Understanding HDR and Exposure Stops

[JasonCA]

Using a camera, you can bracket photos for HDR. Perhaps for 3 photos, you can do -2 exposure, 0 exposure, and +2 exposure. I suppose people refer to this as -2 stops, 0 stops, +2 stops.

Let's say I have one picture only. How do I manually change the exposure 1 stop each? For instance, let's say I have one picture and I want to generate 2 additional pictures: one picture to be -2 stops and the other to be +2 stops.

Let's say I have a picture, and I want to see what it would look like at each stop. How would I generate that from one picture?

[jmac698]

Hi,
To simulate different exposures, just load the pic into any paint program and change the brightness slider.

Unfortunately, this will not completely simulate the effect. Think of the camera eye as having buckets that fill up with light, and once full, they can't register further light. A bright pixel, will fill up to a certain level and be stuck there, resulting in no detail in the white, washed-out areas of the picture. The only way around this is to physically reduce the exposure, and that's done with a faster shutter, smaller aperture, or in some cases, not amplifying the sensor as much (exposure).

Likewise, the dark areas are black plus some noise, and turning up the brightness on those will reveal nothing but a muddy, greyish image. Those pixels need more light to register anything.

Thus HDR combines 3 pictures so that the brightest pixels are taken from the darker picture, and the darkest pixels from the brighter picture, then combines them in a way that shows the full detail in all areas.

There are two things you can do, if your camera supports RAW files, they have more information in them for the light+dark areas. The other option is what is sometimes called "LDR", it's just a special contrast manipulation to even out the brightness throughout the picture. You can find that effect in various paint programs.

If you do physcially take 3 pictures in a row, it should be done on a tripod, otherwise there can be problems lining up the pictures. If there's motion or they can't be lined up by the HDR software, there is a ghosting effect, as the wrong "bright" pixels are taken from the wrong area of the picture, if that makes sense.

[JasonCA]

Hey jmac698,

It's always great to hear from you. Very interesting what you shared.

You said:

Quote:

Originally Posted by jmac698

Hi,
To simulate different exposures, just load the pic into any paint program and change the brightness slider.

Change the brightness slider by how much to be equivalent to +1 stop? Change the brightness slider by how much to be equvillant to +2 stops? Each stop is 20 percent increase in brightness?

For instance, if I have one picture, and I want to make 2 additional pictures from it that are +2 stops and -2 stops...how would I do this?

[jmac698]

Hi,
that question is determined by the Apex standard https://en.wikipedia.org/wiki/APEX_system
There's some formulas there, but basically I think a stop is a doubling of brightness (but is that linear brightness?).
I've got some examples and I can check manually. One pixel went from 80, to 128, to 186 with each doubling of shutter time. The shutter times aren't perfectly accurate though. I'll have to work this out more. But a change of contrast of 50% should do it.

[jmac698]

Ok, I worked it out, but it can only be approximate. To change one stop, you have to change the contrast by 59 (in RGB). This is just for my camera. Here's the data over 8 stops for the same pixel:

Tv        RGB
1/200      11,11,11
1/100      24,19,16
1/50       24,32,21
1/25       66,71,51
1/13      115,106,91
1/6       156,151,122
1/3       206,208,187
.6        248,248,246

If you graph that, you see a straight line in the middle, but bent at the ends. That's from the S-curve tone compression that the camera creates for in-camera JPGs. Every camera is different about this.
For this scene, the auto exposure picked 1/13, so I have from -4 to +3 exposures, or 8 stops. There's some noise in the pixels too.
It would be quite complicated to perfectly simulate different f-stops in a paint program.

[admin]

To change stops in-camera, you alter one of the values: aperture, shutter, or ISO.
Or a combination thereof.

Example: 1/250 @ f/8 ISO 400 = 1 stop different from 1/500 @ f/8 ISO 400

For most people, adjusting stops means adjusting the aperture. Of course, that can changes focal length, hence shutter or ISO instead.
There's not a single answer.
If you're not familiar with cameras, this is learning it backwards -- learn about light source first, learn this later.

You can only do so much with software alone. Shooting it right in-camera is most important.

What kind of camera are you using?