CameraTalk » DOF

One Point Or More To Focus

Peter Ching — Mon, 16 Nov 2009 02:41:51 +0000

USING THE CENTRE FOCUS POINT, especially for close up, thin depth-of-field shots, often leads to focusing errors as well as metering errors.

There are times, of course, when using the centre Focus Point is fine – for example, landscapes and other shots with very wide depth-of-field.

But for group shots and portraits, especially with narrow DOF, you’re better off selecting Focus Points that best serve the composition. Manual selection of the best Focus Point will reduce out of focus and poorly metered shots.

Trying to lock focus and recompose will likely result in focus errors. In fact, doing so misleads many users to believe their lens is back-focusing or front-focusing when, in fact, the problem is poor technique.

Lenses are not perfect, so their plane of focus is not a perfect plane. This means that, for narrow DOF shots, if you focus with your centre Focus Point and recompose, the region of the frame now occupied by the target, may not be in perfect focus for that part of the lens. Result: soft images.

Multiple Focus Points are there for good reason – to help produce sharp, correctly exposed images. Note: histograms will show whether exposure compensation is needed.

This is what Canon, Nikon and other camera manufacturers had in mind when they designed their DSLRs. Exploiting multiple Focus Points is the best way to get the sharpest images with the best metering.

Here’s the how:

Bring camera to eye
Compose and frame shot
Manually select the most appropriate Focus Point (eg, on subject’s eyes)
Hold framing steady, shoot.

If this sounds all too time consuming, a little practise will help make it second nature, such that you’ll proceed quickly and effortlessly, concentrating instead on the composition.

Multiple active Focus Points also come in handy is when using the Dep or A Dep mode – where the camera sets a focus distance and aperture to capture the chosen near and far points (Dep mode), or the range, to a given minimum level of sharpness, between the closest and furthest Focus Points (A Dep).

Multi-Point AF is your friend when tracking something that is moving unpredictably. Aside from subjects in rapid or erratic flight, the same system is a godsend for animal portraiture as dogs and horses, to name but a two, tend to move their heads around quite a bit.

Continuous mode AF with multi-points uses a multi-point algorithm to identify the subject and choose the sensor(s) that are on it. For example, if you pan past a light pole in the foreground, AF won’t generally shift off the main subject. Or if you’re following a plane flying by at upwards of 1000 kph, AF will stay “locked on” even if only a fraction of the subject is in the finder.

Apertures… Open Wide And Say Aaah!

Gabriela Sousa Rocha — Sun, 01 Nov 2009 08:44:13 +0000

Everybody is aware of what it means to have a 70-200mm lens, or a 18-105mm. These days, the emphasis always seems to turn to the zoom range when it comes to buying a lens.

But how about that other set of numbers… the ones after the “F”?

Those numbers refer to aperture. They measure the diameter of the opening inside the lens that allows light into the camera. The “f stop”, as it’s called, can enable you to use faster shutter speeds, help you take great pictures in low light conditions without using a flash, and achieve beautiful depth of field.

Depth of field (DOF) refers to how much of the image from the foreground to background will be in focus. Wide apertures (represented by small f.stops) will give you shallower DOF. I’m sure you’ve seen portraits where the subject is sharply in focus and the background is out of focus.

Wide apertures combined with flattering light will likely lead to amazing photographs.

On another note, to properly freeze the movement when shooting sports or birds, for example, you’ll need fast shutter speeds. How fast your shutter speeds will need to be, will depend on the subject. In good daylight, fast shutter speeds can be achieved even without a wide aperture. But, in lower light, a wide, or “fast”, aperture will be essential.

Taking pictures in low light can be challenging, especially without a flash (in churches and museums, for instance, the use of flashes is often not possible). Consequently, slow shutter speeds are needed for sufficient light to enter the lens.

However, even when using high ISO settings, shutter speed limitations can result in blurred images. In low light, lenses with wide apertures guarantee faster shutter speeds for sharper images.

Wide apertures give you the option of shooting in the available light, even when use of a flash is possible. You can, with faster apertures, photograph in candlelight, which can be very flattering and create fantastic effects.

So when you consider your next lens, think about making it a fast one ─ it will take your photography to the next level.

Filter Essentials

Peter Ching — Thu, 09 Jul 2009 01:04:43 +0000

Photographic filters are akin to sunglasses. They shield your lens similarly to sunglasses protecting your eyes. They also modify light in much the same way as sunglasses do.

Here’s an overview of some of the types of photographic filters available.

UV Filters

UV filters are the Go To filters for most people. Other than a purpose-built protective filter, UV filters have the least impact on image quality. They can be left permanently mounted to your lens so that when the unexpected happens and your new-born projectile-vomits, you need only clean the filter.

They’re called UV filters because they eliminate the haze commonly seen on a summer’s day. Commonsense suggests that colour and sharpness remain untainted because a clear filter attuned to only ultra-violet lightwaves will ignore the slower wavelength colours.

But, as with many applications of commonsense, arguments abound. Detractors point out that anything between subject and lens glass will affect image quality. They also argue that your best defence to protect your lens is your plastic lens cap.

Whatever the argument, the upside is that your $3000 lens will be shielded from untoward debris and will never wear the visible evidence of subsequent scouring efforts. The downside is that a stickler will resort to a 400% image enlargement just so they can offer you a smug “told you so”.

If you who own top-shelf glass, such as Canon’s L lenses and the faster Nikon lenses, you will want nothing less than Hoya’s Pro1D filters to maintain high optical standards. This applies equally to the circular polarising filters described further on.

Circular Polarisers

Unlike UV filters, you’re hardly going to want to leave a circular polariser on your lens fulltime. They WILL have an affect on your images. But it’s an effect you will achieve by no other means.

Take, for example, a landscape shot featuring lots of blue sky and puffy white clouds. Without a filter, your exposure is correct for the trees and farm buildings but the blue sky looks washed out and the clouds have no definition.

Enter the circular polariser. These filters work like Polaroid sunglasses – they redirect, i.e. refract, the angle of light passing through. You will not only see more sky depth and detail, you’ll also be able to eliminate reflections off glass, water or foliage.

Polarisation is most effective at right angles to the sun. At 180 degrees – in other words with the sun directly behind you – polarisation is almost non-existent.

Due the directional nature of polarisers, it’s not a good idea to use them when you want to stitch together a panorama, as the sky in each panel will be at variance with the others.

Above left with a polariser; above right without a polariser.

Above: upper half with a polariser; lower half without a polariser.

Neutral Density Filters

Neutral Density (ND) filters reduce the amount of light through the lens. They’re designed to filter out all colours equally, therefore having no effect on the colours in your images, hence the term “neutral”.

All ND filters are grey in colour – the deeper the grey, the more light is reduced. These filters come in three strengths, giving 1, 2 and 3-stop reductions in light intensity. You can stack a number of ND filters to further reduce the intensity of light.

ND filters have ‘gradings” depending on the amount of light they cut. A higher grading means lower light.

There are two commonly used notations:

ND2 ND4 ND8:
The number designates the density of the filter based on binary numbers 2=21
i.e. 1 stop of light. 4=22 i.e. 2 stop of light, etc.

0.3 0.6 0.9 ND:
Another notation in which every 0.3 will make you lose one stop of light. So a 0.3ND will cut 1 stop of light 0.6 2 stops etc.

ND filters help in at least three situations:

reduce light intensity;
allow slower shutter speed use; and
allow larger aperture use.

If the shutter speed remains the same after adding an ND filter, you’ll use a larger aperture to obtain the same exposure. Likewise, if the aperture is retained after adding an ND filter, a slower shutter speed will achieve the same exposure.

Keep in mind that a larger aperture produces a shallower Depth of Field. So, for example, if you are shooting a portrait in broad daylight and the light conditions demand high shutter speeds, an ND filter will allow you to use bigger apertures to isolate your subject via shallow DOF.

On occasions where you cannot achieve correct exposure even with the smallest aperture and fastest shutter speed, ND filters provide a means. Reduced light intensity allows either a slower shutter speed or a larger aperture. A slower shutter speed can give moving objects a blurred result (i.e., motion blur) to create a sense of motion.

Shooting a waterfall through an ND filter is a revelation. The key to achieving that milky water effect is slow shutter speed.

Above, without an ND filter.

Above, with an ND filter.

You do not want to use ND filters when different objects are lit differently and you need to expose all of them correctly.

Should you buy a Neutral Density filter? If you shoot landscapes, you DEFINITELY need to have an ND or two in your kit. ND filters provide an effect you cannot reproduce in Photoshop.

Filters available from CamerasDirect