Depth of Field Help 14-Aug-2003

Note Depth of field does not depend on the frame size (e.g. 35mm, 6x7cm, digital camera CCD,…). For a given focal length, the frame size does however affect the field of view. The appropriate value for resolution (see below) may need to be adjusted based on the frame size as well as the film or CCD resolving power and other factors.

Focal Length Enter the focal length of your lens here. You can either type numbers into the edit control or tap the drop down arrow and select the focal length from the list. You can also change units if you wish by tapping on the units drop down box. You can customize the list of focal lengths by editing the file FL.txt that resides in the same folder as the program: DOF.exe.

Teleconverter Select any teleconverter you are using (or None if you are not using a teleconverter) from the drop down list. You can customize the list of teleconverter factors by editing the file TC.txt that resides in the same folder as the program: DOF.exe. Using a teleconverter setting other than None automatically adjusts the f stop (see below) so there is no need to adjust the f stop manually -- just use the aperture value indicated on the lens dial. For example, a 300mm lens at f/16 with a 2X teleconverter is equivalent to a 600mm lens at f/32.

F Stop Enter the f stop for the lens here. You can either type numbers into the edit control or tap the drop down arrow and select the f stop from the list. For example if your lens is set at f/16, enter 16. Enter the f stop as marked on the lens -- do not attempt to correct for any extension due to macro focusing as this is already taken into account by the formulas used to compute the depth of field.

Resolution Enter the desired image resolution here. You can either type numbers into the edit control or tap the drop down arrow and select the resolution from the list. You can change units if you wish by clicking on the units drop down box. The resolution you specify defines the maximum amount of blurring you are willing to tolerate at the film plane. Resolution is specified as the number of lines per mm (or other units) on the film that can be distinguished. The higher the resolution you specify, the sharper you are requiring the image to be and the narrower the depth of field range that will satisfy that requirement.

Circle of Confusion A related measure of resolution is the circle of confusion which is defined as 1/resolution. The circle of confusion is defined as the diameter of the circle that an ideal point on the subject is focused to on the film plane. For example, a resolution of 30 lines/mm is equivalent to a circle of confusion of 1/30 mm.

Choosing the Resolution Setting The appropriate resolution setting depends on a number of different factors including: the resolving power of the film you are using, the contrast of the subject, the accuracy with which you focus on the subject, the quality of your optics, how stationary your camera and subject remain during the exposure, diffraction, film curvature, and the maximum size enlargement you expect to make.

Enlargement Size Resolution of 30 to 40 lines/mm is sufficient to make a high quality 8x10 enlargement from a 35mm negative or slide (based on a 10" viewing distance) -- a standard value of about 30 lines per mm is often used to compute depth of field markings on 35mm lenses. Bigger enlargements or closer viewing distances require proportionally greater image resolutions. On the other hand, larger film formats require less enlargement for the same size print, so less image resolution is required to achieve the same quality print of the same size.

Digital Camera Considerations Since CCDs are typically smaller than the 24x36mm frame size for 35mm cameras, digital cameras normally use shorter focal length lenses than 35mm cameras. Make sure you enter the actual camera focal length of your digital camera lens and not the "equivalent 35mm focal length". To compute the theoretical maximum resolution of your digital camera, you need to know the dimensions of its CCD's active area in mm. The resolving power of the camera in lines per mm. (assuming optics are not a limiting factor) is one half of the width of the CCD in pixels divided by the width of the CCD in mm. For example, if the CCD has an active area of 16x24mm and the camera captures 1600x2400 pixels over this area, the pixel density is 100 pixels/mm and the resolving power is 50 lines/mm (since it take 2 pixels to resolve a line).

Film Resolution The best modern general purpose films resolve as many as 200 lines/mm when photographing high contrast test targets under ideal conditions, although for normal subjects of reduced contrast resolution is considerably lower. In any case, to get the sharpest possible images from modern fine grained films requires using a higher number than the standard 30 lines/mm for resolving power. Some film manufacturers have begun rating their film's resolution by publishing an MTF curve instead of listing lines per mm. You can infer the resolution in lines per mm for high contrast subjects by looking at where the MTF curve drops to about 20%.

Diffraction The depth of field formulas used by this program do not take diffraction effects into account, and diffraction limits resolution, especially at small lens apertures. An approximate formula for the maximum achievable resolution based on the diffraction limit is: Resolving power = (1600/F) lines per mm. Where F is the f stop of the lens. Thus at f/16, diffraction effects limit resolution to about 100 lines/mm. Note that the diffraction depends only on lens aperture, not its focal length.

Lens Distortion and Film Curvature Additional blurring, especially in the corners of the frame is caused by various types of lens distortion that result in curvature of the plane of focus or variations in focus for different wavelengths of light. These effects are generally more pronounced with larger lens apertures (lower f stop numbers). Another factor affecting image sharpness is the film not being held absolutely flat in the plane of focus.

Focus At Enter the distance at which your lens is focused. You can change units if you wish by clicking on the units drop down box.

Near Focus This field contains the closest distance that will be in focus.

Far Focus This field contains the farthest distance that will be in focus.

Hyperfocal Distance The hyperfocal distance is the nearest point at which you can focus and still have objects at infinity be in focus. When you set your lens to focus at the hyperfocal distance, objects from half that distance to infinity should be in focus.

Note: All distances are measured from the optical center of the lens, normally located somewhere near its front element.

Technical Notes: This program computes depth of field according to the following formulas:

A = circle of confusion = 1/resolution
D = distance at which lens is focused
F = lens focal length
N = f stop
G = N*A
H = (F*F)/G
near focus = (D*H)/(H+D-F)
far focus = (D*H)/(H+F-D)
hyperfocal distance = H + F

Acknowledgements: Depth of field formulas follow those derived by Nick Sushkin as modified by Harold M. Merklinger, author of The Ins and Outs of Focus, available from the author at:

Harold M. Merklinger
P.O. Box 494
Dartmouth NS
Canada B2Y 3Y8