Depth of field – An insider's look behind the scenes
Maybe the same thing happened to you recently: Upon receiving some rolls of film back from processing, you briefly check the color rendition, and then examine the new images with an 8x magnifying loupe for fine detail, for sharpness.
Every once in a while you may encounter an image with double contours and you know immediately that some bad vibration tricked you. But you may also find images that show stunning sharpness in some areas, whereas detail resolution in other areas does not meet your expectations. When you took those photos you knew exactly what depth of field you were after and with the help of the depth of field scale on your lens or the table supplied with it you set the aperture and the correct focus accordingly. However, the result is disappointing.
If images like this accumulate, you suspect your lens to be at fault and turn it in. You mail it to Contax or Hasselblad or Carl Zeiss. Be assured it will be in good company: "Depth of field is insufficient" is the most common complaint to meet the Carl Zeiss service department today. And there is an upward trend. Why? To find the answer, let's take a short look at the basics:
• In your image the one plane will be perfectly sharp that you set the focus for.
• Everything on either side of that plane will come out blurred, more or less.
• How much a subject detail is blurred, depends mainly on its distance to this plane of perfect sharpness, and on the aperture setting, and the focus setting.
• A certain amount of blur is supposed to be tolerable. According to international standards the degree of blur tolerable is defined as 1/1000th of the camera format diagonal, as the normally satisfac-tory value. With 35 mm format and its 43 mm diagonal only 1/1500th is
deemed tolerable, resulting in 43 mm/1500 » 0.030 mm = 30 µ m of blur.
Imagine the very tip of a pin with a size of exactly zero, located precisely in the plane of perfect sharpness, that means, it is imaged to the film with a size of exactly zero, not widened by any blur. Now, move this pin towards the camera and watch the diameter of its tip increase by blurring. When it has reached 30 µm, halt the pin! It is now right at the inner border of the depth of field. Now, do the same in the opposite direction. Beyond the plane of perfect sharpness you will reach the outer border of the depth of field.
All the photo school books in the world explain that same principle and tell a similar story, although with different words and sketches and images. And all the camera lens manufacturers in the world including Carl Zeiss have to adhere to the same principle and the international standard that is based upon it, when producing their depth of field scales and tables. But here's what the school books don't tell:
A blur tolerable of 30 µm equals a resolution of 30 line pairs per millimeter (lp/mm). The normally satisfactory value was standardized with the film's imaging quality in mind – at the time the standard was defined which was long before World War II! Meanwhile some decades have passed, today's color films easily resolve 120 lp/mm and more, with Kodak Ektar 25 and Royal Gold 25 leading the field at 200. Four-color printing processes have also improved vastly and so have our expectations about sharpness. The depth of field standard, however, has remained unchanged.
This is still absolutely okay as far as the large majority of photo amateurs is concerned, that take their photos without tripods and have them printed no larger than 4 x 6. Be aware that these amateurs represent 90% of all picture takers, so don't expect the depth of field standard to change fundamentally before long, creating a reason for the camera lens manufacturers to introduce new depth of field scales. If you are not satisfied with the results you achieve using the existing scales, tables, and formulas, keep tuned to CLN. We will provide you with information on how to achieve utmost sharpness in photographic images. Let's sum it up for today:
• The international depth of field standard, the basis for all camera lens manufacturers to calculate their depth of field scales and tables, dates back from a time, when image quality was severely limited by the films available.
• Those who use depth of field scales, tables, and formulas (e. g. for hyperfocal settings), restrict themselves – most probably without knowing why – to the image quality potential of an average pre-World-War-II emulsion.
Camera Lens News No. 1, summer 1997