This post is based off of a question from the discussion “Are m43 lenses really so good?” Here are couple of generalized basics about lenses that may assist. As stated it is really pretty hard to find a terrible lens today and the more you spend, typically the better the lens. In essence you get what you pay for, taking into account the artistic, light gathering capabilities, and reach or angle of view of a lens. Most lenses have very decent acuity at 2 to 3 stops from wide open. Most lenses of 35mm formats at f/8 or m4/3 at f5.6 are very close in IQ, close enough that most people could not tell what photo was produced by which lens. So if you like sharp, don’t want to spend much money, only show photos online or on a computer, and don’t print bigger than 11 x 14 – then lenses with a f/3.5-6.7 range or even f/4.0 are typically more than enough if you don't expect to become extremely interested in photography. Pixel peepers will see differences in about any lens – yet seldom will these folks find a lens that's outstanding across the spectrum. Pixel peeping digital images is not worth the time, try limiting views to no more than 100% size and 50% size views may be more worthwhile, prints are far better way to evaluate lenses. The more you move away from the typical standard lens - 35mm’s 50mm or m4/3’s 25-30mm; typically the larger, heavier, and more expensive they become. As an example using 35mm format, moving from 50mm to 14mm or 50mm to 800mm; the larger, heavier, and more expensive the lenses get as you progress to the extremes. And the larger the aperture, the more the cost, size, and weight. And then, to quite a degree, the difference in bokeh when going from F/2.0 > f/1.8 > f/1.4 > f/1.2 becomes decreasing less in differences and the cost, size, and weight differences are not linear. Same goes for moving from 200mm to 800mm. So for many the cost/IQ ratio makes an f/1.8 or f/4 lens a lot more inviting. Some lenses becomes really well known and highly sought after what they can produce. The Canon 85mm f/1.2 and Nikon 85mm f/1.4 may be called soft by some at f/1.2 or 1.4 respectively. But the lenses got their great reputations from the IQ they create wide open. Then set the lenses at f/8.0 and compare them to a 70-200mm f/4 shot at 85mm and f/8.0 – and the images will be very hard to tell apart. You’re paying for the IQ at f/1.2 or 1.4. For most people an 85mm f/1.8 makes the most sense and at a significant cost savings with an IQ that is nearly as good. On the other hand the Nikon 14-24mm f/2.8 and Olympus 75mm f/1.8 became well known for their sharpness across the entire image at all apertures. But none of these are inexpensive lenses. So best suggestions for buying a lens – Do you need the focal length? What’s the smallest aperture you can cope with? The more artistic choices or wider spectrum of DoF and bokeh options typically require more funds, weight and larger size. If you see reviews/opinions outside the generalizations above, dig deeper (both the 14-24mm and 75mm f/1.8 fall in this category). Find reviewers you feel you can trust and weigh what they say accordingly. DxO Mark and SLRgear are always interesting – but sometimes will contradict each other and they are not beyond skepticism. Take all reviews and opinions with a grain of salt. Look for a generalized consensus about the lens you are looking at, stay away from the nitpicky details. Find a local retailer or an internet company that makes returns easy and the least cost if the lens doesn’t work out for you. In this regard a local retailer can become you best friend. A lesson often taught to new photographers is - buy the best lens you can afford and then learn everything about how that lens responds though-out all settings. You will buy fewer lenses, get more consistent results, generally be happier with photography as a whole, and have a lens that you can grow with, along with your experience level. Yet do not expect that lens to do everything – each lens is unique and it is fixed, so you have to adapt to it. That is not an easy question to answer. For all lenses I weight my priorities for cost, size, weight, IQ, ease of use, and flexibility of the lens, and somewhat weight the generalized common traits based off of reviews and opinions. For a pro another consideration is how many photos do I have to sell using that lens to pay for itself? And how long will it take to do that? These criteria help me immensely. One great deal was the 4/3s 50mm f/2.0 macro - a really good portrait lens, a really nice near true macro lens, a large enough aperture that low light situations were not a problem, fairly small, light in weight, and a very decent price = a lot of flexibility. These do not come around often. The Olympus m4/3s 45mm f/1.8 is almost in the same category. Here are some other examples. The Panasonic 25mm f1.4 or the Olympus 25mm f/1.8. One is a premium lens at a commensurate cost. The f/1.8 lens produces about 80-90% of the IQ capabilities of the f/1.4 lens, is the cost for the extra IQ capabilities worth it to you? The discussion between the Olympus and Panasonic 7-14mm lenses aslo has all sorts of opinions. IQ from both is debatable so I fell back on my generalizations above. For me the Panasonic lens has performed extremely well for my needs. I simply don’t want to and can’t justify the cost, size, and weight of the f/2.8 lens. For a Pro that shoots UWA and makes considerable sums from using the UWA f/2.8, it makes sense. For me the Panasonic has paid for itself, if I bought the new lens I’d have to shoot enough to pay for the lens – and gain little. Another discussion was the Olympus 40-150mm vs the Olympus 4/3s 50-200mm. Again an extremely hard decision. My final decision came down to using both the lenses in a local camera shop and then renting the 40-150 and comparing it to the 50-200. Weight and size are negligible with the 4/3s adapter and the 1.4 extender for the m4/3s lens. IQ balances each other out. My 50-200 is paid for and any jobs it is used for, is profit. If I bought the new lens I’d have to shoot enough to pay for the lens – and gain little if anything. If you can’t afford or don’t want to pay top dollar, the 50-200mm is an excellent choice netting you at least 95% of the new lens. Your experiences and criteria is far more valuable than what you read or test results. Rent the lenses if needed to make a choice!