Most of us take photos—even if only with our phones—but not everyone really knows a lot about photography, or about the lens that’s inevitably involved in its capture. If you’ve even been bewildered by a photographer’s lens chat, and you wish you knew half of what they were talking about, then this article is for you. We’ll take a look not only at professional lenses for SLRs, but at how the same factors affect other cameras, and what the future might hold.
If you’ve ever seen a professional SLR lens, you’ll have seen a number on the side followed by “mm”. In traditional 35mm film terms—and as on a Canon 5D—a 50mm lens would be a “standard” lens, providing a field of view similar to your eyes. A lower number, like a 28mm lens, provides a wider field of view, and a higher number, like 200mm, provides a narrow, telephoto field of view, like a telescope.
A long vintage telephoto lens next to a Blackmagic Cinema Camera.
(It’s similar with compact cameras, where you’ll see a zoom ratio like “5x”, but there’s no fixed standard for their starting field of view, so it’s less meaningful.)
But that’s not close to the whole story. The size of the sensor inside your camera determines how much of the lens actually gets used. While the lenses are described in consistent terms, if your camera’s sensor is smaller than a 5D’s, you’re only using the center part of your lens. Effectively, you’re zooming in all the time. Most Canon cameras have a “crop sensor”, meaning you have to multiply the focal length by 1.5 for an equivalent field of view. A “standard lens field of view” now requires roughly a 35mm lens on a crop sensor camera.
This 30mm f/1.4 is a nice fast prime that’s close to a standard lens for crop sensors.
Other sensors, like those found in Micro Four Thirds cameras or Blackmagic Cinema Cameras have even smaller sensors than a Canon or Nikon crop sensor, meaning that it’s easy to find a telephoto lens, but hard to find a wide one.
While most lenses that most of us use are zoom lenses, which span a range of focal lengths, many professionals instead swear by primes, which only offer a fixed focal length and force you to “zoom with your feet”. Restrictions can drive creativity, and forcing a photographer to a particular field of view can be freeing, in one sense.
Zooms are handy, if not perfect.
Another pressing concern is that zooms often compromise image quality. Certainly the cheap “kit” zoom that comes with most SLRs is often a disappointment—soft, or no good in low light.
For around $100, the classic 50mm f/1.8 can’t be beaten.
Prime lenses (like a 50mm f/1.8 or a 30mm f/1.4) usually have better image quality than similarly priced zooms, and in particular, have a wider aperture for brighter images in low light. Which brings us to...
Measured in f-stops, this describes how much light is being let into the camera. A lower number means more light, so while a fast prime might offer f/1.4 or f/1.8 when wide open, a kit zoom might offer f/3.5 when zoomed out, and as little as f/5.6 when zoomed in. Each of these f-stops represents a halving of the amount of light entering the camera:
1.4 2.0 2.8 4 5.6 8 11 16 22
A manual aperture ring is the easiest way to see this in action.
That kit zoom, when zoomed in, offering f/5.6? That’s just a quarter of the light available to a “good” zoom which might be a constant f/2.8 throughout its focal range. While you may well decide to reduce the aperture (let less light in), nobody turns down a fast lens.
Controlling light is not all that aperture does. The wider the aperture, the shallower the depth of field is, meaning that less of the image is in focus. Let’s clear that up.
While you focus on a single object in a shot, there’s an area behind it and in front of it which is also in focus. Beyond those boundaries, the image blurs. Shallow depth of field indicates a narrow area, and deep depth of field means that most of the image is in focus.
Though it seems contradictory at first, we associate a shallow depth of field with a higher quality image. It’s harder to get a shallow depth of field on the tiny sensors in cheaper video and stills cameras, so we’ve seen a lot of professional images with blurry backgrounds and a lot of amateur images with sharp ones. Blurring the unimportant areas of an image is also a great way to focus—apologies—attention on the parts of the image you want people to look at, in both still and moving images.
The shallow depth-of-field shown in this photo of a kit lens couldn’t be easily achieved with a kit lens.
Of course, take it too far and it can be very difficult to focus at all. So sure, get an f/0.95 prime for those deep dark shots, but you’d better nail your focus every time, because it’s going to be very shallow.
Different lenses are designed for different systems; some large, some small, some screw-in, some bayonet. Sometimes, you can adapt one to another, and sometimes you can’t. A most important factor is the distance between the back of the lens and the camera sensor, known as the flange focal distance. A lens must have a flange focal distance larger than the camera it’s to be used on, to make room for the adapter.
A full-size Four Thirds lens can work on a Micro Four Thirds mount with the right adapter.
While you can use Canon EF lenses on a Blackmagic Pocket Cinema Camera with a Micro Four Thirds mount and an adapter, you can’t use Micro Four Thirds lenses on a Blackmagic Cinema Camera with the Canon EF mount. The camera’s snout is too large, the flange focal distance simply too long, and it doesn’t work. A camera with a short flange focal distance is therefore more useful if you want to use a wide range of lenses.
That’s an EOS > MFT, an M42 > EOS and an M42 > MFT.
You still can’t use every lens, because some lenses are fussy. Many Micro Four Thirds lenses need power to focus, to zoom and to change aperture, so you’d need an adapter that can provide those features.
The same issues occur with modern lenses that change aperture electronically rather than with a physical aperture ring—you’re stuck wide open unless you can find an adapter that can change aperture on your behalf. Especially with vintage lenses, the process of fitting the lens at all can be a little hit and miss, and you may need to mess around with a lathe to make adapters work perfectly. Joy.
While many modern lenses can be sharp, remember that the resolving power (resolution) of a lens may be less important than the character it gives to your images. Especially for video, where Full HD is only 2 megapixels, you may not need the absolute crispest images. Instead, maintain a bank of lenses to deal with the situations you’re likely to face: sharp-enough zooms for daytime walk-about use, faster primes for night shots, vintage lenses for special effects or for their manual aperture rings on non-native cameras.
Old lenses can sing on new cameras.
But watch out. Once you realize that the kit zoom isn’t that great, you’ll want to spend more money on lenses than you ever considered possible. It’s great fun, though.
To learn more about digital photography check out this video course from AskVideo: