Understanding Lens Measurements

Many people wanting to learn photography are confused by all the numbers and acronyms involved in the craft and it can be quite intimidating. sports74.ru

Lenses are probably among the worst when it comes to cryptic meanings. Even the name that describes it, Nomenclature is a scary word.

Nomenclature def. — A system of names used in the classification of an art or science or other field or subject

Lets look at a typical camera lens name:

Nikon AF-S DX Zoom-NIKKOR 18-135mm f/3.5-5.6G IF-ED

Each piece of the name has meaning that you need to understand. Camera makers use this nomenclature to pack enough information for you to understand the lens’s abilities. Lets look at each piece.

IF-ED — IF stands for Internal Focus and ED is Extra Dispersion glass (sharper than normal glass).

Here is a Canon example:

Canon EF 28-300mm f/3.5-5.6L IS USM

You can see a lot of information is packed into these code strings. Lets look at another very basic simple example:

AF NIKKOR 50mm f/1.8D

Much like above, we can tell its Autofocus (AF), its Nikon, and its a 50 millimeter lens. There is no range of numbers so this is a fixed or “prime” lens. Prime lenses are generally considered sharper than zooms and are usually cheaper, but are less flexible than zoom lenses.

A prime lens is the simplest lens to work with. It only has one focal length and its maximium lens opening or aperture is always a single value. In the example above, its an F1.8 lens which lets in a lot of light.

Lets break this down so we can understand what the numbers mean.

50mm is the focal length. That is the distance from the from lens element to the film plane is 50mm or about 2 inches. A 35mm lens would have a distance of 35mm. A 300mm lens would be 300mm long. Over the years, prime lenses are generally found in common sizes like 20mm, 24mm, 28mm, 35mm, 50mm, 85mm, 120mm, 135mm, 200mm and 300mm. There are of course smaller and larger lenses.

For all practical purposes, these numbers by themselves are meaningless but when you combine it with the film (or digital sensor) size we can now determine if the lens makes things bigger or smaller than what our eyes normally see. Lenses that make things bigger are called “Telephoto” lenses. Those that make things smaller are called “Wide Angle” lenses and those that approximate what we see naturally are called “Normal” lenses.

The standard by which we measure most things are 35mm cameras. The number 35mm comes from the size of the film where each frame has an area of 24x36mm which is rounded down to 35mm for convienience. Other common film formats include medium format films which are generally 60mm square format. Also APS whch is a 24mm format (16x24mm).

Since 35mm is the most common almost everything we deal with in digital photography will be based on that format.

A 50mm lens is considered “Normal” for 35mm film. Actually anything between 35mm and 70mm is considered normal. Normal is actually calculated as the diagonal of the film size. Now to calculate that will require you going back to high school algebra so to avoid that trama you’re just going to accept that for 35mm film, 50mm is normal.

Anything 35mm or smaller is considered a “wide angle” lens and anything 70mm and larger is considered telephoto. That is if your shooting 35mm film (or a full frame digital camera like the Nikon D700 or the Canon EOS 5D). However most of us are using dSLR’s with sensors that are more the size of an APS file size. Nikon calls this their DX format. With the exception of the Nikon D3 and the D700 all other Nikon’s use a 24x16mm sensor which is almost identical to APS. These camera’s film sensors are 1.5X smaller than their 35mm counterparts. When using 35mm lenses on these DX bodies, you have to factor in this 1.5X multiplier.

Canon cameras with the exception of the 5D and the EOS 1ds line use smaller sensors as well, but they come in two sizes, either a 1.6X sensor in the Rebel and 10d,20d…50d line or a 1.3x sensor in the EOS 1d family. Because 1.6x and 1.3x are harder to calculate, the rest of this tutorial will use Nikon’s 1.5x factor for examples (which is close enough to 1.6x and most of you are not using 1d’s).

Now you go out and by that normal 50mm lens and decide to mount it on your Nikon D80, a 1.5x crop camera. The lens on that body behaves like a 75mm lens would on a full frame (35mm body). This means a 35mm lens becomes the normal lens on a 1.5x camera. 50mm and larger are telephoto, and 28mm and wider are wide angle. Back to the original example above, 18mm-135mm would allow you to zoom from a very wide angle to a medium telephoto distance. But the lens above has a DX in its nomenclature.

This means that it is specially designed for the 1.5X cameras. If you were to put it on a 35mm camera, it wouldn’t let light hit the entire frame. Camera makers can build smaller, lighter weight and less expensive lenses for the 1.5x cameras if they don’t have to cover the entire 35mm film area. If you see a lens marked DX (in Nikons, Canon has another designation) then you really should not use it on a full frame camera.

Since your going to use it, you have to think of the numbers in a different way. 18mm * 1.5 is 27mm and 135mm x 1.5 is around 202mm (we will round down to 200). This lens on the 1.5x cameras will be in effect 28mm-200mm lens which is a medium-wide angle to long telephoto lens.

Now for something really confusing. If a 50mm lens is normal. A 100mm lens is a 2x magnification. A 300mm lens is a 6x lens. If you were to go buy binoculars from a store you would see them rated like 7X or 10X. In camera terms (on full frame cameras) a 7X binocular would be a 350mm lens and a 10X pair would be 500mm. That’s pretty straight forward. But if you were to go buy a video camera that has a 40X Zoom that would imply, in full frame terms the lens would be a 2000mm lens, but thats not the case. The lenses have a wide angle side and the multiplier starts there. This comes into play when you buy point and shoot digital cameras since their lenses are typically measured as a 10X Zoom. They go wider than normal. This means the telephoto side is 10X the widest side. In our example above, our 18-135mm lens is a 7.5X lens but its magnification above normal is only about 2.5X on a 35mm camera or 4x on a DX camera. Confusing isn’t it?

Instead of trying to use these X multipliers, its easier to understand what the mm’s numbers mean and what is wide and what is telephoto.

The next major part of understanding lenses in the aperture of F-stop. This is a number that measures how much light the lens lets through. The maximum F-stop number is calculated by dividing the focal length of the lens by the diameter of the lens Therefore a 50mm long lens with a 50mm wide lens would have a maximum F-Stop of f/1.0. Each full stop of light is 1.4X larger. So you will see standard whole F-stops of:

1.0 1.4 2.0 2.8 4.0 5.6 8.0 11.0 16.0 22.0 and so on

F1.0 lenses are very rare. Primes are generally the lenses with the widest openings and you will typically find them with apertures like f/1.4 f/1.8 etc (1.8 is 2/3 of the way between 1.4 and 2.0). Prime lenses are quite simple in many cases involving only one or two pieces of glass to form the lens. Zoom lenses are quite complex and to keep their sizes reasonable, there are some complex lens combinations at work. These complex lens settings can restrict the maximum amount of light. Most zoom lenses will have a variable maximum f-stop frequently in the f/3.5-5.6 range.

Usually in these cases at the widest lens setting the lens will be open to its widest aperture. So the 18mm-135mm at 18mm will most likely be an f/3.5 lens. But zoom out to 135mm and the widest opening will shift to f/5.6.

Its important to understand that your 50mm f/1.8 lens lets in over 4 times the light as a zoom lens with an f/3.5-5.6 aperture range. That is the difference in shooting at ISO 200 or ISO 800 or shooting with a 1/125th shutter speed and a 1/30th shutter speed. You will find pro-grade zooms that have fixed f/2.8 apertures. These lenses are typically in the $1500 price range, but they are typically as sharp as primes and let in quite a bit of light relative to the consumer and prosumer zooms.

Most all lenses can close down to smaller openings in the f/16 to f/32 range or smaller. This lens opening has a major control for out photo creativity. That is something known as Depth of Field. The smaller the opening (larger f-stop number) the more of the photo will be in focus. An f/11 or f/16 aperture setting will have a lot of the photo, say from 6 feet away to infinity in focus. An f/1.4 opening will only have say an inch or so in focus. If you want a blurred background for your photos, you want to have a more open lens. If your shooting landscapes, you want to stop down to a smaller lens opening. If your lens can only open to f/5.6 your going to have an inbetween depth of field.

But this is only part of the depth of field equation. A wide angle lens will naturally have more depth of field than a telephoto lens will. Thus a photo shot with a 300mm f/2.8 lens wide open at f/2.8 will have a very blurry background. This $4500 lens is treasured by sports photographers as it helps them separate their subjects from the typically busy backgrounds. Where as landscape photographers like wide angle lenses to get more landscape in, but they naturally have more depth of field.

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3 Comments

  1. Mar 27th 6:48pm

    On Nikon glass the “G” stands for “gelded” after what they did to the aperture ring. 😉

  2. Apr 15th 10:02am

    After reading through this article, I feel that I need more information on the topic. Can you share some more resources please?

  3. Jim
    Nov 7th 8:56pm

    Thanks Rob, now my brain hurts. Seriously, thanks for explaining it as simply as possible. It is a jumpled mess of gobbledygook to a beginner and left me just thinking – “The bigger the number the more zoom” and I had to leave it at that. Now I have a basic understanding of what it all means. Thanks!

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