Camera Mount and lens inconsistencies

LENS / MOUNT TOLERANCES / INCONSISTENCIES Common questions regarding lens problems: Why is my lens softer on one side? Is my lens decentred? My lens is faulty one corner is soft. Lens or camera problem? With modern high megapixel cameras, and zooming in 100% any inconsistencies can easily be spotted, Where lenses are slightly out on one side or corner, we have found that it is not necessarily a lens problem. In fact more often then not it is a camera lens mount problem, or just normal inconsistencies between the two. Sample Variation With modern day manufacturing techniques with tight tolerances modern lenses generally have smaller variation. Most manufacturers also optically test them in factory before releasing, so if any were de-centred or outside the specified variance threshold one would hope these would be noticed, and rejected. Historically major third party manufacturers such as Sigma may have had greater sample variation, their new lenses, particularly their ART series are extremely consistent and high performing, in many cases their optical performance are better than their main rivals. Sample Variation (Big brands vs third party). It is easy to assume that the smaller, third party manufacturers deliver lenses that have more variation or are optically inferior. On the contrary, it can be argued that the smaller manufacturers have a reputation to build, and could better test and control the stock that leaves the factory. There is also an interesting theory that customers who receive an expensive Canon lens may not bother to specifically test the lens, just to assume that as its a big brand will perform perfectly without any doubt, whereas that little known brand at half the price needs lots of testing on charts and pixel peeping, as surely it cannot be as good as the main brand, or that particular spectacular review and amazing comment put out there on the internet. This type of reasoning can skew perceived and actual results. Users expectations can easily start to exceed reasonable real world performance of the lens. Charts and testing It is worth noting that tests do not always do justice to the real world results. Those charts you have just photographed will not show the skin tone rendering, how quickly or accurately the AF focusses, the look, flare resistance against the sun, or general handling of the lens. So what if the UWA lens you have is soft wide open, and another of the same model is better wide open? Yours may be better at f/11 or f/16 where you are going to use for landscapes .. or not? For landscapes edge to edge sharpness may be understandably important, but even then you can't go off a chart taken at 2 feet away. If you are going to test, try assessing in the field, at apertures you are going to use with the lens is properly focussed. If an softness issue is apparent then it is time to see where the problem is. Softness on one side or corner. The industrry seeing more and more complaints of one side or corner softness concerns than they were 5 years ago, and is exaggerated from a combination of using Ultra Wide Angle lenses, use on large sensors, demanding high megapixel cameras. Lets investigate why this is. Is it the lens or your camera? Users often rule out the camera as being the cause, or as a contributing factor. Retailers commonly hear things like "It can't be my camera, as my other lenses are fine." "Its got to be the lens as its so soft down one side" "Its got to be the lens as its just one corner" Users have been trying their camera with various lenses for years, and can be insulted by the suggestion that it could be their camera, or a combination between camera and lens that is the issue. When comparing optical performance users need to compare to similar or the same focal length, aperture, and price point lenses and on different camera bodies to fully understand where the issue lies. Of course in many cases this is not easily possible or convenient to test every combination, although time can be saved by resisting to test your new sub 15mm focal length f2.8 UWA lens against your trusty 50mm, or standard f3.5-5.6 zoom kit lens, and then confirming that your cheap kit lens is better in the top corner, at 50mm at f4.5, than your expensive 15mm f2.8 is at f2.8. While you are at it maybe you can compare the sweetness of your newly purchased Granny Smith apple with the Honeydew melon you got last week.... Softness issues on Wide Angle lenses vs Telephoto Issues of corner softness do not tend to occur on telephoto lenses anywhere near the extent of Ultra Wide angle lenses. Why is this? Lets investigate. It is commonly known that wide angle lenses have a large depth of field (greater area that is in focus at a given aperture), whereas telephotos have a shallow depth of field. However this applies to the front of the lens. Logically the opposite occurs at the rear (mount side of the lens), wide angle lenses at the sensor side have a telephoto effect, and telephotos have a wide angle effect. If you are not quite sure that works logically try turning round a pair of binoculars - things now look far away! The depth of field effect is also reversed - at the sensor / mount side there is be less depth of field (depth of focus) at the rear of wide angle lenses, and more depth of focus with telephoto lenses. This explains why there are less problems reported with telephoto lenses. With telephoto lenses mount / sensor intolerances would have little effect, the mount would have to be way off to see much difference from one edge to the opposite side. Where differences are observed with telephoto lenses then it would be far more likely the problem is with decentring of the lens elements themselves. Possible causes for this include manufacturing defects, by physical damage in transit, or accidental damage, ie. dropping of the lens for example. Investigating the issue It is important that as retailers we fully investigate the cases of the issue in order to ascertain where the issue is, how it was caused, and if a replacement is provided that it will perform well on the customers setup, and to the customers expectations, and to offer accurate and consistent advice in order to achieve that objective. Examples We have had first hand experience where customers have said their lens was soft on one side, and after sending back to us for testing we found on our cameras have actually been softer on the opposite side. In one example a customer sent back a Samyang 14mm lens (Canon EF Mount), stating it was soft on the RIGHT, particularly bottom right. We tried the lens on a Canon EOS SLR camera from our stock and found the same lens was softer on the LEFT! This was resolved (with full communication with the customer) by us sending them a copy that was even SOFTER on the LEFT, particularly top left on our camera. On his camera he found it was perfect. This further demonstrates that the lens / mount combination can dramatically affect the performance. Camera Mount inconsistencies There are known camera mount inconsistencies, not only between camera models, but also differences from exactly the same models. On Ultra wide angle lenses there only needs to be a slight difference in mount to cause an issue. For example if the camera mount is not exactly square to the sensor from the resultant images would have on one side or corner which was soft and blurry, leading to the user naturally (and wrongly) concluding that the lens decentred. and not considering that it was in fact caused by the camera mount. From experience, and explained earlier, the effect of the camera mount inconsistencies is greatest and most noticeable in UWA (Ultra Wide Angle) lenses, and the least in long telephoto lenses. However if the lens contains a decentred element within it, dependant on the design of the lens the effect of softness can be pronounced in either telephoto or wide angle similarly. Perfect lens? No lens will be perfect, and the results from the same lens will vary when used on different cameras. If you happen to find a "perfect" lens you would ideally need to keep in on the same body to remain perfectly matched.