[SOLVED] killing ambient light

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keratos

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Daz
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Hi. Recently joined this forum having took up photography two months ago, and having come from a USA form where the lingo doesnt take my fancy, ....

I am outdoor shooting my beloved partner (camera not projectile) with the garden fence (6' panel) behind at about 2 metres in the background. I cant kill the background.

This is what I am doing...

D300 in full Manual mode with SB700 in TTL mode camera mounted pointing foward (reporting 35mm zoom) which is the same as the lens, Nikon 35-f/1.8
Take ambient exposure and stop down on Aperture (flash only concered with aperture as it will sync to speed) until the beloved missus is just , just visible. This required about 5 stops down and its not bright and sunny here, is it ever in the UK?
Now with flash on take the shoot. Wifey nicely lit up but so is the fence and shrubs.
Subject is about 1m away from me and I'm pointing the camera flash toward her.
Inverse Square law tells me that if wifey is 1m away and the fence is 2m behind her, then distance from camera to fence is 3m which means, I think, that the fence is getting 1/9 of the light not accounting for falloff due to the subject being in the way.
The background appears much lighter and the fence, shrubs and flowers are all lit up quite reasonably.

Darn. What is wrong here??
 
If you tell us your shutter speed, aperture and ISO setting, we migt be able to help.


Steve.
 
Ooops

f16/250 (camera on AP auto sync) ISO200 kills the ambient light and just makes the wife visible, just. Add TTL flash and the garden (at least three or so fences 2m away) is lit up?

edit: I'm trying to get one of those shots , outside in daylight, where the subject is lit and the background is black. You know the type .?!
 
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Do it manually. Set you camera to manual mode at ISO100, 160th sec, and start at f8. Adjust your exposure until you've almost got a black picture. Then you can begin to add light from your flash manually. Forgot about ETTL for now and learn what is happening. Your flash does have to be close to your subject. If you still find that light is hitting your background move the wife further away from it.
 
Maybe you can post one of your attempt and a link to somethink looking like what you want to obtain and it will be easier to understand.
 
Dazb75 said:
Do it manually. Set you camera to manual mode at ISO100, 160th sec, and start at f8. Adjust your exposure until you've almost got a black picture. Then you can begin to add light from your flash manually. Forgot about ETTL for now and learn what is happening. Your flash does have to be close to your subject. If you still find that light is hitting your background move the wife further away from it.
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The camera is in manual, full manual, I always shoot in control of A, S, ISO, WB etc. The Manual mode has a power from 1/1 to 1/128. Not sure how this relates but it does also have a GN setting where I enter the distance to the subject, I used this and got pretty much same result as TTL mode.
I have moved the subject away from background , 3m, which is 1/16th of the light excluding fall off. It shouldnt be so illuminated so there is something not right here?
 
keratos said:
The camera is in manual, full manual, I always shoot in control of A, S, ISO, WB etc. The Manual mode has a power from 1/1 to 1/128. Not sure how this relates but it does also have a GN setting where I enter the distance to the subject, I used this and got pretty much same result as TTL mode.
I have moved the subject away from background , 3m, which is 1/16th of the light excluding fall off. It shouldnt be so illuminated so there is something not right here?
Click to expand...

I've never enter a "Guide Number" into a flash. Set the power of the flash manually to a middle of the road power setting like 1/16 and see what happens. Remember to use the lowest ISO.
 
keratos said:
The camera is in manual, full manual, I always shoot in control of A, S, ISO, WB etc. The Manual mode has a power from 1/1 to 1/128. Not sure how this relates but it does also have a GN setting where I enter the distance to the subject, I used this and got pretty much same result as TTL mode.
I have moved the subject away from background , 3m, which is 1/16th of the light excluding fall off. It shouldnt be so illuminated so there is something not right here?
Click to expand...

Agree with Daz, put the flash in manual mode. Power ranges from 1/1 (full power) to 1/128 (least). Not tried flash outside during the day so just theory but I understand that it will take a lot to overpower the sun so would imagine you'd need to be close to full power depending on subject distance from flash, perhaps even then may be underpowered?

Feel free to post up what you have so far.

Jeff.
 
Quoting from a flash article in an old copy of amateur photographer, the guide number (GN) of a flash helps calculate f-stop if distance is known.
The relationship is GN = distance (in m) x F-number.

So say your GN is 32 and subject is 4m away then f8 is needed for f-stop. 32 = 4 x f8.

So I guess, when entering a distance into a flash, the flash knows its own GN so can then tell you what f-stop is needed. (or tell the camera automatically maybe).

The GN also varies according to ISO and whether the flash can zoom in or out or not. (Widen or narrow the beam)

Hopefully Ive not confused myself or anyone too much with that little dollop of (hopefully correct) info!
 
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keratos said:
Inverse Square law tells me that if wifey is 1m away and the fence is 2m behind her, then distance from camera to fence is 3m which means, I think, that the fence is getting 1/9 of the light not accounting for falloff due to the subject being in the way.
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This is not *quite* right I dont think.
What the inverse square law says that if you double the distance from light source to subject, illumination is reduced by a quarter. i.e if you moved your subject from 1m to 2m your subject then receives 1/4 of the light. (1 / 2 squared).
At 3m therefore the illumination is 1/8th. (or 3 stops), and at 4m 1/16th etc etc. (disclaimer, this is by my reckoning and could easily be wrong if Ive misunderstood my source)

I suspect that 3 stops drop off is not enough to take the background down to black.
 
DorsetDude said:
Quoting from a flash article in an old copy of amateur photographer, the guide number (GN) of a flash helps calculate f-stop if distance is known.
The relationship is GN = distance (in m) x F-number.

So say your GN is 32 and subject is 4m away then f8 is needed for f-stop. 32 = 4 x f8.

So I guess, when entering a distance into a flash, the flash knows its own GN so can then tell you what f-stop is needed. (or tell the camera automatically maybe).

The GN also varies according to ISO and whether the flash can zoom in or out or not. (Widen or narrow the beam)

Hopefully Ive not confused myself or anyone too much with that little dollop of (hopefully correct) info!
Click to expand...
The guide number is of no relevance here.

What is relevant is the effect of the inverse square law, but it isn't the distance from flash to the background that matters here, it's the distance from your flash to the subject and then to the background, let's call your subject the front subject and the background the rear subject.
If your subject is 1 metre away from the flash then power has been lost by the time it reaches her, so the power (flash energy) that reaches her then reduces further by the time it reaches the rear subject. In this case the power on the rear subject is reduced by 2 stops (1/4) relative to your subject, not by 3 stops (1/9th) as you calculated.

So, the exposure on your front subject will be 2 stops brighter than on your rear subject, counting the flash energy alone. But there is also the ambient light to consider, the effect of this is controlled by the length of the exposure, so all that you can do is to
1. Shoot at full power
2. Have the rear subject as far from the front subject as possible
3. Have the flash as close to the front subject as possible
4. Use the fastest shutter speed possible

Edit: Dorset Dude types faster than me, he also points out that the light loss from front subject to rear subject is just 2 stops, not 3
 
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Just to add a little bit to what Garry and Keith said (they have your technical side bang on)

The important bit of help photographically is getting the flash closer to the subject, presumably, you'll want to be shooting with a telephoto length (more flattering) which will screw with the distances if you keep your flash on camera (shooting from further away from front subject reduces the relative difference between front and rear subject).

Take the flash off camera, whack it in a softbox and you have a shorter distance between flash and front subject, you also have a nicer light. You can further feather the light and you won't even be pointing it at the rear subject any more. Job done!
 
Sorry guys. I sure your mistaken....

Inverse square is a simple 12yr child math formulae

Light intensity is equal to the inverse square.of the distance from the source distance . 3m away from the.source.is 1/[3*3] is 1/9 and not 1/8 as suggested.above
 
keratos said:
Sorry guys. I sure your mistaken....

Inverse square is a simple 12yr child math formulae

Light intensity is equal to the inverse square.of the distance from the source distance . 3m away from the.source.is 1/[3*3] is 1/9 and not 1/8 as suggested.above
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No.
It's 3 stops from source to rear subject, but that isn't where you need to measure from.
You need to measure from front subject to rear subject, because it's the fall off from front subject to rear subject that is relevant, not from source to rear subject. And that results in 2 stops, not 3
 
OP says, main subject/wifey is 1m from flash/camera, and the fence is 2m behind her. For the flash exposure, in round figures the fence will be three stops down on wifey.

Daz, you were going to post a pic - please do :)

Edit: while what matters is the exposure difference between wifey and fence, that can only be measured using ISL maths from the light source.
 
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My understanding is that to kill as much of the ambient light the OP should have the highest possible shutter speed (1/250). And then reduce/increase (depending if you start at 1/1 or 1/128) the flash power till the subject is exposed and the BG is black.
 
amer369 said:
My understanding is that to kill as much of the ambient light the OP should have the highest possible shutter speed (1/250). And then reduce/increase (depending if you start at 1/1 or 1/128) the flash power till the subject is exposed and the BG is black.
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The OPs problem isn't the ambient, it's the flash spill.
 
keratos said:
Sorry guys. I sure your mistaken....

Inverse square is a simple 12yr child math formulae

Light intensity is equal to the inverse square.of the distance from the source distance . 3m away from the.source.is 1/[3*3] is 1/9 and not 1/8 as suggested.above
Click to expand...
Yep, you're probably right.

You had no idea how to solve your problem. An industry expert corrected your maths, explaining your issue perfectly, but he's probably wrong and it's just a mystery.

As above, get your flash off camera and closer to the subject than your camera, that's using the ISL in your favour, it'll also give you more creative lighting options (not an industry expert, but I know a bit about making ladies look pretty in pictures).
 
This is all getting over-complicated.
Richard is right in that the fall off of light starts from the source of the light, and of course a fair bit of light has been lost before it reaches the front subject, but in practical terms what matters is how much further reduction there has been by the time it reaches the rear subject - and it isn't enough, for the OP's purpose.

And Phil is right, the answer is to get the flash of camera, to a position where it flatters the (front) subject. And if, by doing so, it can also be positioned closer to the front subject, then the fall off of light to the rear subject will be greater too.
 
Just as an aside, am I correct in thinking the inverse square law only applies very accurately with a bare bulb type light source which isn't guided our focussed allowing light to disperse in all directions. The effect of fall off is less pronounced the more the beam is focussed (say on an sb900 at the 200mm setting). please correct me if I'm wrong but my understanding of why the isl exists and something I've read lead me to believe this.
 
velonoir said:
Just as an aside, am I correct in thinking the inverse square law only applies very accurately with a bare bulb type light source which isn't guided our focussed allowing light to disperse in all directions. The effect of fall off is less pronounced the more the beam is focussed (say on an sb900 at the 200mm setting). please correct me if I'm wrong but my understanding of why the isl exists and something I've read lead me to believe this.
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Yes and no ;) Strictly speaking, the ISL applies to a point light source, radiating in free vacuum space or something (not sure without looking it up). If your average softbox adhered strictly to it and we took a reading one inch from the front, at 2in it would be two stops down, and quite obviously that's not true.

But in reality, the way we generally use (unfocused) studio lights, if you meter read a softbox at 1m, then at 2m it's going to be something like 1.5 stops down, and getting close to four stops at 4m. So it's a very good guide.

With smaller light sources like speedlites, exposure sticks more closely to the ISL principles. That's the basis of all Guide Number calculations and they're pretty reliable. GN's are far more skewed by the surroundings/environment, especially at greater distances, than any variance to the ISL.
 
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Garry Edwards said:
No.
It's 3 stops from source to rear subject, but that isn't where you need to measure from.
You need to measure from front subject to rear subject, because it's the fall off from front subject to rear subject that is relevant, not from source to rear subject. And that results in 2 stops, not 3
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Garry, I dont understand you. One cannot measure this in f-stops down unless an incident meter is facing the fence, or, the meter is facing the back of the subject. either way will work provided exposure is metered for the reading. But one cannot arbitrarily say how many f-stops down from source to fence because it depends on what light is hitting the subject in-between the two distances, the angle of attack, incident illumination, ambient light, so much. You are assuming laboratory conditions Garry. You might be close, you might not? I prefer the fall-off argument from velonoir above.

p.s. I am no industry expert, nor is my club colleague at work .... and if they/I was ... I wouldnt be on these forums. I am no way an expert or semi-pro or proficient as you guys are.
 
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velonoir said:
Just as an aside, am I correct in thinking the inverse square law only applies very accurately with a bare bulb type light source which isn't guided our focussed allowing light to disperse in all directions. The effect of fall off is less pronounced the more the beam is focussed (say on an sb900 at the 200mm setting). please correct me if I'm wrong but my understanding of why the isl exists and something I've read lead me to believe this.
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agreed. re: my previous comment above.
What I think we have here is a non-scientific problem in that the fall-off is hard to calculate and is not sufficient to opaque the fence.
So what I need to do is move the subject further from the fence toward me, and I will move backward.
Simple.
I was trying to scientific and apply math, but its not trivial and not a simple question of 2 or 3 stops down - there are more variables at play when outdoors.
So I have resorted to trial and error.
In the end I gave up as it started snowing and raining here !!
 
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Garry Edwards said:
This is all getting over-complicated.
Richard is right in that the fall off of light starts from the source of the light, and of course a fair bit of light has been lost before it reaches the front subject, but in practical terms what matters is how much further reduction there has been by the time it reaches the rear subject - and it isn't enough, for the OP's purpose.

And Phil is right, the answer is to get the flash of camera, to a position where it flatters the (front) subject. And if, by doing so, it can also be positioned closer to the front subject, then the fall off of light to the rear subject will be greater too.
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Garry, come on .... this isnt complicated .... but this is: http://en.wikipedia.org/wiki/Navier-Stokes_equation
took me almost an hour to solve it !http://en.wikipedia.org/wiki/Navier-Stokes_equation
 
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keratos said:
Garry, I dont understand you. One cannot measure this in f-stops down unless an incident meter is facing the fence, or, the meter is facing the back of the subject. either way will work provided exposure is metered for the reading. But one cannot arbitrarily say how many f-stops down from source to fence because it depends on what light is hitting the subject in-between the two distances, the angle of attack, incident illumination, ambient light, so much. You are assuming laboratory conditions Garry. You might be close, you might not? I prefer the fall-off argument from velonoir above.

p.s. I am no industry expert, nor is my club colleague at work .... and if they/I was ... I wouldnt be on these forums. I am no way an expert or semi-pro or proficient as you guys are.
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I'm not measuring in f/stops, I'm calculating in f/stops.
The angle of incidence is close to irrelevant here because it's an on-camera flash, pointing straight at the subject, and the fence is presumably not made of stainless steel or other highly reflective material, so losses due to cosine law simply won't be enough to factor in.
So, the only two factors at play here, which have already been addressed at depth, are distance and ambient light.

Incidentally, the inverse square law applies in a pretty linear way in this situation, with a small light source.
It isn't a point source of light, but it's several million (or billion) point sources close together and all occupying a small space. It isn't operating in free space, but the main point of a vacuum in this context is the lack of pollutants, this isn't normally a measurable problem where such a small distance is involved.

Newton's theories, applicable to astro physics before the invention of flashguns, don't relate exactly to photography but they're close enough for the purpose.
 
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Garry - shooting in daylight here - and in broad daylight the angle of incidence is "irrelevant",you say
what, with the sun shining down and incident light being reflected??? leading to fall off at different angles and strengths, dependent upon time of day and positioning,
I am trialing various settings to let various ambient light in and to move objects around , and so on...
"irrelevant"
oh dear Garry, have you been on those funny fags again

anyway , its solved now - went to manual mode , turned down the power , moved subject closer to me, I moved closer to the subject, and so on...

thank you to everyone for the insightful tips and recommendations

here is one I did with another of my subjects:
Decent alternative to Nikon's SB700
 
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Daz, suggest stop being rude and arguing the toss about irrelevances and just post a pic like you said you would.
 
keratos said:
Garry - shooting in daylight here - and in broad daylight the angle of incidence is "irrelevant",you say
what, with the sun shining down and incident light being reflected??? leading to fall off at different angles and strengths, dependent upon time of day and positioning,
I am trialing various settings to let various ambient light in and to move objects around , and so on...
"irrelevant"
oh dear Garry, have you been on those funny fags again

anyway , its solved now - went to manual mode , turned down the power , moved subject closer to me, I moved closer to the subject, and so on...

thank you to everyone for the insightful tips and recommendations

here is one I did with another of my subjects:
Decent alternative to Nikon's SB700
Click to expand...
When that sun shining down is "5 stops underexposed" it's definitely irrelevant. Completely and utterly irrelevant.
 
Oh no! I've been reading Garry's lighting blog posts and thought they were most informative. Now it seems they're all bluster, and he doesn't know what he's talking about.
Thanks for the heads up keratos. Next time I can't figure out how to light a wee white dog, I'll come to you.
I'll bring along some funny fags, but you'll probably have started without me.
 
RBaker said:
Oh no! I've been reading Garry's lighting blog posts and thought they were most informative. Now it seems they're all bluster, and he doesn't know what he's talking about.
Thanks for the heads up keratos. Next time I can't figure out how to light a wee white dog, I'll come to you.
I'll bring along some funny fags, but you'll probably have started without me.
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typical scot humor (non existent) - cant wait for that referendum YES vote
 
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RBaker said:
Oh no! I've been reading Garry's lighting blog posts and thought they were most informative. Now it seems they're all bluster, and he doesn't know what he's talking about.
Thanks for the heads up keratos. Next time I can't figure out how to light a wee white dog, I'll come to you.
I'll bring along some funny fags, but you'll probably have started without me.
Click to expand...
I've just taken in 2 pallet loads of lady's shoes and 4 dress rails full of clothes, for a fashion shoot starting tomorrow.

I try to help on these forums, but frankly I just don't care when people who ask beginner questions choose to accept only the answers that they want to hear.
I just get on with what I do best, which is photography:)
 
Right, seeing as you seem to have resolved it, I can close this thread now then, before you ignore or insult any more members keratos. Can I suggest that thinking a little more about what you type and being a little more appreciative for the help you are offered, even if you neither agree with it nor accept its relevance will make you far more friends around here.
 
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