| 摘要 |
A projection apparatus includes a light source emitting an illuminating beam, a light valve receiving the illuminating beam to create an image beam, and a projection lens system arranged on a path of the image beam for receiving and projecting the image beam onto a screen. The projection lens system includes four lens elements arranged along the optical axis in order from the screen toward the light valve, namely, first, second, third and fourth lens elements respectively having positive, negative, positive and positive refractive power. The four-piece projection lens system satisfies the condition of 0.79<BFL/TL<0.99; where, BFL is the back focal length of the projection lens system, and TL is the overall length of the projection lens system on the optical axis from a screen side of the first lens element to a light-valve side of the fourth lens element.
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