Camera lens rant.
I've been meaning to write about camera lenses for a while now. The single most important thing about a lens is that the pictures you take with it looks great. If it isn't perfect in every possible way when you zoom in, but look great I would say it is a good lens. For example Leica lenses are not the sharpest in the world, but they look fantastic. And I would say that look better than most if not all of the "perfectly sharp" lenses available. This is because if you are making a "perfect" lens you add more elements or glass to the lens that means more stuff it has to pass through to correct various optical "problems". And then you loose something called micro contrast. Some people want a perfect lens because of reasons that is beyond me (a fancy way of saying: for reasons that don't matter) and some of us just want a lens that produces great images.
Every lens has a focal length. That is the number you read in mm. It is a horrible system because how close you would be to a subject with X mm varies based on how large the film or sensor is. You have something called the crop factor, that is how large the sensor or film in your camera is relative to 35mm film. For example my Fuji is around 1.5(same goes for Sony and Nikon APS-C cameras) and a full frame camera is the same as a 35mm camera; Canon APS-C is 1.6 and MFT (Sony and Panasonic) has a crop factor of 2.0.
Let's take a 50mm lens on all of the systems:
- Full frame / 35mm: 50mm
- Canon Crop: 80mm
- Fuji / Sony / Nikon: 75mm
- Micro Four Thirds: 100mm
We are stuck with the mm system, but it sucks. The mm number on your lens is not just about how much you can fit in your frame or how far away you can stand away from your subject. A beginner would probably chose a lens or focal length based on that. But more experienced photographers would often chose them based on the look the different focal lengths give you.
Longer lenses (larger focal lengths) compresses what you take pictures of while wider lenses (smaller number) kind of makes everything look more "dragged out". This means that a portrait taken with my 8mm lens makes people look fatter than with my 105mm. A longer lens also means that you need a shorter shutter speed to avoid a shaky image, this is because the magnification means all shape is magnified. The rule is usually that the slowest is the same as the focal length equivalent on a 35mm camera.
It will also be a larger separation between subject and background when you use a longer lens. This means that background blur and "bokeh" is more visible the longer the lens is. This plus that people look slimmer with longer lenses are the reason you often want to use a as long as possible lens when shooting portraits.
The next important technical detail about a lens is the aperture. It's that strange number with a f in front of it. It basically tells us how wide a lens can open up. The smaller the number the larger it can open. For example f/1.2 is very wide. When you set your lens to a small number means that less stuff are in focus, While a larger one means that more stuff are in focus. But there is a limit to how large you should go, relative to how large your sensor is. This is one of the reasons medium and large format cameras are popular among landscape photographers. Some lenses also have optical stabilisation, and some camera bodies have it. This can make it possible to shoot pictures with much slower shutter speeds than without it. But you should always disable it when you shoot with a tripod.
Now. Let's get back to what I started with.
The smaller the f-stop number is, the bigger the lens would be, and the more "technically perfect" it is the more elements there are in the lens, often really big and heavy when it is really small, like f/1.2 or smaller. This means that the lens can be very unfordable to hold and use for long periods at a time. But it also means that auto focus will be slower. The reason for this is that there is small motor inside your lens that moves the elements forth and back until it's in focus.
For example Fuji has f/2 and f/1.4 versions of both their 23mm (equal to 35mm) and 35mm (equal to 50mm) lenses. The f/2 version is much lighter and has much faster auto focus speed. This shows the never ending problem when you pick a lens: what's most important to you?
When I talk about "technical perfection" that means that the lens manufacturer add a lot of extra elements or focus more on the lens being "perfect" than producing good pictures. They are basically trying to solve a number of optical anomalies. Like making sure all of the lens always is "sharp". Or that the light spots in the out of focus parts of your pictures have the right shape and a million other things.
Some people (that often are more into the technical specs of the camera) care a lot about this. I personally don't care. The thing I care about is that the pictures look good, and that the colours come out right.