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Ambient (Sky) underexposure with flash

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mmonette

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Hi everyone.

I'm trying to figure out something. I have a two part question...

I'm wondering the following. If I am using a studio battery strobe outdoors and want to underexpose the ambient by two stops and correctly light a model...

Using manual let's say I am getting the following metering for the scene 1/250 sec, f11.

I would change my aperture to f22 to underexpose ambient by two stops. Would I set my studio strobe to output f22 to properly expose model?

If I used a 2 stop ND filter so I could get a larger aperture for the same scene, my ambient would now be 1/250 sec at f5.6.

I would change my aperture to f11 to underexpose the ambient. Would I now set my studio strobe to output f11 for this same scene with 2 stop ND?

Would highly appreciate some advice.

Kind regards,

Mike
 
mmonette said:
Hi everyone.

I'm trying to figure out something. I have a two part question...

I'm wondering the following. If I am using a studio battery strobe outdoors and want to underexpose the ambient by two stops and correctly light a model...

Using manual let's say I am getting the following metering for the scene 1/250 sec, f11.

I would change my aperture to f22 to underexpose ambient by two stops. Would I set my studio strobe to output f22 to properly expose model?
Click to expand...
Yes. The aperture is f/22, so the flash is set for f/22 if you want it to expose correctly. The ambient is expecting f/11, so it gets the surprise.
mmonette said:
If I used a 2 stop ND filter so I could get a larger aperture for the same scene, my ambient would now be 1/250 sec at f5.6.
Click to expand...
But the flash requirement is still f/11 (doing it the hard way, f/5.6 and 2x ND).
mmonette said:
I would change my aperture to f11 to underexpose the ambient. Would I now set my studio strobe to output f11 for this same scene with 2 stop ND?
Click to expand...
No, the flash needs the 2x ND beyond than the f/11 aperture, still f/22.
 
Is the model in the same ambient light as the background? If so, you're still going to get a contribution from the -2 stop ambient on the subject and you would need to account for that by underexposing the flash by 0.2. If you're using a meter that reads ambient and flash simultaneously and you've set the meter at the same 1/250 shutter your camera is set to, then it will account for that for you and you don't have to worry about it.

If the model is in the shade and the background is a sunny landscape, then the flash will account for 90% of the exposure on the model and your example question is applicable.

Without thinking too hard, I would say yes to both your questions. The ND filter would just shift the exposure for both flash and ambient in the same direction and require the same compensation. But I'm sure Wayne can write a treatise on why that is or is not the case. ;)
 
Thank you kind gentlemen!! This helps a lot :)

Going to play around with this next weekend.

Will let you know how I make out.

Mike M
 
Barrie Davis posted his rules for flash, and they are excellent. Learn them and you will never be confused again.

Flash Lighting Properties

Rules for mixing flash and ambient lighting by Barrie Davis.

http://forums.dpreview.com/members/5375205838

From dpreview post:
http://forums.dpreview.com/forums/readflat.asp?forum=1025&thread=37578401

When mixing flash with ambient at "X" synch and slower....

A) Shutter speed controls ambient light level independently of anything else.

B) Flash power and distance controls flash light independently of anything else.

C) Aperture controls flash and ambient together and in step with each other.

Therefore, having used shutter speed and flash adjustment to get an appropriate ratio between them, we go on to use aperture to fine-tune the overall exposure.

Now that digital gives us the chance to alter ISO shot to shot, it might be worth adding..

D) ISO level also works as aperture, and adjusts flash and ambient together and in step.
 
Those are useful rules except that they don't address what to do with a ND filter. That is, once you've reached the sync speed barrier of about 1/250th sec and have lowered the ISO as far as it can go, a modern DSLR will still not be able to darken a sunny sky background while using a strobe fill unless ND filtration is added.
 
The Sage Knows said:
Those are useful rules except that they don't address what to do with a ND filter. That is, once you've reached the sync speed barrier of about 1/250th sec and have lowered the ISO as far as it can go, a modern DSLR will still not be able to darken a sunny sky background while using a strobe fill unless ND filtration is added.
Click to expand...
You then STILL need to increase your flash output to compensate as well.

The biggest problem is that generally speedlights or even low powered strobe are not powerful enough to allow a correctly exposed subject and a darkened background, but people insist that adding ND gels to camera darkens ambient.. they do, but they darken flash by the same amount as well. Nett effect.. no change.

If you can't get your background dark enough when you're using your flash at full power, adding ND won't help in any way... your flash is just incapable.

All ND does is reduce sensitivity - much like lowering ISO. With this comes reduced GN.

--
Ian.

Samples of work: http://www.AccoladePhotography.co.uk
Weddings: http://www.AccoladeWeddings.com
Events: http://www.OfficialPhotographer.com

Theres only one sun. Why do I need more than one light to get a natural result?
 
Hi guys, Thanks again for all the info.

The reason I want to use the ND filter is not that my flash is not powerful enough, it's that I don't want to shoot at f22, I would rather set the camera to f11 and set my flash to output f22 with the 2 stop ND filter. Adding the ND filter was confusing me as to what settings to set my flash output with it on.

Mike
 
Would it be easier to just wait for the right light, i.e., shoot at a different time of day?
 
UKphotographers said:
The Sage Knows said:
Those are useful rules except that they don't address what to do with a ND filter. That is, once you've reached the sync speed barrier of about 1/250th sec and have lowered the ISO as far as it can go, a modern DSLR will still not be able to darken a sunny sky background while using a strobe fill unless ND filtration is added.
Click to expand...
You then STILL need to increase your flash output to compensate as well.

The biggest problem is that generally speedlights or even low powered strobe are not powerful enough to allow a correctly exposed subject and a darkened background, but people insist that adding ND gels to camera darkens ambient.. they do, but they darken flash by the same amount as well. Nett effect.. no change.

If you can't get your background dark enough when you're using your flash at full power, adding ND won't help in any way... your flash is just incapable.

All ND does is reduce sensitivity - much like lowering ISO. With this comes reduced GN.
Click to expand...
Which is why I picked up a dual flash head to use with ND. Works fine so far.
 
Sailor Blue is correct,

Per you example exposure of 250th @ 11 if you want the background darker you would increase the shutter speed. The shutter speed does not affect the flash exposure just the ambient light. If you want to adjust the flash to subject you can do this two ways. Adjust the aperture or leave the aperture the same and move closer or further from the subject.
Sailor Blue said:
Barrie Davis posted his rules for flash, and they are excellent. Learn them and you will never be confused again.

Flash Lighting Properties

Rules for mixing flash and ambient lighting by Barrie Davis.

http://forums.dpreview.com/members/5375205838

From dpreview post:
http://forums.dpreview.com/forums/readflat.asp?forum=1025&thread=37578401

When mixing flash with ambient at "X" synch and slower....

A) Shutter speed controls ambient light level independently of anything else.

B) Flash power and distance controls flash light independently of anything else.

C) Aperture controls flash and ambient together and in step with each other.

Therefore, having used shutter speed and flash adjustment to get an appropriate ratio between them, we go on to use aperture to fine-tune the overall exposure.

Now that digital gives us the chance to alter ISO shot to shot, it might be worth adding..

D) ISO level also works as aperture, and adjusts flash and ambient together and in step.
Click to expand...
--

'Not everything that is faced can be changed, but nothing can be changed until it is faced.'
 
Of course, if your camera sync speed tops-out at 250th with studio strobes then you cannot set it any faster...

And using flash at small apertures requires a LOT of strobe power, or moving the strobe closer to the subject.

If you want to under-expose the ambient 2 stops, then you will have an easier time of it by shooting earlier or later in the day when ambient light levels are lower.
 
Thanks again everyone - great information!! I really appreciate it. I picked up the Profoto AcuteB2 pack (600ws) w/ acute B head so I getting some good output from it. I am going to test this out next weekend with a 2 stop ND filter.

Mike M
 
UKphotographers said:
The Sage Knows said:
Those are useful rules except that they don't address what to do with a ND filter. That is, once you've reached the sync speed barrier of about 1/250th sec and have lowered the ISO as far as it can go, a modern DSLR will still not be able to darken a sunny sky background while using a strobe fill unless ND filtration is added.
Click to expand...
You then STILL need to increase your flash output to compensate as well.
Click to expand...
Well yes. The OP said he would take another measurement presumably to readjust the strobe level.
UKphotographers said:
The biggest problem is that generally speedlights or even low powered strobe are not powerful enough to allow a correctly exposed subject and a darkened background, but people insist that adding ND gels to camera darkens ambient.. they do, but they darken flash by the same amount as well. Nett effect.. no change.
Click to expand...
Well a change in depth of field. And after increasing the flash power to bring the foreground back up, the background will stay dark.
UKphotographers said:
If you can't get your background dark enough when you're using your flash at full power, adding ND won't help in any way... your flash is just incapable.
Click to expand...
You can still move the strobe closer. And where did you get the impression that he’s using flash at full power? He said he was going to adjust the flash until the meter said f11. That would imply he had spare power available to adjust it.
UKphotographers said:
All ND does is reduce sensitivity - much like lowering ISO. With this comes reduced GN.
Click to expand...
Correct and the reason you would do that is if your ISO is as low as it will go- which is usually the case when you're trying to do this type of exposure at midday. Only option then is to go for the ND (polarizer also works) filter.
 
" The biggest problem is that generally speedlights or even low powered strobe are not powerful enough to allow a correctly exposed subject and a darkened background, but people insist that adding ND gels to camera darkens ambient.. they do, but they darken flash by the same amount as well. Nett effect.. no change.

Well a change in depth of field. "

How is an ND filter going to change the depth of field?
--
Best Regards,
Renato

'The world is going to pieces and people like Adams and Weston are photographing rocks.' Henri Cartier Bresson, in the 1930's
 
DennisButler said:
Which is why I picked up a dual flash head to use with ND. Works fine so far.
Click to expand...
The only time you would get an advantage to making a background darker with that is if you used a LESS THAN 1 stop ND filter.

Maximum benefit with two flash units is obtained when used WITHOUT an ND filter and both units at full power.

--
Ian.

Samples of work: http://www.AccoladePhotography.co.uk
Weddings: http://www.AccoladeWeddings.com
Events: http://www.OfficialPhotographer.com

Theres only one sun. Why do I need more than one light to get a natural result?
 
The Sage Knows said:
UKphotographers said:
The Sage Knows said:
Those are useful rules except that they don't address what to do with a ND filter. That is, once you've reached the sync speed barrier of about 1/250th sec and have lowered the ISO as far as it can go, a modern DSLR will still not be able to darken a sunny sky background while using a strobe fill unless ND filtration is added.
Click to expand...
You then STILL need to increase your flash output to compensate as well.
Click to expand...
Well yes. The OP said he would take another measurement presumably to readjust the strobe level.
Click to expand...
No. At no time does adding an ND filter provide any advantage to a flash.
The Sage Knows said:
The Sage Knows said:
The biggest problem is that generally speedlights or even low powered strobe are not powerful enough to allow a correctly exposed subject and a darkened background, but people insist that adding ND gels to camera darkens ambient.. they do, but they darken flash by the same amount as well. Nett effect.. no change.
Click to expand...
Well a change in depth of field. And after increasing the flash power to bring the foreground back up, the background will stay dark.
Click to expand...
If you have 'power' to turn up you have no need for anything else. You just do that, just turn up the power. Least of all use an ND filter, this provides no advantage to darkening a background when using a flash in ambient lighting as it affects both equally.
The Sage Knows said:
The Sage Knows said:
If you can't get your background dark enough when you're using your flash at full power, adding ND won't help in any way... your flash is just incapable.
Click to expand...
You can still move the strobe closer. And where did you get the impression that he’s using flash at full power? He said he was going to adjust the flash until the meter said f11. That would imply he had spare power available to adjust it.
The Sage Knows said:
All ND does is reduce sensitivity - much like lowering ISO. With this comes reduced GN.
Click to expand...
Correct and the reason you would do that is if your ISO is as low as it will go- which is usually the case when you're trying to do this type of exposure at midday. Only option then is to go for the ND (polarizer also works) filter.
Click to expand...
That's not the only option at all.

In the first place, the suggestion of using ND when trying to darken a backround is unfounded as ND doesn't assist in this in any way. The only useful purpose it serves is to allow a more suitable / wider aperture. An ND filter will not add any more power to your flash or reduce background brightness when your lighting is at full output. Adjusting output is the first option, if it's available, if it's not - no amount of ND will help.

Most lenses shift down to around f32 which is adequate to underexpose a background by 3.5 stops at 100 ISO even on the brightest of days in the Sahara..
you might not chose to use f32.. that would be the reason to use an ND filter.

--
Ian.

Samples of work: http://www.AccoladePhotography.co.uk
Weddings: http://www.AccoladeWeddings.com
Events: http://www.OfficialPhotographer.com

Theres only one sun. Why do I need more than one light to get a natural result?
 
Revision said:
Revision said:
Revision said:
" The biggest problem is that generally speedlights or even low powered strobe are not powerful enough to allow a correctly exposed subject and a darkened background, but people insist that adding ND gels to camera darkens ambient.. they do, but they darken flash by the same amount as well. Nett effect.. no change.
Click to expand...
Click to expand...
Revision said:
Well a change in depth of field. "
Click to expand...
How is an ND filter going to change the depth of field?
Click to expand...
A 2 stop ND filter will allow an exposure of 1/250s f11 to become 1/250s f5.6 with no change to the image brightness level, only the depth of field. If flash is used amongst ambient here, the ratio of flash and ambient will remain exactly the same as they were.

--
Ian.

Samples of work: http://www.AccoladePhotography.co.uk
Weddings: http://www.AccoladeWeddings.com
Events: http://www.OfficialPhotographer.com

Theres only one sun. Why do I need more than one light to get a natural result?
 
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