This post is the second in a series about lenses.  For many first time DSLR users, the sheer variety of lenses can be daunting. Furthering the confusion is all the lenses that at first glance seem similar in some respects but vary widely in price.  In this post we will take a look at aperture, how it affects your images and gain some understanding into why seemingly small changes in the feature set of a lens contribute to the large price differences. To start we need an example lens and what better place to start than the lens that most first time DSLR buyers get with their camera, the kit lens.  Called the kit lens because you buy the lens and camera together as a kit.  If you haven’t bought yet you do have the option of buying body only but I rarely recommend that for first time buyers (more on that later).  Both Canon and Nikon offer an 18-55 lens as part of their kit.  Using the Canon as an example(Nikon’s 18-55 is identical in focal length and aperture to the Canon lens) Canon EF-S – Zoom lens – 18mm-55mm – f/3.5-5.6 IS – Canon EF-S f/3.5-5.6 represents the aperture. Aperture is the opening in your lens that allows light to pass through. Because the number is represented as a fraction the smaller numbers represent a larger opening. I am going to repeat this slightly differently – when we talk about larger apertures or the maximum aperture we are talking about the smaller numbers, again this is because aperture is represented as a fraction. The diagram below further illustrates the point.   Shooting at a larger aperture lets in more light and narrows your depth of field(the area that is in focus in your picture).   Using the Canon kit lens above as an example the maximum aperture is a range from f/3.5 to f/5.6.  As you zoom from 18-55 your maximum aperture decreases to f/5.6.   But why do you care about the maximum aperture in the first place? Having a lens with a large maximum aperture has two benefits (f/2.8 – f/1.4 is considered large) 1. Larger apertures let in more light allowing you to shoot in lower light with a fast enough shutter speeds to minimize blur and camera shake. 2. Larger apertures decrease you depth of field allowing you to isolate your subject by blurring the background, that background blur is referred to as bokeh.     Lets look at another example Canon EF 50mm f/1.8 II Camera Lens This prime(doesn’t zoom) lens has a maximum Aperture of f/1.8 – one full stop faster than the kit lens at it’s maximum of f/3.5. And actually a true comparison would be to zoom the kit lens to 50mm where its maximum aperture changes to f/5.6.  Look at the graphic above again – see the difference in size between the f/5.6 aperture and the f/2 aperture?  Pretty significant different in the amount of light that can hit the sensor, in fact each stop halves or doubles the amount of light hitting the sensor.   Translated into a real world example – Let’s say you have your kit lens on and are zoomed in to 50mm – your aperture cannot increase beyond f/5.6 and the suggested shutter speed is 1/15 or a second Shutter speeds that slow are most likely going to give you blurry pictures from camera shake and if anyone is moving they will be blurry too. If you switched to the 50mm f/1.8 lens and changed your aperture to f/1.8, keeping all other settings the same your suggested shutter speed now would be around 1/180 of a second, fast enough to stop most motion and plenty fast enough to eliminate camera shake.  The catch is that at f/1.8 your depth of field is very shallow, or can be at closer focusing distances- There are always trade off but one of the reasons I recommend this lens is it lets you experiment shooting at larger apertures, also referred to as “shooting wide-open” and you can quickly begin to see how controlling aperture can greatly change the look of your photographs. Why is only the maximum aperture prominently displayed in the lens title? Don’t people care about the minimum aperture? Most lenses are sharpest somewhere between f/4 and f/11 – this range for each lens varies slightly and is called the “sweet spot” It is not that no one cares about the minimum aperture it is just a fact that almost all lenses have a minimum aperture that is small enough to cover the sweet spot and then some. And at the very small minimums, usually around f/18 and on your image quality begins to degrade due to diffraction. So that’s why you hear very little about minimum apertures. Just FYI – the Canon 50 has a minimum aperture of f/22. But what if you don’t want to use a lens with a fixed focal length like the Canon 50mm prime above?   There are zoom lenses that have larger maximum apertures than the kit lens and some of the nicer lenses provide a constant maximum aperture for example the Tamron AF 28-75mm f/2.8 allows you to shoot at f/2.8 over the entire range of the lens.   Earlier I mentioned the sweet spot, the aperture range that is going to give you the best quality from the lens, and you may have noticed that f/4 was the largest aperture in that sweet spot range.  Now you may begin to realize why some lenses are so much more expensive.  The Tamron listed above is about $500 and is a decent lens and a good value but if you find that you will be shooting at f/2.8 often and sharpness wide open is critical you need to start considering other lenses, one excellent but expensive example is the Canon EF 24-70mm f/2.8L USM, a lens that is almost 3x the cost of the Tamron.  There are other factors determining that price difference but the fact that the Canon 24-70 is sharper wide open across the entire focal range is part of the price difference.   This is another reason why many photographers who need good low light performance are willing to sacrifice a little bit of convenience to shoot with a prime lens- not only can you get wider apertures they are more likely to be sharper at those wider apertures. Just a bit more about item two above, the shallow depth of field you get when shooting at larger maximum apertures can be both a blessing and a curse. Shooting a shallow depth of field allows you to isolate your subject from the background. The curse is sometimes your depth of field can be so narrow you end up with only part of your subject in focus, this is more likely to happen at longer focal lengths, close focusing and larger apertures. I will dedicate a post to depth of field soon, in the meantime Cambridge in color has an excellent page. A quick video illustrating the difference in depth of field between f/11 and f/2.0 all shot with the 50mm f/1.8. Some examples of excellent primes at several different price points Canon EF 50mm f/1.8 Canon EF 50mm f1.4 USM Canon EF 85mm f/1.8 Nikon 50mm f/1.8D AF (will not auto-focus on D40/60/3001/3000/5000/ for those cameras buy either Nikon 35mm f/1.8G AF-S DX or Nikon 50mm f/1.4G) Nikon 85mm f/1.8D (will not auto-focus on D40/60/3001/3000/5000/) I finished my Focal Length post with the following paragraph, I am going to include it here as well because you might be scared away by the aperture limitations of the kit lens, don’t be! My advice for first time DSLR buyers is to buy the kit lens- you might read on the Internet something to the the effect that the “KIT LENS SUCKS MAN!!” Don’t listen to those people.  It is a decent lens and a solid value capable of excellent pictures in most situations, it often only adds $100 to the price of the camera and unless you have a very good reason to go body only and get a different lens I suggest buying the kit.  As you use it, and learn the type of shooter you are you can start shopping for wider, zoomier, wider maximum aperture or whatever works best for you and the type of shooting you do. Next post will be about all those little acronyms that follow the focal length and aperture numbers, other posts I need to do in this series include camera brand versus off camera brand (e.g. a Canon lens vs a Sigma lens), Pro lenses, filter sizes and ??? you tell me what else you want to know about lenses. Canon Users I have a list of recommended lenses Nikon Users – I am working on one Take a moment to like this post if you found it useful. And if you have ANY questions find me on twitter, no question is stupid and I am happy to help. This post and others like it are supported by your Amazon purchases- Canon shooters go buy a nifty fifty right now Thanks!