Dokument: CLN 02_07 Resolution of camera lenses

Resolution of camera lenses where are the limits – and why?

Some users of Zeiss lenses are so enthusiastic about these optics that they believe Carl Zeiss can do won-ders. Well, this is not exactly true. The fact is: Carl Zeiss has to adhere
to nature's laws just as anybody else. So far, Carl Zeiss's physicists have found no way around it.

One of these laws defines the resolution limit of any optical system, any camera lens, even a perfect one with absolutely no lens errors. It is the law of diffraction. This law states that a sharp point in an object will not correspond to a sharp point in the image thus having a diameter of exactly zero, but rather to a small diffraction disc (physicists call it "Airy disc"). This disc has a certain dia-meter, which varies with the aperture of the imaging system. The smaller the aperture (e. g. f/22) the larger the disc. And the larger the disc, the lower the resolution!

So, even a perfect lens with no lens errors is limited in its resolution! (Such a perfect lens does not exist. Even the most sophisticated lenses on this planet, the Carl Zeiss S-Planar lenses for the semiconductor industry are only approximations to the perfect lens, although very close ones.) Thus the term "diffraction-limited" has become the synonym for lenses that are so good their only per-formance limit is the law of diffraction. According to this law no camera lens used in photography, still or motion, can produce resolutions higher than given in the following table (Approximate, rounded off values for white light spectrum of even energy distribution. In unevenly distributed spectra of gas discharge light sources somewhat higher resolutions may be achieved.)


f-no......................resolution
.............................(line pairs per millimeter)
.45.............................35
.32.............................50
.22.............................70
.16...........................100
.11...........................140
...8...........................200
5.6...........................280
...4...........................400
2.8...........................560

In practical photography other limitations of resolution occur, too. The existing depth-of-field concepts, for example, lead to a limit at 30 line pairs per millimeter, simply because they assume that the image of a sharp point may be considered sharp as long as the unsharp disc it actually is (called the "circle of confusion") grows no larger than 1/1000th of the focal length of the respective standard lens. In 35 mm photography only 1/1500th is allowed. 1/1500 th of the 50 mm standard focal length equals 1/30th mm. So 30 fit into one mm, which means, we are talking 30 line pairs per millimeter (CLN 1 has more details in "Depth of Field – An Insider's Look Behind The Scenes", an article that meanwhile has been reprinted in many publications around the world). This is about the same limit set by diffraction for f/45 which was an aperture setting quite popular with 5 x 7 inch large format photography, producing images that can be viewed without any subsequent magnification.

Today's high quality color films do reach resolutions in the region of 140 line pairs per millimeter with Kodak Ektar 25 leading the field at 200! The full resolution potential of these films cannot be utilized with existing depth-of-field concepts nor f-settings of f/11 and beyond. On the other hand all real lenses on the market today are limited not only by diffraction, but by lens errors also. Some of them quite heavily.

Carl Zeiss has always strived to develop very elegant sets of lens error corrections and to deliver a high degree of lens performance, second to none. So, with normal Carl Zeiss lenses set at f/8 and f/5.6 resolutions at the very limits of the best color films have been reached.

Objects of 4 millimeter in size (approximately 1/6 of an inch) have been imaged from almost 400 meters distance (more than 1.000 feet) with a 100 mm Carl Zeiss Makro-Planar lens at f/5.6 and a Contax RTS III 35 mm SLR camera featuring the unique Contax vacuum pressure plate. Similar results were obtained with Contax AX 35 mm autofocus SLR, Contax RX 35 mm low noise SLR and Makro-Planar 60 mm, Planar 85 mm f/1.2, Planar 100 mm f/2, Aposonnar 200 mm f/2, Vario-Sonnar 28–85 mm and 35–70 mm zoom lenses, even with moderate priced Planar 50 mm and Distagon 28 mm.

Resolutions on the same level have been achieved with Carl Zeiss lenses in Hasselblad medium format cameras, proving that at Carl Zeiss, medium format lenses, contrary to popular belief, offer no lower resolution than the very best 35 mm lenses.

Other camera systems which achieve performances in the same premium class with their Carl Zeiss lenses include the medium format SLR cameras of Rollei fototechnic with the Rolleiflex 6008 integral as their latest top of the line product and the discontinued Rolleiflex 3003, the only 35 mm SLR with inter-changeable film backs since the demise of the Zeiss Ikon Contarex and Contaflex.

The most obvious limitation of resolution in everyday photography, however, is unwanted motion with handheld cameras. Even a seasoned photographer with a very "calm" camera will hardly find resolutions higher than 40 line pairs per milli-meter in his photographs unless he uses an adequate tripod. CLN will detail on this topic in a later issue.

Camera Lens News No. 2, fall 1997
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