28 February 2009

Focussing screens - more than standard

All the EOS cameras feature a ground glass focussing screen. It's the translucent surface you see when you look through the viewfinder. We've explained before the importance of correctly setting the dioptre correction so that the screen is correctly viewed by the photographer. An increasing number of the EOS cameras allow you to change the focussing screen and it's something worth doing if your style of photography suits something more than standard.

The standard focussing screen fitted to your EOS is designed to give a bright clear image in a wide range of conditions and with a wide range of lenses. The maximum aperture of a lens determines how bright the screen can be, with lenses that have f/5.6 apertures giving a darker - harder to see - screen than lenses with faster apertures like f/1.4. The standard focussing screen has a special ground glass surface that effectively amplifies the light so even the slower f/5.6 lenses give a reasonably bright viewfinder.

Grid screen 
The first alternative screen is the grid screen, this one has a set of fine lines etched on it's surface making a grid pattern. These lines can help you with composition and also to keep horizons level and stop you composing with buildings falling over. In some cases they are used for technical purposes too. Brightness and viewfinder performance wise these screens are based on the standard ones so work well with all lenses.

Super precision matte - 'manual focus' screen
The first of the specialist screens is the super precision matte screen, sometimes called the 'manual focus' screen. This one has a different ground surface and as a result shows an almost real view of the actual depth of field that a lens and aperture combination will produce. The standard screen tends to show images with too much depth of field. The downside of this screen is that it gets darker quite a lot faster so Canon recommends it for f/2.8 aperture or faster lenses - exactly the ones you may be doing critical manual focus with.

Changing the focussing screen is quite simple, you remove the lens and release a small catch in the top of the mirror box, the screen drops down and using the tool supplied with each accessory focussing screen you lift out the existing one and replace with the alternates. Don't ever touch or try to clean the surface of the focussing screen, you'll end up buying a new one, but at around twenty to thirty pounds they are not expensive. When you change the focussing screen you need to set the appropriate custom function in the camera, this tells the exposure metering about the characteristics of the screen in use and tailors the metering system appropriately.

All EOS-1D models can have the focussing screen changed, as can EOS 5D, EOS 5D Mark II and also EOS 40D and EOS 50D. Some third parties provide screens and instructions for other models though you may invalidate your warranty if you fit them yourself.

Generally Ec-'X' screens are for EOS-1D models, Ed-'X' for EOS 5D, Ef-'X' for EOS 40D / EOS 50D and Eg-'X' for EOS 5D Mark II.

'X' indicates the type of screen, D = grid screen, S = super precision matte, A = standard screen.

In the case of the EOS-1D models there's a much more extensive range of focussing screens available including microprism and split image screens.

EOS 40D/ 50DEOS 5D Mark II


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