Taking good photographs that make an audience say WOW! can be delivered with crap equipment. It is the photographer that makes the image through that cyclopic eye, not equipment. Good cameras and lenses only help. We over consider the importance of top equipment performance and often disregard photographic skills and true artistry. How many classical photographers’ images make us say that WOW!, but what was the technical performance of the equipment they were using?
If you’re using for example a Leica Summilux with aspherical correction, usually top performers wide open, you are expected to use it in those circumstances where it shines: wide open! Otherwise, when stepping down the lens is not significantly superior to Summicron of Summarit models that will cost you only a fraction of the price. So such lens is not justifiable if you often close your diaphragm for increasing the depth of field. You have to decide if you wish people to look at you and think you are an excellent photographer because you have a fantastic camera or lens, or if you want people to look at your photographs and say WOW!
I would be lying if I wouldn’t state that all of us have this conflict. When I look to a fancy, expensive top lens on my camera body I feel good. OK. But the correct way of thinking should be: well lets make photographs that can justify the quality (and cost) of this piece! And did I really need it in first place?
Are you enlarging photographs for printing and putting on the wall or make exhibits, or do you use photography for Facebook sharing and to show to friends in the computer? The same rationale in fact applies to post-processing. Do you intend to document reality, or do you wish to produce those colourful skies and over vivid lanscapes that return hundreds or thousands of likes on Facebook? In this case the colour rendition of the lens will not matter anyway… Photoshop will take care of it! And you’re a plastic artist, not a photographer.
The great Erwin Puts, the best reference for Leica, refers in his 2002 analysis of Leica M lenses:
‘’ We don’t just compare lenses, we compare the entire chain of performance.’ MTF diagrams are not intended to replace your own practical evaluation of a lens.’’
‘’ Personal experience and expectations, not purely MTF-based considerations, are the basis for the acceptance of a newly purchased lens… ‘’
‘’ MTF is necessary when an objective evaluation of a lens is wanted, but it is not the only criterion for purchasing a lens: methods are only as good as the way a photographer uses them, and information is only useful when one knows how to interpret the data correctly.’’
Avoid websites that base their information on lenses characteristics and their usability based on fancy MTF charts and other technical info. This is generally of no use for the real photographer. If you are a photo hobbyist with interest on technicalities go to an authoritative site. Professional photographers will know these sites. Even Ken Rockwell says that lens sharpness doesn't mean much to good photographers, and go to his section on Why Lens Sharpness Doesn't Matter Anyway to see the line of thinking.
Having made my point, lets advance.
For Nikon the best synthetic resource, to my knowledge, is the Photosynthesis (www.photosynthesis.co.nz), where you can check the characteristics, via focal length and serial numbers, of each and every one SLR lens made by this brand. For Leica the best resource is the Leica Forum (www.l-camera-forum.com), especially the page Wiki (www.l-camera-forum.com/leica-wiki.en/index.php/Main_Page) where you access all Leica equipment, including serial numbers. Needless to say, Erwin Puts’ various books and materials are the ultimate technical description of Leica photographic products, and you can access most of these materials via the Leica Forum Wiki.
Be also critical of reviews that rapidly propose you to buy the equipment via links they provide. While some honest reviewers (and there are plenty out there!) can help you understand better both cameras and lenses, often reviews will tell what they need you to do and not necessarily what you need and want!
Ken Rockwell’s (www.kenrockwell.com) worldwide known website is an excellent resource for lists, characteristics and reviews of major cameras and lenses brands and models. I use it often as information source for my equipment; it is comprehensive and easy to read. My criticism concerns the considerations on usability of the equipment. He takes too much in guiding us to follow his line of thinking regarding photography and generalizing his particular needs, albeit giving you alternatives. While professional photographers (such as sports, commercial, wedding) may have clear standards and high profile needs, general photographers’ needs will strongly depend on tangible things like volume and weight, cost, focal lengths, low light photography, subjects, etc., and less tangible issues like your own background, feeling etc. The worlds of hobbyists and artists are vast and overlap, and there are amateurs out there that have much higher photo skills and credits than many ‘professional artists’.
NEVER decide based on others’ suggestions, create your own thoughts on equipment needs, and think on if what you wish will effectively get you to photograph better. If it won’t, than what you need is not another fancy lens, but to learn and practice photography.