Sensor Size vs. Depth of Field vs. Exposure Time

[Pat in San Jose]

However both the 80mm lens on the 5D and the 50mm lens on the 7D were set at f/4 the same shutter speed would get the same exposure results. However, if you wanted to use an 80mm lens on the 5D and a 50mm on the 7D and if you demanded the same DOF, you would need to open the 50mm lens approximately one stop to f/2.8 which would require a shutter speed of 1/2 the duration of your exposure at f/4.

Thanks very much, Richard, for the extensive explanation. I managed to follow almost all of it, and the excerpt above is where I have a problem still. I tried the DoF calculator you left a link for, and it pretty well confirmed what everyone has been saying: for equivalent field of view, the depth of field is nearly proportional to the f-number divided by the sensor width. That worked out with every example I tried.

In the part I quoted above, you use this principle to obtain equivalent fields of view at the same subject distance by using a 50mm lens on the 7D and an 80mm lens on the 5D, then getting equal DoF by using f/2.8 on the 5D and f/4 on the 7D. Hunky-dory so far. But why would you need twice the shutter speed on the 7D under those circumstances? The physical aperture is essentially the same size on both (and would be exactly the same if the f-stop on the 5D had been 4/1.6 in order to produce exactly the same depth of field. So the same amount of light enters the shutter and the same amount of light impinges on both sensors. So why do you need to change the shutter speed?

My conclusion would be that the 5D will have the same depth of field as the 7D given the pair of lenses that yield the same field of view (1.6 ratio of focal lengths) and setting the aperture of the 5D to be the same physical size as that of the 7D (i.e., f# 1.6 times as large), with no need to increase ISO sensitivity or use a longer exposure time. But no one else seems to think so.

As I mentioned, I really want to understand this, because I'm contemplating buying a camera, but I don't want to go large-format if it means sacrificing DoF.

[Pat in San Jose]

OK. I think I've got a better handle on these concepts, thanks to an outstanding article by Joseph James, at http://www.josephjamesphotography.com/equivalence/. He makes the distinction between exposure and total light on the sensor, exposure being, as Sean explained, the light per unit area. And indeed, to have equal exposure with the 2/3" sensor and the 4/3" sensor, with the same DoF and field of view, you would need to compensate by having 4x as long an exposure time or 4x as high an ISO setting on the 4.3" camera. However, to keep the total light on the sensor (as well as the apparent noise and dynamic range) the same, the exposure times and ISO settings would stay the same. While one could set these parameters manually and have virtually equivalent images, the camera, when it chooses, will normally keep the exposures (not the total light) more or less the same. This is largely because the larger pixels on the larger sensor can hold correspondingly higher charges, the major advantage, in fact, of the larger sensor. So, with the aperture forced to a position that yields the same DoF and angle of view as the smaller sensor, the larger camera will choose a longer exposure time or a higher ISO or a combination of both.

[Colin Southern]

So, with the aperture forced to a position that yields the same DoF and angle of view as the smaller sensor, the larger camera will choose a longer exposure time or a higher ISO or a combination of both.

Yes. People often assume that "Full Frame" is the "nirvana" of formats, whereas the reality is it can work to your advantage or to your disadvantage - or make no real-world difference what-so-ever (as is the case most of the time).