Photography Depth of Field - Does sensor size affect DOF?

With full frame digital SLR cameras you can get shallower depth of field, but the reason is actually not related to the sensor size itself but to other factors.  Some photographers prefer getting large sensor cameras to achieve better depth in their images by blurring the background using low f-stop numbers (large aperture settings). I've read in forum posts that digital cameras' sensor size gives shallower depth of field. How can the DOF change if you use the same camera settings and in the same distance from your subject? - Very curious, I have decided to search an answer to the following question: "Does the camera sensor size affect the DOF?" and "What factors directly affects the DOF?".

Of course there are many other benefits of having a FF camera. It is very tempting from jumping from ASP-C camera to a Full Frame one. Over the years, Full frame DSLRs have become somewhat more affordable for enthusiast and professional photographers. Large sensor means larger pixels and that leads to better light sensitivity, less noise in high ISO and of course wider angle with ultra wide angle lenses compared to ASP-C cameras. With full frame cameras you don't have to multiply the focal range according to the crop factor. You get the same focal range as stated on the lens, and that means that a 24mm lens is a 24mm not 24x1.5=36mm on a 1.5x crop factor sensor. In this article, I want to focus on one aspect regarding ASP-C and FF sensors and what is the relationship between Depth-of-field and digital camera's sensor size?

As you can see from the size comparison illustration above, the ASP-C size sensor (1.5x crop in this example) is smaller than full frame sensor. When you buy an ASP-C camera, you should take the focal length multiplier into account. As I have mentioned above, a 100mm lens on an ASP-C sensor will become 150mm. The image on ASP-C camera will also appear more magnified because of this. FF sensors get to record most of the light diameter. It covers most of the circle of confusion (CoC) area. ASP-C sensor size varies from one manufacture to the other, but the differences are smaller, 15.0 mm x 22.5 vs. 16 mm x 24mmm, for example.

Shallow Depth of Field is one reason why photographers prefer DSLR cameras over Point and shoot. Sometimes people have a misconception about the difference in depth of field with different sensor sizes. If you take a shot with both your P&S camera and your Full frame DSLR, use the same focal length and aperture settings, both cameras will actually produce the same Depth of Field (DOF). That means that the sensor size alone, doesn't affect the Depth of Field.  In order to get a 35mm field of view on a P&S, camera manufacturers utilize wider angle lenses to achieve the same field of view as in 35mm lens. If they have decided to use the same lens on such a tiny sensor such as in P&S cameras, you would get a focal length that is far beyond 35mm.(remember the focal length multiplier from the previous paragraph?).

Sensor size by itself doesn't affect DOF

By using a wider angle to get the same focal length, you actually change one variable that directly impacts the DOF and it is the focal length. There are three main factors that are responsible for changing the DOF: 1) aperture opening, 2) focal length of the lens, 3) camera/sensor distance from your subject. I mentioned the difference between P&S cameras and DSLRs, but it works the same when you compare the DOF that you can achieve with ASP-C or FF cameras at a given focal length. Again, in order to shoot in 35mm in ASP-C, you will need a 23mm (23.3mm to be exact = 35mm / 1.5 crop factor) lens if the sensor has a 1.5x focal length multiplier.  At the same focal length, in ASP-C you will actually be using a 23mm lens to shoot in 35mm, instead of 35mm. As for the same aperture settings and distance from your subject, the larger the focal length the larger/deeper/longer the DOF. That means that's with a Full frame sensor you will get longer DOF for the same focal length because the lens focal length used has to be lower in 35mm figures. DOF actually is a measurement of the length between nearest and far limit distance of acceptable sharpness distances.

Three factors that affects depth of field:

1. Aperture - The larger the aperture (diaphragm opening of the lens), the narrow (shallow) the DOF will be
2. Distance from Subject - The closer you are to the subject, the narrow the DOF will be
3. Focal length - the larger the Focal length, the narrow (shallow) the DOF will be

That also means that if you took an image with a P&S camera and a DSLR with the same focal length, the P&S camera will only record a smaller portion of the image, because the smaller sensor doesn't cover a large area of the circle of confusion diameter. Of course because the recording device (sensor) is smaller, it doesn't mean that the DOF will change. You actually just look at a portion of a large image, that's all.

Remember this:

To take an image at 24mm you will need to shoot in:
1) 24mm (35mm equivalent)  focal length on a Full frame DSLR (24 mm x 1  = 24 mm)
2) 16mm (35mm equivalent)  focal length on an ASP-C (1.5x crop) DSLR (16 mm x 1.5 = 24 mm)

Because the longer the focal length the narrow the DOF, in 24mm shot you will get shallower DOF at that given length. Of course the aperture number (f number) and you distance from the subject is the same.

So what we have learned is that sensor size has an effect over Depth-of-Field, BUT indirectly.

I also want to add that when you take the same shoot of an object with a P&S camera, it might have an effect on how we perceive the depth of field. If you take a photo of an object with a lot of objects around it, by shooting with the same focal length, you will get different pictures. In the point-and-shoot camera you will get an image much closer to the subject. Because you are close to the subject, you won't perceive the DOF compared to a FF shot where all the out-of-focus objects appear in the image. We can better perceive DOF by comparing our main focus object to its surroundings. If you had only a plain white background, you would have no way to determine whether the background was out-of-focus or not.

shallow depth of field also referred to as: small, short, low, narrow, limited, minimum DOF
great depth of field also referred to as: large, long, deep, high, maximum DOF

So Full frame compared to ASP-C DSLRs indirectly allow you to achieve shallower depth of field. Of course in order to blur the background as much as possible, you will need a fast (larger aperture) lens with a long focal range and take pictures as close as possible to your subject.

I gave myself an important lesson today. I love when I can both learn and share the knowledge with you at the same time. If you have anything to add, please comment below.