Sensor sizes and DOF

One factor that affects depth of field is the sensor size. Simply put: the smaller the sensor, the more extended your depth of field in any given image.

This is an approximation and simplification (it also depends on angle of view, point of view, etc)  but it is good for us as photographers.

Clearly, this means that if we want blurry backgrounds, we want large sensors. So what are the choices?

  1. Lower-end (and many higher-end!) point-and-shoot cameras usually have very small sensors. These do not make it easy to get blurry backgrounds!
  2. Then there are “almost-APS-C” sensors such as the “Micro four thirds” format – these are almost as big as a crop camera’s sensor. Micro four third cameras are twice as small as a negative. This is the trend in small cameras.
  3. The next step up is the APS-C crop sensor – 1.6 times smaller than a negative for Canon; 1.5 times for a Nikon. Most DSLRs have this size sensor. Some small cameras now also do (like my Fuji X100).
  4. Next, there is a Canon-only size that is 1.3 smaller than a negative – this is the 1D’s format.
  5. And finally, there is the full-frame sensor – it is exactly the size of a 35mm negative.

The bigger the better – also because a larger sensor gives you lower noice and hence higher ISO capability, and a larger, brighter viewfinder.

And this is why we are seeing today’s wonderful move to larger sensors. So my advice: when buying your new camera, do ask how large the sensor is, and go for the largest one you can afford.


7 thoughts on “Sensor sizes and DOF

  1. Hey Michael

    If I use my DOF Calculator on my iPhone or on the Web, punching in 1.6x versus full frame shows a more narrow DoF for the 1.6x… why?


  2. There is a lot of misinformation around and some defective math. Like the notion that attaching a lens to a body with a crop sensor automatically makes the lens longer due to the physical dimensions of the sensor, the notion that film or sensor size has any effect on depth of field may simply be another load of nonsense.

    Sensor size/film size within limits, certainly within the limits of a four thirds body to a full frame or even medium format probably has no effect on depth of field. I say that because the only difference between a full frame sensor and an APS-C sensor is the area of the image circle that is sampled. If you mount a 100 mm lens on the full frame body, and place it some distance from an object, the focused image will have some depth of field. If you leave the lens in place and mount an APS-C sensor body, the image should still be focused, the depth of field should still be the same, but the sensor does not fill the image circle, that is the only difference. Of course this assumes both bodies have the sensor exactly the same distance from the lens mount. I wonder if that is the case.

    Most point & shoot cameras have a huge depth of field because they are taking a tiny crop from the centre of an image produced by a very short lens. If you mount a 5 mm lens on a full frame body you will have a difficult time keeping your feet out of the photo, but your feet and the distant hills will all appear to be sharp.

    • Ron, that is why “magnification” is the key concept here. A small sensor certainly magnifies lenses, but that is because you use it differently, to crop/magnify more. In the same sense, cropping also magnifies. Keep in mind: “per megapixel”. Check the article on Wikipedia for the math – not defective math, but also not simple math.

      • Well, I disagree that a small sensor magnifies lenses and I can demonstrate that my view is the correct one.

        You start out by saying “One factor that affects depth of field is the sensor size. Simply put: the smaller the sensor, the more extended your depth of field in any given image.” Having spent some time testing, I can now state sensor size, at least the difference between full frame and APS-C has virtually no effect on depth of field.

        You probably have enough different Canon bodies you could duplicate my test for yourself. My photos are at the bottom of this page:

        I do see your point, that with a larger sensor you can get closer to your subject without your subject overflowing the frame, and by getting closer you affect depth of field. That is not quite the same as a larger sensor affecting depth of field simply because it is a bigger sensor.

        • Ron,

          Two cameras next to one another, both 10 Mp cameras, say. Both with a 50mm lens.

          Camera 1, full frame, gives you a 50mm-looking image, 10 Mp.
          Camera 2, crop, gives you an 80mm-looking image, 10 Mp.

          Print both images. Both same size, say 8×10 (since we have the same number of Megapixels). The crop image will look like it was taken with an 80mm lens.

          Same kind of effect with DOF. It’s all about where you are!


          • Yes, Michael, and that works with some models of Nikon because one of their full frame cameras has roughly the same number of pixels as many of their crop sensor cameras so the pixel density is the same ratio as the physical dimensions of the sensor. That is a special case that lead to a generalization that is repeated by camera stores and magazines but is not very accurate across brands or with newer models. It can be misleading, too.

            Take 2 cameras, a Nikon D3S (12.1 Mpx) and a Nikon D3X (24.5 Mpx), both with a 50 mm lens. Take the same subject and print at 300 dpi. The D3X image will look magnified. But, they are both full frame cameras!

            It is not how many mm wide your sensor is, that causes magnification. It is how many pixels per mm.

            Canon have APS-C sensors with several different densities so if you want to determine magnification you have to decide on your reference then work out the pixel densities. Blindly multiplying by 1.6 will approximate angle of view but if you take a picture with a Rebel T3 and another with a Rebel T3i, then look at both at 100%, you will see the T3i appears to have magnified the image. Both have APS-C sensors!

            DOF is dependent on distance to subject. If you take two photos from different places, you probably will have different depths of field. But if you take two photos through the same lens, from exactly the same position, focused on the same object, you can crop with a smaller sensor, or you can crop later with Photoshop, or with scissors, and the depth of field will not change.

            Take a picture and print it unprocessed is very Uncle Fred. Even more so if the camera used had a 95% viewfinder. If all you are only going to take a few photos and display them unedited on facebook, then none of this matters. If your students are going to try to control all aspects of the photo from setting up the scene to the final print it would be useful for them to understand what all the controls really do. As an educator who has previously complained about content in the camera manuals, I would have expected you to want to provide information that was accurate and complete.

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