Please shed some flash on this

[thatguyfromvienna]

Glenn
I'm shooting full manual.
Both my camera and my flash are set manually.
I simply don't like TTL so I always set power output manually.

[Glenn NK]

Because no matter whether the distance to my subject is x, 2x or 3x, I still take pictures with the same ISO, aperture and shutter speed.

I'm shooting full manual.
Both my camera and my flash are set manually.
I simply don't like TTL so I always set power output manually.

To summarize all the information we have:

1) the images are captured in a dimly lit room so that the flash is the primary source of light,

2) the distance varies for different images, BUT:

3) the f/stop remains the same for each image,

4) the ISO remains the same for each image,

5) the power output from the flash remains the same for each image (I'm assuming full power each time),

6) the time of exposure is the same for each image (the shutter speed remains the same for each image),

7) And it seems to be implied that the exposures are equal (that is the histograms are pretty well the same),

I think there is some information missing in all this, or one of the variables is changing (or being changed) to compensate for the change in distance. If not, then the exposure of the images will vary, and without automatic correction in camera, the histograms will be quite different.

Glenn

PS:

the power of light getting to the subject is not 1/3x but 1/x³.

Actually it varies as 1/x2 not 1/x3.

[pnodrog]

Is the flash set to zoom mode? (assuming it has one)

[bambleweeney]

Lex's answer and L.Paul's mini lasers example nail it I think. Makes perfect sense now

[Glenn NK]

Because no matter whether the distance to my subject is x, 2x or 3x, I still take pictures with the same ISO, aperture and shutter speed.

Is the flash set to zoom mode? (assuming it has one)

Lex's answer and L.Paul's mini lasers example nail it I think. Makes perfect sense now

How many zoom positions does it have? x, 2x, 3x?

One thing I haven't seen confirmed is that the exposure level is the same for images taken at different distances with manual flash.

The light from a flash is not a laser; a laser has some special properties, whereas sunlight and regular light sources do follow the inverse square law.

http://en.wikipedia.org/wiki/Laser

[jcuknz]

.
But when I shoot strobes only in an otherwise totally dark room?

The [ an ] answer is the distance light-to-subject not camera-to-subject.

edit ....Actually since I see reference to a zoom lens both remain the same so the exposure does too.

edit #2 The use of the zoom is akin to putting a different sized piece of paper under the enlarger the exposure remains the same. Hence a strip of paper used as a test stri[p is valid for the large sheet of paper for the photo.

[xpatUSA]

A more general view:

Everything we can see emits light - er, because light is defined as visible to the average human.

Even a reflection is an emission of light governed by the laws of reflectance and absorbance and stuff.

Then there are many kinds of light emission. For example these reflective two:

If your flash is directed toward a big white reflecting diffuser (a large Lambertian surface) then, taking the surface as the scene, the distance from the camera to the diffuser's light emission is not very significant in terms of exposure.

On the other hand, aim that same flash at a polished steel ball and, voila, the reflection is a point source and the inverse square law will most certainly apply.

In between these two extremes lies the Real World of lumens, lux, polar diagrams, solid geometry and probably some calculus too

[Glenn NK]

Torch beams, lasers, car headlights or attenuation by fog do not follow the law.

Oddly enough, the lights on all the cars I've owned most certainly seem to follow the inverse square law. And my flashlights (torch lights?) follow the IV law too.

Lasers are a unique class of light.

[PhotomanJohn]

I have tried to stay out of this but Glenn are you saying that your head lights are 1/900 or 0.0011 times as bright at 30 feet as they are at 1 foot? I would think that would make driving at night a real problem.

John

[Colin Southern]

I have tried to stay out of this but Glenn are you saying that your head lights are 1/900 or 0.0011 times as bright at 30 feet as they are at 1 foot? I would think that would make driving at night a real problem.

John

So in summary John, you're saying that the inverse square law for light falloff is wrong?

[PhotomanJohn]

Colin - Absolutely not. The inverse square law states that the intensity of light from a point source in space that freely radiates in all directions without any objects to reflect it will decrease by the square as the distance increases. In other words, totally unfocused light. As has been stated by others, virtually all of the common light sources we have around us such as torches (flash lights), headlamps, speed lights, studio strobes in parabolic reflectors or umbrellas, etc include reflectors that focus the light so that it can't radiate in all directions. Also, we often have things in the environment that reflect some of the light back in.

The inverse square law was not intended to apply to these light sources. The closest thing we have is a bare bulb flash which does fall off at a rate close the inverse square law. The purpose of focusing the light is to keep it from falling off as fast as the ISL states. The more focused, the slower the light falls off. A grid on a parabolic reflector is one of the best focused sources we use and it's benefit is it creates a spot that doesn't fall off at the rate a bare bulb does. Some photographers refer to different sources/modifiers as having a different "depth of light" when referring to the difference in the lighting on the backdrop as compared to the subject when different light sources are used without changing any of the distance.

The ISL can be used as a guide if one remembers that in the real world it defines the maximum rate of light fall-off we will ever see. Many light sources come close enough to the rule that it has become a handy way to estimate the exposure.

Does this help?

John

[Glenn NK]

I know from working with electrical engineers, that they don't want interior lights mounted too high or the intensity of the light won't meet their requirements.

This would imply that the intensity of light does fall off with distance even from a focused light source (such as car headlights).

If the intensity of light from vehicle headlights doesn't diminish with distance then why are oncoming vehicle headlights not nearly as bright in one's eyes at a distance of 1.6 kilometer as they are at a distance of 10 metres? Or in the US system: one mile away vs 33 feet.

Where I used to live (the flat prairie of southern SK), vehicle headlights can be seen for several miles, but at that distance it matters not if they are set at high or low beam.

However - we still haven't answered the problem posed by "thatguy", and repeating myself, I don't think we have all the information from him on the matter.

Glenn

[PhotomanJohn]

I guess this is one of those days when my brain has failed. After finding the hard re-boot button on my head I realize that most of what I said was incorrect. As long as the light from a source is diverging then the inverse square law does apply. It is only in the case where the light does not diverge (laser) that the rule falls apart.

Sorry for my long-winded senior moment. I will now fade away and hope the redness in my face subsides soon.

John

[Glenn NK]

Don't sweat it John - to paraphrase a well known verse, "let he has not goofed cast the first aspersion".
And let me assure you, I will be the last as I've put the odd foot into an awkward place more than once on a public forum.

I'm still sweating the problem however, and would like to find the answer. There has to be some clue that is missing because from what we know, things just don't add up.

Given that the following parameters remain constant:
ISO
f/stop
time of exposure
flash/light power
subject.

Increasing the distance between the flashlit subject and the camera, the light intensity MUST fall off - even if it's not strictly according to the ISL.

What Alexander seems to imply is that the exposed images shot at differing distances from the lit subject have identical exposure.

Because no matter whether the distance to my subject is x, 2x or 3x, I still take pictures with the same ISO, aperture and shutter speed.

The question that keeps coming to mind is, "are the histograms identical or near-identical"?

If they are, then there is another factor that we don't know about. IOW, something is compensating for the fall-off in light intensity with increasing distance. Does the camera have a setting that "normalizes" or corrects the exposure? Is it something else that I've missed?

Glenn

[bambleweeney]

Increasing the distance between the flashlit subject and the camera, the light intensity MUST fall off - even if it's not strictly according to the ISL.

What Alexander seems to imply is that the exposed images shot at differing distances from the lit subject have identical exposure.

Glenn

Not at all, if you're walking towards a building on a sunny day does it get brighter as you approach it or dimmer as you walk away from it ? Substitute flash for sun and studio subject for building and the same rules apply. Don't try this but ... look directly into a flash from 10 feet, then do the same from 20 feet ... I'd bet that it isn't 1/4 as bright ... but the ammount of incident light falling on you will be 1/4 as much due to the spread of light between the flash and you. (good old ISL)
The photons striking you will be just as strong - but there will be less of them (I haven't actually counted them but I believe this to be true ) and so leaving less to be reflected, those that are reflected will not diminish in intensity or as has been mentioned, astronomy just wouldn't work and I'd have wasted money on my flash meter. The only (rubbish) analogy that comes to mind is a garden sprinkler, stand right next to one and you'll be soaked in no time. Move 20 feet away and you'll still get soaked but it will take a lot longer as the ammount of incident water falling on you will be less ... plus you won't appear to get any dryer as spectators move further away from you

[Colin Southern]

Why doesn't someone grab a lightmeter - flash (set to manual) and shoot some targets at set distances (I was going to today, but got sidetracked -- and now I'm off to bed).

Zzzzzzzzzzzzzzz

[tao2]

I'm very obviously too stupid to wrap my mind around this - or I'm just acting on a false assumption.

We all know that light falls off according to inverse-square law which means that when the distance x between light source and subject is three times bigger than before, the power of light getting to the subject is not 1/3x but 1/x³.

Why doesn't the same principle apply to my camera then?
Because no matter whether the distance to my subject is x, 2x or 3x, I still take pictures with the same ISO, aperture and shutter speed.

I'm pretty sure I'm currently just too stubborn to understand this so I'd be very grateful if someone could give me a hand here.

This is Alexander's OP. He's not acting on a false assumption but he should be applying the theory of apertures in conjunction with inverse square law.

The point of the ISL (as applied tae cameras/lenses) is that the camera is focused on a distant subject (infinity). The theory of apertures applies tae lenses used at infinity, where the lens tae image distance is one focal length.

Focusing on a closer object, the light path (lens tae image distance) is increased and the intensity of light decreases at the film/sensor plane. Generally speaking, this effect is negligible; a 50mm lens focused at 0.5m needs only about 4/5mm adjustment. Light fall-off is only a small fraction greater than 1:1.

For example, the light beam diameter entering the lens is equal tae lens focal length divided by the f no. So f4 on a 50mm lens denotes an aperture of 12.5mm. On a 200mm lens it will be 50mm.
Although the diaphragm opening on the 200mm lens is greater, the light transmitting ability is the same. The greater amount of light transmitted by the larger opening is offset by the greater distance travelled. Again this applies tae lenses focused on a distant object.

With a 50mm lens focused at infinity, light travels 50mm from the rear of the lens tae the film/sensor plane. Similarly a 200mm lens has tae project its beam 200mm. This is where ISL kicks in again. The 200mm lens (needing tae transmit 16 x as much light) since the light path is 4x longer, must have an aperture 16 x larger in area. Calculaating these ratios mathematically gives the same 1:16 ratio.

At any given f no. the light-transmitting ability of the lens is mathematically the same, no matter its focal length. That's as far as the ISL applies regarding cameras/lenses. When introducing the abstruse concepts of flashguns, lasers even lens coatings etc., folk are moving intae areas where it's unlikely that ISL would (strictly) apply as regards cameras/lenses. Always bearing in mind that maths deals with a perfect world and photography and maths do not always tie up neatly.

Alexander says above, that he uses the same ISO, aperture and shutter speed but he doesn't say what effect it has on his photos?

[revi]

reason why the inverse square law doesn't seem to apply to the illuminated objects:

if you double the distance form an object to the camera, the number of photons coming from an object and reaching the lens will be divided by four, but the apparent visible are will have been divided by four as well. Result: the number of photons per unit of apparent area remains the same...

Note that the inverse square law assumes a point source.

[Glenn NK]

Not at all, if you're walking towards a building on a sunny day does it get brighter as you approach it or dimmer as you walk away from it ? Substitute flash for sun and studio subject for building and the same rules apply. Don't try this but ... look directly into a flash from 10 feet, then do the same from 20 feet ... I'd bet that it isn't 1/4 as bright ... but the ammount of incident light falling on you will be 1/4 as much due to the spread of light between the flash and you. (good old ISL)
The photons striking you will be just as strong - but there will be less of them (I haven't actually counted them but I believe this to be true ) and so leaving less to be reflected, those that are reflected will not diminish in intensity or as has been mentioned, astronomy just wouldn't work and I'd have wasted money on my flash meter. The only (rubbish) analogy that comes to mind is a garden sprinkler, stand right next to one and you'll be soaked in no time. Move 20 feet away and you'll still get soaked but it will take a lot longer as the ammount of incident water falling on you will be less ... plus you won't appear to get any dryer as spectators move further away from you

Paul:

The inverse square law DOES apply to sunlight. By the way, for those who think the ISL only applies to a point source, consider the size of the sun - it is gigantic - not a point source at all. According to Wikipedia: sun diameter = 1,392,684 km = 865,375 miles.

The reason you don't notice any change in brightness when approaching a sunlit wall is because the distance you travel in getting closer to the wall is piddling compared to the distance the sunlight has already travelled from the sun to the wall.

Please refer to Post No.3: In this example, I've assumed that the camera has moved one mile.

Sunlight has already travelled 93 million miles and the inverse square law has been taken into account on its way here.

. . . the remaining distance it travels between your subject and your camera is so small in comparison to the 93 million miles, that the effect is negligible.

sqrt (93,000,000) = 9643.65076
sqrt (93,000,001) = 9643.65081

Not significant is it?

Now to your flash example; try this; keeping all settings the same (including the lens focal length).

Take a photo at night of someone ten feet (3 metres) away. Repeat this with the same person at 500 yards (metres) using all settings the same. Do you think the image would be under-exposed? Frankly I doubt you'd be able to see the person at all.

[Glenn NK]

One last question for Alexander (the OP):

Did the focal length used remain constant? Using a zoom lens, it is possible to seemingly void the inverse square law.

Anyone that has used 10 x 50 binoculars knows what I'm getting at.

I used to hunt wild upland game at dawn and sunset when light levels were marginal. With binoculars (10 power x 50 mm objective are preferred), it's surprising how much one can see at a distance in poorly lit areas.

I've also tried to shoot my son's hockey games with a pretty powerful flash in an arena that wasn't up to pro standards. It worked great at distance from ten ft (3m) to 30 ft (10 m), but across the full width (85 ft = 26 m), it was pretty dark, and using ISO 400 film it was pointless.

Yes, in spite of what Alexander found, the reflected light from a flash does fall off with distance. I've experienced it thirty years ago, and I sincerely doubt that much has changed.