HDR, explain please....

[Cliffs]

I had a small brainstorm session with myself while watching football on television today.

The subject, HDR photography.

I have read quite a few articles, forum posts and blogs about this subject, yet I can't for some reason shake the thoughts I was having.
I'm hoping someone with way more photographic knowledge than I (that would be just about all of you), can explain why my thinking may be misconstrued.

Okay, I begin....
From what I gather HDR is the combining of more than 2 photos of different exposures into one photo to be post processed.

Now if I shoot 3 photos, one at -2 ev, one at 0 ev, and one at +2 ev, then I open these in a program like photomatix and begin the process to combine them into one, isn't the end result going to match the 0 ev shot?

Am I simplifying what HDR really is?

I mean I look at this as a mathematical equation....-2 ev + +2 ev should equal 0 ev, should it not?

How does the program change this calculation?

Is the difference really that much different?

Any feedback is welcome and thanks.

[PowerPix]

Fairly correct, but think of it this way: under exposed + correctly exposed + over exposed = exposed all around!!!!! What you are doing is just exposing the different parts (shadows, highlights) into one that showcases all the best attributes. HDR images represent more accurately the wide range of intensity levels found in real scenes. The human eye can't even recognize that much dynamic range. Stop the math, it just works!!!

[milosh]

I'm not so experienced with HDR, but I thought I might be able to try to explain.
As Stupid said, your camera can't capture all the dynamic range in front of it in a single exposure, that's why you combine more exposures, to get the details from all the exposures in a single image file.
Or, if you want a mathematical (sort of) explanation, maybe this would work:
Your camera can capture only 2 digits of data. So, if you expose to 0 ev, you will record the data from -1 and +1 digits. Now, if you take two exposures (-2 and +2) and merge them into an HDR, you'll get the data from both -2 and +2 in a single image. Hope it makes sense!

[wulf]

I think milosh is along the right lines but it can be expanded further. The camera has a limited dynamic range. It can capture more exposure steps than -1 -> +1 but noticeably less than the human eye.

For sake of argument, lets say the camera can catch 7 different intensities of light. I don't have exact figures to hand but it gives a reasonable idea. At a centred exposure setting that will be everything from -3 to +3 (including 0). An area that is very dark (< -3) wil come out black and a bright area (>+3) will show as white. By combining that with over and underexposed shots (assuming -2, 0 and +2 as the settings) you now have information which covers -5 to +5. That is as much or more than the human eye can deal with.

The process is actually analagous to what the eye does naturally. As you look round, you build up a picture of the scene around you. Your eyes adjust automatically to varying light levels and the brain composites this information into what you "see". Step from a dark room into sunlight and it takes a noticeable time to adjust. Look into the shadows and shapes will gradually resolve themselves.

Therefore HDR can be a naturalistic process, which is why it can look like it has been painted (artists spend a long time observing a scene and so include details that a glance would miss). That, of course, has nothing to do with the very heavily processed images that are also presented as HDR which are more about garish colours and sharpening halos than seeing more fully into what is there.

Wulf

[fletch]

The human brain is the worlds best HDR processing software. The eye is constantly scanning the scene and recording different parts of it all at once. The image that is finally presented to the concious part of you brain is actually a mish mash of all the different images. Its not just exposure blending either, the brain is constantly adjusting for focus and colour as well.

The key to the eye is it processor (the brain) Its actually a pretty crappy camera.

Since the brain can't perform this trick with photos due to the limitations with prints and screens HDR tries to do the work for it, prior to printing or display. It is taking the 'best' bits from the multiple exposures to replicate what the brain would have done if it were stood their.

[Swisstony10]

It might be easier to explain with a quick example:

Here's a shot I took of a stately home at the weekend. Notice that the sky is a lovely shade of blue, but the front door is in shadow. If I'd exposed to get better light on the front door, I would've had blown highlights in the sky, that blue would've been lost, along with some of the detail on the front of the building. This is the "0" exposure:



However, I took three bracketed shots of the same scene - in addition to the one above, I overexposed one by 1.3 f-stops, which gave much brighter detail on the front of the building, including the shaded area where the door is, and one underexposed by the same amount, which gave depth to the sky. After processing them through Photomatix, I ended up with an image that picked the best bits of each exposure:



I hope that helps to put it all into context a bit better.

Russ.

[aropilot]

I'm going to jump in here and help in a laymans way.

I had a difficult time understanding it all as well. I downloaded the Photomatix HDR for free (it imprints a water-mark on the image) to play with and work with. My photos lately have been specifically take to put through the HDR process. (once I feel comfortable with the program and process I will pay the $99 for the program and it will then remove the auto water-marks from the program)

First I set my cameras (canon XTi & 40D) to take both High quality JPEG and RAW files. Yes, it takes a great deal of space on the CF card but I get to do a good deal of messing around once the images are on my hard drive.

Second I go to the menu (in my camera) and the AEB setting to get the camera to do the setting changes automatically for me. It then will set itself to take the 3 separate pics. All I have to do is pay attention and take 3 photos of the exact same thing - Use a tripod and take them quickly if they are of landscape, slow moving object, etc. Use of Tripod is extremely important!

Third, when I get home and on the computer I download them to their proper file. I actually have a file specifically for all my test pics for HDR. This is so I can easily find them when I go into Photomatix to transfer.

Next I open Photomatix, load in the 3 JPEG images and mess around with some of the basic settings just to learn the process and see some of the changes. As I continue to mess with some of the settings I'm slowly learning what each does and how to improve upon the images and the process overall.

At first I was not sure I was doing it correctly or even really seeing the differences but now that I'm doing it more, I'm really seeing the more vivid results and liking it!

I've not printed any out yet but that is my next step. Good thing Walgreen's is having a sale on their prints again!

I also find it helpful to have a book or two on my desk to assist with my learning curve. Not a specific plug here but I do have a book "mastering HDR" by Michael Freeman to help me grasp what each adjustment can do to the image. I was talking with Rick Sammon a couple weeks ago and he is in process of writing a book on HDR as well.

Hope this helps you and other non technical folks like me out there!

[Cliffs]

Great responses from all of you, THANKS. I have a better understanding now and will have to try some PP for HDR for myself BUT...

if one were to take more shots, not just -/+ 2 ev but some of the in between ev, wouldn't that in essence make for a better composite?

Then, because (wow bad grammar there) one has taken all these extra shots, would the 0 ev really not be needed or is there something in the 0 ev that the other settings might miss?

I know this can be a complicated process and I'm not looking for somebody to write a thesis on the subject of HDR...just using the curiosity factor to discover more about photography.

***************************

swisstony, your images explain the process, thanks but the question I have about the HDR image (and a lot of HDR images)...is that really what you saw before you took the picture or was the door really in the shadows but due to PP/HDR you brought it into the image?
I guess what I'm saying is, some HDR just seem to overdone, they look good but not realistic.

[Swisstony10]

I know what you mean - a lot of HDR images to seem to be done for the sake of the technology that allowed them, rather than for any artistic reason. That's not to say that they're all bad, or that there's anything necessarily wrong with that, it's just that it's not my bag.

Personally, I use Photomatix and bracketed images to give me images that come a lot closer to what I was able to see at the time I took the shot. The second shot of Montacute House's front door (above) is pretty much what was visible to the eye at the time..

[fletch]

When you look at a scene what do you see? Can you see the deep blue sky? Can you see the detail in the shadowy areas of the door? if so then there is no harm in recreating that look using HDR.

One of the reasons HDR often looks fake and computerised is that images lack overall contrast (the dark areas are very light and the ligh areas dark) but have very high local contrast (enhnacing the textures in brick etc). Its partly down to the concept of HDR but mainly down to specifics of certain tone mapping software (photomatix). It can be avoided and its a fine line between natural, fantasy land or not even HDR anymore. It can be fun trying to work it out but it shouldn't be a crutch for crappy light or composition.