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
Good Morning All Last week at the club we held a local PDI competition, Wildflowers and Open, there were some very good entries and especially well done to Keith Rossington who was placed first in both categories. The results are on our website. Thank you to Dave Turner who judged the Open, the first time he had judged live, and he did an excellent job with plenty of helpful comments and some good advice as regards processing our images. That was the last meeting at Church House until September as we now begin our summer break. There is however an LPA Battle, third round at Cleethorpes camera club next Tuesday, It is at 7.30 in their clubroom which is between 49 and 51 Reynolds Street DN35 7TU. Parking is best by the tennis courts. Regards Graham
Last night we held our last competition before our summer break. The PDI (Projected Digital Image) competition had the categories of "Local Wildflowers", and "Open". As this was a members competition, Dave Turner judged the "Open" category, this was his first time judging a competition, he had previewed the images, and made notes, and he gave some excellent critique, which will help members when submitting images in future competitions. After the break, Graham Harrison judged the "Local Wildflowers", a couple of the submitted entries were excellent, but unfortunately did not meet the category criteria so lost some points, flowers always make a good subject for a competition, and Graham had quite a task in judging the images. It was a superb evening with some amazing images, Keith Rossington took first place in both categories, the images he submitted show how much time, and effort he takes with his photography, well done Keith.
Good Morning All Last Sunday I went to the Armed forces day at Cleethorpes, there were not so many people about as I think most wanted to see the Red Arrows on Saturday. There was an excellent display from a group of first world war re-enactors who were mounted and in the uniform of the Lancers as well as a limited air display the highlight been the Spitfire. Last week was also the run for life in Louth, Sam from Colorgrafix did the run in memory of Paul Malley and raising money for cancer research. Sam asked the club for sponsorship so we sent her a donation from us all. Last Friday 21 members and partners enjoyed a meal at the Royal Oak, Cawthorpe, it was good to be able to socialise with each other with a few drinks and a very good meal. I hope everyone had a great night. This week, Wednesday 9th July, we are holding a PDI (Projected Digital Image) competition, with the categories of "Local Flowers in Their Environment (25 miles limit)" and "Open". I am going to judge the Flowers with Dave Turner judging the Open. Regards Graham
Most people are aware of “writers block”, where a writer sits staring at a blank page, and can’t think of a story to write, well photographers suffer from a similar affliction, with a camera sitting on a shelf, and we can’t think of anything to photograph. I believe that change in one way or another can invigorate our photographic creativity, one of our members renowned for his seascapes, has begun taking photos of birdlife, creating some stunning images. My favourite photographic genre has been in fashion and portraiture for the past 6 years, but I’ve lost the “joy” in the genre, so now I want to take more landscape, and macro photography, varying my style. I have just changed my camera system, and now I’m excited to try out the new lenses and watching endless YouTube videos to learn about the new camera system, as well as getting out, and trying the new techniques, these are some of the images from my new camera system. Another method of invigorating your photography is to start a project, think of a subject, and build a portfolio of images, watching how your photography improves as you take each image over time. Joining a camera club is another way to invigorate our photography. Many of our members have stated that their photography has improved since joining, being inspired by the talks, workshops, and regular competitions. The competitions are important both from a learning perspective, but also from the challenges in the subject matter. Recently I was dashing around trying to photograph “Wildflowers in the Environment” for our next competition, a lot of the roadside verges had been mowed reducing the wildflowers locally. Next, I tried some churchyards, there was some “Ladies Bedstraw” flowers in Scrivelsby churchyard, but it didn’t make for a convincing photo, I found some wildflowers along a footpath that did work. I did find another convincing photo in a local church yard, but still I had only two out of three photographs, and the deadline for competition entry submission was fast approaching. Driving out of Horncastle I spotted an ideal wildflower candidate, so quickly turning the car around, parking on the verge, and laying flat on the ground I was able to take one of the best images of the three, and with a lens that I wouldn’t normally associate with this style of photography. In conclusion, dust off that camera, take some photographs, and rediscover the simple joy of photography.
We had a really good turn out for the visit to Rimac last night and the weather was fantastic for taking wildlife photographs. There was plenty to photograph from moths and butterflies to many different birds, there were also plenty of wildflowers as well. I think we all took plenty of photographs so the competitions should have plenty of entries this year. We spent just over two hours photographing and walking round then some of us visited the Prussian Queen in Saltfleetby, where we were made to feel welcome and quenched our thirsts.
Paul Malley, a gentle, genial, giant of a man sadly passed away recently after a long illness. Paul had been a longtime member of the Louth Photographic Society, and had served in several roles, Treasurer at one time, and latterly as a stalwart active member on the Committee. Always ready with helpful knowledge, and advice, Paul was calm in a crisis, and insightful, able to understand both sides of a discussion, and suggest a compromise. An outstanding photographer, dedicated to his craft, he produced some excellent images, some of which are shared here. Rest in peace my friend, you will be much missed by Pam, family, friends, and members of the Louth Photographic Society.
Good Morning All Last week at the club we held our monthly print competition, there was a good entry of just over 40 prints and of a very good standard. Tony Gaskins won the subject, Britain's Coastline with an excellent photograph of Whitby and Dave Tuner was 1st in the Open with an excellent misty morning photograph at Buttermere. Thank you to all that entered, and all the results are on our website. This week, Wednesday 25th June, we are going to Rimac near Saltfleet, I have heard there is quite a bit of wildlife there at the moment, let's hope we are lucky. If we meet in the car park at 6.30pm and set of as a group though we usually get stretched out as we go. I have attached a map, but if you are on a corner and there is a track going towards the sea at the area shown on the map you should be ok. Regards Graham
Our print competition with the categories of “Britain's Coastline”, and “Open” was held last night. Gail Wrigg, the LPA President, was the judge for the evening, and there were some stunning prints submitted into the competition to review. The images were judged fairly, majoring on the technical imperfections of the images, depth of focus, composition, elements within an image that could have been removed, and improved cropping of an image. Gail always connects with the audience with her critique of the submitted competition entries. It's surprising how the print medium looks, and feels so different from an image viewed on a screen, just very alive, and tactile. The competition results have been updated on the Competition page of our website.
Good Morning All, I visited Welton-Le-Wold nature reserve the other evening, a bit like a small Red Hill, there were a lot more wildflowers there than Red Hill but again not many insects, it may be better in the daytime? It is only about two miles from Louth as well. Last week was the LPA battle at Scunthorpe, only Dave Turner went taking our prints with him, not a good result for Louth, 1st Grimsby 258, 2nd Scunthorpe 245 and 3rd Louth 236, quite a difference from the first round but that's LPA battles. The next round is at Cleethorpes, it would be good if a few more members could attend. It will soon be our annual dinner at the Splash, any more deposits this week please and menu choices by midnight Wednesday please, otherwise you may go hungry. This week, Wednesday 18th June, we hold our monthly print competition, with the categories of “Britain's Coastline” and “Open” there has been a good entry so should be a good night. Regards Graham
Good Morning All Last week we visited Red Hill nature reserve, a very good turnout, but the wildlife didn't seem to know we were going, it's quite worrying though the lack of any butterflies or any other insects of any sorts. Hopefully we all got some photographs before we visited the Three horseshoes at Goulceby. I did photograph an unusual caterpillar but couldn't manage to identify it? This week we are away in the LPA Battles at Scunthorpe, for anyone planning to attend it is at the Pavilion, Bramley Crescent, Bottesford DN16 35N this Wednesday 11th at 7.30pm. I don't think I will make it due to work but hopefully some members will attend. There will be no meeting at Louth this week as members will be a way at Scunthorpe. The following week, Wednesday 18th June, we are holding the monthly print competition, if you can email your titles to Dave Turner by midnight Wednesday 11th if you have not already and bring your prints in on the night by 7 pm as no one is there to collect them this week. There are still a few deposits for the meal to come in yet, but in two week's time will be ok. If you can send your menu choices to me if you are attending by midnight 0n the 18th June please, as they need typing out and sending to the Splash two weeks in advance. Well, I think that's all, Regards Graham
