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
A print competition was held last night with the categories of; "Monochrome" and "Open". Diane Seddon from Cleethorpes Camera Club was the judge for the evening, Diane is very skilled, and thorough in her judging, providing useful insight, and critique of the prints. Many of her comments centered around the best cropping of an image to optimise the subject within the frame, and giving the subject "room to breathe" in the framing. Congratulations to David Whitehouse placed first in the "Monochrome" category, with his superb rendering of a face sculpture, and Dave Turner placed first in the "Open" category with his stunning print of the sunrise over Whitby Harbour. Thanks to Diane Seddon for a great evening, and as she remarked at the end of the competition all the prints were of an extremely high standard. The Competition Page has been updated with the highest placed images, but currently the score sheets for the competition are unavailable.
Good Morning All Last week Dave Bennett came to the club again, it has been a while since we have seen his photography but well worth the wait. Dave's photographs were excellent and his story telling along with AV's made it a very good night. This week, Wednesday 15th October, we are holding a print competition, with the categories of "Mono" and "Open", our judge for the evening will be Diane Seddon AFIAP CPAGB BPE3* she is well qualified to judge our prints. The following week we are going to try a retro night again, with Transparencies taken prior to the year 2000, please let me know if you want to take part and have a time slot allotted, time will depend on the numbers wanting to take part. Regards Graham
Last night Dave Bennett came to the club to show us AV's and narrate other sets of photographs on a theme. Dave started with photos along the Humber and then carried on the evening with AV's on nature, flowers and transport as well as talks and photographs showing us Grimsby docks including detailed photographs of fish smoking and engineering. Dave's photography is of a very high standard with strong colours, lot's of use of depth of field and they tell a story. A great night, thank you Dave.
Good Morning All Last week at the club we held a local PDI competition with a category of "5 Minutes From Your Home", we had some very good entries in each category with myself, and Dave Turner judging on the night. All the results are on our website, but well done to Derek for his mobile phone picture of a remembrance seat in Horncastle and Harry for his hellebores in the "Open" category. This week, Wednesday 8th October, Dave Bennett is coming to the club to show us what he has been taking lately, Dave does quite a bit of AV so as usual it will be a very good night. A reminder, send your print titles for the next print competition to Dave Turner by midnight Wednesday and bring your prints in this Wednesday night as well please. If you have any more N&EMPF Exhibition PDI entries send them to me by midnight tomorrow (Monday) please. Regards Graham
Our PDI "Local" competition was held last night, with the categories of "5 Minutes From Your Home", and "Open". Dave Turner, one of our members began the evening with judging the "Open" category, always a difficult category in my opinion because of the variety of subjects entered into the competition. Dave had previewed the images on his home computer, and had notes on each of the images, he provided excellent feedback on each image. After the break, Graham Harrison began judging the "5 Minutes From Your Home" category, this was a very unusual subject category which stretched the creative ingenuity of members, and it was particularly interesting to see what members had submitted. Graham judged the entries very fairly, and provided very good critique of the images. The standard of images submitted was high, which makes judging a competition such as this difficult, and as always is subjective, but a big thank you to both judges, it was an interesting evening. The first placed images, from each category are shown on this post, "At the Going Down of the Sun" by Derek Smith, "5 minutes from your home", and "Hellebores" by Harry Kerman in the "Open" category. The website Competition Page has been updated with the top three placed images, and the scoresheets.
Good Morning All Last week at the club member Tony Gaskins gave us an excellent presentation of his recent trip to Kenya on safari. Tony showed us the animals and birds he had encountered on his trip. It was an evening of excellent wildlife photography as well as a very informative talk, thank you Tony. There is also a report on our website blog. This week, Wednesday 1st October, we are holding a local PDI competition, with the subject categories of "5 Minutes From Your Home" and "Open", Dave Turner has kindly offered to judge the Open with myself the subject, and you have until midnight tonight to enter. Dave Turner is also asking members to send him the titles of their prints for the next print competition, the categories for this are "Mono" and "Open". Regards Graham
Our member, Tony Gaskins presented us with an illustrated talk on his visit to the Kenya game reserves last night. Tony's talk was perfectly timed, and his description of the trip held our attention throughout the evening, the images he captured were indeed stunning. The variety of wildlife he captured with his camera were amazing, all of the photos of birds were incredible, and the fact that they didn't fly off like they do here in the UK. On Lake Naivasha Tony captured a sequence of a fish eagle swooping down to catch a fish, just look at the way those deadly talons are outstretched to capture the fish. The lodge Tony stayed in at Tsavo West was beside a watering hole, look at this capture of a Kirk's Dik Dik, taken from the balcony of the lodge. Images of Hippopotamus, Lions, Impala, Ostrich, Giraffe and Zebra kept us enthralled through the evening. Thank you and well done Tony for an interesting evening.
Good Evening All A little late as just got back late this afternoon from the golfing trip, I didn't take my camera other than phone as I wouldn't have really had time but there was some interesting wildlife about including Swallowtail butterflies which was a first for me. Last week you had the monthly PDI competition, Long Exposures and Open, Derek has done a blog on our website and the competition page has all the results. Some excellent photography on show judging by the top places. This week, Wednesday 24th September, we have member Tony Gaskins entertaining us with one of his talks, Tony is well travelled so as usual it will be an excellent evening of photography. Regards Graham
Last week we held our first PDI competition after the summer break, and what a superb evening it was. There were some excellent images submitted into the competition, and over half of the submitted entries were "held back" for awarding higher points, Tony Houlden was the judge for the evening. This week, Wednesday 24th September, Tony Gaskin will be giving an illustrated talk on his recent visit to the Maasai Mara game reserve in Kenya, it will be an interesting evening, with some stunning photography. Harry will be taking entries for our next PDI competition, with the categories of "Local 5 minutes From Your House", (in this case the "Local" applies to your house, not the 25 miles from Louth) and "Open". Harry is also taking entries for the LPA Competition, he's looking for 6 entries in this competition, from each member. I've been out, and about as usual, this photo is of an abandoned garage forecourt on the road to Spilsby.
This was our first competition after the summer break. The categories for the competition were "Long Exposure", and "Open" and our judge for the evening was Tony Houlden. We began the evening with the "Long Exposure" category, something of a surprise in this section were the amount of night time images, more typically were the images of flowing water, either of the sea or in rivers, Tony judged these images skillfully, and with his usual sense of humour. Dave Turner was placed first with his excellent "Saltburn by the Sea Morning", Keith Rossington took second place with his beautiful night time image of the river at York "From the Bridge", Harry Kerman showed the "River Dee, Cairngorms, Scotland" the flowing peat laden waters stilled in this carefully composed image. After the break, Tony began the judging of the "Open" category, I always think this is the more difficult category to judge because of the variety of images presented, Tony judged these thoughtfully, providing excellent critique. Dave Turner took first place with his monochrome view of "Scarborough Bay", I was pleased to take second place with "Sunset Jetty" taken on my holiday in Corfu, and third place with my monochrome "Haworth Main Street". It was an excellent evening, with some amazing images, Tony's critique was very helpful, highlighting unwanted distracting items in an image, haloing from excessive sharpening, and cropping errors. Thanks to Tony for judging, and Dave Mann for the refreshments.
