Do You Know the Four Factors Affecting Depth of Field?

If you're like most photographers, you think there are three factors affecting depth of field. In that case, you would be incorrect - there are indeed four factors affecting depth of field. Do you know what they are?

In this video Mat Marrash covers the four factors that contribute to depth of field. Moreover, he provides some useful education to better understand how they work together to create the desired effect. In addition, for those photographers who shoot large format or utilize lenses or cameras with tilt capabilities, Marrash discusses how a photographer may take advantage of front or rear movements to affect depth of field in creative ways. 

Personally, the depth of field, tilt, and shift capabilities of a large format camera is why I got into 4x5. The full frame equivalent to the standard 4x5 lens (e.g., 210mm f/5.6) is a 56mm f/1.5 which may not sound like much but the fact of the matter is that most people would agree that there's something particularly special to large format images (particularly when it comes to portraits). When you start playing around with front standard movements, that already razor-thin depth of field can make some very creative looks. 

James Madison's picture

Madison is a mathematician turned statistician based out of Columbus, OH. He fell back in love with film years ago while living in Charleston, SC and hasn't looked back since. In early 2019 he started a website about film photography.

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I would not consider the circle of confusion as a 4th factor affecting the depth of field, as it is an explanation or the result of the depth of field itself.
Also the image at 1:30 in the video shows the depth of field extending to the same length both before and behind the focus plan. That’s not right. The total depth of field is spread 1/3 before and 2/3 behind the focus plan… unless I learned it wrong…

That's only true at one specific focus distance for any particular focal length. As the focus distance approaches unity (1:1 magnification), the ratio of front to rear DoF also approaches 1:1. As the focus distance is increased, rear DoF increases faster than front DoF until rear DoF stretches to infinity while front DoF is 1/2 the focus distance, thus at the hyperfocal distance the ratio is 1:infinity.

Michael Clark:
You are right. On the printed depth of field on lenses that have a focusing ring, the depth of field marks for each diaphragm values are symmetric on both sides of the focusing mark, with the focusing mark effectively at the centre, but the printed distance scale numbers in meters or feet are more spread for short distances than those for long distances.

There are only two factors: aperture and total magnification. Focal length, subject distance, sensor/film size, display size, and viewing distance are all contributors to total magnification. The CoC is calculated based upon enlargement ratio (display size divided by format size) and anticipated viewing distance, as well as an assumption of visual acuity that defines the minimum angle that can be perceived as larger than a point by the viewer.

Respectfully, sensor size has nothing to do with may enlarge the scene by capturing more information but

There's absolute magnification: what is the ratio of an object's size in real life to its size on the film.
There's relative magnification: what is the size of an object in a final print.

If one were to hold focal length and subject distance constant and only change the film size, there may not be a change in absolute magnification in the sense of a macro photographer, but if both images where printed as 8x10's, those two prints will have different relative magnifications. And the different has a direct influence on the choices of a photographer.

Now. this matters more for large format film than for any digital size. A photographer chooses a different absolute magnification for different formats.

On 35mm 1:1 magnification is great for insects, watches, etc.
But for 11x14, 1:1 magnification is for head shots and portraits.

That difference means magnification has something to do with format size.

“On 35mm 1:1 magnification is great for insects, watches, etc.
But for 11x14, 1:1 magnification is for head shots and portraits.”

That’s why for a portrait with a 11x14” camera we lose 2 stops compared with the same portrait taken with a 35mm film camera. On large format photography we have to make that adjustment even for a portrait.