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Posted: Sun Dec 03, 2006 9:48 pm Post subject: Intro to HDRI (bandwidth warning)
(This post was originally made in the photographymonthly forums)
Before I start this marathon intro into HDRI, I have a quick disclaimer I am not the world's foremost expert on either Photography or HDRI. Rather than take what I say as gospel I encourage you to go and play, and find out for yourself what's possible with the technique.
There is a goldmine of amazingly good, and shockingly bad HDR/Tonemapping examples on flickr, so I'll suggest that as a place to start. I've listed some of the main groups that i've stumbled across over the last few months at the bottom.
-----------------------------------------
We all know that cameras only have a limited dynamic range (about 5 stops for a 12-bit raw), far less than the human eye. It's something that photographers have always lived with, and usually can be taken advantage of. There are times however, where the limited dynamic range is a bind, not a blessing. We've all suffered from exceedingly tricky lighting conditions - it's where the serious photographers shine at getting the results they're looking for where the rest of us tend to hope for the best and often end up being disappointed. Although most cameras are going to let you bracket 1/3 or 1/2 a stop either way there will always be the situation where you're still going to lose something, beit details in the shadows or details in the highlights..
or are you?
The basic principle of HDRI is quite straightforward - expand the dynamic range by taking information from a range of differently exposed images of the same subject and saving it in a single high range (usually 32-bit) image.
It might sound like a lot of extra work, but if your camera has an auto bracket mode then half the work has been done for you already. My default starting point is to set my D70 to bracket 2 stops apart using the AEB function. Just think about how much extra detail you could capture using an exposure range like that - there's nothing that can compete using just one exposure. You can bracket closer, but the effect will be less pronounced.
There's nothing stopping you taking more than just the 3 auto-bracketed images either (The D200 can AEB for 5); there's a lot more manual intervention in this case so it's not always suitable, and you'll only see a significant improvement when there's a very wide range to be captured.
I'm going to use a shot from my parent-in-law's farm taken in September as an example. It's not the world's best photo, but should give a good example of a situation where there's a case for using HDR.
The sun was just off to my right and was quite strong with it. Because of where the tractor was parked, I couldn't get a shot from the 'sunny' side, so had to set up in the shadow.
Nothing special about the settings - it's down to personal preference, but in this particular case the shots are taken 3 stops apart.
'Normal' (0ev):
+3ev:
-3ev:
Not particularly inspiring, are they? In fact, before I'd heard of HDRI/Tonemapping, I'd have quite happily binned them all.
The 3 shots are then merged into one HDR image using Photomatix. There are a number of different apps that will perform this task and I'm sure with the mini-boom that's occured regarding HDR there'll be plenty more in the near future
When you load the image up, it'll look something like this:
There's so much range, that monitors can't show it - you'd have to play with photoshop using effects to see the difference in behaviour between it and an 8-bit image. Although the example here has been changed down to 8-bit jpg, what you see here is pretty much how you see it in photoshop when it's a 32-bit HDR.
Now the fun bit.. tone mapping
To get that rediculously wide-ranged picture to something we can actually enjoy, we have to compress the range back down to LDR (Low Dynamic Range) again.
So what's the point?
The thing is, because software has more information to refer to than a straight single exposure, it can make better decisions on where to make the savings - details can be kept so the overall result is still far, far better than if we'd just taken the one photo.
I can't say I've delved particularly deeply into the mathematics of it all - the wiki entry goes into quite some detail regarding tone mapping if you have a spare lunch break to peruse it.
The photomatix cs2 plugin and the standalone app are fairly similar in operation - and there's some significant power at hand which you have to be careful with if you want your photo looking reasonably realistic
With almost the default settings, and a slight contrast tweak, we're left with a reasonable result:
Photomatix allows quite a range of adjustments and some of them get pretty mad - it's certainly worth downloading a demo of the main contenders to see what you think - they tend to be fully functional but leave water marks on the final image.
We've still not achieved an ideal result with the above selection though. The range of light was so great that 3 exposures wasn't enough to capture everything, even at 3 stops apart - the clouds are still blown out!
Fortunately for me, the reality is I didn't take 3 shots as I usually do, I took 7, 1 stop apart :D I've since learned that ideally I should have really taken 9 images 1 stop apart, or maybe 5-7, 2 stops apart in this situation. As I said at the top, I'm still learning about this malarkey myself, but at least I've got a good example of where 3 images @ 3ev increments isn't quite enough!
To take 7 shots I metered, set to manual, then knocked it down 3 shutter speeds so the 'metered' photo was the middle exposure.
For completeness, here is a link to the 7 images (3 of which you saw earlier)
awesome post...make it a sticky venom!!! _________________ The sun cannot set,
Because you are not here,
To see it,
The day cannot end,
It can't be right,
That night will come,
Without you.
any chance of a step by step guide on how to!!?? _________________ The sun cannot set,
Because you are not here,
To see it,
The day cannot end,
It can't be right,
That night will come,
Without you.
you usually get errors if you try to produce a number of images from the same raw file (creating over and under exposed images by means of exposure compensation during raw conversion)
photomatix looks at the exposure information to blend the images, so if all your images contain the same exposure info, it throws a strop.
if this is the case, strip all the exif out and manually set the bracketing range - that should sort you out.
if it's not the above, let me know, although i think the above does cover 99% of all merging errors _________________ There is nothing in life that cannot be improved with either monkeys, pirates or ninjas NPU Member
have a look at the photomatix website, they have a free download of the program (it puts watermarks on finished images - sometinmes it doesn't but that depends a little on what you done - but I go astray.)
It also allows you to play with a single image - it's well worth trying out, and it also gives some very good examples.
Awsome thread,but I use a canon and I still havent learnt how to change settings.also on canon theres no ev (eg)-3ev -2ev -1ev 0ev etc...so what do I do for hdr on canon.I have been wanting to try hdr for some time now.I have heard a lot about bracketing but I dont know wher to start.Any tips.
All of canon's dslrs are listed there, so you'll just need to trawl your user manual for details
That said, you can always meter then switch to manual, and manually take the correct number of exposures - that's what i did for the tractor pics in the original post. _________________ There is nothing in life that cannot be improved with either monkeys, pirates or ninjas NPU Member
Awsome thread,but I use a canon and I still havent learnt how to change settings.also on canon theres no ev (eg)-3ev -2ev -1ev 0ev etc...so what do I do for hdr on canon.I have been wanting to try hdr for some time now.I have heard a lot about bracketing but I dont know wher to start.Any tips.
Chris, the 350D does do it, its AEB (Auto Exposure Bracketing) on the Canon, Its on page 90 in the Manual.
In the menu go to the second page and its the first one (you must have it set to one of the creative modes (P, Tv, Av, M or A-DEP).
If your not sure next time you see me on Yahoo give me a buzz I'll go through it with you.
Generally yes. The effect may not be exactly the same, but should work well for starters. The only real problem with this is you're limited to the dynamic range of the single image you use which could result in clipping or limiting the effect. Multiple original images, when combined, will provide much more range for the software to work on.
But by all means, give it a go - I've had some success with it at times. _________________ see my photography at john.seelbinder.org
OMGoodness what am I doing wrong? I thought this would be cooler with my husband's Harley than flowers but when I did it I used 6 shots on a tripod all at different exposures. Now it's all pixelated LOL I mean from far away it's great except I need to do a little more editing in the wheels and things but the pixel mess is gonna drive me nuts on a print. Help please!
Not too sure how much under/overexposed the shots were, but try it with 3 shots - one at -2EV one at 0 and the other at +2 EV. _________________ -----------------------------------------------------
I think alot of the output depends on the picture you use...
I'd have to agree with that. My limited experience has been spotty. Some images work better than others depending on a lot of factors.
How much dynamic range is in the images (i.e. -1 EV to +1 EV or -2 EV to +2 EV), how many images are used (2, 3, 5, 7?) and the image content. Photomatix has worked miracles with some images, and completely messed up others with lots of paint brush-like halo effects.
I haven't figured out the magic incantation to predict and shoot the right combination yet. So far it's been the luck of the draw.
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I am not the world's foremost expert on either Photography or HDRI. Rather than take what I say as gospel I encourage you to go and play, and find out for yourself what's possible with the technique.
on the flowers. When I took the pictures I bracketed manually but I also moved an inch or two to either side and adjusted my zoom LOL So NOW I have 5 bracket photos that are completely different of the same flowers and I can't use them because it doesn't look right. They won't superimpose correctly because they're different. I was trying to get around setting everything up again but it looks like i'm gonna have to anyway :-( Back to the drawing board uuughhhh
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