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How many shots do you need to make an HDR merge?

The most common answer is three.

The real answer is … it depends. First, it depends on the contrast ratio of the scene. Second, it depends on the exposure value (EV) increments you use between exposures. Third, it depends on the camera you use.

Many scenes only need 2 exposures. Most scenes need 3. Some scenes need 5. Only a few scenes need 7 or more.

How far apart in EV (exposure value) should separate exposures be?

1.5 stops. (Really any value between 1 and 2 stops.) While you won’t get better image quality if you use more shots separated by less exposure value (less than 1 stop), you also won’t compromise it. While you can also use higher increments (more than 2 stops) be careful – you may produce banding in smooth areas, particularly those with gradations.

Do you need to make HDR merges more frequently with some cameras and less with others?

Yes. Cameras that have a greater dynamic range can capture a higher contrast ratio and so don’t require HDR bracketing as frequently. While this can make a difference for images that would require two and occassionally three shots, for scenes with more extreme contrast ratios HDR merges will be necessary for all cameras.

What exposure mode should you use?

In a majority of cases, use Aperture priority mode (fixing aperture) to fix depth of field. If aperture changes dramatically between separate exposures, substantial changes in depth of field will most likely lead to a loss of focus in some image areas.

You can make exposures for HDR merges by bracketing ISO. Noise levels between exposures will be averaged. The final results will have more noise than the lowest ISO and less noise than the highest ISO.

You can also make exposures for HDR merges with shutter priority mode (fixing shutter speed). Try this when shutter speed drops so low that you can no longer eliminate motion blur, either because of subject motion or because you’re hand holding your camera. (But, use a tripod if you can.)

Do you need to use a tripod to make exposures for HDR merges?

No. By setting your camera to auto-bracket and making exposures in quick bursts you can eliminate the need to use a tripod for well lit scenes. Today’s HDR merging softwares do an excellent job of aligning separate exposures. However, in low light or when long exposures are desired using a tripod is usually necessary.

Read more on HDR techniques here.

Learn more in my digital photography and digital printing workshops.


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Today’s cameras (including smart phones) can create great looking HDR images on the fly, but to get optimum results it’s best to do this manually. In camera solutions render artifacted JPEGs and give you little or no control over how the results look. For optimum results, make separate Raw exposures and render them manually. While the technology at work is wizardry, this four step process is easy to practice. It’s an essential skill for all photographers.

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1       Expose

Today’s fast burst auto-bracketing cameras combined with software alignment make hand held HDR possible. However, it’s recommended that whenever practical you use a tripod to eliminate any alignment issues between frames that might arise; it’s necessary if exposures are long.

HDR merges require multiple bracketed exposures. The goal is to produce at least one exposure with great highlight detail and another with great shadow detail. You may need additional exposures in between your lightest and darkest exposures to help smooth tonal transitions between shadows and highlights. The most common number of images used is three, because this is the default number for auto-bracketing on DSLRs. However, there is no ideal number of exposures for all scenes. Some scenes need as few as two, while others need as many as eight. In general, it’s best to have more than you need, not less. The wider the dynamic range of the scene the more exposures you’ll need. Make sure that separate exposures are between 1 and 2 EV (exposure value)(equivalent to one f-stop) apart. It’s typically recommended that you fix f-stop and change shutter speed to avoid depth of field issues, but other changes in EV will work.

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Merge in Lightroom

2       Merge

After producing multiple bracketed exposures, the next step is to combine them with software into a single 32 bit file.

Simply select the exposures you wish to include (You don’t have to use them all.) and use the software of your choice. The software you use to merge exposures will compensate for alignment and ghosting, from motion of either camera or subject. (Lightroom and Photoshop do excellent jobs.)

Rather than rushing to render this file at the same time, save it – you may want to render it multiple times.

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Process in Lightroom

3       Process

As 32 bit files can’t be displayed or printed, the penultimate step is to render them into a new 16 bit file capable of being displayed and/or printed. This is a critical step that has the greatest effect on the look and feel of your images.

You can use the same software that you created a 32 bit file with or any HDR software. There are many HDR softwares to choose from; Lightroom, Photoshop, Photomatix, NIK’s HDR Efex Pro, Aurora, etc. While most HDR softwares are capable of producing fine results (if used carefully), they do not all produce the same results. It’s worth comparing multiple products before settling on your favorite. You may even want to combine the renderings from different softwares in Photoshop.

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Refine in Photoshop

4       Refine

Consider the purpose of the first HDR rendering of an image as a way to create a great base for further improvements. Even in the best HDR renderings, it’s rare that additional post-processing isn’t recommended. This could be as simple as reducing noise or as extensive as dodging and burning. In a few cases, you may even want to use Photoshop’s layers to blend multiple renderings of the same image.

Today’s cameras and software continue to make HDR merges easier and easier. You can get great benefits for just a little effort.

Read more on HDR techniques here.

Learn more in my digital photography and digital printing workshops.

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Alignment XIV

My free October Desktop Calendar features an image from Death Valley, California.

Download your free copy here.

Find out more about this image here.

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Enjoy this collection of quotes on being in sync.

“Tidal rhythms have an effect on our physiology…. When we feel out of sorts, our body is out of sync with the body of the Universe. Spending time near the ocean, or anywhere in nature, can help us to synchronize our rhythms with nature’s rhythms.” –
Deepak Chopra

“Sometimes you are in sync with the times, sometimes you are in advance, sometimes you are late.” – Bernardo Bertolucci

“Be out of sync with your times for just one day, and you will see how much eternity you contain within you.” – Rainer Maria Rilke

“When mind and action are separate, zen is lost. We keep the two in sync by paying attention.” – Philip Toshio Sudo

“There’s something about the rhythm of walking, how, after about an hour and a half, the mind and body can’t help getting in sync.” – Bjork

“Part of the joy of looking at art is getting in sync in some ways with the decision-making process that the artist used and the record that’s embedded in the work.” – Chuck Close

“It happens so quickly it seems like it’s coming from somewhere else. It’s not It just means that you’re in sync with yourself. And whatever your goal is, in terms of hearing a melody or a lyric, the closer you get to it, the faster it comes out and the easier it is to “spit it out”, as it were.” – Harry Nilsson

“Dream big, as long as you do it in sync with your truth, with your heart, your brain. And you are not hurting anybody, go ahead and do it.” – Angelique Kidjo

“I feel that all you can do is give it your absolute best with whatever gifts the universe has given you. And if you make it in some way that other people can recognize, that’s fine. But even if you don’t quote-unquote make it, you’re fine, if you’ve given it your whole heart and soul. You’re totally in sync with your purpose and with the universe. And that’s fine.” – Alice Walker

Explore The Essential Collection Of Creativity Quotes here.

Discover more quotes in my social networks.

View The Essential Collection Of Creativity Videos here.

exposurevaluechart

In photography, exposure value (EV0 is a number that represents a quantity of light. Each increase or decrease in number indicates a doubling or halving in the amount of light; often referred to a stop of light.

Although multiple camera settings can yield the same EV, they often don’t produce the same image characteristics. The shutter speed (time – how long the shutter stays open) determines the amount of motion blur, the f-number (aperture – the size of the hole that lets light into a camera) determines the depth of field, and the ISO (sensitivity – a way to boost image signal) determines the amount of noise.

“The exposure triangle” is practical way of visualizing the interaction of these three variables; shutter speed, aperture, and ISO. To achieve optimum exposure one must balance all three and an adjustment in one requires an adjustment in at least one of the others.

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Read more on Exposure techniques here.

Learn more in my digital photography and digital printing workshops.

evcrcomparisons


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1 EV is equivalent to 1 F-Stop of brightness

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These Contrast Ration (CR) figures are approximate

Dynamic Range

Today, many people think HDR refers to the practice of merging bracketed exposures with software, but HDR actually applies to everything in an imaging workflow – capture, processing, display, and printing.

What is HDR? HDR is an acronym that stands for High Dynamic Range. It’s the opposite of LDR or Low Dynamic Range Imaging.

What is dynamic range? In imaging, dynamic range (DR) is the highest overall level of contrast found in an image. In other fields, such as in the audio industry, dynamic range is used to describe similar phenomena. In audio, DR is defined as the logarithmic ratio between the largest readable signal and background noise. DR is akin to signal-to-noise ratio. In imaging, DR refers to the entire image. Consider an image a signal – and every signal has some noise.

The values used to specify dynamic range can be charted on multiple scales. Whatever language is used to describe this phenomenon, two critical factors must be addressed; the total range of brightness and the fineness of the steps used within the scale.

Two scales are most useful for images – exposure value and contrast ratio. Exposure value (EV) is easier to use while contrast ratios better display logarithmic increases in light intensities. Both refer to the same phenomenon – relative increase or decrease in brightness.

The EV scale makes it easy to compare the ratios rather than the big numbers of logarithmic progressions; each successive EV rating represents a doubling of values. The exposure value (EV) scale has been used by photographers for ages. The International Organization for Standards (ISO) defines EV 0 at an aperture size of 1 and a 1 second exposure time. The same EV can be achieved with any other combination of fstop and shutter speed that produces the same amount of light.

‘The contrast ratio scale specifically delineates values; when you use this rating you instantly see how much greater each step in a progression is than the previous one because the numbers are so much bigger. You can convert EV to contrast ration or vice versa with the right formulas. 2 (power of EV) = contrast ratio (2*8=256 for a contrast ratio of 256:1) or EV=log10(contrast ratio)*3.32 (log10(4000)*3.32=12EV

Dynamic range, gamut, and bit-depth are often confused. Though related, they’re all different. Dynamic range refers to a total range of luminosity values. Gamut refers to a total color capacity, including saturation. Bit depth refers to the number of points of data between values or the fineness of the increments in the scale. Just because an image is wide gamut doesn’t mean it is HDR or has high bit depth, but it will contain more and potentially better data if it does. Just because an image is HDR doesn’t mean it is wide gamut and has high bit depth, but it will contain more and potentially better data if it does. You can’t convert low dynamic range, small gamut, low bit depth information to high bit depth, wide gamut, high dynamic range information. To get it and use it, you have to capture high quality information upon exposure and preserve it throughout your workflow.

How the Eye Sees

In many ways, the human eye is inherently HDR. The human eye can see a contrast ratio of up to 1:10,000 or 14 EV at any one time. And, it can adapt to varying amounts of light, extending that contrast ratio to almost 1: 100,000,000. The eye has a physical response (pupils dilate) and an incredible sensitivity (as few as 30 photons). The eye (like all of our senses) has a nonlinear response; the power of a signal needs to increase logarithmically to produce a response twice as strong. So, highlights are naturally “compressed”. The eye has local adaptation; it applies different sensitivities to different regions in our visual field of view. This last ability is extraordinarily complex. The eye is not a completely separate organ but rather an integral part of our brain. Perception is always accompanied by interpretation. Perceiving value is a coordinated construction process that considers shape, color, light, and filtration. This happens subconsciously and cannot be prevented – but you can become more aware of it. At this point physics, biology, psychology, and philosophy all intersect.

How the Camera Sees

The camera eye is not HDR, yet. While tomorrow’s cameras may become inherently HDR, today’s camera’s can be used to produce HDR images with software. Like the human eye, film has a nonlinear response to light. For film we adjust the EV to fit the amount and contrast ratio of the available light into the most useful area of its curve response. Using film, you expose generally and when compromises need to be made you favor shadows or highlights. Details lost at the point of capture are irrecoverable.

Unlike the human eye, CCDs have a linear response to light. It simply counts photons, with no scaling. Consequently, in linear capture (Raw) half the data in the file is contained in the top EV of the tonal scale and the quality of the data in the lowest EVs is comparatively poor (susceptible to noise and banding). Raw files without conversion look very dark. When converted a tone curve is applied (gamma encoded) to make them appear normal. The images are mapped to an output referring standard.

Today, camera capture, film and CCD, offer roughly 8 EV compared to the eye’s 14 EV. Tomorrow, in this fast evolving field, the EV of photographic capture will be much higher, higher than today’s software solutions offer, and quite possibly even higher than the capabilities of the human eye.

HDR Software – Merges

Comparatively recent advances in software technology allow us to combine the data from multiple bracketed exposures into a single file. Here’s the fundamental difference between a single Raw file and an HDR file created from multiple Raw files. A Raw file captures the scene as a sensor sees it. An HDR file captures the full values of the scene. You can “reexpose” this HDR scene digitally as often as you like by taking a snapshot of it and developing an LDR image for output.

Rather than applying output referring gamma curves to an image, you choose how the tonal values are rendered. Tone mapping takes a scene referred to an output referred state. Once merged into an HDR image, exposure becomes adjustable; you can select any single exposure or any combination of all of the exposures; you can even do this selectively.

The extra data gives you astonishing fidelity and flexibility. You lock into a solution only when you render the image to LDR. If you preserve the HDR image, you can always return to it and rerender it, in another way, at another time. HDR is essentially not bound to the limits of a capture device or an output referring rendering.

32 Bit

HDR merges use 32 bit editing spaces to hold all the data contained in multiple 16 bit bracketed exposures. Creating an HDR file from 16 bit (65,536 shades of gray) capture does not create a true 32 bit (4,294,967,296 shades of gray) file. Few capture devices today deliver a full 16 bits of data. Software, like Photoshop, use 16 bit editing spaces to hold data over 8 bit (256 values). (As an aside, for complex coding reasons, a 16 bit file in Photoshop actually offers 15 bits of data plus 1 value or 32,768+1.) 32 bit file sizes are twice as large as 16 bit files (4 times as large as 8 bit files).

Adjustments made to high bit data display fewer artifacts, but, whether significant or insignificant, there are artifacts nonetheless. Tweak more and you continue to lose values and encounter rounding errors, possibly producing noise and banding. Raw files adjust the appearance of a file during conversion to a gamma encoded color space. Whether 8 or 16 bit, these are still LDR solutions; only whole values exist (128 or 129). HDR floating point solutions allows for decimals (128.1 or 128.15)(You can split the bit.) providing a near infinite number of values. No gamma is needed because you can get finer levels whenever you need them. Image data can then stay in linear space, unchanged by our perception of them. The upper and lower limits disappear. You can assign any value to any pixel and then adjust the relative relationships of other values to it. Depending on how a tonal structure is mapped, values may stray out of the histogram – what’s being used – but they are never lost.

As there are currently no 32 bit devices, 32 bit files must be rendered (interpreted) to be seen.

Displaying HDR

Currently, viewing HDR images is challenging. We use LDR monitors, LCD monitors achieve 9 EV, which can give us only a limited preview of the total data contained in an HDR file. However, technology is constantly advancing. Canon/Toshiba SED monitor technology offers 17 EV (or 1:100,000) and Brightside LCD/LED combination monitor technology offer an EV of 18 (or a contrast ration of roughly 1:200,000). What will tomorrow’s monitors look like?

Printing HDR

Images in print are limited to roughly 6 EV. You can move the smaller window of the printable DR up or down within the larger captured DR; the larger that captured DR the more flexibility you have. HDR offers more options in print. It doesn’t offer higher dynamic range print materials. It’s difficult to create a surface that reflects less than 1% of the light it receives and impossible to create a surface that reflects 100% – though light emitting substrates exist today and will become increasingly available tomorrow. Many new technologies are starting to expand the 6 EV in print limit. There’s still a great deal of room for improvement before we catch up to the eye’s 12 EV.

HDR Is About Creative Control

How far do you want to go with HDR? It’s really up to you. But, you probably don’t want to go as far as you might think. As the brain tries to decode reflected light based on what it sees, extreme ranges of illumination may hinder rather than help perception. The artist who removes excess dynamic range in an image may actually make it easier for you to see the scene, than if you had to view and interpret it. To a large extent, finished images are already fully interpreted. HDR imaging techniques do more than solve technical challenges, they also give you the greatest freedom of interpretation. HDR technology and practices give you maximum control over the look and feel of your images.

HDR imaging (sensors, processing, and viewing) is the future of photography; it has been since the invention of photography and it continues to be today.

Read more on HDR techniques here.

Learn more in my digital photography and digital printing workshops.

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