(i) Technical Field
The present invention relates to focal plane shutters and optical device.
(ii) Related Art
A focal plane shutter is equipped with: an iron core of an electromagnet; and a drive lever holding an iron piece, the magnetic attraction force being generated between the iron piece and the electromagnet. The drive lever drives blades. The drive lever is biased by a biasing member such that the iron piece is moved away from the iron core of the electromagnet. The electromagnet is energized in a state where the iron piece abuts the iron core, so that the magnetic attraction force is generated between the iron piece and the iron core. Thus, the iron piece is adsorbed to and held by the iron core against the biasing force of the biasing member. The energization of the electromagnet is stopped in the state where the iron piece is adsorbed to the iron core, so that the adsorption force of the iron piece to the iron core is lower than the biasing force of the biasing member. Thus, the drive lever moves in accordance with the biasing force of the biasing member such that the iron piece is moved away from the iron core. In this way, the drive lever drives the blades by use of the magnetic attraction force of the electromagnet and the biasing force of the biasing member. Japanese Unexamined Patent Application Publication No. 2004-029277 discloses such a focal plane shutter.
The iron piece of the drive lever repeatedly abuts the iron core of the electromagnet, so that the adsorption force of the iron piece to the iron core might decrease. This is conceivably because the repeated abutment of the iron piece with the iron core might cause abrasion of the adsorption surface of the iron piece, so that the adsorption surface might be made uneven or the adsorption surface might be attached with abrasion powder.
The adsorption force of the iron piece to the iron core decreases in this way. This shortens the period from the time when the energization of the electromagnet is stopped to the time when the iron piece is moved away from the iron core by making the adsorption force lower than the biasing force of the biasing member. Thus, the speed of the blades also changes to increase the error between the actual exposure period and the desired exposure period assumed beforehand. Thus, the image quality might be influenced.
Further, the same problem might occur in a self-holding electromagnet in which a magnet is secured to an iron core, an iron piece can be adsorbed to the iron core in a non-energized state, and the adsorption force can be reduced by energization.