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Different HDR renderings accentuate different artifacts

HDR (high dynamic range) imaging captures extreme contrast ratios and subsequently renders them for LDR (low dynamic range) devices, monitors and / or prints. The very things that make HDR renderings appear natural can make them appear unnatural if taken too far.

Midtone compression

You can’t avoid midtone compression, they get caught in the middle when the relationships between highlights and shadows are compressed. But you can take steps to minimize it by being sensitive to this when choosing compression settings and amounts and by taking subsequent steps to expand it.

Tonal inversions

Some compression routines and settings can be so aggressive that they create inversions or solarizations of specific tonal relationships. Avoid this, there is no subsequent cure. If you like the overall effect of an aggressive setting and the inversion is contained to one area of an image you can render an image twice, once for the overall effect and once for a specific area, and then blend the two together using Photoshop’s layers and masks. 

Saturation Distortions

Saturation changes when lightness shifts but color stays the same. Because HDR produces effects that can be aggressive and localized to specific set of tones, the saturation shifts that accompany tonal compression often appear unnatural. Selectively adjusting the saturation of specific hues, with tools like the HSL panel in Lightroom or Camera Raw, can often convincingly cure a majority of these side effects and hide the rest.

Halos

HDR softwares help restore midtone contrast by accentuating contours. When used aggressively this edge contrast can produce halos.

Over the years, these algorithms have dramatically improved their ability to treat the halo (light line) separately from the line (dark line), suppressing the first more than the second. Sometimes, to avoid distracting halos at the border of skies, you may want to make a second rendering for the sky and blend it with another rendering using Photoshop’s layers.

Exaggerated Texture

The contour accentuation that HDR softwares use to help restore midtone separation can exaggerate texture, for better or worse. For this reason, HDR software routines can be excellent detail enhancers that produce different results than the standard image sharpening tools. Once again, the trick is not to overdo it.

Noise

Noise accentuation in HDR images is a bi-product of tonal compression and the routines designed to exaggerated micro detail. Just like LDR exposures, to avoid unnecessary noise use the lowest practical ISO setting. In addition, use exposures that are bracketed by two or fewer stops. Finally, plan to reduce noise after rendering; recent noise reduction algorithms are capable of producing remarkable results.

Banding

To avoid banding (most visible in smooth gradations) use exposures that are bracketed by two or fewer stops. Process files in a high (16) bit depth. When you see banding appear during rendering, change the settings that produce it; there is no satisfactory cure for banding in post-processing.

The vast majority of objections to HDR imaging come from observations of examples where practitioners were oblivious of or insensitive to these regularly occurring artifacts. When HDR renderings are made with these potential pitfalls in mind, the objections disappear. In many cases, viewers don’t realize HDR software was used in the production of a photograph. Some HDR renditions challenge classic photographic aesthetics and offer new ones. If viewers do recognize that these artifacts exist in images and they are appropriate for the visual statement being made, not unconscious bi-products but conscious choices, then viewers who are open minded may experience a sometimes surprising new window onto the world.

Read more on HDR techniques here.

Learn more in my digital photography and digital printing workshops.

8 HDR Myths Debunked

December 12, 2016 | 2 Comments |

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There are many misconceptions surrounding the practice of high dynamic range (HDR) photography. Here are eight – debunked.

HDR is new

Within the first five years of the invention of photography photographers began bracketing exposure to extend the dynamic range of photography. They used chemistry to process their negatives instead of software to process their files – but they still bracketed exposures to capture contrast ratios that exceeded paper, glass, and film.

HDR is hard

High dynamic range imaging has become so commonplace that cameras and software make it increasingly easy to practice HDR techniques – auto-bracketing, merging and rendering.

HDR requires the use of a tripod

While there are times when the use of a tripod is required, when exposures are long in duration, in a majority of cases current cameras’ auto-bracketing features and softwares’ image alignment algorithms make hand-held exposure bracketing highly practical.

HDR requires exposure bracketing with a fixed aperture

Aperture priority (fixing the f-stop) is recommended during bracketing to avoid changes in depth of field between exposures – and so the appearance of a loss of focus, which in extreme cases can make alignment challenging. However, it’s entirely possible to bracket exposure value by changing aperture, especially when depth of field issues are minimal. Shutter speed (motion), aperture (depth of field), and ISO (noise) can all be used to bracket; each has different consequences.

With HDR, you get better results with more exposures

Using too many exposures (half or quarter stops) can produce almost as many artifacts as using too fiew exposures (more than 2 stops); chiefly banding and excessive midtone compression.

HDR looks unnatural

The appearance of HDR images is flexible. What software you choose to render your image and how you choose to use it is up to you. You can produce a classical or a contemporary appearance with many possible variations in between. Whether the technique becomes invisible or obvious is a choice.

You can only choose the look of one HDR rendering

You can blend different renderings of the same image either globally or locally. (Use Photoshop’s layers and masks.) Again, how you choose to render your images is controled by you, not technology.

HDR is cheating

Really?

Read more on HDR techniques here.

Learn more in my digital photography and digital printing workshops.

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back LCD menu

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top LCD menu

It’s easy to set your camera to auto-bracket. The hardest part of this process is navigating a camera manufacturer’s menu. Once you find it – and do it several times – you won’t forget it.

Here’s how to do it on current Canon cameras – the steps are similar for other cameras but the buttons and menus vary.

First, set the number of frames made in each bracketed sequence. Press the Menu button. Use the main command dial (top) to cycle through the menus on the LCD screen (back) Go to the 4th tab (small camera) > 1st list and then the use the jog wheel (back) to select the 5th item. Press the set button to select it. Use the jog wheel to select the number of shots and press the set button once again. While 3 is the most commonly used, it’s not unusual to use 5 or even 7. Because 3 is the most commonly used number, it’s likely that once you set this, you’ll reset it infrequently.

Second, set the difference in exposure values between shots. Press the Menu button.

Use the main command dial (top) to cycle through the menus. Go to the 1st tab (large camera), 2nd list, 1st item. Use the set button to highlight this function and the command dial (top) to set the difference in exposure values, then press the set button again. Remember! Press the set button at the end of this – if you don’t these new settings won’t be saved.

Remember! Continue shooting in bracketed sequences as long as auto-bracketing is turned on. When you return to shooting single shots, turn auto bracketing off. If you don’t, one out of three exposures will be improperly exposed – because your camera is still bracketing exposure. The quickest way to turn auto bracketing off is to turn your camera off and on, which will turn auto-bracketing off on most cameras.

When auto bracketing is activated, on the display at the top of your camera you’ll see a series of bars indicating how many exposures will be made and at what exposure value.

What about exposure compensation? Use exposure compensation or not. Bracketing is extreme exposure compensation. All that matters is that you create multiple exposures that once combined render both excellent highlight and shadow detail.

For HDR exposures that are hand-held set your drive or burst mode to continuous; the fastest setting you have; One Shot is too slow.

For HDR exposures using a tripod, consider using a cable release and mirror lockup to further reduce camera motion.

Practice turning on and turning off auto-bracketing and it won’t seem nearly as complicated as the first time you do it and in time the habit will become so ingrained that you won’t have to think about it any more. But until turning auto-bracketing on and off becomes second nature, proceed carefully and methodically. The most common mistake is to turn it on and forget to turn it off.

Read more on HDR techniques here.

Learn more in my digital photography and digital printing workshops.

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Today’s cameras have the ability to generate HDR merges on the fly. The problem is that they produce JPEGs with a smaller gamut (lower saturation), lower bit depth (fewer shades of gray), and compression artifacts (noise and jagged edges) and they offer no control over the tone mapping process.

If you want a better HDR file, choose to make multiple bracketed Raw files, then merge and tone map them manually. Remember, aside from exposure settings, in camera settings that affect the look of your image have little or no affect on Raw files, which can be processed any way you want to process them.

In camera HDR JPEGs can offer a fast and convenient preview of potential HDR results. You can get the convenience of one and the quality of the other by setting your camera to produce both JPEG and Raw files simultaneously.

Read more on HDR techniques here.

Learn more in my digital photography and digital printing workshops.

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1 exposure

scenes like this won’t benefit from HDR

Incubation II

 3 exposures

scenes like this may be made with 1 exposure but benefit from more

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5 exposures

scenes like this require HDR

How many exposures do you need for HDR images?

It depends.

It depends on the contrast ratio of the scene you’re photographing.

And, it depends on how many corners you’re willing to cut.

There is a case to be made for rendering all images with bracketed exposures. If the best data in a file is in the top stop of the dynamic range of a camera, then making one exposure per stop of dynamic range in the scene, each weighted to the top stop pf the camera (ETTR), will give you ideal data in every stop of the rendered scene, once the exposures are merged. So, ideally, you’d make one exposure per stop of dynamic range in the scene. In your first exposure place the shadows in the top stop of the histogram (to the right), without clipping. Then in subsequent exposures reduce exposure in one stop increments, making a new exposure each time, until the highlights are placed in the top stop without any clipping. Then stop.

Practically, you don’t need to make one exposure per stop of dynamic range in the scene because detail rendered by data in the middle of the histogram, though potentially not ideal, is perfectly usable. Here’s where theory can overtake practice. Test this yourself and find out whether using many more exposures is really worth the effort. In my own tests, I haven’t seen enough difference to make this a regular practice. If you can’t see a difference, then the difference doesn’t matter.

If data in or above the middle of the histogram is good enough, then some scenes will have so little dynamic range (one, two or three stops) that you only need one exposure. Remember to expose to the right to get the best data. Files like this will look over-exposed until they are processed. Once again, for these types of images, using more exposures and processing them with HDR software routines may actually cause problems like posterization.

Surprisingly, some images that look ‘normal’ and don’t seem to need HDR can actually benefit from additional exposures; images where shadows are close to the left of the histogram; the last three stops to the left have dramatically less data to render tonal values and are prone to excess noise and posterization.

For scenes that exceed the dynamic range of a camera sensor and can’t be captured without clipping in a single exposure, the number of exposures needed to render them depends on the contrast ratio of the scene – some need two, some three, some four, some five …

How about making more even exposures and bracketing in half stops? Theoretically this would work. In practice, you’re likely to find that using too many exposures causes some artifacts like excessive midtone compression and posterization.

The number of exposures you need also depends on the number of stops of difference you choose between exposures. Using exposures one and a half stops apart usually produces results just as good as those made with one stop apart. Using exposures with two or more stops of difference between them may cause posterization, which is most visible in smooth gradations, like skies. Using exposures less than one stop apart usually does not produce better results. In a few cases using too many exposures can produce artifacts like excessive midtone compression and sometimes posterization.

In the end the number of exposures you use matters because if exposures are too close or too far apart results will not be optimal, nevertheless the final number of exposures you need depends on both the scene you’re photographing and the camera you’re photographing it with.

Read more on HDR techniques here.

Learn more in my digital photography and digital printing workshops.

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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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