Today’s Question: A friend has a Canon PowerShot G1X camera and it appears that when we are both photographing the same subject (me a D800E with a Sigma 180mm macro) that he gets a greater depth of field at f/13 than I do at f/22. Is there a way to estimate the f-stop I need to use to get a comparable depth of field using my full frame sensor versus his smaller sensor?
Tim’s Quick Answer: The overall math involved here could certainly be a bit tricky. However, as a general rule if you are adjusting your position to achieve the exact same framing of the scene, on the full-frame sensor you would need to use an f-number that is about double the aperture setting being used on the smaller sensor, bearing in mind that the f-stop is a fraction based on focal length. So, for example, if the G1X sensor is set to f/11, you would need to use about f/22 on the full-frame sensor with the same framing of the subject to achieve the same depth of field.
More Detail: There are a variety of ways you could look at this issue, but I think the key thing to keep in mind is that several factors are impacted by the smaller sensor size. The image sensor on the G1X is considerably smaller than a full-frame sensor (in this case about the size of a four-thirds system sensor). As a result, the framing of the scene will have a narrower field of view compared to the same focal length on a full-frame camera.
It is common with smaller cameras to reference the “effective” focal length of the lens, rather than the actual focal length of the lens. For example, the kit lens for the G1X is often described as having an effective focal length range of 28mm to 112mm. However, the actual lens focal length is 15.1mm to 60.4mm. The smaller sensor is simply cropping the image circle, resulting in the field of view you would achieve with a longer lens on a full-frame camera.
Because of that smaller image circle then, you are able to achieve the same field of view (cropping) for the scene from a greater distance. Focusing at a greater distance results in greater depth of field. This increased depth of field can be a tremendous advantage in some cases, but of course it can be a tremendous (and frustrating!) disadvantage when you are trying to achieve narrow depth of field.
In this type of situation, my personal approach would be to think about the fact that the focusing distance would be greater (or the focal length shorter at the same distance) for the camera with the smaller sensor compared to the camera with a full-frame sensor, rather than trying to think about the difference in aperture setting that would be required. In other words, I would tend to think about the overall concepts rather than trying to do a bunch of math in my head (or taking out my iPhone to use my depth of field calculator).