon EOS 5D.

Nevertheless, that's not the best way to focus a camera fast. Besides, it requires sharp eyesight. That's why many manually focused reflex cameras were equipped with additional tools, like a microraster (in Zenit TTL). A microraster is a system of micropyramids with their tops on the level of the matted Fresnel lens. The height of micropyramids and distance between them do not exceed 0.01mm. Sharp focus in this case drastically changes the image in a viewfinder. Cameras also use wedge focusing devices for sharp focusing. Sometimes, like in Kiev 88, this device is used together with a microraster.   A wedge focusing device consists of two optical wedges in the center of a collecting lens.  If a lens is not focused, a viewfinder shows two skewed parts of an image, to be merged for sharp focusing.




Focusing screen in Kiev 88. The upper photo is out of focus. The bottom photo is focused sharp. Digits denote zones of various focusing devices. 1- wedge device, 2 - microraster, 3 - matte surface.

With the appearance of autofocus cameras, manual focusing has fallen out of usage. That's why optical systems that allow to detect the moment of sharp focus are not installed into viewfinders anymore. They were not necessary - if an autofocus lens is switched into manual focus mode, the electronic autofocus system indicates sharp focus in a viewfinder. You just use your fingers instead of an electric engine to move the lens. The autofocus system is practically completely built into camera cases. In some cameras, like Nikon, Pentax, Minolta, even the lens engine is inside the camera body. The difference between autofocus and non-autofocus lenses comes down to lens movement - in one case the lens is moved by manually rotating the external ring, in the other case it's moved by the engine that rotates the axis inside a bayonet, which latches a lens to a camera. Engine prices have dropped very low now, so each autofocus lens is equipped with its own engine. The above mentioned companies have chosen this way. Canon refused to install engines into cameras from the very beginning and all its autofocus lenses were equipped with built-in engines. As their main operating mode was focusing with an engine, manual focusing, though preserved, was much less convenient than in lenses without autofocus. The stroke of the focus ring became much shorter, there appeared some backlash. In other words, manual focusing in many autofocus lenses is far from convenient. That's why you want to install a classic old lens with manual focusing and still take advantage of modern features, so that camera's automatics indicated the point of sharp focus.   It's no problem to install a lens with M42 thread or with Bayonets B or C, adapters for Canon EOS can be easily found in stores.   But if a camera is not connected electrically with the lens, the system will not be able to indicate sharp focus. The device under review is designed to trick a camera. The author called his device "Dandelion". Here is Victor Lushnikov's page about this device.




"Dandelion" is a plastic plate with a group of contacts. When glued to the adapter ring, it reports focal distances and aperture to a camera. You can program any values. By default, the focal distance is 55 mm and the aperture ratio is 1:1.4.   Your lens in the adapter may have a different focal distance and aperture ratio. But it makes absolutely no difference. Our task is to cheat camera's automatics. If a camera is in aperture priority mode, both the sharp focus notification and automatic exposure will work. And EXIF headers will contain the values, written in Dandelion's chip. Contacts glued to the adapter ring will be enough for new Canon cameras, except for 1D models. Film cameras, Canon EOS D30 and D60, and looks like all 1D models have to be cheated on the mechanic level as well. If you compare a bayonet on the lens and a bayonet on the EOS-M42 adapter, you may notice that one of the bayonet's lugs on the adapter is narrower.





EOS-to-M42 adapter from Poisk-Photo with a glued "Dandelion" and an extension ring for the macro mode. Bayonet's lugs of different lengths are marked with arrows.

We've done it, lest the lug should move the switch when you mount the lens.





Canon D60 (left), Canon 5D (right). Pay attention to the arrowed locations. The 5D model lacks a mechanical switch of the electric lens interface.

If the switch is closed and the lens has no contacts, an old camera may freeze. If the switch is not closed, a camera just does not check if anything is connected to the contacts. Thus, if you plan on using Dandelion with old cameras and an adapter from Poisk-Photo or Jolos, you will have to modify the adapter. That's what I did: I drilled a 0.5mm hole in the bayonet and inserted a piece of steel wire there.




A special L-type holder is necessary to attach Dandelion to adapter rings from Bayonet EOS to Bayonet B, because these rings do not have a surface to glue this device to. In my opinion, in most cases it's more convenient to use two adapters: EOS - M42 with "Dandelion" and M42-B.





Adapter from Bayonet B to Canon EOS (on the right) and a combination of two adapters B-M42 and M42-EOS.

Canon EOS 5D Guide runs that the autofocus system contains sensors of two types: some of them are intended for lenses with the aperture ratio of 1:2.8; the others – for 1:5.6.





Sensors for the aperture ratio exceeding 1:2.8 are marked with red

My experiments show that this system has a safety margin. We managed to get a focus notification with the Volna lens (focal distance - 80 mm) when shooting a contrast test target, lying on a light table, Aperture 16.   





The test target used for focusing.

What's important is the aperture number, not the amount of light falling on a sensor. Focus notifications worked just as well with a neutral light filter NS-8 as without it.  The estimated exposure specified by the camera is 1.6, ISO 100. The threshold aperture probably has to do with a depth of field and depends on the focal distance of a lens. As a rule, the longer-focus lens, the narrower the aperture.

Lenses for the medium-format Kiev 88
Lens Focal distance Threshold aperture
Mir-26 45 mm F:11
Mir 38 65 mm F:11
Volna 80 mm F:16

Unlike lenses for medium-format cameras, Zenit lenses demonstrate worse results.

Zenit lenses
Lens Focal distance Threshold aperture
Zenitar 16 mm F:8
Helios 44 58 mm F:8
Industar 61L/Z 50 mm F:8
Industar 100 110 mm F:11
Pentacon 135 135 mm F:11

This value differs for various digital cameras. The table contains threshold apertures, when sharp focus notifications still worked in Canon EOS 5D. On the average, the threshold aperture for Canon EOS D60 was half-division wider.

This sharp focus notification system is especially interesting in case of catadioptic lenses, as they have only one aperture. So you cannot compensate for focusing errors by closing the aperture and thus increasing the depth of field. A lens/camera combo of Rubinar 500/8 and Canon EOS 5D was comfortable enough for photo hunting.

Miniature
Fragment



It's very difficult to pick up the focus point using the D60 camera with this lens. Even in case of a contrasty test chart, I missed the focus point every second time. It's difficult to catch a moment when a focus indicator lights up in a viewfinder even on a testbed. I guess it's hopeless in photo hunting.

Canon EOS 350D was tested with this lens on real objects, not on the test chart, so I failed to catch the focus point.

If various camera focusing sensors are used, Dandelion facilitates focusing with a skewed lens and subjectively raises precision. But skews and shifts deteriorate threshold sensitivity. Industar 100 offers the threshold aperture of 11, when skewed or shifted - only 8.

On this photo, the stone is shot from a side to show the side surface, Industar 100 is skewed relative to the sensor plane. I got two simultaneous focus notifications for two extreme points.

Miniature




Fragments



P.S.

There is no point in installing Dandelion on wide-angle lenses. But buying it for macro shooting, capturing slides, and long-focus lenses is certainly justified.



Vladimir Rodionov (rwpbb@ixbt.com)



June 5, 2006




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