Wednesday, den 17. October 2007

The ideal digital SLR

Status as of December, 2005

The functionality and image quality of existing digital SLR camera equals, without a doubt, the quality of analogue cameras. But there is still some room for improvements.

Live image
Some applications require a live image. For example, when a camera is mounted on a microscope or the “Wolfenbüttel Book Reflector”, it may not be possible to look through the viewfinder. Or, cameras capturing images of a scientific experiment may be located in another room for safety reasons. If a live image were available on the implemented pivoted LCD panel, as well as the video output of the camera, it would help tremendously. From there it is just a small step to enable the camera to provide video capture. Keeping the development in storage capacity of existing storage cards in mind this might replace some of the video cameras in scientific applications.
Other problems occur if the optical view finder is replaced by an electronic viewfinder. It is common knowledge that electronic viewfinders suffer from low resolution and bad visibility under bright lighting conditions. And there is also an issue of low refreshing frequencies, if the camera is set to a burst mode.

Autofocus accuracy
A study we made in 2004 showed that in many cases the autofocus accuracy of digital cameras is not sufficient. The reason is the required level of precision and adjustment procedures for the “extra” autofocus sensor.

Manual focus
Existing digital SLRs all suffer from lack of ability to manually focus, because the previous screens used to focus in older SLR cameras were replaced by screens without the capability to focus on microprisms or split screen indicators.
Image Quality

The dynamic range, noise and speed of existing sensors are already very good. The best sensor in terms of dynamic range is of course the Fujifilm super CCD version SR. But there is still a lot to do in the areas of colour and luminance. Most cameras still expose the images according to 18% grey which is the technology in the analogue world. For a digital image the exposure should be adjusted to the highlights which should remain unclipped with some restrictions if the contrast in the scene exceeds the range the camera is able to capture. Image processing should be adjusted to the scene-contrast to match the luminance-appearance of the human eye (e.g. the foreground in a sunset scene). The colour as well, should follow a colour-appearance model to create a “pleasing image”. Each camera should have a mode that allows the matching of colours to the original scene, as closely as possible. In many situations, however, it might be better to have a specific output referred rendering in the camera as well. Resolution no longer matters as much as it did in the past because all the cameras with 8 and more Megapixels have a sufficient resolution for 95% of applications. At the moment I am more concerned about the shrinking in pixel size because this leads to higher noise, lower sensitivity, lower dynamic ranges, and last but not least, diffraction problems at lower apertures. Therefore, the pixel pitch should be higher than 5 microns in order to avoid these problems.

It would also be very nice to add at least some of the metadata to the images right after capturing them. This of course is already the case with the technical metadata specified in the Exif standard (CIPA…). But if an interface would enable the photographer to add some describing metadata, i.e. author, date, place, and part of the caption as well, it would be especially useful for journalists.

From the user’s point-of-view, a standardised image RAW format like Tiff/EP or Adobe DNG would be very useful for integrating the image processing into the various workflows required for different applications. Unfortunately, this is more a political issue and there are no technical restrictions for implementing a standard format.

A proposed solution for the live picture and focus problems
First of all, to enable the sensor for fast autofocusing, as well as to achieve sufficient image frequencies for video capture, the sensor used in that camera should have a good windowing capability. This means that it should not be necessary to read out a complete or even a fourth of an image to acquire the data for a good autofocus. It should be possible to strip down the read-out to “a few” selected pixels for exposure measurement and autofocus and for the appropriate video resolution. This windowing would allow fast signal processing and video modes up to the required 30 frames per second.

Using such a sensor would replace the camera’s complete autofocus system, which in turn would lead to higher focus accuracy due to the fact that the same sensor can be used for both focus-measurement and imaging. To increase the focus speed even more, it might be helpful to locate the objects by using an additional active autofocus system for pre-focussing.

Since electronic view finders are not sufficient in camera use in any situation, an optic view finder is still necessary. A mirror with a reflection of 30%-50% has been proposed in order to ensure live preview on the LCD or video output, as well as the use of an optical view finder. Since the glass plate mirror will introduce spherical as well as chromatic optical errors, it is better to mount it on a pivot as is common on a conventional SLR or dSLR camera. The errors may be small enough to still have a sufficient preview and video quality, but for high resolution images blur and colour fringes may appear. To minimize errors, it should be investigated as to whether or not a certain shape of glass on the back of the mirror or a coating will lead to a reduction. It should swing up or downwards to avoid these problems when capturing the final image. Turning the mirror out of the optical path during exposure will also increase the illuminance on the sensor by about one f-stop.

Sony with its DSC-R1 showed that a fast autofocus system based on the imaging sensor in the camera is possible and the principle of CMOS technology allows a sufficient windowing, although it can not be found in the actual sensors. Hopefully we will find these sensors in the future. My idea has been to implement the partially reflective pivoting mirror, especially in combination with such a sensor. If you are interested in using it, please do so and I look forward to being acknowledged as the person who originally came up with the idea.

rays.jpg Figure 1: Principle of a conventional SLR or digital SLR system

Figure 2: Principle of the proposed new system