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The camera in question is a Nikon D3200, it features 24 MPx.

I would like to know what changes are brought about on the sensor to enable it to shoot at lower-resolutions.

  • Does the camera shoot in a crop mode, ignoring some pixels to bring resolution down?
  • Or does the camera merge multiples pixels to behave as a larger pixel?

I'm quite certain the former is correct, however I would like to be sure about this.

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  • \$\begingroup\$ I'm sure most cameras won't crop, the latter seems more correct \$\endgroup\$
    – Janardan S
    Commented Mar 16, 2017 at 12:04

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Let's say that with your 24 megapixel digital camera, you have selected an output size of 6 megapixels.

Usually what this means is that the camera captures the scene at the full 24 MP, performs some internal processing to shrink the number of pixels, and writes out a 6 MP image. The shrunken image has the same field of view (framing) and aspect ratio as the full-size image. This mode of shrinking images is by far the most popular because it's the most intuitive and gives the most desirable output.

Does the camera shoot in a crop-mode, ignoring some pixels to bring resolution down? Or does the camera merge multiples pixels to behave as a larger pixels?
I'm quite certain the former is correct, however I would like to be sure about this.

We are quite certain that the latter idea is correct.

However, some cameras do crop the full image to create a smaller output image with a different field of view. For example:

  • In video mode, some DSLR cameras will crop the sensor for various reasons - extra zooming, matching the 4K video resolution to the sensor's native resolution, etc.
  • Various cameras (smartphones, Panasonic MFT, etc.) have options for different output aspect ratios - such as 4:3, 3:2, 16:9, etc. This necessarily involves cropping the sensor's image.
  • On Nikon DSLRs when a DX lens is mounted on an FX camera body, the camera will crop the image by default to match the smaller image circle.
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  • \$\begingroup\$ Also using any digital zoom will probably crop the image. \$\endgroup\$
    – vclaw
    Commented Jan 27, 2017 at 13:03
  • \$\begingroup\$ Can you please elaborate on how pixels are merged to behave as a larger pixel? I would like to create a second question following that line of thought. (Does lowering resolution improve ISO quality on the same sensor?) \$\endgroup\$
    – HelloWorld
    Commented Jan 28, 2017 at 0:46
  • \$\begingroup\$ @user2440943 en.wikipedia.org/wiki/Multivariate_interpolation \$\endgroup\$
    – Nayuki
    Commented Jan 28, 2017 at 0:53
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    \$\begingroup\$ And the answer to your second question, before you even ask it, is yes. If you average four pixels, you get about the same noise as you would from a pixel with four times the area, give or take (ignoring situations where the smaller pixels result in lower full well capacity, resulting in blown highlights). Of course, this approach limits your ability to do more sophisticated noise reduction later that might give better final results with less resolution loss. \$\endgroup\$
    – dgatwood
    Commented Feb 5, 2017 at 5:33
  • \$\begingroup\$ @Nayuki- I currently own the d7100 which does not use the whole sensor, but a smaller area of it. \$\endgroup\$
    – HelloWorld
    Commented Jan 25, 2018 at 9:48
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Does the camera shoot in a crop-mode, ignoring some pixels to bring resolution down?

No. The camera always uses the full size of the sensor - this is why RAW files are always at full resolution. The only thing that happens when producing lower resolution JPEG output is that the JPEG is scaled down as it is produced.

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  • \$\begingroup\$ I wasn't aware that this only applied to JPEG. The motivation behind my question was to find out if shooting in lower-res would potentially bring me better ISO quality. Thank you \$\endgroup\$
    – HelloWorld
    Commented Jan 26, 2017 at 21:46
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    \$\begingroup\$ Raw files are always at full resolution - except when they aren't. Such as with Canon's M-Raw or S-Raw options. \$\endgroup\$
    – Michael C
    Commented Jan 26, 2017 at 22:35
  • \$\begingroup\$ @user2440943 Then why not ask your real question? \$\endgroup\$
    – user29608
    Commented Jan 27, 2017 at 8:34
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    \$\begingroup\$ I'd disagree with that 'full size" claim. In fact, I have two different cameras both of which use a reduced portion of the array when shooting high-speed stills or high-speed video. In general, check to see if the RAW file in your camera is in fact full-res, in which case it is JPG-downconversion. \$\endgroup\$
    – Carl Witthoft
    Commented Jan 27, 2017 at 12:56
  • \$\begingroup\$ @CarlWitthoft- that is interesting. I know Sony DSLRs do that in their burst mode. But I have no idea if that applies to RAW as well. I would be interested to hear more about this. \$\endgroup\$
    – HelloWorld
    Commented Jan 28, 2017 at 0:41
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Some cameras have a "crop mode", but when used, it changes not only the resolution but also the framing. On a DSLR (e.g. D7200), the optical viewfinder always shows the full image, so a crop mode will take a picture of only a part of what can be seen in the viewfinder.

On a camera without optical viewfinder, the viewfinder can show the frame that will actually be used for the final picture, but it is also smaller than what the optics show. In other words, this is a digital zoom.

In both cases, you'll notice it if your camera uses a crop mode. The normal way to shrink images it the other (downsampling).

In the end, "shooting at low resolution" does not really have advantages other than giving smaller files: both cropping and downsampling can be done after shooting (especially if you shoot RAW).

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