Filters and lenses

Filters and Lenses

What does the ø symbol on my lens mean?
After the focal and aperture ranges, the other measurement you’ll see on most dSLR lenses is preceded by ø and describes the diameter of the screw mount on the front of lens barrel. Check this number each time you head out to buy a filter or hood as you can’t guarantee that it will be the same for each lens in your collection, even if they are all designed to be used on the same camera.
If you only buy one filter…
…make it a circular polariser. This is the perfect beginner’s filter, and one that will have the biggest effect on your day to day photography, giving holiday skies a vibrant blue tone and accentuating the contrast between the sky and passing clouds to afford your images greater texture. Although you can add blue to your images in Photoshop or a similar post-production editing tool, the effect is never as believable when done that way as it is when shot using a lens.
Don’t confine it to skies
Polarising filters also cut through glare and reflection. Use it to shoot through windows and water.
Don’t forget about white balance
When using a filter set your the white balance on your camera to the appropriate conditions, rather than auto, to stop the camera compensating for the filter in front of the lens.
Don’t rush out to buy a skylight filter
Putting a clear filter on the front of your lens to protect its surface sounds like a great idea. After all, your lens was an expensive investment. The end of your lens is stronger than you might think, however, and easy to clean if you don’t let the dirt build up. Dispensing with a skylight filter will not only save you money, but also avoid the chance of introducing light problems due to increased reflections or the slight reduction in the level of illumination reaching the sensor.
Cheat’s macro mode (add-on filters)
Dedicated macro lenses are expensive, but you can quickly and easily improve your existing lens’ macro credentials by using screw-on magnifiers. They’re not a perfect solution as they decrease the level of light coming into the lens, but for occasional work they are very effective, easily sourced and cheap.

Choose a manual lens over a powered one
Some compact interchangeable lens cameras come with a choice of powered or manual zoom. The former is a great lazy option, allowing you to press a button to get the framing you’re after, but the latter is often cheaper and almost always quicker to use as it moves at whatever speed you turn it, without being hobbled by the speed of an internal motor. You can also often make finer and more predictable changes when zooming manually than you can with a powered zoom rocker.

Shoot slowly, zoom quickly… At the same time
If you’re shooting a static display, add some interest by turning the zoom control while shooting with a fairly slow shutter speed (you can only do this with a manual zoom, as a powered lens will be locked off when shooting). This works particularly well when shooting cars and other forms of transport as it gives them a sense of motion.

Try a prime lens for more creativity
Shooting with a fixed focal length — a prime lens — will make you think more carefully about how you want to frame a subject to tell a particular story. It will often also get you a cleaner, sharper result.

What do the measurements on my lens mean?
Lenses are measured in terms of their focal length, which broadly describes the effect they have on incoming light and the way it is focused on the sensor. A short focal length, such as 24mm, doesn’t have a very high level of magnification, so will focus a broad vista on the sensor. A long focal length, such as 240mm, has a high level of magnification, like a telescope, and so will fill the sensor with just the central part of the view.

Understand your lens’ true dimensions
Unless you’ve paid for a high-end dSLR, or a professional camera such as the Leica M9, your pocket snapper’s sensor will almost certainly be smaller than a frame of 35mm film, the standard point of reference against which all focal lengths are measured.

The 35mm in a frame’s name actually relates to the space between the top and the bottom of the film strip, which as well as the frame itself also contains some border areas and the sprocket holes used to move the film through the camera. A 35mm frame is positioned lengthwise on this strip, with its shortest dimension — top to bottom — perpendicular to the film’s direction of motion. As such, neither the height nor the width of the frame measures 35mm, but instead 24x36mm.

To understand how the stated focal length on any lens will affect the shot captured by your camera, you need to factor in the multiplier effect, which converts the size of your sensor to the size of that 35mm piece of film. The multiplier is often between 1.5 and 1.7 but varies between manufacturers and models.

So, if you’re buying a lens for the Canon EOS 600D with its 22.3×14.9mm sensor you’d need to multiply the stated focal length of the lens by 1.6. This would make a 50mm lens, commonly used in portrait photography, act like an 80mm lens, thus increasing the effective zoom and narrowing the amount of the scene seen in each frame. On a Nikon D5100, which has a slightly larger sensor (23.6×15.6mm) you’d need to multiply the lens’ measurements by 1.5, in which case an equivalent 50mm lens would act as though it were a 75mm unit.

Save money by opting for a smaller sensor
This means you can, technically, save money by opting for a smaller sensor, as you’ll be able to buy less powerful lenses to achieve the kind of results you would otherwise only get with a longer, more expensive zoom.

Use zone focusing
Related to point 6 — f/8 and be there — if you have a lens with both f-stop and focal measurements on the barrel, understanding how they relate to each other can help you take great spontaneous photos with a high degree of confidence.

In the image below we’ve set our aperture to f/5.6, as indicated by the red line pointing to the 5.6 reading on the lower gauge. We’ve then set the range on the yellow gauge to around 1.2 metres by positioning this at the top of the same line. We can now use the green scale to understand how far away from the camera our subjects need to be if they are to be accurately focused.

By following the lines running from the two green entries for 5.6 on either side to their measurements on the yellow scale, we can see that so long as we’re more than 1m away from our subjects they will be in focus (the green 5.6 on the left is linked to around 1m on the yellow scale, while the green 5.6 on the right is linked to the infinity symbol, which is like a number 8 on its side). Anything closer than that will be blurred.

This gives us a great deal of freedom to snap whatever we want without making any further adjustments, so long as it’s no closer to us than 100cm. To create a more intimate effect, adjusting the distance ring so that 0.4 sat at the top of the red marker would mean that only those objects between around 36cm and 50cm would be kept in focus.