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很多图像传感器可以输出多个分辨率的图像，如果选择低分辨率格式的图像输出，对于图像本身会有什么影响呢？

传感器本身还是使用全部像素区域进行感光，但是在像素数据输出时会进行所谓的降采样（down-sampling）。具体的降采样方式各个传感器可能略有不同，但是得到的效果都是另实际的PPI（pixels per inch）数目下降，即输出图片的实际像素数小于传感器最大像素数。

可以参考以下网址的一个回答

If I set my camera to lower resolution, does that mean more sensor area per image pixel?
In terms of PPI, no. In fact, your effective PPI in the final picture is reduced. When you choose smaller image sizes in camera, when the picture is taken, the full sensor is still used. However, when the camera’s processor goes to write the image to a JPEG file, the image is first downsampled. This has the effect of being able to discern fewer fine details in the reduced picture.

Nikon’s D3200 specifications page gives us the following (I added the sensor conversion to inches):

Image sensor: 23.2 x 15.4 mm CMOS sensor (0.913 x 0.606 in)
Image size (pixels) [L]: 6,016 x 4,000 (24,064,000 pixels)
Image size (pixels) [M]: 4,512 x 3,000 (13,536,000 pixels)
Image size (pixels) [S]: 3,008 x 2,000 (6,016,000 pixels)

Thus, depending on whether L, M, or S is chosen in the camera settings, the D3200 will perform as either a 24 MP camera, 13.5 MP camera, or 6 MP camera. In all 3 cases, the entire CMOS sensor area is used to take the image. In other words, regardless of L/M/S setting, it is still a crop-sensor (1.5 crop factor) camera.

For the Large setting, no downsampling is performed. Thus the effective PPI of the sensor image is: 4,000 pixels / 0.606 inches = 6,601 PPI.
For the Medium setting, a 4:3 downsampling is performed. Thus the effective PPI of the sensor image is: 3,000 pixels / 0.606 inches = 4,950 PPI.
For the Small setting, a 2:1 downsampling is performed. Thus the effective PPI of the sensor image is: 2,000 pixels / 0.606 inches = 3,300 PPI.

Returning to your question, but addressing “Sensor Pixel Area” (I read that as effective pixel (sensel) size),

Would setting it to 13.1MP allow for a greater Sensor Pixel Area ... in my final picture?

Yes. At the following image sizes, your effective sensel size is approximately:

Large setting: 14.8 µm²
Medium setting: 26.4 µm²
Small setting: 59.3 µm²

The area covered by each pixel in the final image is the size of several pixels on the sensor. Fewer pixels in the same area means there is more area covered per pixel. Or to put it simply, if you reduce PPI, then you increase the area covered by each pixel. Pixel/inch inverted is inch/pixel. 100 ppi means the same thing as 1 inch per 100 pixels which means the same thing as .01 inches per pixel. 50 ppi means the same thing as 1 inch per 50 pixels which means the same thing as .02 inches per pixel.