The larger the reflector or smaller the light source is, the longer the distance that needs to be used.
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For example:
Moon Blaster: 120W Hi Pressure Mercury short Arc, 9” reflector
16.1 Million CP (Measured by ma_sha1 @ 100 meter distance)
16.7 Million CP when measured @ 10 meter distance
Mega Blaster: 120W High Pressure Mercury short Arc, 4.5” Aspheric lens
5.0 Million CP (Measured by ma_sha1 @ 100 meter distance)
5.5 Million CP when measured @ 10 meter distance
Maxa Beam: 75W Xenon short Arc, 5” Hi End Reflector
2.3 Million CP (Measured by ma_sha1 @ 100 meter distance)
2.3 Million CP when measured @ 30 meter;
1.9 Million CP when measured @ 10 meter.
Military short Arc: 150W Xenon short Arc, 6” Hi End Reflector
0.8 Million CP (Measured by ma_sha1 @ 100 meter distance)
1.1 Million CP when measured @ 10 meter distance
The conclusion was that about 30m is good, 10m is not far enough for these large diameter lights.
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My point is that if you focus it too close you will end up with a diverging beam past the focal point, which will not give the longest possible throw.
You can read here about how LED lights can be focused for the best throw without having to take lux measurements very far away: