OK, so you have just bought yourself a nice new digital camera with super duper auto focus, so you can skip this step, right? WRONG!
Autofocus cameras have made getting well-focused photos much more reliable and indeed, often more precise than the human eye can consistently achieve in many situations, BUT there are many situations where the AF will be fooled or just fail to achieve a focus lock such as:
lack of subject contrast:
most AF mechanisms rely on detecting contrast in the subject, often only vertical and thus need vertical lines to be present.
unfortunately, faces tend to be low contrast subjects and most AF cameras will preferentially focus on anything else in the picture but not the face - a major dilemma as for portraits, one usually needs the eyes to be the most sharply focused part of the image.
the lower the light level, the longer it takes for most AF mechanisms to focus & thus the longer the image must be kept still in the viewfinder before a lock is obtained. Moving the camera or having a subject who moves will often result in failure of focus lock.
many cameras have a low light AF illuminator to assist but these usually only work for up to 2m or so.
a moving subject is difficult for most AF mechanisms, although high-end cameras often have a "predictive motion" AF mode (eg. Canon's AI Servo mode) that helps with this.
examples of history of capturing birds in flight with a 400mm lens at f/2.8:
manual focus - perhaps 1 in focus in 1,000 frames
Canon A2 35mm film with AF - perhaps 1 in focus in 10 frames
Canon EOS 1N 35mm film with 5 across AF sensors - faster initial AF acquisition, start AF on central AF point
Canon EOS 3 35mm film - 1st 45 point AF camera - better AF on erratic flight but seemingly poorer on central birds
Canon EOS 1D digital - 45 point AF cameras - 10-30% razor sharp images
success depends partly on:
ability to keep central AF sensor on the bird's eye in flight
keeping subject framed properly
ability to pan to match the subject's speed
ability to pan smoothly
adequate light and contrast on the subject part being focused
bright object in the image such as a reflecting surface, bright light or backlighting:
these will fool the AF mechanism nearly all the time, the camera usually targets the brightest object to focus on.
subject not in the centre of the image:
many cameras will focus on what is in the centre of the image, but this is not what is needed usually.
newer cameras have a number of AF points which the camera will try to guess which is the most relevant - but it is only a machine and can't always guess what you are thinking.
poor focus not only loses detail and gives a blurry image but reduces contrast making it flat looking & poorly tolerates enlargement.
understand depth of field so that you can better judge what to focus on and what aperture setting you need to achieve what you want.
it is much easier to blur a photo in PS than it is to increase its sharpness and detail - aim for as sharp as possible in the key subject region.
How to focus when your AF system is having trouble such as in low light:
1st thing to try is ensure your AF point is on a contrasty subject (eg. eyes and not skin) and hold the camera still while it tries to lock AF.
ALL current AF systems will fail to lock when subject lacks contrast and your lens aperture is too small (eg. f/5.6) for the light conditions to allow AF to be achieved.
Obviously pro dSLRs have more sophisticated AF sensors that are less likely to fail to lock and wider aperture lenses (eg. f/2.0-2.8) allow more light to hit the AF sensors.
Just accept this and be prepared to work around it.
there are two main methods I use:
use AF lock:
turn of continuous AF and set the AF trigger to a button on the rear of the camera INSTEAD of the shutter button
most dSLRs allow this via the menu system
this then allows you to aim the centre AF point at a bright contrasty subject at the same distance as your subject then LOCK AF (you will need to keep camera still while it is gaining lock).
using a lens with a wide aperture makes gaining AF lock more likely.
then when you take your photos, your camera will not try to AF each time.
NB. most modern lenses require you to re-focus if you change the zoom setting so make sure you have the lens at the desired zoom setting before locking AF.
use manual focus:
most modern dSLRs have limited help in the viewfinder for doing accurate MF, but some signal MF confirmation if you have the shutter button half-pressed or AF lock button pressed while you manually focus and focus is achieved - however, if this was possible, then you should be able to use the AF lock method above unless of course you are using a MF lens.
as a last resort, use the live preview live magnification feature of your dSLR - you did buy a dSLR with this feature didn't you?
set camera to MF
go to Live Preview mode
move the central marker to your desired subject then go into live magnification mode
adjust MF until it looks sharp
if you are finding that camera is moving to much, use a lower magnification (eg. 7x not 10x) or activate your image stabiliser momentarily (Olympus dSLRs with IS built in allows this).
Quality of focus or the apparent sharpness of the subject on the image:
this depends upon:
f/ratio & depth of focus of the optical system (see below)
accuracy of the focus
quality and resolution of the optical system
changes with f/ratio
limitations due to physical constraints of diffraction at small apertures
optical aberrations such as chromatic aberration will increase the size of the Airy disk
resolution, sharpness & contrast of film or sensor
relative subject movement during exposure
quality of seeing - variations of light refraction due to atmospheric changes during the exposure
eg. ever tried photographing a plane landing on a hot airport tarmac?
this comes into increasing importance the greater the subject distance is (ie. the more atmosphere one must go through), the greater the magnification used and when there are extreme adverse seeing conditions.
Auto-focus methods for digital cameras:
consumer & prosumer digital cameras such as the Olympus C8080WZ rely on digital auto-focus systems that utilise data from segments of the "film" sensor and then in an iterative process, computer algorithms determine the sharpness of the edges of an image and by re-imaging after adjusting the focus of the lens, eventually a set focus point is assigned. If it is unable to determine a focus due to either insufficient light or contrast, or by movement of the camera or subject, the camera usually opts for a default focus - in the case of the Olympus C8080WZ, this is about 3m.
digital SLRs tend to have a more advanced auto-focus detection mechanism which tend to be much faster and more accurate than the consumer and prosumer digital cameras, but even these still can fail to focus appropriately as outlined below.
TIP: if you have doubt about the focus, check the image on playback on the LCD screen and zoom in to 5x (10x is better but this is not available on most prosumers). If the subject looks sharp at 5x, then it probably will be adequately sharp for most purposes. If it looks a little blurry at 5x then it will probably be not sharp enough.
several issues that may result in poor focus:
insufficient light to set focus:
the "film" sensors in consumer & prosumer cameras, as sensitive as they are, are not nearly as sensitive as the dedicated AF sensors used in SLR cameras which are positioned behind the mirror.
low light situations thus prolong the duration of setting focus which is exacerbated by camera or subject movement.
many cameras including the 8080 thus use AF-assist lamps which are red lights which come on to light up the subject
unfortunately, this lamp may be obstructed by lens adapters and has limited range
some cameras can utilise the AF-assist lamps on certain external electronic flash guns
insufficient contrast to set focus:
most cameras require vertical lines on which to focus
subjects such as non-textured surfaces and smooth skin such as a person's face, can be difficult to focus on & thus focus may take a long time, not be set, or another subject chosen by the camera upon which to focus making studio portraiture with these cameras often difficult as the camera often focuses on the background behind the face, especially if it is textured and brighter than the face.
the AF solution is to use spot-focus & orientate the camera so that one has a vertical line in the spot focus region which is at the same distance as the desired subject.
TIP: if the camera insists on focusing on the background of a portraiture, if you can't go to accurate manual focus, consider temporarily holding a black cardboard behind the subject while focus is being obtained, or decrease the light on the background (although this may not help if using an AF-assist lamp which hits the background).
camera chooses the wrong subject to focus on:
in spot focus mode this may occur due to either:
not setting focus with the spot focus region centered on the desired subject (ie. one should move the camera to the correct position, half-press the shutter button & hold it there to set the focus, then recompose before taking the photo).
if using a tripod with a stationery subject, most cameras allow you to move the spot focus region around & thus avoid the necessity of the above procedure, but this then requires adjusting it for each variation in composition.
insufficient subject contrast as outlined above
bright specular highlights in front of or behind the subject
here is an example of the autofocus problem as demonstrated using the Olympus C8080WZ:
subject at 2.5m sitting ~1.5m in front of a new synthetic moderately finely woven professional white background, auto-focus via spot focus on subject indoor in low-medium light (light from north window), but external flash used for exposure.
ISO 100, f/6.3 with focal length 36mm (equates to 140mm in 35mm film)
note that no matter what I did, I could not get the camera to focus on the subject's face, while the brighter white background was behind her, choosing different parts of face did not help, nor did changing to iESP mode.
see this image for a 100% crop of a section which shows face out of focus while weave on background is clearly evident (I changed the RAW development to darken the background using Photoshop's vignette setting).
I was able to resurrect a similar image by applying Gaussian blur to background and a gentle soft filter to main subject sparing her eyes which had an unsharp mask applied - see here
in future I will use a black background temporarily while I set focus and save as manual but this means it will have to be refocused if subject moves or if I change zoom.
"intelligent" AF modes (eg. iESP for Olympus):
these modes attempt to analyse the image to select the most appropriate object to focus on, this will often be the brightest, closest object with most contrast - unfortunately this may not be the subject, but for most "snap" shots it works, although often relying on the usually wide depth of field of consumer digital cameras, and the fact that the images are usually not enlarged significantly to show the inaccurate focus.
camera AF is not aligned properly:
this causes it to focus behind the subject (back focus) or in front of the subject (front focus)
in addition to the CCD sensor AF detection, the Olympus 8080 has the ability to use a 2nd option to help with setting AF, passive range-finding phase detection via a special AF sensor located above the lens:
Olympus E series SLRs:
sensors are located above the mirror
metering and WB is via sensors above the mirror thus mirror comes down temporarily prior to exposure to determine exposure and WB in Live Preview mode B.
Canon EOS SLRs:
TTL-SIR (through-the-lens Secondary Image Registration)
Canon is generally regarded as being the leader in AF technology and this is the main reason it's film cameras became more popular than Nikon ever since the early 1990's, and remains so in the digital SLR world.
traditional 35mm film SLR cameras such as the Olympus OM series had to rely on manual focus and thus much effort was put into the design to ensure manual focus was as fast & accurate as possible:
bright optical viewfinder which displayed exactly what you were imaging through the lens
options to use a variety of dedicated focusing screens to optimise for various types of lenses such as:
the standard matte screen with central microprism and split image focusing devices for most situations
an extra bright clear screen for specialised purposes such as telescopes & microscopes
a manual focus ring on the lens that provided smooth focusing while indicating the precise distance at which the lens has been focused.
a depth of field indicator on the lens to indicate the approximate range in which the subject will appear to be acceptably sharp
a depth of field preview button on the lens that allows one to see how sharp everything will be at the selected lens aperture
an infra-red adjustment marker to allow for the different focus of infra-red light
enter the AF consumer digital camera world & almost all of these aids have disappeared making manual focus difficult & inaccurate:
the optical viewfinders tend to be small and without manual focus aids, although some are true optical TTL viewfinders (eg. Olympus E20 or the true digital SLR cameras)
the more expensive prosumer cameras with electronic viewfinders (EVF), whilst giving a TTL image, tend to be low resolution and again with poor manual focus aids apart from a digitally magnified central area which can be temporarily activated but still very difficult to use accurately.
the LCD screen is also difficult for the same reasons as the EVF
absence of manual focus ring on the lens, being replaced by a scrolling device on the camera back
worse still, most of the new AF zoom lenses have change in focus when you zoom in or out & thus one must set focus AFTER selecting your zoom position.
even worse, some cameras (eg. the 8080) will not manually focus at infinity at wide angle to mid-zoom focal lengths by scrolling the focus as the manufacturer compromises the lens design and assumes DOF will be sufficient in these situations (but unfortunately is not at wide apertures)
see below how to use Moire patterns to help MF on these cameras.
the Olympus E330 was the first dSLR camera to offer an excellent manual focus mechanism by using a 10x magnified live preview on an LCD screen in its B mode which makes it perhaps the best camera for macrophotography work or for photography using manual focus lenses.
personally, I would NOT buy an expensive prosumer camera again that did not have good, easy, accurate through-the-lens manual focus to over-ride those situations where AF fails, so I bought the Olympus E330, Olympus E510 & Canon 1D Mark III - all with Live Preview for accurate manual focus.
Moire pattern and manual focusing a digital camera:
Critical focus zone / depth of focus:
NB. this is a different concept to depth of field.
the critical focus zone is the maximum amount of error in lens to film distance for good focus
in other words, it is the allowable movement of the sensor or lens that allows the diameter of the cones of converging & diverging light to still be less than a given diameter (such as the diameter of the Airy disk).
it is ONLY dependent on the focal ratio (and pixel size if this is bigger than the Airy disk), and NOT focal length as one would expect
depth of focus = 2 x circle of confusion x f ratio
see Circle of Confusion (CoC):
the problem is that historic CoC are for film resolution, for digital astrophotography, the equation is changed by replacing CoC with the linear diameter of the Airy disk produced by the optical system (although in practice it should be modified by the quality of seeing as measured in arc-secs)
the equation for the linear diameter of the 1st interspace of the Airy disk is:
d = 2.44 x lamda x f ratio
where lambda = wavelength of light
if one uses lambda = 650nm, then d in microns equals:
2.2 for f/1.4; 3.2 for f/2; 4.4 for f/2.8; 6.3 for f/4; 8.9 for f/5.6; 12.7 for f/8; 17.4 for f/11;
the following formula is an approximation by Ron Wodaski by substituting the Airy disk formula using lambda of 450nm:
critical focus zone in microns = 2.2 x (f ratio)2
using this, we get:
|CFZ in microns at light = 450nm assuming CoC = Airy disk diameter||4.3||8.8||17||35||69|
|depth of focus in microns assuming CoC of 20 microns||56||80||112||160||224|
in practice, the real value is somewhere between these as the size of the Airy disk if enlarged by longer wavelengths, optical aberrations, subject movement and effects of poor seeing.
if your pixel size is greater than the Airy disc then pixel size should probably be used instead.
furthermore, Nyquist theorem shows that you need a pixel size less than half the Airy disk to image it adequately.
it is thus obvious from this that at wide apertures we need to be precise with focusing, and in some manual focus systems, the mechanism may not allow such precision with ease, hence astrophotographers often purchase electrically driven fine focus mechanisms to assist with critical focus.