The optical viewfinder
A small technical masterpiece is located inside every SLR camera – the optical viewfinder. How exactly the viewfinder works and what happens when the photographer presses the shutter-release-button, is the focus of the following article.
Through the lens
The characteristic, that distinguishes SLR cameras from almost all other camera-types, is, that the viewfinder picture corresponds exactly to the picture that the photographer is looking at "through the lens" (TTL).
When using rangefinder cameras, the photographer looks at the subject through a rangefinder that is detached from the lens. As the range-finder is located sideways of the lens, the image in the range-finder is inevitably different from the picture taken by the lens. This is often termed as parallax.
The first SLR cameras used a mirror to project the image upwards – the photographer looked on the subject through a funnel. This system provided a bright picture, and had the advantage, that it could be implemented very cost-effectively. The main disadvantage was, that the picture was displayed upside down.
With a penta-prism, the picture is displayed right way round and right way up.
The pentaprism (Penta is Greek for five), is the heart of every SLR viewfinder. The accuracy of the prism decides the brightness and sharpness of the viewfinder image. The pentaprism does not affect the final image quality, it only reproduces the picture to ensures an accurate representation.
The focusing screen
The picture that is generated by the lens, is projected onto the focusing screen, that is located at the underside of the prism. Some focusing screens have a certain structure to make image composition, or – in case of manual-focus cameras – focusing easier for the photographer.
Some cameras allow the focusing screen to be changed.
Minolta uses a special focusing screen for its bigger Dynax-models. The so-called honeycomb metering focusing screen provides an extraordinarily bright and clear viewfinder image.
The viewfinder of an SLR harbours many more functions. As the picture is deflected in the viewfinder, the viewfinder also has to be where interpretation of brightness and focusing takes place.
The autofocus-measurement and exposure-metering both are done in the viewfinder.
The autofocus system of the current Dynax camera models uses several autofocus sensors. To show the photographer which autofocus-sensor is currently responsible for the focusing, the active AF-sensor is highlighted in many Dynax cameras.
What happens during exposure?
The shutter-release button of a camera has two pressure points.
If the button is pressed slightly only, the brightness-metering and focusing is started (if the standard autofocus-setting is used).
While the mirror is projecting the image into the viewfinder, the aperture is fully opened. The autofocus of a SLR-camera is based on a contrast-metering method, the higher the contrast in the subject, the faster the autofocus is able to determine the focus point. Thus the autofocus system of an SLR is faster, the greater the illuminative power of the used lens is.
Once the focus and exposure are determined, the aperture is closed back down.
The mirror is flipped up and releases the shutter. Once the shutter is opened in front of the film or CCD, the image is captured.
As soon as shutter release time is over, the mirror is again dropped back downwards and the camera is ready for the next shot.
1. Looking through the viewfinder. The autofocus determines the focus, the camera meters the exposure.
2. The mirror of the viewfinder is folded up and releases the shutter, the aperture shuts down.
3. The shutter opens, exposure takes place.
4. The shutter closes, exposure is finished, and the aperture opens up.
5. The mirror folds down, the camera is ready for the next exposure.
To work comfortably with an SLR, the photographer has to posses a lot of technique and know-how.
The viewfinder is one of the most complex parts of the camera, without it, pictures as we know today would not be possible.
* This content is taken from the konicaminoltaphotoworld.com website that was unfortunately taken offline. I take full responsibility for offering this here on dyxum but I honestly believe that this contests SHOULD be available to all Konica Minolta customers; this is the least they can do !