High dynamic range?

PNA · Aug 31, 2006

markc said:
But yeah, there are some amazing HDR images out there. The good ones are really, really good.

How can you tell those from any others??? I understand your example......

No doubt my technical skills are limited and just pushing buttons is not the answer to finely tuning a photo.

markc · Aug 31, 2006

acsonpg said:
How can you tell those from any others??? I understand your example......

Do you mean from the HDR images I don't care for or normal images?

The good HDR images have a full tonal range and good contrast, and while they can look rather etheral, you won't find areas that should be lighter than another actually looking darker. Those are the kind of things you don't always notice right off the bat, but the brain still picks up on them and says "wrong!" Like if a shadow area under a bridge has the same darkness as a tree's shadow. The bridge shadow should be darker.

Telling a good HDR image from a regular image is more about being familar with the dynamic range of film and sensors. If you see a wide exposure range in an image, it's probably HDR. But the wider the range, the more "blocked up" it will be also, since the step between each color becomes larger. A 3 ev range across 256 levels is .01 ev per level. A 10 ev range across those 8 bits is .04 ev per level. You can end up losing fine detail as it all becomes one color, since you are compressing 10 ev to fit in the same space as 3 ev.

ksmattfish · Aug 31, 2006

"Merge to HDR" is a technique that takes 3 or more different exposures of the same scene, and uses them to create a 32 bit image file with a higher dynamic range than would be possible in a single exposure with the gear/media/materials. For DSLRs with a normal dynamic range of 6 stops or so it's possible to get a file with a dynamic range of 10 to 12 stops with 3 exposures bracketed at 2 stops apart.

This 32 bit file with the higher than normal dynamic range is not really viewable on most monitors or any print, because monitors and prints have a maximum displayable dynamic range of about 5 stops. To create an image that is printable, or viewable on a normal monitor, the 32 bit file is converted to a 16 or 8 bit image file, and the dynamic range/contrast is compressed using a variety of methods depending on your software.

The part of this that most people notice actually has little to do with high dynamic range. It's the appearance or manipulation of the local contrast that makes it look the way it does. It's completely possible to manipulate the heck out of the local contrast in a regular dynamic range photo, and get similar looking results. Burning and dodging, adjusting lighting ratios, the zone system, graduated ND filters, USM (in the darkroom or PS), contrast masking, tone mapping, and the various methods of converting from 32 bit HDR files are all techniques for manipulating local contrast.

BW film has a higher potential dynamic range than most other photographic media, but getting 10 to 12 stops on a BW neg is still within the normal range for a single exposure. Merge to HDR with 3+ different exposures should allow for dynamic range beyond what is possible in a single exposure. Not that we can see it all at once with current technology.

I think people like it because the photos with heavy local contrast manipulation look like paintings (ala Photomatix). Painters have been using local contrast manipulation for centuries. But just like any technique, it doesn't have to be taken that far. Local contrast can be manipulated to bring out more details in the shadows and highlights, and still look like a "normal" photograph to the viewer.

ksmattfish · Sep 1, 2006

One thing that maybe should be pointed out. I'd say that in both examples above (the helicopter and the glass vase) the contrast range in the scenes don't appear to be beyond what is normally capturable with a DSLR. In any scene with a "normal" dynamic range there are numerous ways to lower contrast, open up the shadows, lower the highlights, etc..., although if you like using merge to HDR that's fine too.

To really see what merge to HDR can do find a scene that's blowing past both ends of the histogram. Like a dark interior room with windows open to a bright sunny day. Look for a shot that's impossible to get anything decent in a single exposure. That's when merge to HDR really shines.