Category Archives: Craft

Things I’ve Learned About the Craft of Photography: Focusing

Field_20150823_1138

Overcast Morning, Pebble Beach, The Sea Ranch

(Craft of photography: part 4)

As I’ve said before, one of my objectives is to capture the rich texture and details in the landscape. I have an above average emphasis on resolution and image sharpness. I’ve previously described the importance of shooting technique, aperture selection to avoid resolution-robbing diffraction, and lens selection. All are important but we must insure the lens is critically focused if we are to achieve an acceptably sharp image that maximizes the detail rendered on the sensor.

In a modern DSLR, there are basically three approaches to focusing:

  • Auto-focus using phase detection AF
  • Auto-focus using contrast detection AF
  • Manual-focus using live view with the camera’s display enlarged on the area of interest

Phase detection AF (PD-AF) is the classic SLR auto-focus method and has been around since the earliest days of auto-focus film cameras in the late-1980s. The principle advantage of PD-AF is speed: cameras focus rapidly and generally follow moving subjects quite well. The industry has continuously improved the method and the latest cameras are able to focus in exceptionally dim light, as low as -3 eV for the current generation.

The principle disadvantage of PD-AF is the location of the phase detection array: it’s normally located in the pentaprism, not on the image sensor. Focus is being controlled over a light path that is not the path of light that will be captured by the sensor. Manufacturing tolerances in the lens (e.g., the lens element to flange distances) and in the camera body (e.g., placement of the mirror and placement of the phase detection array) contribute to focus errors. The better consumer and virtually all professional DSLR bodies allow you to compensate for these errors on a per-lens basis (that is by calibrating a specific lens to a specific body).

The second disadvantage of PD-AF is the location and number of AF “points.” Today’s better DSLRs provide 50 to 60 AF detection points – some working horizontally, some working vertically, some working in both horizontal and vertical modes. AF points are of necessity clustered within roughly the central half of the sensor area. Want to set the focus point outside the area? Aim the camera, lock focus and then recompose with attendant errors in focus as you recompose.

The last disadvantage of phase detection AF: the focus sensors are not necessarily exactly located where the display shows them to be. You may think you are focusing on a given point but the camera could be locking on a high contrast area in the image a bit away from the displayed point.

If you are a nature photographer shooting moving subjects such as a bird in flight, today’s phase detection auto-focus is a godsend, providing a much higher “keeper” rate than most people ever thought possible with manual focus or with earlier phase detect AF. If you are a landscape photographer dealing with a static landscape, you can do better.

Contrast detection auto-focus (CD-AF) has been in DSLRs since the advent of “live-view” circa 2007-2008 and is the principal method of auto-focus in mirrorless cameras. Contrast detection uses the photo-sites on the image sensor itself to drive focus. The contrast across adjacent pixels is maximized when a subject is in focus. Contrast detection eliminates the errors that accrue from out of path focusing in phase detection. By design, contrast detection automatically compensates for manufacturing tolerances in the lens and camera body. Lastly, we can set the focus point to be any place in the image; no longer are we limited to using phase detection points in the central part of the image.

The first drawback of CD-AF is focus speed: it frankly is much slower than phase detection. This is true both in terms of the way you place the AF point (on the rear display) and in terms of the amount of hunting the AF system does searching for the point of maximum contrast. The nature photographer capturing moving birds would be hampered; focus speed is not an issue for the landscape photographer. In my experience, the second drawback of CD-AF is required minimum light levels: it frankly needs a bit more light than the best phase detection systems. This can be an important consideration in dim dawn and dusk shooting conditions – some of the prime shooting times for my practice of landscape photography.

Lastly, we have my preferred focus method: manual focus using live-view. With the camera’s display zoomed into the part of the image I want to use to set focus, I manually rotate the focus ring until I achieve optimal focus. This takes practice and patience: there is a (small) time lag between moving the focus ring and changes to the display. While this lag has become very short in the latest cameras, it is still not zero. With practice, you learn to compensate and can generally nail focus. This method shares some of the advantages of CD-AF: we are using image information from the sensor itself to judge focus. Some cameras use the lens’s “taking” aperture to drive live-view. To the extent this is true, you can inherently check for depth of field on the display. If your “taking” aperture provides considerable depth of field, you may find it hard to focus accurately. Open the lens to its widest aperture, set the focus manually, then set the lens to your desired shooting aperture.

Make no doubt, manual focusing with live-view is much slower than auto-focusing and suitable only for static subjects and frankly only when you have the camera mounted on a tripod. Too, like CD-AF, it does not work well in very low-light conditions such as those at the time of civil sunrise and sunset (generally 30 minutes before sunrise and after sunset). But as the sky starts to brighten as we near sunrise, the method is my preferred approach to achieving spot-on focus accuracy in my images.

Things I’ve Learned About the Craft of Photography: Lens Selection & Image Sharpness

Old Corner Post, Stornetta Public Lands, California Coastal National Monument, Mendocino County

Old Corner Post, Stornetta Public Lands, California Coastal National Monument, Mendocino County

(Craft of photography: part 3)

One of my objectives is to capture the rich texture and details in the landscape. Thus, I have an above average emphasis on resolution (short of capturing diffraction effects) and image sharpness.

When an image is sharp, you tend know it at first glance, even when it is reduced to a two-inch thumbnail size. Fine detail is well rendered, without artifacts. Edges are crisp without being pumped in a post-processing sharpening algorithm. Detail is subtly apparent without shouting at you. The image pleases the eye.

What do I think about when selecting a lens in the field?

Fixed focal length lenses, or prime lenses, are simply sharper than zoom lenses at the same focal length, nearly every time. Zoom lens design has progressed markedly over the last 30 years. We are at the point where image center sharpness is frankly just about the same for zooms and primes of equal focal length. But, primes tend to hold that critical sharpness across the frame to the edges and corners.

Longer focal length lenses, whether primes or zooms, are simply not as sharp as moderate focal length lenses. Telephoto lenses have a lot of glass in them. Moving closer to your subject and using a moderate focal length lens will produce a sharper image than standing back and using a telephoto lens.

Choose the lens with lower geometric distortion. This especially applies to barrel distortion seen in wide-angle primes and on the short end of wide-angle zooms. We have excellent algorithms today to correct for geometric distortion, both in-camera and in post processing. But, correction inevitably stretches parts of the image, effectively reducing available resolution especially out towards the edges and corners.

Wide-range zooms are noticeably less sharp on the long-end. The ratio of the longest to the shortest focal length of a zoom lens is called the zoom ratio. A 24 – 70 mm zoom has a zoom ratio of 70/24 or about 2.9. Lenses with zoom ratios of about 3 or less tend to hold up well across the full range of focal lengths. When that ratio stretches to 5 (or even 10 or more for “super zooms”), we see marked fall-off in lens sharpness out toward the long end. Do not expect to obtain the same sharpness with an 18-200 mm zoom at 200 mm as one does with an 80-200 mm zoom at 200 mm. A 200 mm prime will be sharpest of all.

If using zoom lenses, try to stay 10% away from either end of the focal length range. For my 80-200mm zoom, I try to use it between 88mm and 180mm. My own tests of multiple zooms show that the last 10% of focal length tends to bring a disproportionate loss of sharpness.

Use only top-quality filters and avoid stacking filters. You spend a lot of money on your lenses. Choose the best filters with the best anti-reflective coatings.

What do I actually use the most in the field? I’ve evolved my practice to keep the kit I take into the field as simple yet flexible as possible. This is especially true when I’m out hunting for new subjects (vs. returning to a spot and subject I’ve shot before). I’ll talk more about this in a later post but I find keeping the kit simple enables me to concentrate on the images rather than fiddling with too much gear. As such, I tend to carry a kit with a single body, a normal zoom, and a fixed focal length telephoto lens. That kit is one of the following:

  • Nikon D750 FX body with 24-120 mm f/4 zoom and 180 mm f/2.8 telephoto. I try to keep the 24-120 mm lens no longer than 85 mm or so.
  • Nikon D300 DX body with 16-85 mm f/3.5-5.6 zoom plus 180 mm f/2.8 telephoto. When using the zoom, I try keep the focal length shorter than 50 mm or so.
  • Nikon D300 DX body with 17-55 f/2.8 zoom, 85 mm f/1.8 prime, and 180 mm f/2.8 prime.
  • All filters are from B&W and I opt for their multi-coated filters when available.

Photographing along our coast, I do not always have full flexibility to “zoom with my feet.” I’m not fond of walking off the edge of the ocean bluff. Hence, the framing flexibility of a mid-range zoom is quite useful. Still, I value the extra edge in sharpness from prime lenses and if the subject is especially texture rich and accessible enough for me to zoom with my feet, I will take a set of prime lenses into the field in lieu of the zooms mentioned above.

Resources:

I tend to use three resources for evaluating and comparing lenses.

Photozone, a Germany-based website (published in English) with excellent test and evaluation data of lenses. Consistent test methodology and open, full disclosure of test results. I especially like the ability to look at how well lens resolution holds up at various focal lengths, at various apertures and across the image frame. The major limitation I see is that tests are usually with a single copy of a lens and thus lens to lens variation is not accounted for.

DxOMark is probably best known for camera sensor tests . But they also present interesting data on lens plus sensor performance. I find their Perceptual Megapixel measure to be useful though coarse measure of lens resolution. Its principal limitation is that it condenses a lot of information to a simple scalar measure.

My own lens tests. I use a consistent subject target and shoot with my own cameras to visually compare the performance of lenses I own.

Things I’ve Learned About the Craft of Photography: Diffraction Limits and My Choices for Shooting Aperture

Continue reading

Things I’ve Learned About the Craft of Photography: Image Sharpness & Shooting Technique

Foggy Morning, Marine Railway, Historic Lifeboat Station, Point Reyes National Seashore

Foggy Morning, Marine Railway, Historic Lifeboat Station, Point Reyes National Seashore

This is the first of a series of posts through which I hope to share some of the things I’ve learned about the craft of landscape photography with digital SLR cameras. Two terms are operative. First is craft which refers to techniques and approaches employed to produce an image (rather than the aesthetic questions of what to capture and express in the image). Second is landscape which is wholly different from other types of photography (photo journalism, wedding or portrait photography, etc.) in terms of subject (inanimate), lighting (generally the sun), depth of field (generally to be maximized) and other considerations.

From the perspective of craft and like most landscape photographers, I want to produce images that are well-composed, well-lighted, critically focused, and sharp. While there are technical definitions of sharpness, in my mind maximizing the sharpness in an image enables me to reproduce much more of the fine details and texture in the subject. Sharp images catch the eye and the sharpness is apparent even in small prints or screen images. You don’t need esoteric measures to know the image is sharp.

The sharpness of an image is fully determined by the time the shutter closes. I believe it is basically determined by two factors: i) shooting technique and ii) lens choice. I will talk about lenses in a later post. Suffice it to say that careful shooting technique with an average lens produces an image that is noticeably sharper than one shot with the finest lens but without regard to technique.

Careful technique starts with use of a tripod. Sharpness will be lost if the camera and lens are not held absolutely steady during exposure. No amount of ‘vibration reduction’ or ‘optical image stabilization’ in lens or camera body will beat the use of a rock solid tripod.

Through trial and error (common among photographers when it comes to tripods), I have learned to use the most rugged tripod that I can carry into a shooting location. Most of the time this means I use a Gitzo series 3 tripod, meant to support many times the weight of the gear I have mounted on it.

The camera and lens need to be solidly attached to the tripod. I have adopted the Really Right Stuff BH-55 ball head as my standard. The 55 mm (about 2.16 inch) diameter ball is again intended to support many times the weight I place on it. I use Really Right Stuff L-brackets designed specifically for the camera models I use. In addition to being solid, the dovetail mounting system makes for rapid camera mounting and change of perspective (switching from landscape to portrait mode).

Single Lens Reflex cameras are great: what you see in the viewfinder is (almost) what the sensor is going to see at exposure time. Pressing the shutter release on the camera does two things: 1) raises the reflex mirror to clear the optical path and 2) opens and closes the shutter. The drawback of the SLR is that quickly raising the mirror invariably means the mirror will slap against the pentaprism housing. This slap induces vibrations in the camera body that persist for many, many tens of milliseconds before they damp out.

I always use the mirror lock-up function on my camera (and would not own a camera that does not have mirror lock-up). An exposure requires two steps: the first actuation of the shutter release raises the mirror while the second actuation fires the shutter and allows the mirror to return to the down position. Have the patience to wait three to five seconds between raising the mirror and firing the shutter and you will be rewarded with noticeably sharper images!

Keep your hands off the camera! No matter how steady you think you are and no matter how carefully you press the shutter button, you are going to rock the camera. Use a remote shutter release.

Finally, wind will vibrate even the most rugged tripod and camera. Try to avoid shooting in really windy conditions. If you must, try adding some weight to the tripod (partially suspend your camera bag) and by all means remove the neck strap (aka wind sail) from the camera.

What if the shooting location is a long hike from the car or you are traveling by air and you are not able to carry a large, heavy tripod? Do the next best thing and carry a smaller, lighter but well constructed tripod. I have many sharp images shot with a 200 mm focal length lens mounted on a Gitzo series 1 tripod. To gain stability, I carry a one kilogram (two-pound) bean bag, hang it from the center post of the tripod and use extra care in my technique.

With digital photography and tools like Photoshop, we have the ability to examine our images in fine detail (at 100% where camera pixels and computer display pixels are mapped one-to-one). I urge you to run a controlled experiment. Start with an image that uses none of the technique I’ve outlined and then take a series of images adding successive levels of care. Look carefully at the images – you will quickly be convinced that shooting technique really does matter.

Aside from significantly improving sharpness, the techniques I’ve talked about here all slow down the process of shooting. And that is a good thing. Our best landscape photographers, to a person, all tell us that slowing down brings a much more contemplative approach to photography, producing images that are much more compelling and evocative.

Note: I published an earlier version of this post on my old website in 2011. I think the material is even more important today. For better or worse, camera sensor pixel density continues to grow. 24 megapixel is the entry point for full frame DSLRs with top of the line cameras now boating 36 and 50 megapixels. To achieve prints as large as these sensor sizes suggest are possible, you MUST have a rock-solid, absolutely steady camera. Any camera shake whatsoever will limit the size of your prints. If you want to continue to hand-hold your camera, save your money and avoid the highest resolution cameras. Conversely, if you are able to eliminate camera shake, you will be able to print even modest resolution images (e.g., 12 megapixels) to sizes larger than you might expect.