Apertures And Depth of Field On Cameras With Different Sized Sensors

Old Fashioned Soda Fountain, Great Falls, Montana (click to enlarge)
Olympus E-M1, Panasonic 14-140mm f/3.5-5.6 lens @ 15mm; 1/60th sec. @ f/6.3; ISO 800

A recent past post highlighted that identical lens apertures coupled with identical shutter speeds result in the same exposure no matter which cameras, sensor size or film you are using.  You can find that post here.  Let's now move to another aperture related topic, on which I get questions, that also confuses some—depth of field on cameras with different size sensors (or different formats of film).  Unlike the previous topic there IS a difference in depth of field at the same aperture with different sized sensors.

First, for all of you who are very technical, scientific as well as insist upon utmost accuracy, I'll now make the statement that depth of field is an illusion.  Yes, an illusion. The only thing that is in true focus in your image is the exact plane at which the lens is precisely focused.  What we call depth of field is that portion of the image in front of and behind that precise plane of focus that "looks" acceptably sharp.  Typically that zone of acceptable sharpness extends 1/3 of the distance between the camera and focus point toward you (in front of the exact plane of focus) and extends back 2/3 focus distance behind it.  I won't go into the term "circle of confusion" but if you are interested in the above technical explanation, look up that term in reference to depth of field.  But, as I said, technically only the exact plane at which the camera is focused is the only thing in precise focus.  It may not look that way to our eyes, but technically it is.  Now that I got that out of the way, let's talk about the more practical aspect of depth of field.

Depth of field renders differently for cameras and lenses with different sized sensors or film.  Typically, the larger the format with any given focal length lens and aperture, the shallower the depth of field.  It is relatively easy to translate equivalent depth of field on different sized sensors.

For example, f/8 on a micro 4/3 sensor camera will have the same depth of field as f/16 on a full frame sensor camera at the same aperture and focus distance.  That is a 2 stop difference, again at identical apertures and focus distances, because the micro 4/3 sensor is about 1/4 the size when compared to a full frame camera.  F/8 on an APS-C camera will have the same depth of field as f/11 on a full frame camera.  That is a one stop difference because the sensor of a full frame camera is roughly twice the size of an APS-C camera.  It is the depth of field that varies with different sensor or film sizes.  Depth of field is determined by three parameters; focal length, aperture and the distance at which the lens is focused.  

An example.  The field of view of a 25mm lens on a micro 4/3 camera is approximately equivalent to the field of view of a 50mm lens on a full frame or 35mm film camera.  At the same aperture and focus distance, the depth of field on the 25mm lens is about twice the depth of field on that 50mm lens.  What varied?  The focal length of the lens.  The shorter the lens, with everything else being equal, gives greater depth of field.  That makes the difference. 

Now let's take a micro 4/3 camera and attach a 25mm lens to it and set it at f/5.6.  Let's also attach a 25mm lens set at f/5.6 to a full frame or 35mm film camera.  Now we have matched focal lengths.  On the micro 4/3 camera the 25mm is considered a "normal" lens. On the full frame camera, the 25mm lens is considered a very wide angle lens.  When creating identical images, the full frame camera will be much closer to the subject as it is a very wide angle lens.  The micro 4/3 camera will have to be farther back to capture the same field of view.  The wide angle lens on the full frame camera will have twice the depth of field of the micro 4/3 lens.  What changed in this example?  The focus distance.  

I probably should stop here as it seems I'm having a harder time committing this to writing than it is to understand the concept verbally.  This is all about the laws of optics and physics.  Science.  Which, by the way, is the basis of photography.  Without science, we wouldn't have photography.

Final example to sum up both posts in a practical sense.

If you have two cameras set up on tripods side-by-side, one a micro 4/3 camera and the other a full frame camera, both with a 50mm lenses attached, both set at 1/125th sec. @ f/8 and ISO 200, as well as both focused at the same distance, the amount of light reaching the sensor, thus total exposure, will be the same.  However, the depth of field on the micro 4/3 camera will be twice that of the full frame camera.

Physics.

P.S.  Here is some bonus confusion!  But you should know this if you are a photographer.  If you have a manual flash with a constant output, or use a continuous light source such as LEDs or bulbs of any kind, and the light lands on your subject, which is for example 10 ft. away, then at 20 ft. your subject will only receive 1/4 of the light it received at 10 ft.     Shouldn't it be 1/2 of the light?  No, due to the Inverse Square Law.  Each time you double your distance from your light source, only 1/4 of the light will illuminate your subject. Double the distance, open the lens 2 f/stops.  Good to remember if you are working with manual flash as well as a continuous light source.  

Sorry to confuse some of you....

Thanks for looking. Enjoy! 

Dennis A. Mook 

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