Canon EOS 5D Mark II - EOS 5D Mark II 21.1MP Full Frame CMOS Digital SLR Camera, Service Manual

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Appendix
3
●Viewfinder brightness
Normally, the easier it is to catch the point of focus, the darker the viewfinder brightness
will become. As shown in Fig. 007, this applies even to the Super Precision Matte focusing
screen. Compared to when a f/1.8 lens is used, using an f/2.8 lens results in the viewfinder
brightness decreasing by 1 stop. With an f/5.6 lens, the viewfinder brightness drops by about
2 stops.
This is caused by the focusing screen's high-power microlenses. A bright light flux entering
the microlenses at a sharp angle will be directed to the viewfinder's light path. However,
if a dark light flux enters the microlenses at a less sharp angle, it will stray outside of the
viewfinder's light path. (Fig. 008, left diagram).
On the other hand, since normal focusing screens have weaker microlenses, the opposite
occurs. The dark light flux is directed to the viewfinder's light path, while the bright light flux
strays away (Fig. 008, right diagram). In other words, the standard Precision Matte and other
common focusing screens only take in the dark light flux regardless of the lens f-number.
Therefore, the viewfinder tend not to look dark even with slow lenses attached.
1.8
0.0
-0.5
-1.0
-1.5
-2.0
-2.5
2.8 4 5.6
Lens f-number
New Laser Matte
V
ie
w
fin
de
r
B
rig
htn
es
s
Standard
Precision
Matte
Laser Matte
[Stop]
Super Precision
Matte
＊ The vertical axis indicates the viewfinder brightness, while the horizontal axis is the lens f/number.
The graph shows how the viewfinder brightness changes in accordance with the lens f/number.
The flatter the lower right part of the line is, the less dark the viewfinder will be when a slow lens is
attached. Or the steeper the line is toward the right, the darker the viewfinder will become.
Fig. 007 Correlation of viewfinder brightness and lens f/number
Eye
Eye
f/2.8
light flux
Microlens
f/5.6
light flux
Super
Precision Matte
Microlens
f/2.8
light flux
f/5.6
light flux
Standard
Precision Matte
Fig. 008 Dispersion characteristics of Precision Matte