Light Values and Exposure: Highlight and Shadow

Exposure control comes from appreciating and then recording the light in the scene as you want to render it. There are two key values that determine your success in doing so: highlight and shadow.

Here is a recording that delivers a full range of tones. The highlights are the side of the shed wall and the tower. There is no need to "open" the shadows within the dark area of the shed as that seems natural. The details under the silos are "readable" with both tone and texture. This represents a range of brightness values that can be handled by sensors (and film) with no need for any special exposure techniques such as HDR or highlight biasing or compensation.

 Highlights are the brightest parts of the scene; shadows are the darkest. Highlights can be broken down into two main types that concern us here--the principal, or textural highlight, and the specular highlight. The principal highlight is the brightest part of the scene in which detail, or texture is to be recorded, while the specular highlight is just bright tone (brightness) with no detail or texture. It may "read" on the image and later print as “pure” white (in a print, the brightness and color of the paper base.)

A principal highlight might be a freshly painted picket fence in which you want to show the grain of the wood, or grains of sand at the beach that you want to render in textural fashion. This highlight needn't be white; it is just the brightest value you are recording with detail or texture. For example, a bright yellow or orange can be the brightest part of the image, and exposing it correctly means that it will not be washed out or overexposed. (You can underexpose it slightly to add saturation, but you may underexpose darker parts of the scene when you do so.)

Spectral highlights are formed by light reflecting off a shiny surface and usually cannot be reproduced with detail; exposing for them "drives" the darker areas into deep underexposure. Thus, no detail other than the spectral highlight becomes visible. The setting sun off this beach created spectral highlights in the tidal area and in the ocean. This can be used for special effects, but no sensor or film can "hold" the light in the darker areas and they become lost through underexposure.

 A specular highlight can be the glint of light off an afternoon lake, or the glare from a glass-and-steel skyscraper. There is no recording of detail in this tone--it is pure light that “shatters” the light upon reflection, a kind of interference that cannot be recorded with texture or tone without the remainder of the image becoming very underexposed.

There are times when you "previsualize" a scene and want to use a bright highlight to make a bright area the main and usually only detail that is recorded. This "locks" the scene into one rendition. Here the bright formation was read by spot metering off the bright rock. This made the other areas much darker than they appeared in the original scene, an interpretive approach that is worth investigating. 

 The other end of the brightness scale is the shadow area. The key term here is "significant shadow detail", or the darkest part of the scene in which you want to record visual information. There may be darker parts of the scene, but these will record as tone (deep gray to black) without textural information or detail.

These terms, (principal) highlight and (significant) shadow are important to keep in mind when making exposures. While we do have a wide recording range with digital cameras (less so on film), seeing the image in terms of a tonal spread, and how that spread will record on film, is key to making good images. And, applying HDR (multiple exposures that bracket plus and minus and then combining them in software later) or working with highlight tone priority algorithms on advanced camera (which in effect suppressing the highlight value) certainly expands the “dynamic range,” or exposure latitude.

For example, say you are photographing a white car in bright sunlight. You take a reading off the white hood and roof and get f/16; you read the shadow cast by the car and get f/4--this presents no exposure problem that cannot be resolved with minimal processing. However, let's say you also take a reading from the tires on the car in the shadow area, and get a reading of f/2; this is clearly out of range. In this situation do not expose for the brightest area, but back off a bit and expose at f/11 or f/8.5 or, better yet, use highlight tone priority or HDR to get the most out of the light values you record.

The question then becomes, is it important to record detail in the tire tread in this photo, or can the tires record as pure black, with no visual information other than tone? If the answer is that the tires may record as pure black, the significant shadow detail is the subject information in the cast shadow of the car. If no, then other steps, already mentioned, must be taken to record both the bright car and the tire tread. 

The thing is—you can’t fight mother nature (in this case the physics of the exposure system and the range the senor or film can record). But keeping significant highlights and shadows in mind when you expose will lead to much better results in both exposure and printing and help you see with a photographic eye.   

How to Prevent Sensor Dust Problems

Digital sensors are like little static electricity machines that attract minute and sometimes grain-sized dust particles of dust. You can “spot” the particles out later (which record on the image as black dots and streaks) using cloning tools and various healing brushes and blur tools, but having to spot images in this way is painstaking and not a good use of time, especially when the problem can be eliminated or diminished with a bit of care.

Manufacturers have not denied the problem and many cameras now incorporate dust “cleaning” systems that turn on every time you start up the camera. Generally these “shake” the sensor in the hopes of vibrating the dust off; sometimes this works OK and other times you might have to use more drastic measures. The best bet is to try to prevent dust from catching on the sensor in the first place.

All told, dust is pernicious and can work its way inside the camera in more ways than one: here’s some advice about keeping it to a minimum.

1) Always store the camera in a case or bag. It might look handsome on a bookcase or shelf but you are needlessly exposing it to ambient dust.

2) Never change a lens with the camera turned on. Having a charge across the sensor plane when it is exposed will act as a dust magnet.

3) If possible, change lenses inside a vehicle or changing bag, especially if there is any wind. This is when many dust problems occur. Turning your back on the wind and shielding the camera when changing lenses might help, although doing so when shielded by a coat is better.

4) If shopping for a camera, get one with a dust reduction (shake) system and that bills itself as having extra seals to prevent dust. These “dust-proof’ and “weather proof cameras” are worth it. And if you take lots of outdoor nature shots, especially in dusty environments, it’s a good idea to get well-sealed lenses as well.

This photo made with a loaner camera looks "innocent" of dust 

...but close inspection of the sky area reveals a dust nightmare. 

As an exercise I wouldn't want to repeat I enlarged the image to 100% on the monitor and worked with the clone tool and healing brush to eliminate all the lines and particles. Needless to say I won't be loaning from that service again. If you find similar problems send your camera to a reliable service center or the manufacturer and get that dust removed.

Checking for Dust

If you look through the viewfinder and see dust you might not have dust on the sensor—it might be on the lens itself or in the DSLR viewfinder prism or mirror. Check the lens. If the prism has the dust it can be bothersome, but that dust will not show up on your pictures. If the mirror has dust use a gentle cloth, gently, or compressed air.

Inspection

The only way to know if you have a sensor dust problem is to take a picture and inspect it by using the magnifying tool in your computer software—it may not how up on your small playback LCD. Download and enlarge your images at 100% (though that might be too late for a day’s shoot.) Another good test is to shoot a blank white sheet of paper, download the image, and move it around it to check the entire enlarged frame—this will make the dust spots quite visible.

What If You Have Dust?

If you do have dust on the sensor try restarting the camera (if it has a dust reducing system) a few times to see if you can have the system “shake” it away for you. If your camera has a “clean now”  feature you can use this instead of restarting it.

You can try cleaning the sensor yourself with commercially available kits, although I would caution you to be very, very careful if you go this route, as damage can easily occur. The best route for really bothersome dust (but isn’t it all) is to send the body back to the manufacturer or a certified repair shop for cleaning: this is the safest route. Many such services will do an overall check of the camera, including "pixel mapping" (identifying "dead" pixels) as part of an overall CLA (clean, lube and adjust).

Spotting with Dust Delete Programs

What if you have a precious and special image that simply can’t be reshot and you want to clean it of dust? Some manufacturers offer “dust delete” software for their camera cameras.

Here’s a typical dust delete procedure.

1) First, find where the dust delete setup is in your camera menu.

 2) Activate it and the self-cleaning procedure will start.

 3) Photograph, and fill the frame with a solid white object, like a sheet of copy paper. This photograph will not be saved to a card. The menu screen will confirm that is has “captured” the dust “data.” You then load and use the camera manufacturer software to automatically spot out the specks in your image. 

Note that this does not remove the dust but ID's the offensive dust and samples neighboring pixels to "spot" it out. You may have to go in and do additional spotting later. Yes, you can save one or a series of images this way, but don't go into the field without having the camera serviced.

Yes, you can spot and blur and the healing brush in software like Photoshop, but this can be painstaking for really dusty images. If you’ve ever tried to do that, you know what I mean. So, the best protection is to prevent dust from getting on the sensor in the first place, and checking periodically if the dust buildup is getting really, well, dusty.

 

How to Prevent Memory Card Corruption

There’s nothing more frustrating than going into the field and later discovering that your memory card did not “take” your take. 

While there are software programs that can help recover files that do not read when you try to download (google: recover damaged image files), or even files that do not show up in playback on your camera right after you shoot, the best bet is to prevent conditions that can cause corruption in the first place. 

Here' are two images "recovered" from a corrupted card. This showed up as one file after I ran the card through a recovery software program. Interesting, I suppose, but certainly not what I had in mind.

Here are some quick tips for preventing memory card corruption:

1)   Don’t carry memory cards loose in your pocket or camera bag. Dust, dirt, and lint can “clog” the card’s pins and interface areas, and cards can be bent or otherwise damaged by other items in your bag. Use a dedicated card carrier available as an inexpensive accessory that has slots that hold cards separate from one another.

2)   Inspect the card for damage. The chief problem is bent pins, sometimes caused by forcing the card the wrong way into the slot in the camera. Look for insertion guides and if it does not slide in smoothly, back it out carefully. If you do have bent pins the card may be irreparably damaged. My experience is that trying to fix it yourself is not very rewarding. Bent pins make it just as difficult to read from a card reader. If you must, try to fix the bent pins with a needle nose plier or tweezer. My best advice: Toss it, but only after you try to retrieve images later.

3)   Make a few shots prior to going out in the field. Check that the test shots show up in playback. If they don’t, you are playing with fire.

4)   Always swap and load memory cards by first turning off the camera to avoid static discharge.

5)   If you have more than one camera, dedicate the card to one camera and do not swap it to your other camera. When you first use a new card in your camera be sure to use the format function; dedicating a card for use in that camera will remove any possibility of a mismatch. While this is not always a big problem, different cameras may have unique file structures. The formatting command is usually found in your setup menu. (Note: if you are using an older card for a new camera always check for images first, as formatting will erase those older images.)

6)   If you are working in the field and nothing comes up on playback, first try turning off the camera and reloading the card. However, it is better to remove the card and replace it with another and deal with the problem card later.

7)   If you do get corrupted or unreadable files, you can try out downloadable file recovery software. My experience with some of these programs is mixed, but it could just work for you—it’s worth a try!