Here is an eternal question, one of the few that is inevitably asked by any perspective photographer. At least those that invest in a camera with interchangeable lenses.
This is a topic that applies to all camera systems, rangefinder or single lens reflex. I will be making comments on 35mm cameras only, as those of you that have medium format cameras would’ve already made this decision years ago.
The first question to ask if you just invested in a DSLR is should I buy a zoom lens or fixed focal length lens (or prime lens). My recommendation nowadays is invariably a fixed focal lens. Of course if you just brought your first rangefinder camera, then you have no choice but to get a fixed focal lens and you can skip the next couple of paragraphs.
Ironically, despite my current recommendation the first lens I brought for my Canon 5D Mark II was the Canon EF 24-70/2.8 L Zoom Lens, it wasn’t a poor choice, in fact it was a very reputable Zoom Lens with relatively good performance as Zoom Lenses go, it is a lens in the stable of many professional photographers for its performance & versatility. What I didn’t know then that I know now, is that no zoom can match the quality of an equally priced fixed focal length lens. I discovered this fact to my chagrin when I purchased my first fixed focal lens, the Canon EF 35/1.4 L, boy was I freaked out when I first saw the photos. The sharpness and resolution of the fix focal lens just blew my zoom away and the creative option of a fast lens was something I would fall in love with.
The 24-70/2.8 Zoom Lens was an excellent first lens choice, as its performance peaked between 35mm & 50mm which means it covers the standard focal lengths well, with still the wide 24mm option and the tele 70mm option when needed. (All zoom lenses have a certain focal length where its performance peaks and it is usually not the extreme ends, in this case 24mm and 70mm, but somewhere in between)
I still own my 24-70/2.8 L Zoom Lens, its flexibility is still useful on certain occasions where changing lenses and carrying two cameras isn’t practical. But If I only have one camera and one lens, it would be my Canon 5D Mark II and that very EF 35/1.4 lens.
This leads me to the second thing to consider (the first if you are using a rangefinder), and that is which focal length to choose first. My recommendation is 35mm for any full frame camera, this focal length is slightly wide and more flexible than 50mm as your first lens in its application. There are many advocates for 50mm as your first lens, it is a personal choice and many of my favorite photos I’ve taken were with a 50mm lens. You can’t really go wrong with either, but 35mm is no doubt more flexible and that is why it is the more common focal length lens on point and shoot cameras.
The reason to buy a lens between 35mm and 50mm is because these are considered standard focal lengths, as oppose to wide-angle or telephoto lenses. Standard focal length lenses closely emulate our normal field of vision and are the most useful lens for general photography. Most photographers will tell you that 80% or more of their photos were taken with either a 35mm or 50mm lens and there is good reason for this.
Also fortunate is that these standard prime lenses are the kit lens that comes with many cameras or if brought separately they are the cheapest and most often the sharpest lens in any system’s lens range. Standard focal length lenses are easier to make, with simpler builds and hence cheaper.
Remember to take into account the crop factor of your camera if it has one, so for EF-S cameras like the Canon 450, you need to multiply the lens’ focal length by 1.6x to get effective 35mm focal dimensions, in other words you’ll need to purchase a 21mm lens (x1.6 = 35mm). Here you’ll strike some problems with DSLR with crop sensors, you will find that 21mm fast prime lenses are inordinately expensive and not as good as standard 35mm fast prime lenses. You will probably have to compromise and buy a EF-S or AF-S zoom lens that cover 35mm and 50mm, for example Canon EF-S 17-55/2.8 or the Nikon AF-S 17-35/2.8. The problem with these lenses are as mentioned lesser performance compared to primes and their relatively slower apertures. I would consider F/2, F/1.4 as fast but F/2.8 is a little pedestrian.
For full frame Canon, Nikon or other DSLR, your default choices are your brand lenses as these retain autofocus and are good enough quality. There are Zeiss made versions in Canon or Nikon Mounts, the ZE or ZF 35/2 biogon which is a fine lens, but you will lose autofocus which I think is too costly a loss for a standard focal length that you use all the time.
For a full frame m-mount rangefinder camera, like the Leica M6, Bessa R2A or the Leica M9, there are many many options too numerous to list, these cameras can fit virtually hundreds upon hundreds of lenses build since the 1950s. Lenses that I can safely recommend with almost zero risk of regret are the various Summicron 35/2, the six lens element version is probably the most cost effective and the Voigtlander 35/1.4, 35/2.5 or the discountinued 35/1.7 ultron and the Zeiss ZM T* 35/2 Biogon are all very fine choice.
For micro 4/3 cameras like the Olympus EP-1 the Olympus 17mm prime lens or the Panasonic 20mm prime lens are both a good start (I like the Panasonic better as it has better optics in this specific case). The Panasonic m4/3 lenses works fine on the Olympus m4/3 body.
For the Leica M8 with a 1.3x crop factor I recommend getting the Voigtlander 28/2 or a Summicron 35/2 (whichever version that suits your budget, the 6 element version is good value, but the 7 element is my favorite, but the newer lenses with aspherical elements are good for digital as well but pricey. ) as fine first lenses. There exists Leica 28/2, 24/1.4 & 21/1.4 lenses, but I can’t recommend these due to their relative cost and size on the camera. Size may not be an issue with everyone, so I recommend trying these out in a store on the camera before laying down funds. For me any lens that blocks the viewfinder is a big minus. If you can afford a M8 and 21/1.4 lens, you are much better off with a M9 and 35/1.4 asph lens. Another disadvantage of using a 21mm or 24mm on the M8 is the need for an external viewfinder which can decrease the spontaneity of your shots.
For the Epson R-D1 with a 1.5x crop factor sensor, I will stray from my recommendations a little bit, I have brought a Zeiss 25/2.8, Various 21mm lenses for my R-D1s but none have given me the fast prime lens feel I am looking for. My recommendation is a Voigtlander 28/2 lens or any fast 35mm lens, the latter becomes a little too tele for my tastes but makes a great portrait lens. Once again using any lenses aside from a 28/35/50mm will require an external viewfinder which is cumbersome. Those lucky not to wear glasses, have the option to use a 25mm lens instead using the whole viewfinder as an estimate of the view, with the caveat that 25mm lenses are F/2.8 or slower usually.
There are some who will advocate a wide angle lens or even a telefocal lens, but they are in the vast minority.
Another question to consider is the speed of the lens, or the widest aperture the lens can be used. Speed comes with an associated cost and in most situations size and weight as well. Also fast lenses tend to perform not as good as the slower lenses at slower apertures. In the world of digital ISO, where using an ISO 800 or more doesn’t degrade your photo too drastically, the need for ultra fast lenses has lessened, the lost in quality using a fast lens wide open doesn’t quite equate to the increase noise at using high digital ISO. So extremely fast lenses like F/1.0 or F/0.95 which cost a monumental amounts of money is relegate for specialty functions, like shooting in the dark with unique bokehs (background blur). I contest that a F/2 lens is often sufficient with a good quality F/1.4 lens as slightly better. I have found f/2.8 or f/4 standard (35mm or 50mm) prime lenses as too pedestrian and lack creativity of faster lenses that allow you to throw the background further out of focus.