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Focus Screens Part 1

This week’s blog post I wanted to write about a specific part of the camera system that you may or may not be familiar with, the Focus Screen. The Focus Screen is a small piece of flat, translucent material, either ground glass or a fresnel lens. It is located inside the front of your camera where the lens mounts and help you acquire focus and frame your shot through your camera’s viewfinder. This first image is a Canon Eg-A Focus Screen.

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There are various types of Focus Screens for most cameras and they can be either etched or non-etched. There are even Focus Screens which are made specifically for using low light lenses.

Some cameras have plain Focus Screens and others have etched ones that denote the focus points and even have a grid for maintaining your “Rule of Thirds”. Many of the newest mid-range and professional DSLRs have a plain Focus Screen with a monochromatic LCD overlay that reveals the focus points when you depress the shutter button halfway.

One of the interesting things about your Focus Screen, aside from the fact that many people do not realize their camera has one, or that it’s easily replaced, is the fact that if the Focus Screen picks up dirt, many photographers think it’s their sensor that’s dirty when that is not the case. If you are someone like myself that has an OCD thing about even the smallest spec of dust in my field of vision, the Focus Screen can be a source of annoyance. This next image is an image of a dirty Focus Screen, courtesy of Ken Rockwell.

cimg3298-finder

Have you ever looked through your viewfinder and noticed little black specs or even tiny black strings? Well, chances are if you have seen this, it is your Focus Screen that is the culprit and not your sensor. Especially if you see the dirt, but it does not show up in your photos.This next image is a cross section of  a DSLR, Number 5 is the Focus Screen.

slr_cross_section

Focus Screens can be a pain to clean and there are specific methods to do so, which I will go into in my next post. For now, either try to ignore the dirt in your field of view if you can, or go on-line and order another Focus Screen and replace your old one. You can generally buy new ones for around $20-$35 a piece, but look up which model your camera came with originally and buy the same one. Most all Canon cameras come with what’s called the Precision Matte “Eg-A” Focus Screen and the new one will come with a special tool to remove the old one and put in the new one. Whatever you do, do not touch the Focus Screen with your fingers. Next week’s post will be on how to remove and replace your Focus Screen as well as how to clean it PROPERLY.