With the advent of 4K television transmission has jumped substantially in size. Ultra-high-definition television (also known as Ultra HD television, Ultra HD, UHDTV, UHD and Super Hi-Vision) today includes 4K UHD and 8K UHD, which are two digital video formats with an aspect ratio of 16:9. They are with us in the UK mainly broadcast via satellite. The images that we see on our home screens are vastly compressed from their original (most probably) uncompressed capture.
Image compression is the application of data compression on digital images - the objective is to reduce redundancy of the image data in order to be able to store or transmit data in an efficient form. Image compression can be lossy or lossless. Lossless compression is sometimes preferred for artificial images such as technical drawings, icons or comics. This is because lossy compression methods, especially when used at low bit rates, introduce compression artefacts. Lossless compression methods may also be preferred for high value content, such as medical imagery or image scans made for archival purposes. Lossy methods are especially suitable for natural images such as photos in applications where minor (sometimes imperceptible) loss of fidelity is acceptable to achieve a substantial reduction in bit rate.
Digital capture and resolution is limited by the speed at which data can be transmitted through the circuit and stored on tape or disk. It is useful to note if recorded to a form of tape, it will take a compression hit, depending on the format: the new Sony SR format is reputed as having 2:1 compression. The only uncompressed format would be made by transferring directly to disk. However this is limited by the sheer volume of data but can be argued this is now feasible, with the advent of relatively cheap storage devices or hard drives (however one must think about backups, tape, DTF, LTO or even more disks).
Most HD cameras record as 4:2:2 YUV compressed onto tape, in this mode the compression is high. Alternatively you can record to a VTR deck, which will be 10 bit 4:2:2, not great for film work as you will have already thrown half the information away. Different cameras record different levels of compression using different algorithms to do so. If the production pushes this kind of recording, a few quality assurance tests under shooting conditions maybe advisable - under intense grading or FX work, the compression may degrade the image to cause issues, such as clipping, blocking or other problems. However one has to ultimately ask the question is it good enough for the job?
The output can also be recorded directly to a disk recorder (Directors Friend or S.two) and this will avoid compression completely. However this can disrupt some idealistic shooting conditions as the camera becomes tethered to the recorder - like an umbilical, you only have a finite length of rope. This may not be suitable for certain types of photography, so the benefits can outweigh the negative. A film camera never suffers from compression and its recording medium is always connected to the camera.
Newer generations of cameras are also beginning to offer RGB 4:4:4 capture, however the decision between compressed and non compressed (disk) remains in the productions own hands. Compressed data recording is a new addition to the capture workflow, the two main systems being introduced use wavelet compression across the entire frame therefore the quality potential is greater than in-camera tape systems using block based compression.
Most HD cameras are considered a tool for television, high end programs and documentary making. They capture exactly what you see in the view finder - they capture nothing outside of this area. Which means if something accidentally appears in shot, you live with it, resize and soften the image, or completely ignore it. They also do not capture any extended dynamic range, additional black or white information, that can be utilised during post production. For instance the image is WYSIWYG, 'what you see is what you get' and during grading there is a tendency if you are not careful to turn the blacks grey (milky) and make the whites look flat.
To improve the dynamic range on HD cameras, you can alter the way in which the sensor (CCD) captures the image. The cameras normal capture mode is set to give us immediately viewable 'normal' (high contrast) images. However adjusting the cameras gamma, to capture a lower contrast and wider dynamic range will allow for greater flexibility and additional headroom when colour correcting.
However the downfall of this method is the image definitely has to go through a colour correction pass to be broadcast, if the end deliverable is intended for television. However if the project has the flexibility to be graded for film, this is a logical step as it will only need a film grade and the rest of the deliverables can be derived from this master, thus saving money and time (one grade only).
It would be worth thinking about this route when shooting as it would allow the most amount of flexibility for post production, however it would not be the best image for instant viewing. This technique ultimately would give the greater final result in post, plus if your production was snapped up for theatrical release you ultimately already have the work done, the data, graded for film output can be sent directly to the film recorder.