Obscura [Project]

High Dynamic Range (HDR)

What is HDR?

HDR means ‘High Dynamic Range’. Using software like Photomatix you can create images with a more detail in the highlights and shadows than you can with a normal photo from todays digital cameras. Its similar to the old technique of exposure blending. Taking one photo for the sky and one for the ground, then merging them both together in Photoshop. HDR takes it a step further by increase the amount of detail in the image and allows you to create some unique photos. You can use it carefully to create natural looking photos or you can use it creatively to create atmospheric and emotive photos. The choice is yours as to how you process the end result.

Tools Needed

A RAW editor such as Aperture, Lightroom or Photoshop. Secondly, the HDR program like Photomatix. 

Step 1: Source images

There are two main ways to create the source images needed for HDR. You can either use AEB, auto exposure bracketing, on your camera to take 3 images while you are out, or you can use RAW to take 1 image and then use a RAW editor to produce 3 shots back at your computer. For best results I recommend using a tri-pod and auto exposure bracketing.

Auto Exposure Bracketing (AEB)

The main advantage with AEB is that you can get better source images with less noise. For example, a sunset. You would normally get really dark shadows and enhancing them later from 1 single RAW file would increase the noise. With AEB you can take a completely seperate image for the shadows and one for the highlights to preseve the detail and keep the noise to a minimum. The disadvantge to this approach is that anything moving in the image will become blurred and repeated as it moves across the 3 images. To start, you will need a camera that has the AEB mode and a tripod. You will also need a tripod so that the 3 shots you take can be perfectly aligned later. Go to the menu on your camera and set the AEB mode to -2 / +2 stops.

Compose your shot, and set the camera up as your normally would to take a picture. Now after you take a picture you will see the exposure compensation level drop down to -2. Take another picture and you will see it goes up to +2. You will notice the shutter speed or aperture changes too. This allows you to get the three shots for the shadows, a balanced image and for the highlights. You now have the 3 shots required to produce a HDR image.

Using a RAW Editor

The other way to produce the 3 shots needed is to take 1 photo and adjust in a RAW editor such as Aperture, Photoshop or RawShooter. The main advantage to this is that you can produce a HDR shot with moving subjects such as people or cars. The disadvantage is that if you use an image with very dark shadows and you’re trying to boost the exposure, the result will be quite noisey.

Load your photo into the editor, then set the exposure level to -2 and save the image as a 16bit TIFF without any metadata such as EXIF info. This is the important part. If you save it with EXIF info you will find that Photomatix relies on that info to produce the HDR shot. The problem with this is that the shutter speed and aperture values will be the same across the 3 images and Photomatix won’t know which image is -2, 0 and +2. Set the exposure level to 0, save that and then set it to +2 and save that image.

Step 2: Generating a HDR image

Open the 3 images into Photomatix. From the HDRI menu select Generate HDR. Photomatix will ask you if you want to use the 3 opened images or load in some. Make sure it has “Use opened images” selected and click ok. Now Photomatix will confirm the exposures for each image. 9 out of 10 times it will get it right, but just incase make sure that each image has the correct exposure settings. In the case of this guide they should be +2, 0, -2. Click ok, then check that “use standard response curve” is selected. If you are using 3 images taken at the scene tick the “Align LDR images before generating HDR image” option just so that Photomatix aligns the images for you. Click ok and after a few minutes you will see your HDR image. It won’t look exactly right just yet. Some areas will be slightly over-exposed and it’ll look odd.

Step 3: Tone Mapping

This is the magic part. Using the Tone Mapping feature in Photomatix will convert your HDR image into something usable. Goto the HDRI menu and select Tone Mapping. You will see how your photo looks more like a standard HDR image. The sky will be nicely exposed, as will the ground. The trick now is to adjust the settings to get a nice balanced image. You could be creative and go for something a little more fun if you feel like it. For the image in this guide I wanted something special and yet not too far from the real world. I’ll start with a quick run down of the various options.

Luminosity
Adjusts the brightness of the shadows. Moving the slider to the right has the effect of boosting shadow details and brightening the image. Moving it to the left gives a more natural look to the tone mapped image.
The optimal value depends on the image and the effect you want to achieve.

Strength
Controls the strength of local contrast enhancements. A value of 100% gives the maximum increase in local contrast.
The optimal value depends on the image and the effect you want to achieve.

Color Saturation
Controls the saturation of the RGB color channels. The greater the saturation, the more intense the color. The value affects each color channel equally.

White Clip – Black Clip
From watching the way the histogram changes, the white clip adjusts the highlight contrast and the black clip adjusts the shadow contrast.

Micro-smoothing:
Basically this setting should be called “How arty do you want your shot?” At 0 you can get the cool arty style HDR images with all the detail in everything from walls to clouds you didn’t know where there. However, if you want a nice simple blended exposure photo set it to 30. 95% of the image should be perfectly exposed as if you spent hours putting them together in Photoshop. This is a great new addition to Photomatix as it allows people to use the software as they see fit. If they want a nice photo that is perfectly exposed then they can get that just as if they used ND Gradient filters on their lens. However, if like me they want something a bit more unique they can drop the micro-smoothing down and get something dramatic.

Light smoothing:
Never set this below 0 as you will get horrible results.

Microcontrast
Controls the accentuation of local details. The default value (High) is the optimal value in most cases. However, this control may be useful in the case of a noisy image or when the accentuation of local details is not desirable (e.g. seams of a stitched pano in a uniform area may become visible when local details are too much enhanced).

Generating a HDRI from a single RAW file

Using the latest version of Photomatix, 2.3.1, you can load a single RAW file and generate a HDR image. Simply goto File -> Open and then select the RAW file. Photomatix will load the image and generate a HDR from it. You will still need to tone map the image after. The best method is still to take 3 bracketed images on site.

Aperture

Aperture is the size of the opening in the lens when a picture is taken.

When you hit the shutter release button of your camera a hole opens up that allows your cameras image sensor to catch a glimpse of the scene you’re wanting to capture. The aperture that you set impacts the size of that hole. The larger the hole the more light that gets in – the smaller the hole the less light.

Aperture is measured in ‘f-stops’. You’ll often see them referred as f/number – for example f/2.8, f/4, f/5.6, f/8, f/22. Moving from one f-stop to the next doubles or halves the size of the opening in your lens (and the amount of light getting through). Keep in mind that a change in shutter speed from one stop to the next doubles or halves the amount of light that gets in also – this means if you increase one and decrease the other you let the same amount of light in – very handy to keep in mind).

One thing that causes a lot of new photographers confusion is that large apertures (where lots of light gets through) are given f/stop smaller numbers and smaller apertures (where less light gets through) have larger f-stop numbers. So f/2.8 is in fact a much larger aperture than f/22. It seems the wrong way around when you first hear it but you’ll get the hang of it.

Continue to Depth of Field to understand how aperture applies to photography

ISO

The “normal” range of ISO is about 200 to 1600. With today’s digital cameras you can sometimes go as low as 50 or as high as 102,400. The number chosen has two important qualities associated with it. First, it sets the amount of light needed for a good exposure. The lower the number, the more light required. The more light that’s required, the more likely a slow shutter speed will have to be used. That means low ISOs, like 100 or 200, are most often used in bright situations (like sunlight) or when the camera is mounted on a tripod. If you don’t have a lot of light, or need a fast shutter speed, you would probably raise the ISO.

Each time you double the ISO (for example, from 200 to 400), the camera needs only half as much light for the same exposure. So if you had a shutter speed of 1/250 at 200 ISO, going to 400 ISO would let you get the same exposure at 1/500 second (providing the aperture remains unchanged). This is why high ISOs are so often used indoors, especially at sporting events. Needing a fast shutter speed to stop action, photographers regularly choose ISO 1600 or above.

The other important quality tied to ISO is the amount of noise in the image. In the days of film, as you used film with higher ISO values (often referred to as ASA then), your images had more visible grain. Film grain is what made up the image, and higher numbers resulted in larger grain, which was more obvious. Most people found visible grain objectionable and so photographers worked to avoid it when possible.

In digital cameras, raising the ISO means a similar decrease in quality, with an increase in what’s called “noise.” It’s the digital equivalent of grain and results in a sort of “chunky” look to the image. Very early digital cameras had objectionable levels of noise at ISOs as low as 800. Today most digital SLRs can make good quality images at ISOs up to 1600 and above. However, several variables affect this.

One important factor affecting the amount of digital noise in an image is the size of the pixels used on the sensor. Large pixels result in less noise than small ones. That’s why digital SLRs perform much better at high ISOs than compact cameras. The SLRs have larger sensors and larger pixels.

Another factor is the amount and type of noise reduction being applied in the camera. Because all pixels collect some noise, every digital camera runs processing on every image (although with a NEF, or RAW, file that can be changed later) to minimize that noise. Newer cameras use newer technology to reduce that noise, with the result being less noise at similar ISOs than what earlier cameras could achieve.

All of this means photographers are constantly doing a balancing act. They want to keep their ISO low for high quality images (low noise), but also they need a fast enough shutter speed to get a sharp picture. That’s why there’s such high value placed on groundbreaking cameras such as Nikon’s D3, D700 and D3S that allow photographers to shoot at higher ISOs with less noise than ever before.

Auto ISO was introduced into digital cameras several years ago to help photographers manage that balance. Turning on that feature allows the camera to push the ISO up when it decides the shutter speed is getting too low for a good picture. Even better, newer Nikon cameras have added “ISO Sensitivity Auto Control” to the menu choices. This takes Auto ISO and lets you have some say about what happens. Using it, you set the limit for how high it can go (800? 3200?) and at what shutter speed it should start raising the ISO (1/125? 1/30?). The amount of control this feature allows means more photographers will start taking advantage of it.

A solid understanding of ISO will help you make smart decisions about how to set your camera. And that, in turn, will lead to better pictures.

Shutter Speed

Shutter speed is the amount of time that the shutter is open.

In film photography it was the length of time that the film was exposed to the scene you’re photographing and similarly in digital photography shutter speed is the length of time that your image sensor ‘sees’ the scene you’re attempting to capture.

Shutter speed is measured in seconds – or in most cases fractions of seconds. The bigger the denominator the faster the speed (ie 1/1000 is much faster than 1/30).

In most cases you’ll probably be using shutter speeds of 1/60th of a second or faster. This is because anything slower than this is very difficult to use without getting camera shake. Camera shake is when your camera is moving while the shutter is open and results in blur in your photos.

If you’re using a slow shutter speed (anything slower than 1/60) you will need to either use a tripod or some some type of image stabilization (more and more cameras are coming with this built in).

Shutter speeds available to you on your camera will usually double (approximately) with each setting. As a result you’ll usually have the options for the following shutter speeds – 1/500, 1/250, 1/125, 1/60, 1/30, 1/15, 1/8 etc. This ‘doubling’ is handy to keep in mind as aperture settings also double the amount of light that is let in – as a result increasing shutter speed by one stop and decreasing aperture by one stop should give you similar exposure levels.

Some cameras also give you the option for very slow shutter speeds that are not fractions of seconds but are measured in seconds (for example 1 second, 10 seconds, 30 seconds etc). These are used in very low light situations, when you’re going after special effects and/or when you’re trying to capture a lot of movement in a shot. Some cameras also give you the option to shoot in ‘B’ (or ‘Bulb’) mode. Bulb mode lets you keep the shutter open for as long as you hold it down.

When considering what shutter speed to use in an image you should always ask yourself whether anything in your scene is moving and how you’d like to capture that movement. If there is movement in your scene you have the choice of either freezing the movement (so it looks still) or letting the moving object intentionally blur (giving it a sense of movement).

To freeze movement in an image you’ll want to choose a faster shutter speed and to let the movement blur you’ll want to choose a slower shutter speed. The actual speeds you should choose will vary depending upon the speed of the subject in your shot and how much you want it to be blurred.

Focal Length and Shutter Speed - another thing to consider when choosing shutter speed is the focal length of the lens you’re using. Longer focal lengths will accentuate the amount of camera shake you have and so you’ll need to choose a faster shutter speed (unless you have image stabilization in your lens or camera). The ‘rule’ of thumb to use with focal length in non image stabilized situations is to choose a shutter speed with a denominator that is larger than the focal length of the lens. For example if you have a lens that is 50mm 1/60th is probably ok but if you have a 200mm lens you’ll probably want to shoot at around 1/250.

Going out and experiment with your camera is the best way to learn.