One of the most common questions I get is: “how do I choose the right lens for what I am shooting?” It seems like there are a million choices they are all in millimeters and they have f numbers attached to them. What the heck is the difference between a prime and variable aperture zoom/telephoto lens? Do I need a UV filter? Is it safe to swap lenses in the field? What is image compression, and do I need it? Let’s see if we can demystify some of these topics and clear things up.

The basics

Why are lenses measured in millimeters? Historical precedent, when lenses were first developed, they were often used in conjunction with film photography. The distance between the lens and the film was measured in millimeters, and this measurement, focal length, became the standard way to describe a lens’s properties.  This convention has stuck around even with the transition to digital cameras. Not too bad right? We are starting off easy.

Generally, smaller number range lenses (40mm, 35mm, and even 14mm) equate to a wider field of view. One of my favorite lenses is a 17-40mm zoom which allows me to shoot wide for a fisheye style look all the way up to 40mm. which is a bit farther than the 35mm standard focal length. The higher numbers are going to give you a more zoomed in view, 70-200mm is a standard zoom range for a lens I commonly use for portraits and sports. Along with the mm range, you will usually see an aperture value (f4, f5.6 and even f4-6.3) a single number means that the lens can stay at that aperture value throughout the focal range. The 70-200mm lens I mentioned earlier has an aperture value of 2.8 throughout. This means that I can shoot a portrait and use an aperture of 2.8 to blur my background (bokeh) regardless of where I am in the zoom range of the lens.

Variable aperture

When you see a range aperture listed on a lens like f4-6.3, it means the lens will automatically force your camera to switch to a minimum of that aperture as you move through the zoom range of the lens. In many cases (but not always) a variable aperture will be found on a more budget friendly lens, while the single aperture variety will be found on more expensive variety lenses.

An additional side note: the f number on the lens is generally to let the purchaser or user know about the minimum aperture that lens can accommodate. Shooting at f8, 11, or 16 works just fine on almost every lens. Not confused yet? Let’s keep going!

The body

Before we go any farther, it should be noted that the kind of camera body you have will affect how the lens you are using works. A crop body like the Canon t series will effectively magnify the lenses put on it by 1.6 times, meaning a 100mm lens on a crop body camera will give an effective zoom of 160mm. This is why many sports shooters use crop cameras because they get more range with a lens compared to a full frame camera body like the Canon 5d series.  

What is a prime lens?

A prime lens is a fixed focal range lens. 35mm 85mm and 50mm are common prime lens lengths. These lenses don’t zoom, so the photographer is required to use sneaker zoom or move closer or farther from the subject to adjust the framing. My favorite primes are the 85mm for portraits and 50mm for landscapes, both on a full frame camera body.  

How do you get a blurry background?

This is the second most common question I get when talking about lenses. The blurry background look is achieved with a wide-open f-stop like 2.8. This is why the Canon 70-200mm f2.8 lens is commonly referred to as the go-to portrait lens. The 2.8 aperture can stay constant throughout the focal range and give a nice blurry background, but the tradeoff is that the lens will set you back about 1900 USD. The creamy bokeh makes this lens my wife’s favorite lens for portraits.

Changing safely

New photographers are often concerned about changing their lenses while out and about taking photos. Taking the lens off the front of your camera exposes it to the outside elements. If you are in a humid situation, a cold situation, a rainy situation or a dusty situation, care should be taken before taking that lens off your camera. It should also be noted that not all camera bodies are weatherproof, you should check your instructions to see if your camera has weather sealing before taking it out to shoot in poor weather conditions.

In general, it is safe to take the lens off your camera while outside, the sun will not usually affect your camera operation. Care should be taken when there is a possibility of water getting into your camera. I hike in the Rocky Mountains all the time and swap lenses while wandering around the woods with no issues. However, if it is snowing, I generally keep the lens on. In that sort of situation, I will go with a 24-105mm lens which will give me a variety of focal ranges to use and not have to swap in the bad weather.

A disclaimer

We are about to talk about general use cases for lenses. It should be noted that there is no hard and fast rule that says a lens must be used for a specific purpose. An example of this would be that macro lenses are widely used for portraiture. The 100mm macro lens has great sharpness and gives a flattering look when used as a portrait lens.

Portrait lenses

The most common focal range for portrait shooters is 85mm when it comes to a prime lens the 24-70mm zoom and 70-200mm zoom lens. Again, these aren’t the only lenses used for portraiture, but they are the most commonly used.

Landscape lenses

Landscape shooters predominantly use wider and longer lenses for landscapes (myself included). Good choices for landscape lenses include 50mm, 17mm, 40mm, 70-200 and the 24-105mm range.

After years of buying things I don’t need and things I thought I needed, I have settled on a set of lenses that covers every situation I face while shooting portraits, landscapes and products. I use the 17-40mm L wide zoom and the70-200mm 2.8 L for landscapes. The 85mm prime, 24-70mm L zoom and 70-200mm 2.8 L for portraiture and the 24-105mm, the 70-200mm 2.8L and the 100mm macro for food, and product work. The lens that spends the most time on my camera is the 24-105mm f4L because it has a wide range of zoom, and offers reasonable blur in my backgrounds if I choose to shoot a portrait on the go. I’ll often start a shoot with this lens then swap if I need more zoom, more width or more bokeh depending on what I am shooting.  

Filters

The “should I use filters on my lenses” debate is an ugly one. There are people who are firmly on both sides of the fence and will vehemently defend their position until they die. So, I’m happy to offer my opinion and will fend off the haters in the comments. The arguments for: the filters protect your lens, they reduce flares (UV filters) and they protect your lens because lenses are oppressively expensive to have the front element replaced on… yes that one is mentioned twice intentionally. Arguments against: Anything in front of your lens degrades quality, you don’t need them unless you need them, replacing the front element is cheaper than missing a good shot or messing up a good shot with a cheap filter.

My stance is I use them. When you buy your camera in a store they will often offer you an upsell of a cheap UV filter to protect your lens. I do firmly believe that there is no reason to buy a 2000 USD lens and put a 20 dollar filter in front of it. This is why I buy good quality lenses like B&W. They are about 100 bucks and I always keep them on my lens because I’d rather replace 100 dollars of filter than have to go through the hassle of sending in my lens to get it fixed. But perhaps the biggest reason I use UV filters is because I am clumsy. I have scratched my share of filters walking through the woods after forgetting to put my lens cap on or from my camera tumbling off a rock and hitting the ground. I should be more careful, but I’m not and the price of my not being careful is about 100 a shot to replace a scratched UV filter.

Conclusion

What do you think? Did this article help you? Did I forget something? Feel free to drop me a note here on my site or at the social links below, I’d love to hear from you.  Buying a lens can be traumatic if you don’t really know what you are looking for. I can help drop me a message with your camera make and the lenses you are looking at – and don’t forget to tell me what you are shooting.