This is the first part of the tutorial and will cover some basic definitions and terminology you need to get started with HDR photography. In the second part of the tutorial we will cover the workflow to create your first HDR image!
What is HDR?
HDR is photographic technique used to achieve a greater dynamic range of luminosity than what is possible with standard digital photography.
Think about HDR as what you see every day through your eyes and can't be captured properly by photographic gear.
In order to explain how HDR works we need to introduce some basic terminology
Exposure
It is a number that describes an F-Number/Exposure time pair. A difference of 1 EV corresponds to a 2-exposure stop.
Any increase of the exposure value by one equals doubling the exposure, any reduction by one equals halving the exposure.
The human eye can perceive about 20 EV of dynamic range in ideal circumstances. That corresponds to a real-life scene with a contrast of 1.000.000:1. This ratio tells the difference between the brightest (e.g. the Sun) and the darkest (e.g. deep shadow) point of a scene.
Conversion light value to contrast range: 2^EV = Contrast.
A standard camera takes pictures with a limited exposure range, referred as LDR (low dynamic range) photographs. The result is a lack of details (information) in the bright and the dark areas.
Regarding camera's dynamic range (examples):
- A Canon 5D MK IV has a dynamic range of about 12000:1
- A Nikon D750 or Nikon Z6 has a dynamic range of about 22000:1
- A Sony A7III has a dynamic range of about 26000:1
Bracketing
The advantage of HDR is to be able to represent many levels of light intensity in an image. To obtain this, we need several shots of the same scene taken with different exposure times, this is called bracketing. The goal is to darken the bright areas and to brighten the dark areas to retrieve the details. These brackets are then merged together with a dedicated HDR software.
Tone mapping
HDR images use more bits per pixel than conventional images and allow a much higher dynamic range to be stored. HDR files have 32 bit/pixel and can be saved as Radiance's RGBE format or as ILM OpenEXR format for example.
You maybe know that it is not possible to display an HDR photo on a typical monitor without a special conversion step known as tone mapping. Primary purpose of tone mapping is limiting luminosity of HDR image so it fits in the range that monitor is capable of displaying correctly. The tone-mapped image becomes LDR image (Low Dynamic Range).
LDR pictures can be stored in 8-bit bit/pixel (e.g. JPEG) or 16-bit/pixel (e.g. TIFF).
This leads to a conclusion that HDR is in fact a trick, something allowing us to overcome limitations of current devices. It uses photo with much wider luminosity range and it maps it back to the space, which is possible to be displayed on our monitors.
Pseudo HDR
You might see some example of images created from a single RAW file. This is something called pseudo HDR and it differs from HDR that can only be obtained with bracketing.
Example
In this photo, we exposed correctly for the highlights. You can see the detail in the sky but the shadows are black.
In this other photo, we exposed correctly for the shadows. Now you can see the shadow details, but the sky is white.
Now we have taken 5 brackets and combined them in a HDR Software. As you can see, the image has a lot more information with clear details of both shadows and highlights.
You completed the first part of the HDR tutorial. Go to the second part to learn about the HDR workflow and how to create your first HDR image!
Photos and tutorial credits: philta22