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Focusing a Film Camera --
Designing and Using a Focusing Device
for Cameras with Removable Pentaprisms

Suggestions about focusing in general, the Olympus camera, Astrofocuser, Takahashi and knife-edge methods provided by Michael Hart are now in a section called "M. Hart on Astrophotography" elsewhere on this web site. Information on the Canon camera and several focusing devices for it is included below.

A major problem with most standard focusing screens is that they are too dim for focusing already dim astronomical images. Generally the standard screen must be replaced with a brighter version. A variety of interchangable screens are provided by  Nikon and Canon among others. While these special screens help for framing and course focus, they still do not easily allow for the critical focus necessary for astrophotography. Nikon and Canon provide a focus screen with a small clear spot in the center that has a cross pattern etched on the focusing surface. This helps concentrate the eye so as to insure that focusing is being done on the front surface of the screen. They also both have available a 6 or 8X vertical tube-like magnifiers (called chimney magnifiers) that provide an enlarged image to aid focusing. But these magnifications are not quite strong enough to insure critical focusing except with relatively bright objects.

At magnifications of 15 to 20X it is generally possible to focus by object contrast as well as focus by point source. Many find focusing by object contrast very difficult simply because of the lack of contrast in extended objects due to the brightness of the sky background. Then, point source focusing is more useful. Using a simple magnifier of power as high as 20X requires an eyepiece lens of approximately 12 mm focal length. Power in a magnifier is considered to be 250 divided by the focal length of the magnifier lens in millimeters. Such a lens when used directly at the surface of the focusing screen is not very convenient  with any camera because it has to be within 12 mm of the screen surface. This mechanical/optical arrangement is inconvenient at best. A magnifier with a  relay lens to get long focus relief can be devised for these cameras. But the most common method is to use a long focus microscope as described below.

It is essential to focus exactly on the front surface of the focusing screen. The ability to do this will depend on the type of surface used. Finely divided ground glass screens or clear screens with cross hairs are available for some cameras. Several of these will work. In order to do a good job of focusing,  one must be able to concentrate the eye on this surface, keeping it in focus. It is necessary to critically focus the viewing magnifier on the pattern on the front of the focusing screen first so as to hold the attention of the observers eye to that focal plane.   It is sometimes possible to use the defocused light of a bright star to illuminate the screen and thus effect eye focus. The image of the object being observed must be focused on the the front surface of  the focusing screen. At 20X or so it is quite easy to focus since a star will flair a little just as it approaches focus, then become very small and flair again outside focus. With extended objects the contrast will sharpen distinctly at the sharp edges of the image. Generally, if there are suitable stars within the frame, point source focusing will be easier and more accurate.

It is generally felt that a magnification of at least 15X and preferably 25X, or even more,  is required in order to focus a star image on the screen of an SLR camera. Getting this amount of focusing magnification while retaining the maximum brightness of the star image is difficult. A long operating distance microscope has been intensively investigated as an aid to focusing a film camera for astrophotography. This device enables focusing at relatively high magnifications while maintaining great ease of use. It is described here as applied in two ways to the Canon camera. Clearly similar applications are possible with other cameras that have removable pentaprisms.

It is assumed in this discussion that the reflex viewing focusing screen is parfocal with the actual film plane so that the front surface of the focusing screen represents the focal plane of the actual image that falls, during exposure, on the film plane. This is a reasonable expectation with a high quality SLR camera that is in good adjustment. I have checked many Canon and Nikon cameras an have not found any that fail this requirement.

The long working distance microscope will have both a good working distance and good retention of the brightness necessary to effectively focus on a star image. Basically the optical system used for this application consists of a small telescope and a transfer lens which forms a virtual image of the object which can then be observed through the telescope.   Such a device is diagrammed in the following figure.

Focusing drawing

The above diagram is easily understood by observing that the object, the camera focusing screen, is at a distance from the transfer lens equal to the focal length of the lens. This positioning causes the transfer lens to create a real image of the focusing screen at infinity. The optical rays emerging from the lens toward the right are in fact parallel.   This type of image is exactly the image that the telescope is capable of observing. The telescope is focused at infinity and forms an image that the observers eye can see. The total magnification of this arrangement is that of the transfer lens combined with that of the telescope. In the particular device shown below, the telescope has a magnification of 8X and the transfer lens has a magnification of 3.3 times. This gives a total magnification of about 26 times.

This total magnification is about right to effect careful focusing on the focusing screen of the camera. The lenses in the device shown are quite generous in size and thus deliver a bright image to the eye. The structure of this form of focuser is really quite clever. It provides a significant magnification, a good working distance and and a bright image. All of these features are desirable for focusing dim images. Additionally, the fact that viewing is at right angles to the telescope OTA and with good clearance between the OTA and the observers head is a considerable advantage.

The optical device is shown mounted with a custom made tube on a Canon camera. This adapter was devised from a spare Canon viewfinder and a small intermediate tube which holds the optical device at an appropriate distance from the focusing screen. To give the best possible focus and brightest image the focusing screen is of the type that has a clear central spot and a cross hair engraved on the bottom of the screen exactly in the focal position of the film.

Focus Devices

The small telescope and transfer lens is available from Edmund Scientific Industrial Optics Division. It is called a "portable direct measuring microscope."  The particular unit used has an adjustable eyepiece lens which enables critical focusing on a graduated scale and a course/fine focus adjustment at the position of the objective lens of the telescope. The position of the transfer lens is also adjustable with respect to the telescope. It is a very nicely designed and well made unit. With all of the various adjustments, the magnifier can easily be set to give a very sharp image of the front surface of the camera focusing screen. In addition to the 75 mm working distance, the microscope has a 9 mm diameter field of view. (this is a generous and adequate size)

An alternate optical system can easily be devised using the small telescope and the magnifying viewer normally available for the camera. In the case of the Canon, there is available a very fine 6X viewfinder with adjustable focus. This is not just a simple magnifying lens but is a compound, color corrected lens which gives a magnified view of the entire focusing screen. I have thus made another focusing system which uses the Canon 6X viewer and the small Edmund telescope without it own transfer lens.. Since the magnifier has a power of 6X and the telescope has a magnification of 8X the total magnification is 48X.

This device is shown on a Canon camera at the left and removed from it on the right in the two photographs below. The device is especially easy to use since it has a very deep mounting cup which sits firmly on the top of the finder. It stays in place and thus is a hands free attachment   After focusing, the telescope and its adapter can just be lifted off of the finder and the entire screen seen at 6X magnification for framing.

Focus Device 2 Focus Device 3

I have found both of these focusing attachments to be totally suitable for accurate focusing of astronomical images for film photography. It should be possible to make devices similar to these for any camera that has a removable pentaprism.

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