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Ambient light control question

flash lighting light exposure

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11 replies to this topic

#1
Spodeworld

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Hi,

 

OK.  Let's say I am in Aperture Priority mode and am using a flash.

 

If I want to lower the ambient light due to the background being too light without impacting the exposure on the subject with the flash, do I use Exposure Compensation or Exposure Compensation + Flash Exposure Compensation.

 

I get it that there are many who would do this in manual, but I am trying to understand the approach for a semi-auto mode like Aperture Priority.

 

Thanks!

 

Steve



#2
yauman

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If you are using a TTL Flash (ie one compatible with the Nikon CLS system - including the onboard flash) you HAVE to shoot in manual to get a good picture - but you are not really in Manual mode - you are actually transferring the auto exposure from the camera to the flash.

 

Set camera to manual - set speed to 200 (or whatever is highest syn speed of you camera.)  Set the aperture to the depth of field you want for your image composition.  The Nikon CLS will communicate with your speed lite and will tell it what the settings are (including zoom focal length.)  The speedlight will than figure out the amount of power to get proper exposure -so you are really in auto-mode - auto exposure by the speed light.  Now if you want to change the exposure, change Flash Compensation.  The power the speedlight regulate is by the time the light is on - in the case of the SB700, from 1/1000 sec for full power to 1/40,000 sec for 1/128 power, that's why the shutter speed doesn't matter! for the subject but it will for the background.  Change shutter speed to change how the background is expose - speedlights do not have the range to light the background - remember the 1/R^2 rule - 2x the distance, 1/4 the power, 3x the distance 1/9 the power. So, the shutter speed will determine the background exposure.

 

Hope this helps.



#3
akanarya

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you may need measure the backlight and lock, and then high speed synchronization mode of the flash for the subject, If I didnt misunderstand.



#4
Spodeworld

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Thanks, I appreciate the good feedback. It is very useful.

 

But, there was something else I was try to understand too. 

 

Let's say for a minute I decided that I am going to use Aperture Priority (even though Manual would be better).

 

If I wanted to bring down the ambient exposure but not affect the flash exposure, would I just dial down EC?  Or, would I have to dial up FEC to compensate for the dialed down EC too?

 

Thanks



#5
Russ

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Faster shutter would cut down ambient. So would lowering your ISO.



#6
etphoto

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Faster shutter would cut down ambient. So would lowering your ISO.


He is using AP so the camera will choose the shutter speed. Reducing the ISO is the quickest and easiest way to lessen the ambient light in the background being picked up.

#7
yauman

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Let's say for a minute I decided that I am going to use Aperture Priority (even though Manual would be better).

 

If I wanted to bring down the ambient exposure but not affect the flash exposure, would I just dial down EC?  Or, would I have to dial up FEC to compensate for the dialed down EC too?

 

Here's something to keep in mind when using a flash - and you'll be able to solve ALL problems associated with using a flash - regardless of shooting mode!

Without a flash, we have the "exposure triangle" - F-stop, Shutter Speed and ISO.  With flash you have added two more parameter - flash power and distance of flash to subject (not distance of camera to subject!)  The amount of light thrown on subject decreases by the 1/R-square rule so your ambient background unless you are shooting in a 8x8 closet will NOT be lit by the flash.  So, while your subjects exposure is affected by f-stop, flash power, flash distance, & ISO, your background ambient exposure will be controlled by the exposure triangle again - F-stop, Shutter Speed and ISO.

 

When you are on Aperture Priority and you dial in EC values you are telling your camera to change shutter or ISO to affect the overall exposure of the whole image - which includes the ambient background.  If you change FEC, you are only telling the Flash to raise or lower it's output power to some value other than the calculated power based on data from the camera - and remembering that flash effectiveness has very limited distance, only your subject's exposure will be affect and the ambient background much less or not at all (unless you are in an 8x8 closet!!!)

 

Truth be known, I have never successfully shot with aperture priority with my D7100 or D90.  I think Manual with the Nikon CLS system is the only way to go because basically with the CLS TTL you are really in aperture priority - you keep the shutter speed at highest sync speed of the flash, keep the ISO at lowest possible and start shooting with the desired aperture - aka "aperture priority" -  the flash will do the rest for you. When it gives you the beep warning of not enough power, up the ISO by a stop etc.  

 

I shoot exclusive with 2 speed lights (SB700s, both off camera in soft boxes) for all my events and model photography, indoors or out. 

 

Hope this makes sense! 



#8
Spodeworld

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Lots of good stuff here.

 

I should have tried to state it more directly, but I'm just trying to understand how the camera is set up to respond to only changing EC when a flash is also used and the camera is set to Aperture Priority.

 

So, in Ap Priority mode with flash, if I dial in EC, does it only impact the ambient exposure?  Or, does it impact both the ambient and the exposure from the flash on the subject?

 

Thanks



#9
yauman

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Lots of good stuff here.

 

I should have tried to state it more directly, but I'm just trying to understand how the camera is set up to respond to only changing EC when a flash is also used and the camera is set to Aperture Priority.

 

So, in Ap Priority mode with flash, if I dial in EC, does it only impact the ambient exposure?  Or, does it impact both the ambient and the exposure from the flash on the subject?

 

Thanks

 

Ok, simply put, if you dial in EC, you are dialing in more (+EC) or less (-EC) exposure to your whole image. The camera does this by either slowing the shutter speed or upping the ISO - leaving the Aperture unchanged.  If you dial in FEC, you are forcing the flash to put out more (+FEC) or less (-FEC) power - in which case only the part of the image affected by the flash will have different exposure.  As you can see I avoided the use of "ambient" light as it's not alway a useful term here.  If say i have a low ceiling and shooting in my dining room in the daytime or with the room lights on and I point my flash to the ceiling for a wonderful soft bounce flash lighting then changing either the EC or the FEC changes will have the same effect on the resulting image.  However, if I  repeat the exercise with the room light off, ie in darkness, changing the EC will not have as much effect as changing the FEC unless the camera change ISO rather than shutter speed when changing EC - Anyone know if the Nikon up ISO first or slow down shutter first when adjusting for EC change?  Now, change the flash to point to a subject - then changing the EC will definitely change the exposure of the background ambient as well as the subject (ie the whole image) but changing the FEC will only change the exposure of the subject and not the background/ambient.

 

Sorry, I'm not trying to be vague but as you can see, it depends on the situation and how you are using the flash - point straight at the subject with background out of flash range (anything more than 6-10 ft away) or bouncing off a wall or ceiling or straight at subject through a soft box.  If I remember right, you were asking about the SB400 which is a very weak flash so I would say that in most cases, changing EC will change the exposure of the whole image including the background/ambient but changing the FEC will only change the exposure of the subject.  



#10
Spodeworld

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Thanks yauman.  The situation I was most thinking about is when you are pointing the flash at the subject (I know how bad that can be)....for example, if you are outside and want some fill flash.  Based on what you said, lowering the EC will affect the ambient exposure as well as the light from the flash on the subject.  So, the background would go darker as would the subject.

 

So, under that situation, if you wanted to darken the environment but keep the subject properly exposed with the flash (assuming proper exposure was achieved on the subject with the flash prior to dialing in EC), you would lower the EC and up the FEC.  Just lowering the EC would darken everything if I understand you correctly.  I know manual is the better way to go, but I am just trying to understand the camera's behavior.

 

If I misinterpreted this, please let me know.

 

Thanks so much for all the effort you put in to your explanations.



#11
yauman

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Yes, you got it!

 

Now you have discovered the inherent weakness of trying to use flash in Aperture Priority mode.  You end up having to fool around/fiddle with two different parameters to try to "cheat" the camera to get to do what you want out of it - if you are going to fiddle you might as well just shoot in manual so you only have to fiddle with just one parameter - the shutter speed.

 

In this fill-in flash situation, all you have to do is select your aperture based on your DOF need (you do that anyway in A-priority mode) then using the light meter (shows up in the viewfinder when you are in manual mode), set the shutter speed to get the correct exposure - then speed up 1 or 2 stops to get the darken effect you want - than use the flash to fill in the subject and expose the subject properly.  Voila - fiddle with shutter speed and you got it and leave the EC and FEC alone!

 

Hope that helps.



#12
Spodeworld

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Excellent yauman....that is very helpful.  Thanks so much.







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