To check the sensor dusts: set camera to manual focus mode and focus to infinite; set camera mode to aperture priority and set aperture to f/22 or the smallest; set zoom to 50mm equivalent or so. Open Notepad on your computer and maxmize it so you get a white screen. Put your camera possible to the white screen, take a shot while moving the camera slowly from the left to the right. Now open this image in XnView (or Photoshop), click Image, Map, Automatic Levels. Then you'll see something like above. The black dots you see are the dusts on the sensor, a dot on the left upper corner will appear at the right bottom on the sensor, vice versa.
To remove the dusts, you need these:
Don't use an Alcohol that contains either too much or too less water. Never touch the cotton tips to keep them from oil. Shake the swab as hard as you can to drain out extra alcohol before its touches the sensor. Move the swab to the edges of the sensor to minimize the trace.
This is how it looks now:
I also use this same method to clean lenses. Use a blower before and after above procedure. You only need to do such cleanup once, afterward it is almost impossible to get such factory dusts again. From now on, any dusts should be easily blow off.
Abstract in Chinese
我发现无镜相机出厂时传感器上都带有少许很细微的尘埃，吹不掉，要用酒精棉才能擦掉。
把相机拨到A模式，光圈22或最小；用手动对焦，对到无穷远处；用中等焦距（减少暗角）。对着白色的电脑屏幕，在慢慢移动相机的过程中按下快门。不对焦和移动的目的都是为了不把屏幕上的灰尘照下来。用XnView或Photoshop打开这张照片，做auto levels，如果传感器不干净的话，就可以看到上面第一张照片上的那些黑点。黑点和灰尘的位置是倒过来的，比如左上角的黑点对应的是传感器右下角上的灰尘，反之亦然。
酒精的纯度不要太高也不要太低，否则清除能力都不好也容易留痕迹。脱脂棉最好有一头是比较尖的。沾上酒精后，使劲甩掉多余的液体。不要让手碰到棉球，以免沾上油。把灰尘擦到传感器的边缘后抬起棉球，以减少留痕。棉球变得比较干了的话，效果会变得很差，再沾点酒精或换一根。
在这之前和之后都用吹气球吹吹。以后一般不再会粘上这样的厂家带来的特殊尘埃了，普通的灰尘吹吹就掉了（千万别用嘴吹！）。 我用同样的方法清洁镜头。
