Focus on Focusing
Derek Smith • 1 June 2022
Digital cameras use different methods of autofocus to obtain sharp focus on the subject.
DSLR cameras use a phase detect focus module below the sensor to compare light from a sub-mirror split into two separate images by prisms and micro-lenses in the AF sensor unit, each aimed at two lines on the AF sensor corresponding to the active AF point. If the images hit the two lines precisely, the subject is in focus, any distance between the two images tells the camera how much the lens is out of focus, the lens is driven until the images match, and accurate focus is thus achieved. In “Live View” many DSLR cameras use the contrast detect focusing pixels directly on the sensor.
Mirrorless cameras can use a contrast detection focus method, a phase detect autofocus, or a combination of both called a hybrid autofocus system, but without mirrors, or prisms, all the focus points are located directly on the sensor.
Contrast detect autofocus is the simplest system, it compares contrast areas within the subject, and moves the lens focusing elements backwards, and forwards until the highest contrast is achieved. Contrast autofocus is extremely accurate, but can be slow.
Phase detection autofocus (PDAF) pixel pairs are incorporated into the digital sensor and consist of partially covered sensors receiving light from either side of the lens, left, and right pairs. The camera compares both, and when they match, knows that the subject is in focus. If the subject is out of focus, it calculates how much it needs to adjust the lens to bring the subject into focus and drives the lens there.
Hybrid autofocus uses the phase detection to achieve close focus and finalises accurate focus with contrast detection.
Now, knowing all the technical details of camera autofocus is good, but how best to employ that knowledge? The best advice I can give is to get to know the various settings of autofocus that your camera has, and I shall use some images to illustrate those settings that I use.
Portraits – a lot of digital cameras have “face and eye” detect modes, and I’ve used this mode to good effect for photographing this model. My camera model has four autofocus selections, single point, zone, wide & tracking, and finally “all”, for this shot I used the wide & tracking selection, with the “face and eye” detection turned on to focus accurately on the eyes. Marika was photographed with the Fuji X-T20, and the XF50mm f/2 (75mm ff) lens f/4, 1/80, ISO 400
Derelict building – for this shot I used single point autofocus, I was taking practice images with my new Fuji X-T3 camera, and the XF35mm f/2 (50mm ff) lens, f/11, 1/52, ISO 1000. This photo is a technical failure, not being as sharp as it could have been, I needed a faster shutter speed, and should have either widened the aperture to f/8 or increased the ISO to 1600 to eliminate the shutter shake.
Robin – I was very pleased with this image, taken with the Fuji X-T3, and the XF70 – 300mm (105 – 450mm ff) using single point autofocus on the eye of the bird. Photo details: 434mm ff equivalent, f/8, 1/280, ISO 800.
Cadwell – my first outing with the Fuji X-T3, and the XF70 – 300mm lens, using the wide & tracking selection, and continuous autofocus, following the bikes with the camera as they come past, a technique known as “panning”. Photo details: 126mm ff equivalent, f/5.6, 1/300, ISO 200.
All images were captured in Aperture Priority mode, with the aperture and ISO adjusted to give the correct shutter speed (hopefully) for the scene. I very rarely use the “All” selection of autofocus as the camera focus can sometimes lock onto unwanted areas in the frame, a blade of grass in the foreground of a landscape perhaps, or the nearest tree. For a landscape scene, I usually use a single point autofocus point and focus a third of the way into the scene.
For macro images autofocus can be of use, but many photographers prefer to use manual focus, be aware that many lenses are “focus by wire”, which means that the lens focus ring does not control the focusing element within the lens directly, so there may well be a lag in obtaining focus. I find that manual focusing aids such as magnification, and / or focus peaking work well, and instead of turning the focus ring “rock” the camera physically slowly back and forwards until sharp focus is obtained.
DSLR cameras use a phase detect focus module below the sensor to compare light from a sub-mirror split into two separate images by prisms and micro-lenses in the AF sensor unit, each aimed at two lines on the AF sensor corresponding to the active AF point. If the images hit the two lines precisely, the subject is in focus, any distance between the two images tells the camera how much the lens is out of focus, the lens is driven until the images match, and accurate focus is thus achieved. In “Live View” many DSLR cameras use the contrast detect focusing pixels directly on the sensor.
Mirrorless cameras can use a contrast detection focus method, a phase detect autofocus, or a combination of both called a hybrid autofocus system, but without mirrors, or prisms, all the focus points are located directly on the sensor.
Contrast detect autofocus is the simplest system, it compares contrast areas within the subject, and moves the lens focusing elements backwards, and forwards until the highest contrast is achieved. Contrast autofocus is extremely accurate, but can be slow.
Phase detection autofocus (PDAF) pixel pairs are incorporated into the digital sensor and consist of partially covered sensors receiving light from either side of the lens, left, and right pairs. The camera compares both, and when they match, knows that the subject is in focus. If the subject is out of focus, it calculates how much it needs to adjust the lens to bring the subject into focus and drives the lens there.
Hybrid autofocus uses the phase detection to achieve close focus and finalises accurate focus with contrast detection.
Now, knowing all the technical details of camera autofocus is good, but how best to employ that knowledge? The best advice I can give is to get to know the various settings of autofocus that your camera has, and I shall use some images to illustrate those settings that I use.
Portraits – a lot of digital cameras have “face and eye” detect modes, and I’ve used this mode to good effect for photographing this model. My camera model has four autofocus selections, single point, zone, wide & tracking, and finally “all”, for this shot I used the wide & tracking selection, with the “face and eye” detection turned on to focus accurately on the eyes. Marika was photographed with the Fuji X-T20, and the XF50mm f/2 (75mm ff) lens f/4, 1/80, ISO 400
Derelict building – for this shot I used single point autofocus, I was taking practice images with my new Fuji X-T3 camera, and the XF35mm f/2 (50mm ff) lens, f/11, 1/52, ISO 1000. This photo is a technical failure, not being as sharp as it could have been, I needed a faster shutter speed, and should have either widened the aperture to f/8 or increased the ISO to 1600 to eliminate the shutter shake.
Robin – I was very pleased with this image, taken with the Fuji X-T3, and the XF70 – 300mm (105 – 450mm ff) using single point autofocus on the eye of the bird. Photo details: 434mm ff equivalent, f/8, 1/280, ISO 800.
Cadwell – my first outing with the Fuji X-T3, and the XF70 – 300mm lens, using the wide & tracking selection, and continuous autofocus, following the bikes with the camera as they come past, a technique known as “panning”. Photo details: 126mm ff equivalent, f/5.6, 1/300, ISO 200.
All images were captured in Aperture Priority mode, with the aperture and ISO adjusted to give the correct shutter speed (hopefully) for the scene. I very rarely use the “All” selection of autofocus as the camera focus can sometimes lock onto unwanted areas in the frame, a blade of grass in the foreground of a landscape perhaps, or the nearest tree. For a landscape scene, I usually use a single point autofocus point and focus a third of the way into the scene.
For macro images autofocus can be of use, but many photographers prefer to use manual focus, be aware that many lenses are “focus by wire”, which means that the lens focus ring does not control the focusing element within the lens directly, so there may well be a lag in obtaining focus. I find that manual focusing aids such as magnification, and / or focus peaking work well, and instead of turning the focus ring “rock” the camera physically slowly back and forwards until sharp focus is obtained.
Marika
Photo By: Derek Smith
Falling Down
Photo By: Derek Smith
Robin
Photo By: Derek Smith
Cadwell Park
Photo By: Derek Smith
Louth Photographic Society
Last night, Steve West gave a fascinating illustrated presentation on the long history of cinema, and stage plays in Louth. In the late 1890s some films were shown at various locations, in 1910 a “tent” cinema was showing films on Westgate Fields. During 1913 a site was purchased of 2 retail units, on Eastgate, the retail units were demolished, and the Palace cinema / theatre was constructed on the site ready for opening in 1914. During the interwar years many plays were staged at the Palace, and Steve has collected many of the original programs from this era. The current Playhouse was constructed in Canon Street in 1921 in the “art deco” style, some poor quality housing was demolished to make way for this new cinema. The Playhouse, and the Palace both continued in operation until 1960 when the Palace was sold, and the building was converted into a Fine Fare supermarket. During the 1970s when Bingo became popular, part of the Playhouse was used as a Bingo hall, with the reception area restyled to suit the new usage. The Playhouse was converted into a multi-screen cinema when purchased by the Parkway Cinema group in the mid 1990s. Louth is extremely fortunate to have retained a viable cinema in the town, many thanks to Steve West for a very polished presentation.
Good Morning All Sand racing starts again this week at Mablethorpe, this Sunday is practice day, times are on their website. Last week at the club we held a local competition, with the categories of: "Straight out of the camera" and "Open", there was a very good entry in each subject with Tony Gaskins having the task of judging the unedited ones and Robin Begley judging the Open. There is a full report on our website but well done to Harry for first place in the Straight out of the camera and Keith Rossington for first place in the Open. This week, Wednesday 14th, Steve West will be our speaker, talking and showing PDI's on the subject of the Louth cinema. The following week will be a print competition, with the categories of "Mono" and "Open", you need to bring your prints in this week and send an email to Dave Turner with your titles. Regards Graham
At our meeting last night we held a PDI (printed digital image) competition, with the categories of "Local Unedited", and "Open". As this was a members competition one of our members judged the competition, Tony Gaskins was our first judge for the "Local Unedited" category, these were .jpg images taken straight from the camera, without any editing, apart from the resizing for projection. Presenting an image such as this in a competition is quite a difficult task, the photographer has to ensure that the correct exposure is chosen, the composition, and subject matter has to be precise, everything done in camera. Normally for competition entries, a degree of post processing would be involved, simple edits like toning down highlights, or even removing extraneous objects from a scene, emphasising a moody sky, all not allowed. Tony Gaskins did an excellent job with his judging, providing clear and concise critique of the images presented, he "held back" 8 images for further consideration, awarding higher points, and finally choosing the top three placements. After the break it was Robin Begley's turn to judge the "Open" category, as these were post process edited images the standard was higher than the unedited images, but surprisingly not markedly so. Robin is potentially hoping to become a N&EMPF approved judge, and he had a different style, able to suggest edits that could have improved an image, again his critique was clear and concise communicating well with members. Robin chose 8 images for further consideration, awarding higher points, and finally selecting the top three placements. It was a fabulous, and interesting competition with excellent images, many thanks to both our judges, it's not an easy job, probably nearly as bad as being a referee in a football match!! Many thanks to both our judges, well done.
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Roger Hance Memorial Lecture 16th October
Last night Lee Dixon from Cleethorpes Camera Club gave his first ever presentation to a camera club, which we were privileged to view. His presentation consisted of portraiture, creative, and his genre of choice; wildlife. Lee is an extremely talented photographer, with a highly creative portfolio, his portraits are quite atmospheric, unusual, and capture the essence of the model, and scene. For Lee’s creative work, he buys props from charity shops, often very cheaply, reusing those props in various scenes. For his composite work he will often take one of his photos of a room, and blend it with a subject in Photoshop using “blend” modes. For wildlife he visited Lake Kerkini in Greece to photograph the Pelicans, if the sky is a little on the bland side, Lee will often over expose the image resulting in a striking white background to the subject. On safari in Africa Lee showed us some amazing images of the wildlife, the close-up images of elephants, the wildebeest migration, and this stunning image of a cheetah. Lee was ably assisted by Mike Bennett. More of Lee’s work can be found on his website: https://leedixonphoto.co.uk/ Lee his kindly given permission for his images to be used in this post. Many thanks to Lee, and Mike for an amazing evening.
Good Morning All Last week at the club we held our monthly PDI competition, with the categories of "Objects in the Air" and "Open", there was a very high-quality entry in both competitions and judge Mike Bennett had quite a job on his hands scoring the entries. Well, done to Dave Turner for first place in the Objects in the Air and Tony Gaskins for first place in the Open. All the results are on our website. This week, Wednesday 30th September, our speaker will be Lee Dixon from Cleethorpes Camera Club showing us his natural history, Creative and many other photographs he has taken. It will be the first time Lee has shown his photographs at our club. If anyone else wants to enter PDI's in the 2027 N&EMPF Exhibition, the deadline is this Wednesday at midnight. Harry is still taking entries for the PDI competition, with the categories of "Local Straight from Camera" (unedited .jpg), and "Open", to be held on Wednesday 7th October. Regards Graham
At the meeting last night Mike Bennett judged our competition with the categories of “Objects in the Air”, and “Open”. The standard of entries was excellent, and it gave Mike a tough task of judging the images, Mike gave some very good critique of the images displayed, the feedback helps members to develop compositional skills. Dave Turner was awarded first place for his image of the “Tiger Club Display Team” in the “Objects in the Air” category. Tony Gaskins was awarded first place for his image of the “Spectacular Rustgill” in the “Open” category.
Good Morning All, Last week at the club we watched the N&EMPF PDI Exhibition for 2026, it was a very good evening viewing very high standard images from all over the East Midlands. Dave Turner and I did manage to get images accepted; there is a full report on our website. This week, Wednesday 23rd September, we will hold our monthly PDI competition, with the categories of "Objects in the Air" and "Open", please send entries to Harry by midnight tonight. Harry is also accepting images for the PDI competition, with the categories of "Local Straight out of the Camera (only resizing permitted)" and "Open". Regards Graham
Members were shown the 2026 N&EMPF PDI Exhibition video last night with all of the accepted PDI images in the first part, and after the break the awarded PDI images. There were some stunning images on view, and Louth was successful in having 3 PDI images accepted, Graham Harrison with his image of a Stonechat, and Dave Turner with his portrait of Paul Rudd, and his monochrome image of Kings Cross railway station "Going Down". Overall it was a good inspirational evening.
