Lenses: the background
Lens terminology
Focal length: how much magnification to expect from a lens. See detailed explanation below.
Zoom lens: one where the focal length can be varied.
Focal range: the range of focal lengths in a zoom lens.
Kit lens: normally a zoom lens, sold with a DSLR or mirrorless camera, as a starter piece
of equipment. Very commonly 18 - 55mm, which usually has an approximate equivalent focal length of 29 - 88mm, see diagrams below.
Prime lens: lens with a fixed focal length, often offering a less expensive way to have a lens capable of a wide aperture. This is the alternative to a zoom lens.
Lens Ratio: lenses always list the lowest possible aperture the lens is capable of. In many zoom lenses the minimum F number changes with the focal length; in which case lens ratio should be read alongside focal range. See more info in the example below.
Fast lens: slang term for a lens capable of a very wide aperture (low F stop), so called as they allow for faster shutter speeds.
Lens fit: different camera manufacturers use different mounts to attach the lens to the camera body. If you do not buy a lens the same brand name as your camera, you will need to ensure that it is of the same 'fit'. e.g. Nikon fit / Canon fit.
Lens diameter: awkwardly even lenses for the same camera will be a different size at the front, needing a different size lens cap or filter. The diameter is listed on the side or front of the lens Ø.
Crop factor: a necessary adjustment required if you wish to compare lenses from different cameras with one another. This is because the lenses of the same focal length behave differently if the sensor in the camera is a different size. See detailed explanation below.
Optical vs Digital zoom - terms used for compact and bridge cameras.
Optical zoom: ‘Real’ zoom using the glass in the lens (notice the lens itself often moves).
If a lens is listed as 60x that means that at its longest length it is 60 times longer than at the shortest focal length.
Digital zoom: cropping the image, to see a part of it closer up. This means there’s either no more detail or ‘created’ detail. Avoid digital zoom where possible.
Zoom lens: one where the focal length can be varied.
Focal range: the range of focal lengths in a zoom lens.
Kit lens: normally a zoom lens, sold with a DSLR or mirrorless camera, as a starter piece
of equipment. Very commonly 18 - 55mm, which usually has an approximate equivalent focal length of 29 - 88mm, see diagrams below.
Prime lens: lens with a fixed focal length, often offering a less expensive way to have a lens capable of a wide aperture. This is the alternative to a zoom lens.
Lens Ratio: lenses always list the lowest possible aperture the lens is capable of. In many zoom lenses the minimum F number changes with the focal length; in which case lens ratio should be read alongside focal range. See more info in the example below.
Fast lens: slang term for a lens capable of a very wide aperture (low F stop), so called as they allow for faster shutter speeds.
Lens fit: different camera manufacturers use different mounts to attach the lens to the camera body. If you do not buy a lens the same brand name as your camera, you will need to ensure that it is of the same 'fit'. e.g. Nikon fit / Canon fit.
Lens diameter: awkwardly even lenses for the same camera will be a different size at the front, needing a different size lens cap or filter. The diameter is listed on the side or front of the lens Ø.
Crop factor: a necessary adjustment required if you wish to compare lenses from different cameras with one another. This is because the lenses of the same focal length behave differently if the sensor in the camera is a different size. See detailed explanation below.
Optical vs Digital zoom - terms used for compact and bridge cameras.
Optical zoom: ‘Real’ zoom using the glass in the lens (notice the lens itself often moves).
If a lens is listed as 60x that means that at its longest length it is 60 times longer than at the shortest focal length.
Digital zoom: cropping the image, to see a part of it closer up. This means there’s either no more detail or ‘created’ detail. Avoid digital zoom where possible.
Did you know? A true zoom lens, also called a parfocal lens, is one that maintains focus when it's focal length changes. A lens that loses focus during zooming is more properly called a varifocal lens.
From the info printed on the side or from of the lens we can understand how the lens will perform. On the left the focal range is 150-600mm, so we know this is a telephoto zoom lens and that the length varies significantly. On the right the lens has a more modest range of 24 - 70 and we know this will give a much wider view.
Lens Ratio tells us the min aperture. On the right hand lens 1:28 tells us that the lowest aperture will always be f2.8 when using this lens. On the right the ratio has a range and therefore needs to be read alongside the focal range, meaning in this case that at 150mm the min is F5 whereas at 600mm it is F6.3.
Lens Ratio tells us the min aperture. On the right hand lens 1:28 tells us that the lowest aperture will always be f2.8 when using this lens. On the right the ratio has a range and therefore needs to be read alongside the focal range, meaning in this case that at 150mm the min is F5 whereas at 600mm it is F6.3.
Why similar lenses vary so much in price...
Several things can affect the price of a lens: focal range; manufacturer & their reputation; quality of glass and lowest available aperture (lens ratio).
I am commonly asked why 2 lenses of similar focal range by the same manufacturer would vary so much in price. The answer is because of the lens ratio i.e. the lowest aperture possible. The more expensive lens will invariably be a "faster" one, capable of lower f numbers.
As we know we control the amount of light entering the camera by balancing the aperture with the shutter speed. A wider hole in the lens means that the shutter need not be open so long; and we know that having the shutter open too long causes problems with movement in our images. This makes the ability to opt for a wide aperture very valuable, especially when shooting fast moving subjects in low light.
Larger apertures require more glass and therefore cost a lot more. They are generally also heavier for this reason.
I am commonly asked why 2 lenses of similar focal range by the same manufacturer would vary so much in price. The answer is because of the lens ratio i.e. the lowest aperture possible. The more expensive lens will invariably be a "faster" one, capable of lower f numbers.
As we know we control the amount of light entering the camera by balancing the aperture with the shutter speed. A wider hole in the lens means that the shutter need not be open so long; and we know that having the shutter open too long causes problems with movement in our images. This makes the ability to opt for a wide aperture very valuable, especially when shooting fast moving subjects in low light.
Larger apertures require more glass and therefore cost a lot more. They are generally also heavier for this reason.
See also...
Diving in to the details
We define lenses buy their 'focal length'
As you can see in the diagram below, focal length is a real measurement...
...however we normally discuss it in terms of the view it will give you through the camera.
You can also imagine the view from above.
50mm is a key focal length. This gives approximately the same magnification and perspective as the human eye, so it can be considered the mid point, lower than that being wide angle and higher than that being a telephoto lens. Many photographers who shoot street scenes and travel photos choose to use a 50mm lens so the images look very close to how the place appears in real life!
A note about the view of a 50mm lens
|
This focal length gives the most ‘realistic’ looking images.
Travel or street photos feel authentic; the scene looks as it would have when you were there. It was used by renowned street photography Henri Carrier Bresson. At this length distortion is minimised so images look neat with clean straight lines. Depth of field doesn’t have a strong tendency at 50mm so can be readily manipulated with your aperture choice. It’s easy to overlook the “standard” focal length but this would be a shame. Rather than thinking of it as a starting point to move away from consider sticking deliberately to this perspective. |
A 50mm equivalent lens is often available as a prime, fixed length, lens. It is usually an inexpensive, lightweight lens with very low apertures. A fab, fun useful piece of kit. |
Why we talk about "equivalent" focal lengths and the complication of "crop factors."
|
For many years film cameras that held 35mm film rolls were the most common type of camera and we take our standard focal length scale from how lenses worked for them (including all the diagrams seen so far on this page).
Some people use a digital camera that is called a full frame camera, these tend to be larger and traditionally were more expensive so often professional equipment. These big cameras have a sensor the same size as 35mm film, so the lenses act exactly the same. Meaning a 50mm lens taken from an old film camera and placed onto a full frame digital camera would give the same view and the produce the same photo. Take away: there is an industry standard scale of focal lengths, used to compare lenses to one another. |
35mm film inside an SLR
|
Some complication creeps in because many folks use digital cameras with a slightly smaller sensor, the sensor has been cropped down to fit inside, and exactly how much smaller it is is referred to as the 'crop factor.' You can find out the crop factor of your own camera in the manual or with a quick online search. A common crop factor is 1.5.
This becomes important because a different size sensor catching the view through the same lens gives a different picture, as shown below.
This becomes important because a different size sensor catching the view through the same lens gives a different picture, as shown below.
This differing sensor size translates to the same focal length delivering a different view depending on the camera it is on.
Using your crop factor and a quick bit of maths will tell you what to expect from the lens.
For example: Using 24mm lens on a camera with a crop factor of 1.5 behaves like a 36mm lens (worked out by multiplying the focal length by the crop factor - 24 x 1.5 = 36).
The term used for this conversion is finding the "equivalent" focal length. Meaning that you are describing what the real focal length equates to on the standard 'full frame/35mm film camera' scale.
Using the lens's equivalent focal length is like translating it to the universal language any camera user can understand.
For example: Using 24mm lens on a camera with a crop factor of 1.5 behaves like a 36mm lens (worked out by multiplying the focal length by the crop factor - 24 x 1.5 = 36).
The term used for this conversion is finding the "equivalent" focal length. Meaning that you are describing what the real focal length equates to on the standard 'full frame/35mm film camera' scale.
Using the lens's equivalent focal length is like translating it to the universal language any camera user can understand.
|
In practice:
Many digital cameras come with a Kit Lens which is listed as being a 18-55mm. At first glance the numbers would suggest that it is quite a wide lens. However once we know to convert it we can see it will give great coverage- an effective focal length of 29mm to 88mm (covering the central and useful 50mm value and giving some scope either side). |
|
Crop factor & focal length summarised
Becoming conversant in crop factors and equivalent focal lengths just takes a bit of time and experience but in all honestly does not come up all that often.
Times to take care:
If I am in any of the above situations I try to just ask for or roughly calculate the equivalent focal length of the lenses. Then it is really straight forward to make comparisons and understand where on the field of view scale we are.
I introduce crop factor in my lessons before we go one to play with perspective and explore lenses and focal lengths. I do this because I think in a class of learners it is important to know straight away that you cannot necessarily get identical shots to those of your peers despite using similar lenses. I merely want my groups forewarned that there is more to these numbers than meets the eye; grasping all the finer points is not essential.
Times to take care:
- If you are looking at equipment that is suitable for various cameras and you want to know how it will behave on your camera and compare to lenses you already own.
- If you are comparing lens length and the resulting images with people using different cameras.
- If you buy a second camera and hope to use the lenses from your first.
If I am in any of the above situations I try to just ask for or roughly calculate the equivalent focal length of the lenses. Then it is really straight forward to make comparisons and understand where on the field of view scale we are.
I introduce crop factor in my lessons before we go one to play with perspective and explore lenses and focal lengths. I do this because I think in a class of learners it is important to know straight away that you cannot necessarily get identical shots to those of your peers despite using similar lenses. I merely want my groups forewarned that there is more to these numbers than meets the eye; grasping all the finer points is not essential.
