Depth-of-Field
ON THE SECRET DEMISE OF THE CREATIVE APERTURE

Article from Leica World Magazine (March 2006)
 
When changing from the 35 mm format to digital cameras many photographers only consider the focal-length factor. But what actually happens to the depth-of-field? Our background report describes the dramatic situation.


THE CHANGE from the analog 35 mm format to digital cameras was greeted with widespread acclaim. But mixed in with the hymns of praise to the new technologies we sometimes hear comments to the effect that digital pictures seem "somehow different" after all - feelings that are shared by professional photographers as well as experienced amateurs. And it is not only the subtle nuances that are striking. As we shall see, it is also about the momentous loss of fundamental creative possibilities. What we are talking about is selective focus or short depth of field, one of the most powerful visual design factors in modern reporting, advertising or travel photography. Digital cameras of advanced design provide a regular paradise of possibilities, but even this cannot make up for the loss. Numerous (sub-) menus and customi¬sed functions suggest that the user is equipped for all eventualities. But with all these options clamouring for attention, the photographer's view for essentials can become blurred, especially when newly inven¬ted terms secretly cloud photographers' perceptions of well-known interdependencies. What is meant here is the term "equivalent focal length" - a term that was meant to be consumer-friendly. We shall come to the fatal consequences of this linguistic confusion shortly.

35 mm focal length or the force of habit
To begin with, let's make sure that we are broadly familiar with a few physical basics to enable us to better understand the extent of the cala¬mity. The 35 mm format has been used extensively for decades and this has led to automatisms in the way a photographer thinks and acts. Just two characteristics of a lens - its focal length and lens speed - are all that is required to enable halfway experienced photographers to anticipate the effect on the image and estimate the depth-of-field. The depth-of-field depends on the focal length and the aperture. The lon¬ger the focal length the smaller will be the depth-of-field, the shorter the focal length the greater the depth-of-field (assuming the same aperture in each case). The focal length is not dependent on other fac¬tors such as type of camera or film format. Whether 35 mm or large-format camera, a focal length of, say, 80 mm f/2 will always remain the same. One might therefore call it a factor that depends only on the lens. The same applies to the focal length and the lens speed. The focal length designates the distance from the centre of the lens to the focal point on the film/sensor surface (this is equivalent to the angle of view). The lens speed is the ratio of the largest aperture and the focal length. The largest aperture in our example of 80 mm f/2 would thus be 40 mm (40:80 = 1:2). We shall see later that these values, which relate solely to the lens, are subject to substantial change with the change in format. But let's stay with the familiar 35 mm format for a little bit longer. Here, the normal lens has a focal length of 50 mm. This focal length is determined by the diagonal of the film / sensor format, which for the 35 mm format is actually 43 mm. The format diagonal gets longer with the classical 6x6 medium format, 80 mm being the normal focal length of the lens in this case. Both these standard lenses provide approximately the same image. The depth-of-field is, however, reduced in the case of the 80 mm lens compared with the 50 mm lens (at the same apertures in each case), because the depth-of-field, as sta¬ted at the beginning, depends on the focal length. This is why fashion photographers who intend to work with extreme selective focus use the medium format even if the reserves required for enlargement are actually not necessary. Many photographers might still be relatively accustomed to this train of thought but when they change from the 35 mm format down to digital cameras it seems to disappear into thin air.

TECHNOLOGY
Small wonder when one considers that the manufacturers, after all, put four or five sensor sizes on the market almost simultaneously. Somehow you cannot help comparing the situation to a changeover to a new currency. Instead of the usual specifications such as 28-90 mm, the buyer is confronted with numerical values like 5.2-16.7 or 7-22 or 14-45, all equivalent to 28-90 mm in the 35 mm format. And lenses that are expressed in terms of four to five new "currencies" sometimes using very "odd" numbers are downright annoying. This is where the term "equivalent focal length" can be useful. It provides the numerical values converted from the "digital focal length" into the familiar 35mm values. So it seems we can fall back on our long years of experience in photography, particularly as the lens speed was apparently not affec¬ted by the change in currency.

Objective Error
Once the terms "lens speed" and "equivalent focal length" have become part of our mental photographic equipment, it seems reasona¬ble to think that we can also estimate and use the depth of field. This is exactly where the mistake arises. As stated at the beginning each focal length has its typical depth-of-field. Digital pictures, however, are crea¬ted, not by using the equivalent focal length, but the actual focal length, which is comparatively short. This is why digital cameras pro¬duce great depths of field, even if halfway rapid initial apertures seem to contradict this. As already indicated the speed of a lens is defined via the focal length. Given a 7 mm focal length, the widest aperture suffi¬cient to achieve an impressive f/2.8 is about half the size of a shirt but¬ton. This is how surprisingly small lenses provide amazingly high lens speeds. But how does this affect the depth-of-field?

Solution: the equivalent aperture
Peter Karbe has given this matter considerable thought: The head of the optics development division of Leica-Camera AG has held works¬hops and lectures on this topic. We have summarised his most impor¬tant results in tabular form. Karbe advocates supplementing the term "equivalent focal length" with the term "equivalent aperture". The term "equivalent aperture" indicates the aperture that would be invol¬ved using the 35 mm format after appropriate conversion from the digital format. This makes the aperture setting just as accessible to nor¬mal calculation as the focal length. The calculation is performed sim¬ply by multiplying the (digital) aperture setting by a format factor. The format factor is the figure required to multiply the sensor measure¬ments in order to arrive at the 35mm format. With digital reflex cameras it is also known as the "depth-of-field multiplier". A veritable aperture such as f/2.8 then degenerates, as an equivalent aperture, into a paltry f/5.6 (with 4/3") or a catastrophic f/11 (with 2/3") - bearing in mind that this is the initial aperture. This alone makes it clear that with small sensor formats is it scarcely possible to create pictures using selective focus. This general observation is of course also true of the digital Leica cameras. The small sensor formats, on the other hand, have their advantages as well. They make it possible to use smaller len¬ses and cameras which can be transported more easily and manufactured at more favourable prices. But talking about the gap in the design repertoire still won't make the problem go away and leads to yet another consideration. If we regard photographic equipment like a tool box from which one takes the special tools required for a particular photo¬graphic concept, this is where "analog lenses" come into their own again. Photographers specialising in the 'reporting' or 'people' genre, whose concepts call for an extremely short depth of field, will find the perfect tool in the Leica 50 mm f/1.4 Summilux-M ASPH., or the Leica 35 mm f/1.4 Summilux M ASPH. With a wide aperture setting and focused at two metres, the 50mm lens provides a depth-of field of just 14 cm. Even the 35mm wide-angle lens has "selective power" that is quite substantial. Focused at two metres the depth of field is a mere 28 cm, in other words, roughly the diameter of a head. So separating peo¬ple or motifs from their background is spectacularly successful. The two lenses are thought to be state of the art and most likely the world's best lenses in their class. The equivalent Leica-R lenses are also just as good, only slightly softer at open apertures. If one allows oneself to be guided by the idea of a toolbox during a photo production or concep¬tual series, JVI lenses (and silver image) appear to be the ideal solution (if there is no full-format sensor available). Subsequent scanning makes it easy to process the films digitally. This thought is not back¬ward-looking but rather originates from an open way of looking at things. Peter Karbe's idea of the "equivalent aperture" is at any rate likely to open the eyes of quite a few digital photographers to what is different about their pictures when shot with fast lenses

Heinz-jurgen Kruppa

Interview with Ansgar Pudenz
EMOTION VIA SELECTIVE FOCUS
The repertoire of photographer Ansgar Pudenz (www.ansgarpudenz.com) ranges from food and people to travel and transportation. One of his hall¬marks is the virtuoso use of selective focus.
LW: Mr. Pudenz, the way you use the short depth-of-field can almost be described as lyrical. How do you use selective focus and to what effect?
PUDENZ: A short depth-of-field is a means of dramatising a picture, like the spotlights in the theatre. The eyes are taken in one direction. I can use it for interpreting my subjects - instead of the objectivity that you get when everything is in focus there is a more emotional effect instead. The observer becomes involved, literally sucked into the picture. The part of the image in focus can either stand for the whole as in food photography or stand in the way of it, for example with reports. It is certainly one of the most powerful and effective artistic means that the photographer can use.