[GainfulShrimp]

What is HDR and Tone Mapping?

Because of the limited dynamic range of camera sensors, exposing a shot correctly often means losing details from either highlights or shadows, or both in extreme cases (depending on the scene).
High Dynamic Range imaging allows more of these highlight and shadow details to be retained, by combining several exposures of the same scene at different shutter speeds. The highlights are used from 'underexposed' shots and the shadow information is taken from 'overexposed' shots. By combining five or more exposures, a huge dynamic range is possible.
Tone Mapping is used as a method of scaling (i.e. mapping) the high range of luminance values contained in an HDR image into a range which looks good on devices which have a limited dynamic range - like monitors and printers. A tone reproduction operator is used during tone mapping to retain perceptual qualities of the original scene - such as contrast and fine detail.


About this tutorial

I'll use Photomatix Pro 2.2.1 to combine several shots, taken on a tripod, into an HDR image - Exposure Blending. I'll then use the Tone Mapping feature of Photomatix on the generated HDR image, saving it as a 16 bit TIF. Final touches and webification will be done in Photoshop CS2. There are some other options for non-tripod shots discussed later, too.

I've bought the full version of Photomatix, but a trial version is available.

I've split this into three main steps: taking the photos, blending them to make an HDR image and finally tone mapping the HDR image. Then there's some more of my waffle to round it off.

1. Taking the photos

I used a Canon 20D on a decent tripod to take several shots of our living room and french doors. A very boring subject, but the view outside the window included some bright stuff outside and the shadows in the room were relatively dark... so it was a good scene for demonstrating what HDR can do elsewhere with more interesting subjects.

Because the idea is to capture as much detail as possible and use the full dynamic range available from the sensor, I shot in RAW format (I always shoot in RAW, in truth, but it was a really good idea on this occasion). I selected ISO100 to get nice noise-free images. I used mirror lockup (aka MLU - selectable from the Custom Functions menu) and self-timer to help reduce any shake introduced by my finger on the shutter or the mirror action. (Use a remote shutter release instead of self timer, if you prefer.)

I wanted a large depth of field - to get both the room contents and outside scenery in focus - so I selected f/16 aperture in M (manual) mode. The camera's evaluative metering recommended 1/2sec exposure time.
I wanted to cover a wide range of exposure values: -2EV, -1EV, 0, +1EV, +2EV. One stop/EV increase/overexposure requires doubling the light to the sensor - i.e. doubling the exposure time. Likewise, decreasing by one stop (underexposing) is achieved by halving the light to the sensor - i.e. halving the exposure time. Aperture should remain constant at f/16, so that each picture has an identical depth of field.
So, given our rough estimate of 1/2sec exposure time, these are the manual settings that I used:

+2EV = 2 secs
+1EV = 1 second
0EV = 1/2 second
-1EV = 1/4 second
-2EV = 1/8 second

This resulted in five shots, which looked like this:

2 stops overexposed (+2EV): 2secs, f/16, ISO 100


1 stop overexposed (+1EV): 1secs, f/16, ISO 100


correctly exposed: 1/2sec, f/16, ISO 100


1 stop underexposed (-1EV): 1/4sec, f/16, ISO 100


2 stops underexposed (-2EV): 1/8sec, f/16, ISO 100


In the next part of the tutorial, I'll combine the five images using Photomatix's Exposure Blending feature.

2. Creating the HDR image

The RAW files were converted into unsharpened 16 bit TIFs (low constrast, otherwise default settings - use your favourite RAW processor). In Photomatix, select Generate HDR from the HDRI menu. This opens a dialog to open your images. I clicked browse, selected my five images and clicked OK.
A window then pops up called "Exposure Values for Generation HDR Image", asking for confirmation of the Exposure Value guesses made by the software:



In this case, the guesses are correct: 2, 1, 0, -1, -2... so I just click OK. On the next windows, I select Use standard response curve (recommended) and click OK:



After a few seconds processing, the HDR image is displayed. Because the colour space of the display can't show the full range of tones included in the image, an 'HDR viewer' window is displayed so you can check details from anywhere on the image, like this:



Now we've got our HDR image, we can use Tone Mapping to process the dynamic range back into something we can easily display as a normal 8 bit JPEG...

3. Tone Mapping

Still in Photomatix, from the HDRI menu, select Tone Mapping. A settings window appears together with a preview of the tone mapped image:



You can read the online help or experiment with each setting to find out what it does. I used the default settings here apart from setting output pixel depth to 48bit, then clicked OK.

I saved a 16 bit TIF and then opened the file in Photoshop to apply some finishing touches.

Tone mapped image:

http://matthewh.com/images/tutorials...2336-01-TM.jpg

After sharpening, levels and curves adjustment:



So that's it - a bit pants but it shows what can be done with a more interesting subject and some imagination, at least...

Next section discusses further stuff to try and how to use Tone Mapping on single RAWs.

Problems encountered and things to try next time

There are a couple of snags with my image above - apart from the fact that it's an awfully boring arbitrary shot of my living room - (1) parts of the sky are still blown and (2) Ann has turned into a two headed monster due to her moving slightly in between my multiple exposures.
To address the first issue, you would simply take more shots to feed into Photomatix to make sure you covered both the highlights and shadows more comprehensively.
The second issue is trickier to resolve and demonstrates a problem with this technique - namely it only really works well with static subjects!

Something else to try is using an image stabilized lens with a steady hand, autobracketing (set it at +/-2EV) and continuous shooting mode... which allows you to get almost-aligned shots without the use of a tripod. I achieved a reasonable boring HDR pic this way, but a tripod is preferable for sure. (Also, notice the cat at the bottom of this pic moved between the three frames, so it now looks like a ghostly cat...) During the 'response curve setting' step of Exposure Blending, you can check a box to tell Photomatix to attempt to align the images before it blends them. This seems to work reasonably well.

If you use Photoshop CS2, you could use it to generate HDR images (look in the File > Automate menu), but it's not as easy as using Photomatix because CS2 insists on using many images and is much more picky.


How to use these techniques on moving subjects (or single RAW files)

It's possible to treat a single RAW file as an HDR image... which you may think is cheating, but it actually works quite well with certain images.
The dynamic range of a RAW file is quite a bit more than that of an 8 bit JPEG, so it makes sense to try to squeeze the maximum amount out of these files using Tone Mapping.

To do this, you should try to choose a subject with a reasonable dynamic range to start with - don't expect miracles - remember we're not creating an image with a dynamic range bigger than the sensor, like we can with Exposure Blending. Then shoot in RAW, overexposing slightly: err on the side of losing a bit of highlight detail in the sky but keeping as much shadow detail as possible. You need a nice, clean, noise-free image for this, so use as low an ISO value as you can.
During RAW conversion, bring down the exposure slightly to make up for the slight overexposure and save as a 16 bit TIF again.

Open the TIF in Photomatix, and select Tone Mapping from the HDRI menu. You'll need to be a bit more careful tweaking the settings when using a single input file as the source to get the best results. Here is an example I created from a single RAW file:

Result using my normal RAW workflow:
matthewh.com gallery - France 2005/20050719-161024-01

Result after tone mapping and some curves adjustment:
matthewh.com gallery - High Dynamic Range (HDR)/Montaubin

Good luck with your HDRs and please do share the results!

Cheers,

GS

[stepheno]

Thanks for sharing your tutorial GS. It will be helpful to members

[silkstone]

That's very good GS - many thanks.

[Angela]

Thanks GS! That is gong to be incredibly helpful!

[j sotelo]

I'll keep it in mind I have Photomatix Pro 2.4 but it should work the same.

THANKS !

[GainfulShrimp]

Cheers chaps.

I think I'd better take a look at the latest version of Photomatix Pro then... it's not something I've thought of updating since I initially bought it quite a while ago. :o

[fionaB]

great tutorial......... will put photomatix on my list of "wants" have never been into HDR but i think its time to start experimenting with the dark nights coming in

Fi

[Dave]

How to make an impact on the site .... great tutorial

[Steve]

Absolutely fantastic HDR tutorial. Thank you very much for sharing it with us.

Reputation points duly added

[P-E]

Nice one

[simpbow]

Great tutorial, got trial software to use so thiswill be a great help.