The situation was this. A young photographer was shooting basically a table top and was having problems with getting the image as sharp from front to back as she wanted. Another reply suggested that she change to a longer focal length lens and shoot from farther back. I rebutted that remark, saying that as long as she kept the size of the object she was shooting the same size on the sensor, the longer lens would not affect depth of field—it won’t. I suggested that she simply move back a little and then crop out the same image. The original poster took issue with that remark stating his qualifications as a product photographer that knew what he was talking about. This is my rebuttal to his rebuttal.
Depth of Field, DOF, is affected by three things; aperture, distance and focal length of lens. However it is governed by the size of the image on the sensor. Regardless of what anyone says, DOF can only be changed by changing the size of the image on the sensor or by changing aperture—zip, period, tada—that is it.
Optics are ruled by the laws of physics--not conventional photographic wisdoms no matter how many times you have heard it repeated it doesn't check out when you do the math. Physics can be confirmed quite easily mathically.
Assuming there is no question that changing aperture changes DOF let’s look at distance and focal length. We are assuming in all cases that the aperture remains the same throughout.
Scenario 01: You can increase DOF by simply moving farther away because with the same focal length increasing the distances makes the image smaller on the sensor. You can decrease DOF by simply moving closer because with the same focal length lens you have made the image larger. Image gets smaller DOF increases. Image gets larger DOF decreases.
Scenario 02: You can increase DOF by switching to a shorter focal length lens and leaving the camera in the same position because you have made the image smaller on the sensor. You can decrease DOF by switching to a longer focal length lens from the same camera position because you have made the image larger. Image gets smaller DOF increases. Image gets larger DOF decreases.
Scenario 03: If you change both distance and focal length so that the image is the same size on the sensor it does not matter how far away your are or what focal length lens you are using the DOF will be the same. Image size doesn't change, DOF remains the same regardless of distance or focal length of lens.
The following is calculated using DOFMaster for a Nikon D700 camera with a full frame sensor at 0.03mm circle of confusion—all pretty standard. http://www.dofmaster.com/dofjs.html
I am going to step through a series of focal lengths. In each case I will double the focal length of the lens and double the distance. This will for all practical purposes keep the size of the image on the sensor the same in all of the shots.
25mm @ f/16 shooting from 2 feet—DOF 2.25’50mm @ f/16 shooting from 4 feet—DOF 1.89’
100mm @ f/16 shooting from 8 feet—DOF 1.82’
200mm @ f/16 shooting from 16 feet—DOF 1.8’
400mm @ f/16 shooting from 32 feet—DOF 1.8’
800mm @ f/16 shooting from 64 feet—DOF 1.8’
The focal length changes from 25mm to 800mm and the shooting distance varies from 2 feet to 64 feet--pretty drastic changes, yet the DOF remains virtually the same in each case.
Okay, let’s assume the young photographer had a 150mm lens which they used to shoot from 12 feet away. The DOF would be 1.81’ (1’ 9.7”), same as the 100mm at 8' or the 200mm lenses at 16'. Now assuming that instead of 12 feet away she shot at 13 feet away. By increasing the distance by 1’ the DOF would be changed to 2.13’ (2’ 1.6”—with the 150mm lens by moving back 1 foot she would have gained 3.8” of additional DOF) because now the image was smaller on the sensor than it would have been at 12'.
Let’s also assume that the original shot posted to the forum was done with the 50mm lens from 4’ which would have given a DOF of 1.89’ (1’ 10.7”). She could have gained the same amount of increased DOF by using the 50mm lens at 4.31’ where the DOF would have been 2.21’ (a gain of 3.84” in DOF). That means that rather than change lenses all she had to do was to back up less than 4 inches and she would have achieved the same increased DOF.
Keep in mind, it was not changing the focal length of the lens that changed the DOF. In both cases, with the 150mm and with the 50mm, it was increasing the distance and reducing the size of the image on the sensor that increased the DOF.
What would have changed would be spatial distance and perspective in the photograph. The shot using the 150mm lens would have reduced the convergence of the table top making it appear flatter or less deep. The 50mm lens would have produced the table top with more or less “normal” perspective.
Changing focal lengths does not change DOF. It does change spatial distance and perspective.