How to choose your lenses
This post is for those who don’t know much about lenses. My hope is that at the end you will know slightly more. Certainly not all there is to know, but at least enough that lenses stop being a total mystery and instead become a thing you have to research.
What is a lens?
Dumb question it may seem, but I’ll answer it anyway because often people assume they know stuff, and then they find out they don’t.
A lens (from a DSLR perspective) is a tube with optical elements inside whose function is to focus light onto the sensor at the back of your camera.
It is important to know this part because the first thing to know about lenses is that not all cameras have the same size sensor in them. This means that lenses are all made to suit a specific sensor.
A full frame sensor is the same size as a frame on a roll of 35mm film (oddly 35mm wide) so lenses for full frame cameras are designed to focus an image onto an area that large.
An APS-C sensor is smaller than a full frame sensor. (See here for sensor sizes). So lenses designed for this sensor cast a smaller image onto the back of the camera. This means that if you put them on a full frame camera, they will not fill up the entire sensor area. (Often full frame cameras will detect this and crop their sensor down to only the area covered by the crop sensor lens).
What is a Lens mount?
This is where the lens fits onto the camera. It is important because you need to know what lens mount your DSLR body uses in order to choose a lens.
For example the full size Nikon cameras use a F mount. This has been the case for many years and so most old Nikon lenses will fit current Nikon bodies.
But Nikon have Full Frame lenses and APS-C lenses. They designate the APS-C lense with a Dx on the lens body.
So both full frame and Dx lenses will fit your Nikon, but you’ll get the sensor issue already mentioned.
Canon have an EF mount. Their APS-C lenses are called EF-S. So not all Canon lenses fit all Canon bodies.
This is true for Sony, Fuji, Olympus etc.
So do your reading and make sure you know what lens mount is used on your camera and what sensor size your camera is.
What is Focal Length?
In rough terms think of focal length as the amount of zoom or magnification a lens has. It is measured in millimetres because it is an actual length. From the front lens to the focal plane of your camera. This may not translate to exactly that length on a camera because of tricky optical stuff happening inside.
A 50mm lens is about the same zoom as your eyeball.
Notice that there is a limit to how far left and right you can see when you look straight ahead. A 50mm lens on a camera is the same. The amount you can see to the left and right is called the field of view.
Shorter lenses give you a wider field of view and less magnification.
Longer lenses give you a narrower field of view and more magnification.
So that’s the first thing to ask yourself. How much magnification do you need for the type of shooting you plan to do?
Lenses that zoom in and out have that range written on them, 70-300 for example.
Lenses that don’t zoom and are a fixed length are called prime lenses and will have just one length written on them, 50mm for example.
Less than 50mm we call short lenses. More than 50mm we call long.
Really short, like 8mm distort the world to offer a massive field of view and are often called fish eye lenses.
Zoom in and out is not all the focal length does in your picture though.
What is Foreshortening?
Foreshortening is the tendency for things to change their relative distance to each other along the axis of the shot (…. huh?)
The shorter a lens the longer something will look. The longer the lens the shorter it will look. I’ll show you.
Here is my old van. This is at a focal length of 19mm.
Notice how long it looks.
Here is my old van at 200mm. Notice how my van looks shorter. It also looks squarer. This is what focal length does. It is why a cricket pitch on TV looks so short when shot from behind the wicket (which will make no sense at all to those of you who don’t know anything about cricket).
So focal length matters not only because it determines how much of the world you’ll see in one go, but because it also determines how much you can zoom in on the birdie in the tree, and how much it will distort the world when taking portraits or shooting a garden etcetera.
What is Aperture?
Aperture is the hole inside the lens that lets the light through.
This is measures as a ratio of the size of the hole to the length of the lens. This focal ratio is abbreviated to ‘f’. The hole can be set to different sizes. This is called your f-stop.
A lens will have a range of f numbers on it that represent the maximum the hole can open inside your lens. Obviously the hole has to be able to close as well to let less light in. This is part of how you get correct exposure of your shot. You change the aperture and the shutter speed.
So a lens may have a ratio of f4.5 to 5.6 for example on a zoom lens. On a prime lens there will be one number, eg 50mm f1.4.
The range is because although the maximum hole in a lens can’t change with focal length, it might be 20mm wide, when you write it as a ratio of the length of the lens, 20mm is a bigger fraction of the length at 70mm than it is at 300mm.
It is all maths you can ignore. Just know the number range is the aperture range of the lens.
How big a hole is how much light can be let in. If you need to shoot in low light then you need to think about a lens that has a low f number.
What matters is that the aperture sets your depth of field in your shot, and aperture also pretty much sets the cost of your lens. A smaller f number means the lens has to be larger and have more glass and so cost more.
A 70-300 f4.5-5.6 might cost $800.
A 70-300 f2.8 (across the entire range) might cost $3200.
So why bother with a smaller aperture? Depth of field.
What is depth of field?
Depth of field is how much of the world is in focus either side of the part of the world you focus on.
We talk of depth of field as being shallow or deep.
Shallow happens at small f numbers. Deep happens at large f numbers.
Shallow is a big thing with portrait people.
So you can see that focal length and aperture range matter the most when choosing a lens. There are a couple of other factors.
Other factors with lenses.
Lenses have a lot of optical elements inside. They are not always glass. Plastic and flourite and other elements are used.
Lots of shiny inside a lens adds up to a lot of reflections. This means internal elements often have special coatings to stop the shiny.
Lens elements can also distort the image at the corners and can create colour issues at the edges. Coatings and tricky stuff is done inside lenses to counteract chromatic aberration etc. This all costs extra money.
Some lenses have sensors that detect movement and try to counter it. This is known as Image Stabilisation on some lenses, or Vibration Reduction on others. Research what they call it for the type of lens you are buying.
The lens in the picture has a focal length range of 70mm to 300mm.
At 70mm it can do an f-stop of 4.5, at 300mm, 5.6. The AF means Autofocus and the S means it has a motor in the lens. VR means Vibration Reduction which means it is a little less likely, but not impossible, to blur your images. On the side (not shown) are switches to turn on and off autofocus and the VR function. This is an F mount lens designed for a full frame camera, which we only know because it doesn’t have a Dx on it.
Lenses also might or might not have a motor in them. Some use the power of your camera to drive a motor to do the autofocus. Other lenses rely on your camera to turn a little cog for autofocus. Your camera body may or may not be able to drive the lenses with the little cog.
In Nikon land for example a D700 has the little cog but a D5500 does not. This means some autofocus lenses will only work on certain cameras.
So you need to decide how close you want to get to birdies. This is focal length. You need to know that focal length also affects field of view and foreshortening.
You need to know what depth of field you want. This determines the aperture range you need and how big a hole it will burn in your pocket.
You need to think about light. If you are shooting in low light then perhaps a low aperture is needed. Eg you might be shooting dancers or a wedding and they won’t let you use your flash.
You need to know what mount is on your camera and what sensor size you have.
And that’s about it. There are many lens manufactures who all use different quality optical elements etcetera so do your research. With a Nikon camera you might buy Nikon, or you might buy Tamron or Sigma or Samyang or etcetera. The same with Canon, Pentax, Fuji, Olympus etc.
What is true for all lenses though are the basics I’ve written about above.
Focal length, Aperture range, Mount system, type of optical elements and image stabilisation.