Getting it Right in the Digital Camera : Focus

Showing posts with label Focus. Show all posts

Thursday, April 18, 2019

Introduction to Digital Photography, An Overdue Course Syllabus

Upper Garwin Falls, Wilton NH

Green River Falls, Guilford Vt

I am in the middle of my Introduction to Digital Photography Class at Keene Community Education. I have been offering this 4-week course, twice a year, for several years and I continue to be amazed at what seems to be an endless supply of local people who are desperate to learn about how to use their digital cameras. The class has never failed to be fully booked immediately after the course is announced.

The program includes 8 hours of class time and two photo shoots to nearby locations. Eight hours is not enough, but during that time I try to cover a broad range of topics. In the first class, I start with a discussion of the features offered by various classes of cameras and how to approach learning the mind-boggling array of features. I show how and where to upload and archive images and the advantages of different image file types (emphasizing RAW of course). It is a lot to pack into the first two hours.

Rockport Ma

In subsequent classes I cover the control of exposure, approaches to composition and the opportunities and challenges of shooting in various kinds of light. Somewhere in there I try to squeeze in discussions of image sharpness, focus and control of color balance.

Old Home Day, Chesterfield, NH

It is a lot to explain and in preparing my classes I have drawn heavily on the discussions I have written in many of my blog articles. My students often ask whether I have a course syllabus and I generally direct the to my blogs. Most of the information and examples are there, but it is not always easy to find. Included here is a list of links selected from over 400 blog articles that cover the key areas of my course. They are organized to roughly match the order of the classes. Hopefully these chapters will also provide a helpful introduction to the amazing world of modern digital photography for those who will never be able to participate in any of my classes.

Central Square, Keene NH

Nuts and Bolts

My first class is generally the most challenging for the students and myself as it includes such a broad range of topics. The relative advantages of large sensors and small sensors, lossy vs lossless file types, and the long list of approaches to image archiving are just a few of the topics covered.

Intro to Digital Photo Class, Spring 2018

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Exposure

Exposure is not just about the brightness of the image. How we combine Aperture, Shutter Speed and ISO also effects important aspects of the imaging, including depth of field, portrayal of motion and image noise.

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Composition

Lower Purgatory Falls, Wilton NH

The “Rules of Composition” should never be allowed to restrict your own creative vision, but they can provide some helpful starting points. The rules are all based on what looked balanced and beautiful to the eye of others, and what works for your eyes has equal validity. Picasso said “We all know that Art is not truth. Art is a lie that makes us realize truth”. Your compositions should be a path to realizing your own personal truth.

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Using Light

In New England, the light is always changing. Whether it is the warm beauty of the “Golden Hour” or the harsh brilliance of the midday sun, it is important to recognize the conditions, and plan your shooting to respond to the challenges and the opportunities that they provide.

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Sharpness and Focus

The apparent sharpness of and image is not only related to proper focus, but also the steadiness of the camera. These are important topics, but I never know where I will be able to slip them into my presentations. It usually depends on how many questions I must answer and how many images I end up critiquing.

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Final Thoughts

Here are some of my random thoughts about art, photography and the state of digital photography’s place in the world of fine art and media. It is the kind of “artsy” talk I pledged to never use when I started my blog more than 10 years ago.

Jeffrey Newcomer
www.partridgebrookreflections.com

Sunday, April 22, 2018

Aperture Size and Depth of Field, The Jesus Factor

Garwin Falls, Small Aperture / Deep Depth of Field

I often say that much of what I know about photography comes from teaching classes and writing my weekly blog. That is particularly true when I get asked questions that I can't easily answer.

Whence Cometh f Numbers?

f numbers, Where Dragons Dwell

A few years ago, I was observing one of Steve Hooper’s great photography class sessions at Keene State College’s Cheshire Academy for Lifelong Learning. A student came up to me after the class to ask a seemingly simple question, “Why do the f numbers form such an apparently random sequence, and why do they go in the wrong direction.

Of course, I have always known the f stop sequence. I understood that f/8 represented an aperture that is half the size of f/5.6, but I never felt that I needed to understand the origin of this, seemingly nonsensical progression.

Suddenly, it bothered me that I had such a minimal understanding of this fundamental aspect of camera function. The result of this revelation was a two-part search. The first to understand the explanation myself, and secondly to find a way to explain it to future students. It wasn’t easy, especially because the explanation involves MATH!, including inverse ratios and squares, but it was worth the effort. Not only did it clarify the weird f stop sequence, but it also explained other mysteries of lens function. You can check it out in my Exposure article from November 2014 (I hid my explanation at the bottom of the article!).

It was a valuable process of exploration and discovery, all coming from a simple question, and a question that I never thought required an answer. Just one example of how my need to explain photography to my students has led to my own education. And, this week, it happened again.

Introduction to Digital Photography - The Question

Abby at f/3.5, Nice Shallow DOF

I am currently in the middle of my Introduction to Digital Photography course at Keene Community Ed. This week’s class was all about Exposure. I was explaining how aperture size impacts the amount of light reaching the sensor, and also affects depth of field. You know the drill, wider apertures lead to brighter images but also result in shallower depth of field. But then the question came, “Why does a wide aperture result in a small depth of field?”

Peggy's Cove Sunset, Nova Scotia
Small Aperture: Deep Depth of Field

I responded with my usual insightful answer that I use when I don’t know how to explain something, “Because if does! Jesus made it that way!” Surprisingly, that seemed to be a sufficient explanation for most of the class, but it is more likely that they just realized that they couldn’t expect any more enlightenment from me.

But the question was asked, and I knew that I couldn’t expect any sleep until I understood the answer sufficiently to explain it to my students. Happily, the solution is straight forward and in no way related to a deity. I thought I had a superficial understanding, but I found a diagram from Wikipedia that I was able to modify to demonstrate the phenomena more clearly.

The upper and lower figures show the imaging of three objects at different distance from the lens. The top is seen through a wide aperture and the bottom through a smaller. In both cases the lens is focused on the center dot (2) and dots 1 and 3 are further and nearer respectively. For me the essential thing to note is that, regardless of aperture, all three dots are brought to a point of sharp focus (purple dot), but only 2 (blue) reaches that focus on the surface of the sensor. The rays from Dot 1 (red), which is the most distant, converges on a point in front of the sensor. By the time the rays reach the sensor plane they have spread, and therefore the image does not appear sharp. The focus point for the near dot (3) (green) is at an imaginary point converging at the back of the sensor, and therefore also leaves a blurred image.

In the top figure, the wide aperture gathers more light, but the wider opening results in a more drastically splayed distribution of rays, leading to a bigger area of blur. These areas of blur are often referred to as “Zones of Confusion” (Green Bracket) and, when the zone is big enough, the subject will appear out of focus.

In the bottom figure the smaller aperture leads to a more columnated distribution of the rays. This leads to a darker exposure, but also the most drastically splayed rays are blocked. The focus point for each dot is the same but, because of the small aperture, the rays reach the sensor with a smaller area of blur, and the “Zone of Confusion” can be small enough that our eyes may see them as sharp. Furthermore, with a small aperture, the subjects can be further from the point of perfect focus before the dot spread (the zone of confusion) is enough to appear blurry.

This is why a large aperture has a smaller depth of field.

QED.

Shifting Sensor Plane

Shifted Sensor for Increased DOF

One more thing to notice from this diagram. Because all of the dots reach a sharp focus point somewhere, it can be seen that the sensor can be angled so that all of the points can be captured in sharper focus ( see figure). This is why view cameras and tilt-shift lenses can record greater depth of field, by shifting the back or, more often, the lens to intersect with a broader range of focus points. Nobody asked, but I’ll throw that one in for free.

I can’t wait to lay this stuff on the next meeting of my class! That will teach them to ask questions I can’t answer!

Sunday, June 14, 2015

Tips for Sharp Focus

Finding Focus

Over the years I have published a number of articles about achieving sharp focus in digital photography. I have spent much time discussing the use of focus stacking, blending several variably focused images to obtain otherwise impossibly wide depth of field. It is a perfect example of how digital photography has opened creative options that are not possible with film. In special situations, focus stacking is a powerful tool, but most of the time sharp focus has more to do with careful adherence to good technique, which applies to both the film and digital processes. This week I would like to discuss just a few tips that may improve your photographic focus. Let's start with the digital cameras and those giant LCD screens

LCD Focus

Zoomed Live View Focusing

It may be surprising, but some people actually still gaze through a viewfinder to compose and focus their images, but with the proliferation of larger sharp LCD screens many cameras have dispensed with the optical viewfinders all together. The LCD provides an excellent real-time view of the scene and, when zoomed in, can be helpful in the fine adjustment of focus, especially in low light, but the viewfinder is better for routine focusing when a quicker response is necessary.

Although helpful in some special situations, manual LCD focusing is general awkward. The screen works best to compose the image while relying on auto-focus to control; the focusing.

LCD Brightness
One major challenge LCD focusing is the difficulty seeing the image clearly in bright light. It is a simple thing, but adjusting the LCD brightness to its maximum level can make a difference. Just remember to have a spare battery on hand, since the bright screen will more quickly drain power from the camera.

Through the Viewfinder
If your camera is still blessed with a good viewfinder, it offers some definite advantages for manual focusing. It is less bothered by bright light, but I find that, on occasions, I still have to shield the eye cup from glare using my hand or hat visor. Here a few other points to consider on your search for perfect focus.

Diopter Adjustment

If, as you gaze through the viewfinder, you never seem to get the scene to appear sharply in focus, the first step should be to adjust the diopter. Most cameras have a knob on the side of the viewfinder which adjusts the optics to match your vision. Start by focusing the best you can and then turn the knob until the view becomes clear. The adjustment will vary from person to person and with or without glasses. My diopter knob tends to easily get knocked out of focus, so I need to readjust every so often.

Using Focus Points

My vision isn't as sharp as in my youth and the act of manual focusing is additionally complicated by my variable focus glasses. The result is that I have come to depend more on my

camera's auto-focus capabilities. The accuracy and adjustments of auto-focus are different on every camera, but in general, modern digital cameras do a remarkable job at nailing the focus. The key is to grab your manual and take the time to understand how your particular camera focuses and then practice until the adjustments become automatic. The key issues include: the locations of the focusing points, how to select the individual or groups of focus points and which points are most accurate. Not all focus points are of equal accuracy. On my Canon 5D Mark II the

center point is a "crossed" type, which means that it is sensitive to both horizontal and vertical lines. The other focus points are sensitive to either horizontal of vertical lines and are about one half as sensitive as the central focus point. When possible, I use the center point, but at times the peripheral sensors work better to nail the focus when the subject is away from the middle of the field. I have previously discussed the use of the full field of focus points to allow Hand-Held Focus Stacking . Auto-focus is steadily improving and new cameras may have more focus points and a greater number of more accurate sensors. Grab that manual and figure out how your auto-focus does its magic and then practice, practice.

Following Focus

Once you have mastered your camera's auto-focus peculiarities, the next step is to read on in the manual about the various focus modes. My camera has a "One Shot" mode used to hold focus on stationary subjects, as well as two action modes, "AI Focus" and "AI Servo" used to maintain focus on moving subjects. It is a profound mystery to me that, when in AI Servo mode, the camera is able to

adjust focus as a subject changes its distance from the camera. Even more remarkable, when in AI Focus, the camera can detect when a stationary subject starts moving and follow it from there. It doesn't always work perfectly, but the fact that it succeeds so often is a miracle. Many cameras have these capabilities and if yours is so blessed, do it the honor of learning how to use it. Just think of the tack sharp images you will get as that ravenous Lion hurtles toward you with ever shortening points of focus centered on his gleaming white teeth!

Finding infinity
Although the broad concept of infinity may be impossible for our feeble minds to fully grasp, when focusing a camera the location of infinity seems quite comprehensible and finite. By definition, infinity focus is the point of focus beyond which all more distant objects remain sharp regardless of how far away.

Mount Monadnock - Infinity Focus

It would seem that all you should need to do, to focus on infinity,

would be to rotate the lens barrel as far as it will go toward the little infinity squiggle. Older lens had a hard stop at the infinity point, but today, most lens can rotate beyond infinity. As Buzz Lightyear would say, "To infinity and Beyond!". In auto focus the lens is designed to stop at infinity, but when manually focusing it is possible to go too far so that proper focus needs to be pulled back and confirmed visually. Lens manufacturers give various reasons for the removal of the hard stop at infinity, including compensation for changes in the glass related to variable

temperature, reduction in wear on the auto-focus mechanism and, undoubtedly most important, reduction in the price of manufacture. They argue that the ease of manual focus is less important now that most people are using their very capable auto-focus systems. Regardless of the reasons, the simply rule is to be careful when manually focusing to infinity. Again read your manual and experiment with the settings. In good light it is easy to focus visually on a distant object and then check the lens to see precisely where infinity lies. Many lens', including my workhorse 24-105mm, have a little check mark at the infinity point.

Be aware, on many zoom lens (those described as "Varifocal"), the focus will vary as the focal length changes. Zooms that maintain focus while zooming are called "Parfocal". My 24-105 is of the Varifocal variety, and so I can't zoom in to focus and then expect to be able to hold sharpness as I pull back.

Hyperfocal Distance

The hyperfocal distance is the closest point of focus beyond which everything will appear sharp. This distance varies depending on the focal length and aperture. The hyperfocal point is key to capturing those dramatic landscapes, with both foreground detail and distance vistas in sharp focus. There are various precise and a few approximate approaches to determining this distance, but I well save this more involved discussion for another blog.

Focus Stacking

As I mentioned at the beginning, Focus stacking is a technique in which multiple, variably focused, images are blended to create a potentially limitless depth of field. I've discussed this approach in previous articles, but it should be noted that this is another area in which digital photography has expanded creative capabilities and broken bonds to the physical limitations of our expensive optics.

Focus Stacking I

Focus Stacking II

Hand-Held Focus Stacking

Hopefully these few tips will be of help in achieving consistently sharp focus. The key is to use careful technique to control both the sharpness of your focal point and also the size of the full depth of field.

And finally if you feel that you have done everything right and your images are still uniformly fuzzy, make sure that you are holding the camera steady. Perfect focus can't compensate for even a little camera jiggle. Get a tripod! and more than that, actually use the damn thing!!

Jeffrey Newcomer
Partridgebrookreflections.com

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