Understanding Lighting - Strobist

[michaelnotar]

the 4 stops of less ambient exposure comes from 2 stops faster TV and 2 fstops smaller than ambient (60@4), he was at 250@f8.

with ambient being 4 stops UE, it has no effect on exposure. from the additive exposure compensation we get that it being 4 stops differant, it only contributes 1/16 stop exposure. if it was 3 stops, less it would contributre 1/9stop, 2 stopss = 1/4 stop etc.

flash is only alluminating both the subject and background at this point.

if you want to alluminate the background more and keep the subject expsosure the same, bring the subject closer to the wall, while keeping the flash to subject distance the same which keeps the subject exposure the same. ie, in the origional set up, if the subject is 8ft from the wall and the flash is 8ft from the subject, the from the flash has to travel 16ft to the wall. light falls off at the inverse square of the distance which translates into f stops, f8-16 is two stops, your background will be 2 stops darker than the subject in this case.

move the subject 3 ft from the wall and keep the flashs 8 ft from the subject, the flash goes 11ft to the wall, F8-11, 1stop, the wall will be 1 stop darker than the subject.


thats the first situation/section in the article

[michaelnotar]

i dont get hiss term: working above the ambient.

just one guy's term. i havent heard it in any professional environment.

in the shot of the camera: camera is set at 250@4 = AMB 5 stops EU
so normal amb exp = 1/8th @4. when hes at 1/40 @ 4, that EU amb by 2.3s stops.
additive light principle again, roughly 2 stop diff = 1/4 of exposure gain from amb
actually less in this case.

then he uses longer TVs to boost amb exp higher.

[michaelnotar]

after reading the article, i note he starts with flash exposure, then amb.
i work the other way, amb first. its MUCH easier. why not see what light is there, then add more light AS NEEDED.

[David_Buzzard]

The first shot with the flash, the rear of the room is being lit by the spill from the flash. The ambient light on the rear wall doesn't start to show up until he gets down below 1/25 of a second. The reason that the camera isn't getting any brighter is that there is probably less (or none at all) ambient light on the camera than there is on the background wall. If you shot this scene below 1/25 without flash, the camera would probably be in silliouet.

You can control the exposure on ambient light be adjusting either the shutter speed ,the aperture, or adding more ambient light. While with flash, the shutter speed is more or less irrevelent (you can be under the flash sync speed, or if you're using an old Nikon D1 series camera, you can litterly have the shutter speed shorter than the flash duration, but lets ignore those for this example), and you can only change the exposure by the aperture, or by turning up the flash.

David Buzzard's technical Blog

[KevinStecyk]

Originally Posted by michaelnotar

#3 is wrong perhaps a typo. to adjust flash, change power of flash (1/2-1/4 etc) and/or change the distance of the flash to the subject.

From the Strobist:

Strobist: Lighting 102: Assignment - Balance

1. To alter flash and ambient levels together:

Remember, flash cares about the aperture and ambient cares about both the aperture and the shutter speed. So to lighten or darken both at once, we adjust the aperture.

If, for instance, our whole scene (flash and ambient) is too dark at f/8, we could move to f/5.6. This would lighten everything by a stop -- flash and ambient alike. Conversely, we could darken the entire scene by going from f/8 to f/11.

The next subject.

Originally Posted by michaelnotar

"I just discovered the answer to the 4 stops. He increased the time in small increments over one stop and there was no change in the background light, except for his last change. Thus, he was 4 (=3 + 1) ƒ stops over ambient. As others have mentioned, 3 stops and no difference. So in this case where the very last adjustment made a tiny difference, he knew he was 3 ƒ stops away at that point. He starting point was an additional ƒ stop. One question solved."

i think your 3 stop reference is differant the one i meant. i was talking about additive exposure compensation. are you familiar with this.?

Okay, four stops is back on the table again. As far as your additive exposure compensation, I trust you are referring how to add the quantity of light from different sources. Why not enlighten us, pun intended?

Originally Posted by michaelnotar

" So I think flash plus plenty of ambient light does affect the flash lit subject."

if both sources illuminated the subject, then the subject will be brighter. perhaps the small change in brightness is from one of the light sources that contributed only fractionally to the exposure.

dramatic changes in TV will NOT change flash exposure. all cameras sync less than 1/1000th sec or less, most around 1/250th. the time a flash produces light or the flash duration is around 1/1000-1/2000th sec. so the camera shutterspeed can not go truncate the flash exposure since the flash is on and off in a 1/4-1/8 of the time the shutter is open. so flash power, distance to subject and aperature are left to control its exposure.

Perhaps I misspoke. That is, I should have wrote what you wrote, more specifically:

Originally Posted by michaelnotar

if both sources illuminated the subject, then the subject will be brighter. perhaps the small change in brightness is from one of the light sources that contributed only fractionally to the exposure.

That's the point I was trying to make. The camera is subtly brighter in the later pictures. As the time is increased, the subject's brightness is subtly increased as well. The subject's brightness is not solely dependent upon the flash exposure alone.

[Dennis_Vied]

Kevin, you were looking for a simple answer. Here's my input.

The reason he knew he was 4 stops above ambient was that he was using a shutter speed 4 stops too fast for the ambient light level. He gradually moved the shutter speed slower toward the normal exposure for the ambient light level. He metered the scene for ambient, and a certain shutter speed and aperture were called for. He set the aperture, but for test purposes, he set a shutter speed 4 stops too fast.

In the meantime, the flash exposure was determined by the lens aperture. So, in each exposure, the camera was close enough to be affected by the flash, but the background was not. So, in each exposure, (except for the last few with very slow shutter speeds), the camera received pretty much the same exposure, while the background exposure increased with the slower shutter speed. Therefore, the camera looks about the same while the background gets brighter, (or closer to a normal exposure).

Does this make sense?

[KevinStecyk]

Originally Posted by David_Buzzard

The first shot with the flash, the rear of the room is being lit by the spill from the flash. The ambient light on the rear wall doesn't start to show up until he gets down below 1/25 of a second. The reason that the camera isn't getting any brighter is that there is probably less (or none at all) ambient light on the camera than there is on the background wall. If you shot this scene below 1/25 without flash, the camera would probably be in silliouet.

Thank you David for replying. In looking at the last photo in the article, it seems that the whole room is being lit by the light coming through the patio doors. It looks as though the back of the room is receiving more light than the camera area. So perhaps you are correct that the camera would have been just a silhouette without the flash.

Originally Posted by David_Buzzard

You can control the exposure on ambient light be adjusting either the shutter speed ,the aperture, or adding more ambient light. While with flash, the shutter speed is more or less irrevelent (you can be under the flash sync speed, or if you're using an old Nikon D1 series camera, you can litterly have the shutter speed shorter than the flash duration, but lets ignore those for this example), and you can only change the exposure by the aperture, or by turning up the flash.

Let me ask you this question.

First, take a photo of a subject with minimal ambient light with no flash. The ambient (background) and subject receive equal ambient light exposure throughout this discussion. The photo is completely dark. Next, use your flash to light your subject but have the background dark. Next, increase the time only so that both the subject and background are well lit. Does the additional ambient light over and above the flash make the subject brighter?

That's really the nub of my question.

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