This is a guest post by Boris Buschardt, whom you will have seen mentioned in this blog a few times. Boris is a world class landscape photographer who runs wild-places.com. Be ready to be awed when you visit that website.
In June I spent two weeks in Namibia. It was the first real test for my recently acquired Zeiss Apo Sonnar 135mm lens. Normally I use wide-angle or standard lenses for my landscape images, but Namibia is a location where a short tele lens works really well for landscapes.
Technically the 135mm Apo is a nearly perfect lens. Color, sharpness, micro contrast and build quality are top-notch. I would even rate it slightly higher than the great 55mm Otus.
Contrary to many other lenses you could use it wide open even on a 36 MP camera (in my case a Nikon D800E) without making any compromises. It can for example be used for landscape images with limited DOF and a very pronounced background separation. The following images were all shot wide open at f/2.
Sossusvlei (c) Boris Buschardt
Blutkuppe (c) Boris Buschardt
Moon Valley (c) Boris Buschardt
But even this lens improves stopped down. Modern aspherical lenses which are calculated for great sharpness wide open nearly always show a slightly nervous bokeh with artifacts in high contrast OOF areas. Even lenses like the Otus or the 135mm Apo can’t avoid this completely. But stopped down to f/4 – f/8 these artifacts are gone with the 135 Apo and the bokeh smoothens slightly while still keeping an interesting structure.
Often the 3D effect of an image is also improved by stopping down to f/5.6. The main object gets more definition and slightly more texture while the separation from the background is still very good. The following images were shot at apertures between f/5.6 and f/8.
Sossusvlei (c) Boris Buschardt
Sossusvlei (c) Boris Buschardt
Deadvlei (c) Boris Buschardt
Sossusvlei (c) Boris Buschardt
Sossusvlei (c) Boris Buschardt
Moon Valley (c) Boris Buschardt
Welwitschia mirabilis, Welwitschia Trail (c) Boris Buschardt
Blutkuppe (c) Boris Buschardt
Blutkuppe (c) Boris Buschardt
Stopping further down is not recommended. This lens gets significantly less sharp even at f/11 due to diffraction. The 55mm Otus is at bit easier in this regard. While f/11 is fully useable with the Otus it should really be avoided with the 135mm Apo.
Of course the 135 could also be used for wildlife photography although you have to get pretty close to your subjects and a longer lens is often preferable. The following images are slightly cropped.
Sossusvlei (c) Boris Buschardt
Flamingo colony, Walvis Bay (c) Boris Buschardt
Cape Cross (c) Boris Buschardt
The biggest problem besides size and weight of the 135mm Apo is the keeper rate. More than 90% of the images I had shot handheld in Namibia with this lens were unsharp for two reasons:
Motion blurring: even with exposure times as short as 1/500 sec about 50% of my images were blurred, at 1/250 sec nearly all images were blurred, 1/1000 sec is pretty safe to use.
Misfocus: if you use this lens wide open or slightly stopped down perfect focusing is mandatory, otherwise all your images will look unsharp. If for some reasons precise LV focusing is not possible (one reason could be that you are using some kind of old fashioned DSLR like me 
stop down to at least f/8 or forget it.
Even some of my images shot from a tripod were unsharp due to motion blur. I used a Gitzo Series 2 tripod with a BH55 head in Namibia which was apparently not sufficient for this lens in windy conditions. The 135 Apo in combination with a 36 MP camera is extremely demanding regarding your technique. A very stable tripod, mirror lock-up and LV focus are a must with this lens.
But despite this I really love this lens and it will be from now on part of my standard landscape kit (together with the Otus 55mm and the Zeiss 21mm).
If you want to read more about my travel experience in Namibia take a look at my blog:
http://wild-places.com/2015/07/06/namibia-first-impressions/
Boris
Posted on DearSusan by pascaljappy.