Right this is unlikely to be a topic of interest to the majority of people (in most cases it voids the warranty and has a small chance of destroying the equipment), but I think it would be good to at least give it a mention. I should note that I have no affiliation with any of the companies / groups mentioned & if you decide to undertake any of the modifications mentioned below it is solely at your own risk.
The first modification is split screens / focusing screens, these are available from the manufacturers for some of the higher end cameras, and allow easier use of manual and older lenses. They are however also available for many of the lower end cameras. For instance I have replaced the screen in my 550D with one from here: http://www.focusingscreen.com this has given me much better results when using manual lenses (especially when combined with software changes below).
Backing this up when using old lenses with M42 fittings etc. is the use of 'chipped' adaptor rings, these come in a variety of forms some of which you can program to give the EXIF the settings for your specific lens. The second advantage of the 'chipped' adaptors is to tell the camera to allow the AF indicator (LED/beep), this gives you a quick method of verifying focus (especially if you set AF to use a single point).
I am also aware for the astro photography market the ability to remove some of the (UV) filters on the CCD/CMOS sensors, this allows UV images to be taken. I have noted this being available but have no experience of it.
The last section is on additional software (firmware) that allows additional features above those supplied by the manufacturer. The only project that I am aware of is Magic Lantern, which is for use with a number of Canon cameras. The aim of this project was to provide additional video functionality for use in the film industry.
The project has since evolved and includes a number of useful features for still photography, such as;
- focus trap - When using manual lens, when focus is detected the image is taken (allows images to be taken when you cannot see through the lens to focus)
- HDR - Allows setup of number of images and degree of exposure stepping
- motion detection - will take a photo when a change in image or light levels is detected (good for lightning photography)
- Interval timer - set time between shots, may be used in conjunction with the following
- Bulb timer - allows bulb exposure up to 60 minutes (this is much longer than canon exposure control)
- audio & sensor trigger - allows remote trigger on noise level and face sensor (useful if you don't have remote release handy)
Magic Lantern currently supports: (Canon)
5D mark II
550D / T2i / Kiss X4
600D / T3i / Kiss X5
500D / T1i / Kiss X3
coming soon: 60D & 1100D / Rebel T3 / Kiss X50
Magic Lantern details:
Web page / wiki http://magiclantern.wikia.com/wiki/M..._Firmware_Wiki
Development / Discussion group http://groups.google.com/group/ml-devel
All the versions for download are here https://bitbucket.org/hudson/magic-lantern/downloads
Hopefully this may be usefully to people, feel free to add any other tweaks.