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
The LPA Battles competition was held last night at Axholme Camera Club, in Haxey Church Hall. I attended as the Louth Photographic Society representative, bringing the Battles prints with me. It was a closely fought contest with some excellent prints from Axholme, Barton, and ourselves. The judge for the evening was Steve Dormer, from Doncaster, and he was very relaxed in his judging of the prints, providing excellent critique on each print. At break time, Steve had held back a total of 13 prints for further assessment and award of higher points, Axholme, and Louth had 5 each prints held back, with Barton 3 prints. The attached photo shows all the held back prints on the print stand. At the end of the evening Axholme took 1 st place with, 266 points, Louth came 2 nd with 258 points, and Barton 3 rd with 251 points. As the prints were taken off the stand, one print remained, it was a Louth print titled “Sleeping” by David Whitehouse, this print was awarded 20 points, and the judge Steve was very taken with the image, well done David.
Good Morning All Last week members walked along the Swaby valley, we have done the walk before, but it is now very grown up with nettles, grass and brambles. Not put off Chris led us through the valley to Swaby, there were six of us plus three more who caught us up in Swaby. The butterflies we saw as I am finding out this year just don't want to settle but we did manage some photographs before returning and going for a drink in the Massingberd Arms. This week there is an LPA Battle at Axholme on Wednesday starting at 7.30, if anyone would like to attend, please email me for details or look on Axholme's website. We are not having a meeting in Louth this week, probably as well the way the football went last night and with the next match on Wednesday evening. Regards Graham
Last night our members enjoyed a visit to the Swaby valley nature reserve. We crossed over two fields from South Thoresby, then followed the course of the Swaby Beck chalk stream through the valley, the public footpath was bounded by dense vegetation that hindered our progress at times. There was plenty to see around the reserve, myriad butterflies flitted around in a very lively manner, making for difficult photography, I spotted several small grasshoppers, and soldier beetles in the long grass, I tried to photograph a ladybird, but it was gone by the time I focussed on the creature. It was a very enjoyable evening, with good friendly company, and our camera skills were needed for the macro photography, these are a few of my photos from the evening.
Good Morning All Last week's outing was re-scheduled due to a clash with a certain world cup match, the good news was a victory for England, however we intend to still hold the outing on Wednesday 22 nd July. I did go to the Festival of Flight at the Shuttleworth collection, Old Warden last weekend. It was too windy for some of the old Biplanes, but we were treated to a four-and-a-half-hour display featuring many planes that I had never seen fly before. The De Havilland Comet was built to compete in the 1934 race from Britain to Australia, only three were built and it's good to see it flying, the aircraft design was quite advanced for 1934. This week, Wednesday 8th July, we have another camera club outing to the Swaby Valley, meeting at 6pm. Swaby is just off the A16 heading from Louth to Skegness. Regards Graham
Last night’s Louth Photographic Societies outing to Kirkby gravel pits nature reserve was cancelled as some members preferred to watch some sporting gentleman by the name of Harry Kane. (Congratulations to the England team) The reserve is small, and simply laid out, a public footpath leads along a tree lined avenue to the river Bain at the back of the reserve. Historic sand and gravel extraction has left a large lake in the centre of the reserve, several ducks and waterfowl inhabit the lake and there is a bird hide on the shoreline. Returning to the road, there is a path running parallel to the road bounded by a hedge, insects of various types can be found in this area, damselflies, butterflies, and hover-flies are often prolific. Our outing to Kirkby Gravel pits is now scheduled for Wednesday 22nd July. Parking is on the verge by the roadside. Kirkby Gravel pits are fairly close to where I live, and it’s reserve that I often visit, these are some photos from a visit in July last year.
Good morning, All, Last week at the club we had our monthly print competition, Trees and Open. There was a very good entry with a high standard of photography on show. Tony Gaskins was first in the Open and I managed a first in the trees, all the results are on our website and there is also a blog on the evening. This week, Wednesday 1st July, we are having an outing to Kirkby Gravel Pits, if we meet at 6.30 at the reserve, though you can arrive earlier as there are usually dragonflies etc around the entrance. If we walk straight ahead first, then anyone who arrives late can find us. I have struggled to find a map, but you can see the location on the Lincolnshire wildlife Trust website. Kirkby Grave Pits is also a good reserve for birds. Tattershall Road 1 mile south of Kirkby-on-Bain Woodhall Spa Nearest postcode LN10 6YN. Please note - postcodes are for the nearest registered address as we are unable to get postcodes for nature reserves.
A print competition was held last night with the categories of "Trees" as the subject, and "Open". Our judge for the evening was Gail Wrigg from Barton who did an expert job. The first category to be judged was the "Open" with the high standard of prints making the choice of top placement difficult, and Gail held back 9 prints for further evaluation. There were some chuckles from members around one of Richard Hildred's prints of wooden electricity poles titled "In my previous life I used to be one of them" an "in" joke from Richard, which totally baffled Gail. After the break Gail judged the "Trees" category, with 7 prints held back this time for awarding of higher marks, the beautiful colours of some trees, especially the autumnal images contrasting with the stunning monochrome infrared prints from Richard Hildred. It was a great evening, with excellent prints on display, and the website Competition page has been updated with the full results.
Good Morning All Last week we visited the nature reserves at Fir Hill quarry and Muckton wood. I have written a blog report on the evening which is on our website. You can always try something a little different like I did using camera movement. This week, Wednesday 24th June, a print competition will be held, with the categories of "Trees" and "Open" and is to be judged by Gail Wrigg. The following week we have an outing to Kirkby Gravel pits. If you want advice on camera settings etc the outings are the perfect opportunity to ask other members while we all have our cameras. Regards Graham
Each year Louth Photographic Society begins a new project, one photo taken each month on a specific subject, that's 12 photos presented on "Project Night" in January. Last year the subject was "Prime Time" one photo each month taken with the same fixed focal length lens. This year the subject is water, the subject sounds easy enough, and I've completed 5 photos already, but I'm struggling a little with a photo for June. I'm currently on holiday in Corfu, an island surrounded by water which should be easy, but I'm yet to find the "definitive" photo for the project. These are my photos so far this month: Best regards Derek
Good Evening All
