Please shed some flash on this

[Colin Southern]

Sorry Colin, I think we are talking at cross purposes partly because I did not make my self clear. I was referring to the light falling on a macro subject that was being progressively moved away from a large light source. It seemed easier to visualize it that way than moving a wall. So the exposure varies according to the distance of the light source but not in accordance with the inverse square law and I see Ted has been kind enough and energetic enough to prove it.

Yes, you can think of it as an infinite array of point light sources but moving away when are very close as you move from the illumination from one point you move into the illumination from another. I am not suggesting there is no reduction in light intensity but only that at this point the (effective) lighting does not follow the inverse square law.

Unfortunately, Ted's "proof" was fatally flawed; please see my response above.

To be honest, I think I'm done here; I've explained all of this many times the best ways I know how -- hopefully some other brave sole will pick up the torch and run with it (may god have mercy on their sole).

On the bright side, I'm thinking of producing my own series of light meters complete with a camera to subject distance input; heaven knows how we missed that "important" variable in the past, so I hope it'll be a good seller. I think I'll start by marketing it to snake oil customers ...

[xpatUSA]

Your testing methodology is fatally flawed. When you moved the camera to the furthest position you were metering an area that was 4 times the area of what was being metered the first time. Four times the area multiplied by 1/4 the lux arriving at the sensor equals exactly the same exposure. The difference is probably due to unevenness of the lighting.

What??

While I'm digesting the rest of the response, I should have made it clear that I moved the camera along with the card so as not to alter the spatial and angular relationships between the camera and the card. In each case, the card filled the frame. Spot metering, BTW.

"I'll be baaack . . . ."

[Colin Southern]

I too agree that we are talking at cross-purposes. In fact, I needn't have bothered with a trip to the shed. I just put a white screen up on the monitor, thereby simulating the hypothetical Japanese studio wall. Put the camera hard up against the screen and then retired a good few paces with no change in exposure whatever. Not what one would expect with a universal inverse square law in effect.

Aaaarrrggghhhh!

It's EXACTLY what one would expect. The further you go back the more the light drops off but the more screen area comes into the field of view to compensate. At 20cm you might have 500 pixels on the monitor contributing to the exposure or metering. At 40cm you're going to have 2000 pixels all contributing 1/4 of what the 500 originally were.

WHY CAN'T PEOPLE UNDERSTAND THIS?

[Colin Southern]

How bad is it when I feel the need for alcohol ... and I don't drink alcohol!

[Donald]

I think we've reached a point in this thread at which we can conclude that there is not going to be a meeting of minds. Clearly there are people convinced of their own position and are not up for that position being questioned or challenged.

I suggest we all move on to the next subject and leave it to the readers who will see this thread in the future, to make up their own minds as to who has presented the most convincing evidence as to the correct understanding of the question.

[xpatUSA]

I'm done too, Donald. Looks like we're all saying the same thing just in different ways.

[Colin Southern]

Two points of interest in closing ...

1. I went for a glass of dark grape juice and a slice of tasty cheese in the end.

2. I grabbed a 600EX-RT - zoomed it to 200mm and fired it at my Sekonic 758DR Lightmeter at 1m, 2m, and 4m - the readings were F44, F22, and F11, ie falling off according to the inverse square law exactly as expected. I encourage any doubters to reproduce the experiment.

/done

[Cantab]

I think we've reached a point in this thread at which we can conclude that there is not going to be a meeting of minds. Clearly there are people convinced of their own position and are not up for that position being questioned or challenged.

I suggest we all move on to the next subject and leave it to the readers who will see this thread in the future, to make up their own minds as to who has presented the most convincing evidence as to the correct understanding of the question.

I've been following this thread and its vigorous discussion with interest but agree with Donald that everything that could be said has been said. (But having said that, I can't resist saying I agree with Colin.)

[xpatUSA]

Looks like we're all saying the same thing just in different ways

Sorry for posting again but I may have found the answer in the 'five times' rule:



Do notice the "area source line", which is what I was always talking about, and the "point source line" which I believe is what Colin is talking about. The graph shows clearly that a) both views are correct and that b) for any Real World lighting there is always an error with respect to The Law.

Notice also that my surmise that there is more to the subject than just the ratio of distances is completely vindicated.

The book itself is excellent and, as a reference, is valuable and very clearly written, unlike some of our posts in this thread!

It's free from this site:

http://www.intl-lighttech.com/servic...ement-handbook

Let there be light :-)

[PhotomanJohn]

Ted - That is very interesting and makes sense. A practical application is when we use a large soft box very close to the subject. The soft lighting is coming from both the light coming from different angles from the large surface but also because the intensity isn't falling off across the face as fast as one might expect. Actually, both of those may actually be the same effect!

(Sorry Donald, I couldn't resist. )

[Colin Southern]

Ted - That is very interesting and makes sense. A practical application is when we use a large soft box very close to the subject. The soft lighting is coming from both the light coming from different angles from the large surface but also because the intensity isn't falling off across the face as fast as one might expect. Actually, both of those may actually be the same effect!

(Sorry Donald, I couldn't resist. )

It's often said that soft boxes should be placed as close as possible to the subject if maximum softness is desired, but in practice past a certain point (with models anyway) doing that generates hotspots due to the exponential increase in light intensity between the shortest distance between model and light source.

[xpatUSA]

Ted - That is very interesting and makes sense. A practical application is when we use a large soft box very close to the subject.

Thanks, John. Pardon my ignorance, but how big is a large soft box?

[edit] Never mind, I found a big one at Adorama - 5ft octagonal, by golly:

http://www.adorama.com/FPSB60R.html

An approximately "circular source of uniform radiance"?

Placing a model's face close to that one would certainly put it in the area of Radiance Conservation on the graph I posted, making hotspots less likely rather than more, I would have thought.

[Glenn NK]

I too agree that we are talking at cross-purposes. In fact, I needn't have bothered with a trip to the shed. I just put a white screen up on the monitor, thereby simulating the hypothetical Japanese studio wall. Put the camera hard up against the screen and then retired a good few paces with no change in exposure whatever. Not what one would expect with a universal inverse square law in effect. Kept walking back and at 8 or so feet the exposure required by the camera started to rise but not by much - 1/20 sec fell to 1/10 sec at 12 feet (aperture priority, spot metering). Had I walked out thru the kitchen into the front yard (too bloody cold for that) I'm sure the exposure would have tended toward "the law" but would never have approached it.

Ted:

I read Alexander's post over again (this time carefully ).

I noted that he was using a zoom lens, which I somehow missed in the initial reading (so much for reading comprehension ).

I also tried pretty well what you did (plain bright white screen). Interesting.

If you would, please repeat your tests, ensuring that only the white screen is in the image. If you don't have a zoom lens, then two or three primes with different focal lengths should suffice.

Alexander also said he kept the ISO, shutter speed, and f/stops constant, and that the flash to subject also remained constant - which ensured that the incident light falling on the subject did not change. By using a white screen, this should duplicate the effect of a uniformly lit subject.

The only thing Alexander varied was the distance between subject and camera.

Thanks.

Glenn

[PhotomanJohn]

It's often said that soft boxes should be placed as close as possible to the subject if maximum softness is desired, but in practice past a certain point (with models anyway) doing that generates hotspots due to the exponential increase in light intensity between the shortest distance between model and light source.

Colin - Thanks for the insight. As annoying as this thread has been to some, it got me to think about something that I thought I knew a lot about. I now know more and will remain open to learn even more at a future date.

John

[pnodrog]

Some further reading .....

http://www.cs.toronto.edu/~jepson/cs...s/Lighting.pdf

http://optics.synopsys.com/lighttool...nationfund.pdf

http://www.cs.cmu.edu/afs/cs/academi...w/lec/lec8.pdf

[Colin Southern]

Thanks, John. Pardon my ignorance, but how big is a large soft box?

[edit] Never mind, I found a big one at Adorama - 5ft octagonal, by golly:

http://www.adorama.com/FPSB60R.html

An approximately "circular source of uniform radiance"?

Placing a model's face close to that one would certainly put it in the area of Radiance Conservation on the graph I posted, making hotspots less likely rather than more, I would have thought.

Ding, round two.

Putting a large softbox close to a model is more easily visualised as bringing something like a soccer ball close to a large illuminated surface (ie a round surface (ball / head) being brought to a flat surface). One point on the ball can be brought close enough to touch (or almost touch) the surface and thus it has the highest intensity of light on it whilst other parts of the ball receive more of their light from parts of the surface that are further away (and are thus dimmer according to the inverse square law). If you're not convinced, hold a basket ball close to a gas heater and see which part starts smoking first. Also, the convex nature of the sphere/head combined with skin oils also promotes a degree of specularity, which doesn't help either.

Further away, the ratio of the different distances are much smaller; closer they can get far more significant. Regardless, they're all still following the inverse square law (infinite number of point light sources) (as maintained all along), but with the difference being that the distance is no longer being as simple as the linear shortest distance between the object and the lightsource; as the object gets closer you effectively have many vastly different distances to many lightsources. ALL of those lightsources still follow the inverse square law exactly, but the distances change in a non-linear relationship.

But all of this is getting way OT. Getting back to Post #1 (thanks Glenn) the answer is that, as expected, the exposure doesn't change because the relative energy density received at the sensor doesn't change; a change to the distance (thus intensity) is compensated exactly by an inverse change to the area it's received by - and all is nicely aligned in the multiverse.

[PhotomanJohn]

Ding, round two.

Further away, the ratio of the different distances are much smaller; closer they can get far more significant. Regardless, they're all still following the inverse square law (infinite number of point light sources) (as maintained all along), but with the difference being that the distance is no longer being as simple as the linear shortest distance between the object and the lightsource; as the object gets closer you effectively have many vastly different distances to many lightsources. ALL of those lightsources still follow the inverse square law exactly, but the distances change in a non-linear relationship.

I would like to put forward this explanation of the effect that Ted mentioned when close to a large light source. Lets assume we have a large soft box (like my 70" hex) and assume the light intensity is equal across the entire surface and each point on the surface acts like point source of light as Colin mentions. We will discuss the light intensity at points along a line perpendicular to the surface at the center of the soft box. At any point along that line, the value measured will be the sum of the contribution of all the "points of light" on the surface with each point's contribution being defined by the inverse square law as Colin points out.

If we compare the light intensity at 6", 1 foot and 2 feet we see that the light from the center point on the surface is reduced to 1/4 as we double the distance at each step. The distances to the light points off to each side of center have increased but have not doubled because our measurement points are not moving directly away, but moving on a diagonal. The further from the center, the less the percentage of increase in distance at each measurement point. The contribution from each of these points fall off by the ISL but since the distance traveled did not double going from 6" to 1 foot then to 2 feet its intensity did not drop off by a factor of 4 each time. So the difference in the light intensity at 1 foot compared to 6" is less than the ISL would predict because only the light in the center fell off at the ISL rate while the light from other areas of the surface did not.

As we move farther away from the surface then each time we double the distance the distances to each light point come closer to doubling. As mentioned in Ted's article, when we get to 5 times the diameter of the source the overall light falls off as defined by the ISL.

I know a diagram would have helped but I hope most followed my description.

John

[Colin Southern]

I would like to put forward this explanation of the effect that Ted mentioned when close to a large light source. Lets assume we have a large soft box (like my 70" hex) and assume the light intensity is equal across the entire surface and each point on the surface acts like point source of light as Colin mentions. We will discuss the light intensity at points along a line perpendicular to the surface at the center of the soft box. At any point along that line, the value measured will be the sum of the contribution of all the "points of light" on the surface with each point's contribution being defined by the inverse square law as Colin points out.

If we compare the light intensity at 6", 1 foot and 2 feet we see that the light from the center point on the surface is reduced to 1/4 as we double the distance at each step. The distances to the light points off to each side of center have increased but have not doubled because our measurement points are not moving directly away, but moving on a diagonal. The further from the center, the less the percentage of increase in distance at each measurement point. The contribution from each of these points fall off by the ISL but since the distance traveled did not double going from 6" to 1 foot then to 2 feet its intensity did drop off by a factor of 4 each time. So the difference in the light intensity at 1 foot compared to 6" is less than the ISL would predict because only the light in the center fell off at the ISL rate while the light from other areas of the surface did not.

As we move farther away from the surface then each time we double the distance the distances to each light point come closer to doubling. As mentioned in Ted's article, when we get to 5 times the diameter of the source the overall light falls off as defined by the ISL.

I know a diagram would have helped but I hope most followed my description.

John

Spot on.

But - keeping this in the context of the original post, the lighting to subject distance is fixed - so it doesn't matter how the light from the source to the subject behaves - only the light from the subject to the sensor. And contrary to some arguments postulated previously, light reflected from an object has no prior memory of how it got there; it could be lit by the sun millions of miles away or it could be lit by a shoot-through umbrella 20 feet wide. Sure - the differences in those two extremes of light source will have a big impact on the appearance of the subject, but that's irrelevant - ALL were talking about here is "does the exposure change when the CAMERA to subject distance is changed" and the answer is no - it doesn't. Once the light reflects off that subject then the inverse square law applies.

[xpatUSA]

Ted:
If you would, please repeat your tests, ensuring that only the white screen is in the image. If you don't have a zoom lens, then two or three primes with different focal lengths should suffice.
Thanks.

Glenn

Sorry Glenn, going away for the holiday - so preparing for that

Did dig this out of the aforementioned book though:

Irradiance From An Extended Source [big soft box, e.g.]:
The irradiance, E, at any distance from a uniform extended area source,
is related to the radiance, L Lamberts, of the source by the following relationship,
which depends only on the subtended central viewing angle [framing, e.g.], θ, of the radiance detector [camera, e.g.]:

E = π L sin^2(θ/2)

So, for an extended source with a radiance of 1 W/cm^2/sr, and a detector
with a viewing angle of 3°, the irradiance at any distance would be 2.15 x
10-3 W/cm^2. This assumes, of course, that the source extends beyond the
viewing angle of the detector input optics.

(my emphases)

Colin has mentioned a football (the spherical kind) placed in front of an 'extended source' such as that in the above quote. Since the football has a 'viewing angle' of 180° or more, the above quote is not relevant to his example and his explanation of that will suffice, I'm sure.

Happy Hols, All . . .