Lens Primer - The Basics


  1. Introduction
  2. Focal Length
  3. Aperture
  4. Lens Mounts
  5. Lens Hoods
  6. What's In a Name?


On the surface, M and LTM lenses appear pretty straightforward. You have the focal length and aperture which defines the most fundamental aspects of a lens. The controls are equally as simple; a focus ring and an aperture ring. Unlike their SLR counterparts, there's not too much to them as they lack autofocus, stabilization or in the case of high-end professional telephoto lenses, any number of features such as stabilization mode, focus preset, autofocus range, etc. But there's a lot more beneath the surface.

Focal Length

When considering a lens, the focal length is usually one of the first aspects of a lens to consider. One way to look at it, and the most direct description is, "the distance from the lens' rear nodal point to the film plane when focused at infinity." Though optically, it's more useful to think of it as the magnification of a lens - longer focal lengths have a greater magnification over shorter ones. This has important ramifications on how a lens will be used and how it affects things like the sense of perspective and depth of field, which we'll get into later. For now, it's easiest to think in terms of the angle of view that a given lens will provide you. Lenses in our case are described for the size of a 35mm negative, or 24x36mm - digitally known as "full frame" as with the M9. The angle is typically measured diagonally, from one corner of the negative to the other. The angle of view then, can be seen for these common focal lengths:

Focal Length of Lens Angle of view for 35mm
15mm 111º
21mm 91º
24mm 84º
28mm 75º
35mm 63º
50mm 47º
75mm 32º
90mm 27º
135mm 18º

The following illustration will help to visualize this as it relates to the M system and lenses. Click on the image below to see a larger version:

It might be a little hard to wrap your mind around the angles and how it might relate to a scene - let alone compare - so let's take a look at it from a visual perspective. Click on the image below to see a larger version, overlaid onto a scene:

The M8 you'll notice however, has a "crop factor" of 1.33x due to its APS-H sized sensor, which is smaller than full frame (27x18mm). What does this mean? It's important to realize that a 50mm lens used on a full frame/film camera will not give you the same angle of view as a 50mm lens used with an APS-H sized sensor. A 50mm lenses therefore, would be roughly equivalent to a 67mm lens on the M8 as 1.33 x 50mm = 67mm. The Epson R-D1 has am APS-C sized sensor, resulting in a crop factor of 1.5x - which would make that same 50mm lens equivalent to a 75mm. The reason it's called a "crop factor" is because a 35mm lens will project an image (circular in shape, called an image circle) on the medium (film or sensor) large enough for a 35mm negative. Smaller sensors can't capture it all, thus the image is cropped.

As the sensor size shrinks - the image is subject to a crop factor which can vary from 1.33x as on the M8, 1.5-1.6x for Canon/Nikon DSLRs to 2x for Micro Four Thirds. That same 50mm lens would effectively have a focal length of 100mm on a 2x crop body! One point worth noting is that the crop factor does not apply to the aperture of a lens and what makes smaller formats interesting. A fast lens such as the Voigtländer 50mm f/1.1 Nokton would remain an f/1.1 lens!

As mentioned earlier, a lens' focal length can have a dramatic impact on the spatial feeling of your image - namely perspective and depth. A longer focal length appears to compress space and make objects seem closer together. Conversely, a wider angle lens exaggerates space and increases the apparent distance between objects. Another aspect of focal length is depth-of-field. Longer focal lengths appear to have less depth of field while shorter focal lengths have more. These effects are created by camera to subject distance and magnification and are a bit of an optical illusion. We'll get into this further in the Creative Controls article.


The other important aspect of a lens is the maximum aperture, or how "fast" a lens is. In basic terms, the aperture is a variably-sized opening internal to the lens that controls how much light passes through the lens. Though it's not as simple as it sounds. One very important aspect of the aperture as it relates to photography is the control of depth of field. For a given lens, the more open the aperture, the thinner the depth of field is and the more light that is transmitted. Conversely, the more the aperture is closed down (known as "stopping down") the deeper the depth of field becomes, at the expense of the amount of light transmitted. It's this action of controlling the light which requires you to increase or decrease the shutter speed to compensate, when trying to maintain the same exposure. This is the basic mechanical description of the aperture, also called an iris or diaphragm.

So what does it mean to have a "fast" lens? Generally, lenses are "fast" the more they open up and thus transmit more light and require a faster shutter speed. It's important to realize that apertures (or "f/stops") are a ratio - so two different focal length lenses won't have the same physical opening size for the same aperture necessarily. At the risk of getting too technical, this ratio is described by the formula N = (f / D). That is, N is the f/stop, f is the focal length and D is the diameter of the entrance pupil (or front element). So for example, a 50 mm focal length lens with an aperture of f/2 will have a pupil diameter of 25 mm. Using this formula, it's easy to see why "faster lenses" get bigger the faster they are (e.g. an Elmar 50mm f/2,8 as compared to a Noctilux 50mm f/1). Each increase of one stop in speed requires twice as much light! Generally, lenses are considered fast if they have maximum apertures greater than f/2.8 or f/2 - that is, an f/1.4 lens (e.g. Summilux 50mm f1,4) is considered fast.

Besides the necessary increase in size, there are other issues that come into play the faster a lens becomes - notably, aberrations - which must be corrected to maintain a high-quality image. It's easier and cheaper to design an excellent slow lens than a good fast one. This is because corrections typically imply more complex optical designs through additional lens elements, more exotic materials (e.g. flourite) or a unique element shape (aspherical). That's not to say you can't have a fast lens that's every bit as good as a slow one at a given aperture - just that the size, weight and price will often go up as a result - unless ultimate image quality is compromised in some way (which is not always a bad thing).

Lens Speed Pros Cons
Fast (e.g. f/1.4) Faster shutter speeds possible Can be used in low-light conditions Greater range of depth-of-field control Larger, heavier more expensive lenses Requires more corrections
Slow (e.g. f/2.8) Smaller, lighter, cheaper lenses Requires less corrections Slower maximum shutter speeds Restricted to brighter scenes or faster films Less depth-of-field control options

Another aspect of the aperture is the number of blades that comprise the diaphragm. Simpler designs can have maybe five or six blades. More complex diaphragms can have up to 15! Not many lenses can claim this, though certain vintage Leica lenses have just that. More common numbers run from eight to ten or so. The edge and shape of the blades also plays a part. The purpose of all this is to control how round the opening is as the lens is stopped down. It's important to note that wide open, all lenses have a perfectly round aperture, though technically, the diaphragm isn't even in the light path. As you stop down, closing the diaphragm - is where the roundness of the opening matters. Essentially, it affects the "bokeh" or foreground/background blurring of out-of-focus areas. There's no hard and fast rule that more blades and rounder openings make for better bokeh per se, as other optical design factors play a large part in the overall result - but generally, it helps. Where the effect is most obvious is the shape of out-of-focus specular highlights, where it is generally preferred to have them appear smooth and round - not ones that look like hexagonal "stop signs." Consider this extreme example, as seen on a vintage Leica Elmar 5cm (50mm) f/2.8 lens:

Leica and some third-party lenses typically have half-stop positions while others such as the Zeiss ZM line have detents in third-stops. The benefit of half-stop detents is that it corresponds precisely with the shutter speed dial, which is also in half-stops. By having third-stops, you have more control (which is often critical with shooting slide films) but have to keep in mind that a corresponding compensation via shutter speed will be a bit off. The most common scale, half-stops then - would look like this (with full-stops in bold):

1 1.2 1.4 1.7 2 2.4 2.8 3.3 4 4.8 5.6 6.7 8 9.5 11 13 16 19 22

Using a lens wide open affects image quality - as does stopping it down beyond what is known as the "diffraction limit." Based on the 35mm platform - this generally occurs on apertures smaller than f/8 to f/11 on full-frame media. In simple terms, diffraction is caused by the spreading of parallel rays of light - which increases as the aperture is stopped down (due to increasing angles of the light rays). This causes a loss of sharpness of the image. What this means is that even though your lens can stop down to f/22 - unless you absolutely need either the light control or maximum amount of depth-of-field, you shouldn't really do so. Lenses also have what is known as their optimum aperture, or where they present the best possible image quality and sharpness. As a guideline this is often about two stops down from wide open - for example, selecting f/4 on an f/2 lens. We'll expand on both of these in another article.

Lens Mounts

Naturally, there are a bewildering array of lens mounts on the market - typically, each camera manufacturer has their own proprietary format and third-party lens manufacturers need to design their lenses with these in mind. However, in the context of the M system, there are really only two that you need to be concerned about. They are the newer bayonet mount, "M" and the older screw/thread mount, "LTM" (for Leica Thread Mount). All Leica bodies with an M designation (e.g. M3, M4, M9, etc.) have the bayonet, or "M" mount. Older bodies use LTM. Other manufacturers such as Voigtländer and Zeiss also use these two basic mounts.

Generally, modern lenses and bodies have the M mount. It's faster to use over LTM, requiring only a short rotation to mount and unmount a lens - and clicks into place with a notched locking mechanism. Not just Leica, or compatible - but generally all modern cameras have bayonet mounts of some kind now.

LTM lenses don't necessarily mean old - Cosina/Voigtländer for example, still make brand new LTM lenses. But Leica on the other hand, generally stopped producing LTM lenses quite a while ago. These are typically considered "vintage lenses." Why would you want to use such a lens? For one thing, these vintage lenses often have a very unique signature, or rendering. Some just like using vintage lenses for the "retro" aspect. Depending on the popularity of such lenses, they can also be had for very reasonable sums of money. It's also worth noting that "old" does not necessarily mean that they're poor performers - just because they don't have the technology of newer lens designs and materials - which often leads to what is known as "modern" or "clinical" rendering. That is, they're too highly corrected and "perfect."

So how do you use such a lens on an M body? Several companies manufacture LTM/M adapters, which are basically small rings with an LTM compatible thread on the inside and a bayonet flange around the outside. The higher quality adapters are made by Leica and Cosina/Voigtländer to name a few, but many are available from other parties, including no-name Chinese copies. One important aspect of these adapters are the framelines that they bring up and come in three versions - 28/90, 35/135 and 50/75. Therefore it is important to match the adapter to the focal length of the lens (or framelines you wish to bring up). A typical LTM/M adapter, seen here with 6-bit coding for digital bodies:

One other important thing to note for digital users (M8/M9) is that there is enough of a flange area to place your coding marks. Cosina/Voigtländer addressed this issue by creating the "Type II" adapter, as the original "Type I" adapters cannot be coded. The reason is because the original LTM/M adapter design didn't leave enough metal on the flange where the coding marks must be placed - it's typically notched in this area. On film cameras, this obviously has no impact.

Finally, some adapters (mostly non-Cosina/Voigtländer) will mount your lens "off-axis" or "off-center" of the focus/aperture index mark on the lens. This is normal behavior, though some find this undesirable. It is a feature - so you can read your lens settings with a light meter mounted atop the camera, which was the norm for most LTM mount cameras.

Lens Hoods

Lens hoods, like using UV filters for protection - can be a personal thing. However, it is generally accepted that using a hood makes sense. Primarily, it is designed to block stray light from the sides (that are out of your frame) from entering the lens and causing unwanted reflections and light contamination. That's why each lens or range of focal lengths has a unique and different hood - the depth of it is determined by the angle of view, or focal length of the lens. Unlike a UV filter, using a hood will never degrade or impact your image quality - it can only help. Which brings up another useful function of the hood, and that is physical protection. It does this by providing a sort of "bumper" and keeping things from directly touching the front element like stray fingers and foreign objects. Though it's worth pointing out that only metal and plastic hoods offer this - collapsible rubber hoods to a much lesser degree.

Unique to rangefinder lens hoods are the "vents" or areas cut out of the back of a hood. If you're new to rangefinders, you might find them odd or wonder what they're for. You'll notice that these notches are usually only located in line with the viewfinder/rangefinder windows. This is so they can work properly, and obstruct less of your view through the viewfinder. Some hoods have these notches all the way around, possibly because of the way they mount to the lens and just make it quicker to do so without worrying about alignment. Or just as a design feature. Here's a typical rangefinder lens hood; this one from Zeiss for the 25/28mm ZM lenses:

One note about lens hoods on lenses used with digital cameras that have a crop factor. You can often use a deeper hood, since the effective angle of view is reduced. For example, using a 28mm hood on a 25mm lens (e.g. the 25/28 seen above vs. the 21/25 hoods from Zeiss). Sometimes you can push it a bit further - but you should verify that you're not causing mechanical vignetting in your images by doing so.

Some lenses come with hoods - with others, they're an accessory and an additional expense or perhaps a different hood version is available over the one provided with the lens. In any event, it is generally wise to use a hood even if the optical characteristics of a lens (flare control) make them optional. Consider getting and using a hood if at all possible - though in some cases you might not want to. For instance, when portability is very important, such as being able to slip the camera into a large coat pocket. Or when you're trying to be as discrete as possible such as when street shooting. Of course, in low light situations a hood becomes less useful optically, but not entirely - and still offers physical protection.

What's In a Name?

You may have noticed that rangefinder lenses are named a little differently than your average SLR lens. They have names like "Elmarit" and "Biogon." There's no rule to it and it varies in meaning from one manufacturer to another. In the case of Leica, the names generally group a certain speed or class of lenses together (though perhaps more so post-1960) as seen below:

Lens Family Name Speed of Lenses
Noctilux f/0.95, f/1.0, f/1.2
Summilux f/1.4
Summicron f/2.0
Summarit (-M) f/1.5, f/2.0 (f/2.5)
Elmarit f/2.8
Elmar, Super-Elmar, Summaron, Summarex, Hektor, Thambar... f/1.9 ... f/6.3

Zeiss on the other hand names the lenses by optical design. That is, "Planar, Biogon, Distagon, Sonnar and Tessar" are all optical designs used in the particular lens. Voigtländer tends to name their lenses a little in-between. For example, "Nokton" lenses tend to be pretty fast (faster than f/2) and sound reminiscent of Noctilux or nocturnal. "Ultron" lenses also tend to be faster lenses, around f/2 or faster. Another class of lens that's especially compact are in the "Color Skopar" family. Then there's the design names such as "Heliar" which generally refers to longer focal lengths (50-75mm). But wait - it gets better...

Zeiss and Leica are German companies. Thus they tend to use a comma (,) rather than a period (.) when expressing values such as aperture. For example, f/1,4 rather than f/1.4. You could really use either convention. Leica used to use the comma more, but most new lenses use periods - unlike Zeiss which still uses commas. Voigtländer used to be a German company but the rights to the brand name were bought by Cosina, a Japanese company. The lenses, boxes and documentation are printed with periods but the (German) website uses commas (as is the norm there).

Zeiss and Leica generally use only the whole part of the aperture where possible (e.g. f/4) rather than specifying the decimal part on all lenses (e.g. f/4.0) - Voigtländer mixes both randomly.

It's worth pointing out that Voigtländer lenses are just that - not "Cosina/Voigtländer" which is often abbreviated as "CV" on forums and on the web. For the same reason you don't say "Cosina/Zeiss" when referring to ZM lenses (they're assembled in the same factory).

Further confusion arises from the order in which manufacturers list specifics of their lenses. Zeiss likes to abbreviate these, such as "1,5/50" whereas Leica and Voigtländer stick to a more conventional format of "50mm f/2." Zeiss and Leica state the lens name/design/family first, then these specifics - whereas Voigtländer does the opposite.

The most confusing aspect is likely the various features found on a specific lens, and each manufacturer has a different way of noting them. Voigtländer doesn't usually list them as part of the official name, though it's printed on the lenses and in the documentation. Zeiss doesn't list anything as they don't have aspherical or apochromatic lenses, but they do list their standard multicoating, "T*" on every lens... Leica is perhaps the worst offender, putting these features either in front of the lens family name or tacked onto the end of the name. Perhaps one of the worst such examples would be the "APO-Telyt-M 135mm f/3,4 ASPH" lens, with bits strewn throughout!

it's best to think of the various pieces that can make up a name and understanding what they mean - before long, your brain will automatically decipher these on-the-fly regardless of order or semantics. Here then, are common abbreviations as seen in lens names from various manufacturers:

Feature Description
ASPH Aspherical (Leica uses both, though Aspherical is typically found on older lenses, ASPH on newer)
APO Apochromatic
FLE Abbr. for "Floating Lens Elements" - unofficially used to distinguish between Summilux 35mm versions
-M Used by Leica to denote a newer, M mount lens generally
C Used by Zeiss to denote a "compact" or "classic" lens formula or Voigtländer to denote "Classic"
P Used by Voigtländer to denote a "Pancake" lens
II Used by Voigtländer to denote the version of a lens (in this case second, or "Mark II")
T* Used by Zeiss to denote their standard multicoating
ZM Used by Zeiss to denote the lens mount
LTM Abbreviation for "Leica Thread Mount" - unofficially used but often mentioned