Summary: I have investigated the focus shift that occurs in the Zeiss C Sonnar 1.5/50 ZM lens at close range (one meter). It seems that that wide open the point of best focus is shifted about six centimetres in front of the intended focus plane. The Leica Summilux ASPH 1.4/50 lens does not exhibit similar behaviour. Despite the focus shift I find the Sonnar a useful lens for certain purposes. Background: I have the 50mm Summilux ASPH but wanted a normal lens with a more classical rendering, particularly with respect to the out of focus areas (not that the Summilux is bad in this respect). The new Sonnar from Zeiss seemed like an exiting product and after having seen some examples of photos I acquired the lens. I must say that I am already fond of the Sonnar. During the first roll and a half I managed one shot that I really like myself . This picture is more of a family snap , but I really like the way the background is rendered. A pleasing rendering of out of focus areas often goes hand in hand with strong spherical aberration. Indeed, the lovely Rodenstock Imagon soft focus lenses are designed to take advantage of spherical aberration in a controlled way. Here is an example of a portrait taken with the Imagon. It is recommended that the Imagon should always be focussed stopped down since the amount of spherical aberration allowed will cause the focus to shift. Erwin Puts has also documented focus shift in the Noctilux. I then came across the following note from Zeiss: "C-Sonnar T* 1,5/50 ZM Information about special features for dealers and users The C-SONNAR T* 1.5/50 ZM is a very special lens; based on a classical lens design concept from the 1930's. The additional letter "C" in the name of the lens expresses this designation. This lens design helps to achieve pictures with a special artistic touch. This lens 'draws' your subject in a fine, flattering manner and is therefore ideally suited for portraiture. It renders a sharpness that is slightly rounded, being less aggressive than in contemporary lens designs, but at the same time not soft in its rendition. Many famous portraits of glamorous and prominent people during the 1930's used this technique to great effect. These images are characterized by portraying the person in a shining, nearly celestial way. This effect is very well balanced and not exaggerated; therefore many viewers see it in a subconscious way. The trained observer, however, understands the underlining technique and enjoys the results. This lens design exhibits some additional effects, which should be understood to achieve the maximum benefit from the C-Sonnar T* 1.5/50 ZM: Because of the above mentioned classical characteristic of the lens the best focus position in the object space can not be kept exactly constant for all f- stop settings. The passionate photographer might notice a slightly closer best focus in his pictures than expected. When stopping down the lens to f/2.8 or smaller this effect is minimized, so the focus position will be as expected. In order to balance the performance at full speed and other f-stop settings the lens is adjusted with above described characteristic. The special features of the C-SONNAR T* 1.5/50 ZM are best used in emotional, artistic, narrative images, portraits or atmospheric landscapes. For documentation or technical subjects CARL ZEISS recommends to stop down the lens at least to f/5.6 or to use the PLANAR T* 2/50 ZM lens." All this made me think that it would be useful to investigate the amount of focus shift so that I would be able to understand the properties of the lens better, especially when used as a portrait lens. Not attempting to be scientific I set up a simple test (described in more detail below). I marked a strip of paper in such a way that when photographed at a 45 degree angle the major tick marks of the scale would be projected as one centimetre apart, and also separated one centimetre in depth. With the camera on a tripod I focussed on the zero mark of the scale (I also checked the distance to be one meter with a measuring rod), and took a series of pictures at various apertures. Discussion: I have collected the results in this folder. It contains composite images of the scales from the Sonnar (800x1875pix, 400kb jpg), the Summilux ASPH (800x1875pix, 400kb jpg ), and a combined image from the two lenses (1600x1875pix, 850kb jpg ). I have also included a number of pictures that shows a couple of bottles situated 25-35 centimetres behind the point I focussed on. From Dudak's calculator the theoretical depth of field for these lenses wide open at one meter is about three centimetres, and at f5.6 it is about eleven centimetres. From visual inspection I judge the point of best focus for the Sonnar wide open to be six centimetres in front of the intended focus plane. No (or only a very slight) effect is present in the Summilux. Stopping the Sonnar down moves the point of best focus closer to the zero mark but it is not until f4.0 that it is close, and it is not until f5.6 that we see a depth of field that is roughly symmetric around zero. The Sonnar is no match for the Summilux at the wider apertures, but at f5.6 there is not much between them. (It might be the case that the Summilux exhibits ugly rendering of the near out of focus area - see the -8 at full aperture.) I would also say that the sharpness of the Sonnar at the point of best focus wide open is less than the sharpness of the same lens at f5.6 and at the point where the theoretical depth of field ends, i.e. the Sonnar is not sharp at all wide open at one meter. (On the other hand - as with other soft focus lenses - the depth of field might appear greater.) Conclusion: The Sonnar is a useful lens when you want a pleasing rendering of the out of focus areas and stopped down it is sharp, but if you want the best performance wide open you should go with the Summilux ASPH. If you want to use the Sonnar as a soft focus portrait lens at close range you must take the focus shift into account. A useful trick could perhaps be to focus on the ear of the subject! The test setup: Using a graphics program I put major tick marks on a strip of paper 1.4 centimetres apart. I also included equidistant minor tick marks. The strip of paper was attached to a wooden box that was placed on a table at an angle of 45 degrees relative to the film plane. The projected image of the major tick marks would thus be one centimetre apart and they would also differ by one centimetre in distance from the film plane. The lenses were mounted on a M4 body equipped with a cable release and placed on a tripod. I put the zero mark of the scale in the centre of the frame and focussed as accurately as I could. The scene was lit by household tungsten lights and the exposure time on Fuji Acros ISO 100 film was one second at f5.6. For the alternative exposures I took care to change aperture and shutter speed without disturbing the focus setting. The film was professionally developed (in XTOL) and scanned by me using a Minolte DSE 5400 v I at full resolution and Vuescan software. I applied a mild amount of grain reduction and capture sharpening using Neat Image before downsizing the images. I have applied levels and curves adjustment in Photoshop. All settings in Vuescan, Neat Image, and the levels and curves adjustments are identical between the exposures. No additional sharpening was done before saving jpeg:s. I judge the posted images to quite accurately reflect what I see under a ten times loupe. I would appreciate comments on both method and results.