When you already have a selection of fast and slow lenses or just considering either buying a more expensive fast or a relatively more economical slower lens of a given focal length, what factors do you have to consider? Its more complicated than you think, but I will walk you through all the factors involved in your decision, both as to which lens to buy and which lens to take out and use.
First and foremost is the cost factor, fast lenses are invariably more expensive than their slower lens counterparts. For example a Canon EF 24/2.8 is more than 4x cheaper than the corresponding Canon EF 24/1.4 L lens. So if you can’t afford it, well no amount of reading is going to help you, you had better buy more lottery tickets or go and work harder. I do have some practical advice though, for wide-angle lenses wider than 24mm it is generally not worth the expense of the faster lens, for one thing you are not going to benefit much from the better out of focus bokeh of a larger aperture, let it put it to you even more succinctly, “what are you going to do? put the camera 12 inches in front of someone’s face so you can get better bokeh?” The depth of field of wide-angle lenses are much deeper and I find myself stopping down my Canon EF 24/1.4 II lens to get as much depth of field as possible, as well as sharpness. Lenses are usually sharper stopped down 2-3 stops and if you want as much resolution as possible you’ll have to step hard on the brakes of your fast lens anyway. As usual, the slower lens will generally have better performance at smaller apertures, faster lenses have bigger and more complicated lens elements and lens element arrangement and it is more difficult to control things like distortion, vignetting, chromatic aberration and uniform sharpness across the field; a prime example is the infamous Zeiss ZM 21/4.5 lens for Leica M-mount, stopped down to f/8 or f/11 and it is probably the best performing 21mm lens for 135 format cameras ever made, it is compact, light and cost 10x less than the Leica 21/1.4 asph lens. This Leica lens’ performance improves all the way to f/8 or f/11 but at these smaller apertures the Zeiss 21/4.5 will still out perform it. Sure the Leica is three and a half stop faster, but how often are you going to handhold a wide-angle photo in low light situations or put the lens up a foot in front of someone’s face to take advantage of a better bokeh? Certainly not with a wide-angle lens, unless you want an unflattering clown size nose on your subject! So unless you really need a fast wide-angle lens for a specific purpose, I’ll say you can probably save quite a lot of money, by buying slower F/4 or greater wide-angle lenses and using a tripod in low light situations.
The existence of these fast wide angles are a relatively recent invention and it is arguably to accommodate the existence of crop sensor bodies. So take pricey Leica 24/1.4 asph example, on the 1.3x crop body of the Leica M8 it works out to be 31.5mm, so you’ll have a handy 31.5/1.4 lens for the short-lived Leica M8. You’ll be better serve to buy a full frame Leica M9 and the Leica 35/1.4 asph lens. Not only does this lens have better performance, it is more compact as well.
I have already elucidated other factors in picking a fast or slow lens in the above paragraph. Faster lenses tend to be bulkier & heavier than their slower brethren. Slower lenses tend to perform better (or in rare cases the same) as their faster brethren at slower apertures. The faster lens does have a trump card up its sleeves and that is you can throw subjects in the foreground or background further out of focus with the result of a more abstract and creamy blur. This ability to isolate your subject is crucial in many sorts of photos and increases your creative options.
So to answer the question which lens, the fast or the slow one, that I take out to use today: it depends on what sort of photos I intend to take. For examples: If it’s a bright sunny day out side I may select my slower but lighter to carry and better performing lens. If its dark and I am using film and limited with my ISO, I would bring my trusty fast lens and live with the burden. Here is a more complicated example: If it’s a bright sunny day and I absolute must have an abstract bokeh for a certain shot to isolate the subject, I would bring my fast lens and bring a ND filter to decrease the light reaching my sensor/film to maintain a reasonable shutter speed. If I am not so fit, and I am going on a hike, I may choose to bring my Canon EF 24-105/4 instead of my heavier Canon 24-70/2.8 lens.
You get my drift? I pick the right tool for the job. It’s not arbitrary, it’s an intelligent decision based on what I intend to photograph.
To answer the question of which lens to buy, fast or slow, it depends on your budget, buy both if you can afford to. Now that you understand that there is a niche for both lens types in different sorts of photographic conditions, you can be justified owning both. In the last example of choosing between a Canon 24-105/4 lens or the faster and better Canon 24-70/2.8 lens, I would pick the latter lens if I can afford either and can only pick only one. The logic here is with the latter lens you’ll have the option of stopping down the lens if you need to, the performance whilst stopped down may not be as good as the slower lens, but it would be a better compromise then losing the creativity of owning a faster lens (There are other considerations in picking between these two lenses but that is for another article, for another day.) If you can’t afford the fast lens, needless to say, you have no choice anyhow.