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by kiwijohn on Mon Nov 13, 2017 2:15 pm
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Hi Everyone,
I was after some information on this flash option available on Nikon flashguns. (and also probably Canon systems?) 
Nikon seems to be a bit vague about it in their descriptions and I wondered if anyone out there was an expert in these things.

I use it a lot in forest bird photography as it pleasantly blends natural with flash light and does not result in a "midnight effect" - black backgrounds.
I am mostly operating at distances of 3-8 metres from the birds.
Shooting in RAW, I also underexpose by 2-3 stops in-camera and turn the flash system itself down by a further stop.
As you can imagine this results in some pretty dark images, but Nikon NEF's are amazingly tolerant and shadow areas don't seem to suffer greatly when I adjust the exposure back up to where it should be in ACR later.
With careful adjustment I can produce a really subtle effect where it is not obvious that flash has been used at all unless you really know what to look for, like catchlights in eyes and slight leg shadows. Feather details really "pop" with tiniest barb details clearly visible.
Importantly, using 1/8 or 1/16 of the "normal" TTL BL light intensity does not seem to bother the birds, and even shy species will remain doing whatever they were originally doing even after a series of perhaps 8+ shots of them in this flash mode.
I use a Better Beamer to limit the spread of the flash beam so that the entire forest is not illuminated with a wide angle beam. Batteries last forever with this useful gadget.

So my questions are:
1. Does the BL mode of TTL flash involve a certain "% reduction" in flash output compared to the "normal" TTL option? (presumably it does, in order to let the natural background illumination "show through")?
2. What would this % reduction of flash output normally be? (probably varies with conditions - so what would the approx range of variation be?)
3. Does the flash sync speed vary to achieve this balance - (a slower sync speed would allow more background light to dominate in a picture) I will sometimes get a "post flash trail" when a bird is moving as it feeds/courts etc. because of the slow flash sync speed.
4. Do both TTL and TTL BL use "monitor pre-flashes" to calculate the final flash output? I know TTL BL does.

Thanks!
John Sibley
 

by Anthony Medici on Mon Nov 13, 2017 3:41 pm
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I will answer the questions posted though I'm not sure how much that will help since I do not think you really understand what the camera and flash are doing based on the descriptions given above. 

1 & 2. It is not a % difference. The closest approximation for what it is doing is that it is setting the flash exposure compensation to an additional -1 EV. In other words, TTL set to -1 EV flash compensation will be approximately the same as TTL BL set to 0 EV flash compensation. 
3. You are using the term flash sync speed incorrectly. Flash sync speed does not vary. It is the fastest shutter speed that can be used that will allow a full powered flash to record on 100% of the sensor/film. (The shutter is fully open when the flash goes off hence lighting the entire area.) I believe the term you are looking for is the shutter speed for the camera's ambient light exposure. TTL BL doesn't impact the camera ambient exposure. However, since it is trying to be 1 stop below the camera's ambient exposure, it does reduce the amount of light coming from the flash based on the camera settings.
4. All modes in the flash except manual exposure produce pre-flashes.

So I will take this description...
Quote:
Shooting in RAW, I also underexpose by 2-3 stops in-camera and turn the flash system itself down by a further stop.
As you can imagine this results in some pretty dark images, but Nikon NEF's are amazingly tolerant and shadow areas don't seem to suffer greatly when I adjust the exposure back up to where it should be in ACR later.

... and try to describe what is actually happening.

You are setting the camera's ambient exposure to underexpose 2 stops. You don't say if you are doing this with one of the automatic modes or whether you are setting the exposure manually. Assuming you are doing it with one of the automatic settings, that means the camera exposure compensation is set to -2. You are using TTL BL on the flash and using an additional flash compensation of -1.

For the settings above, the camera will set the shutter speed, aperture and ISO as best as it can, within the limitation you've preprogrammed through the various custom settings, to give you an exposure for the ambient light that is 2 stops too dark for the conditions. The flash honors the -2 for the camera, the BL gives the flash an additional -1 compensation and your setting of the flash compensation to an additional -1 means that the flash will light to give the total exposure of -4 stops based on the same metering it used to set the camera. Or the camera is 2 stops too dark and the flash is 4 stops too dark. Of course, the difference between the camera's exposure and the flash's exposure is 2 stops.

So assuming I didn't run into a limitation (shutter speed that can only go so low or so high, ISO that can only go so low or so high, the aperture limitation which can only open so far and the flash which can only go so dim or so bright), the expectation from your settings is that the flash is firing to light 2 stops darker than the ambient light. Of course, that assumes you are using an automatic mode to get your camera's exposure to -2 using in camera compensation. Since the flash uses the camera's compensation setting as well as the flash's compensation setting, there could be a significant difference in how the flash reacts if you are setting the camera's exposure manually and the compensation of the camera is not set to -2.

As a side note... Canon cameras and flashes do NOT react the same way to changes in the setting as Nikon cameras do. And there are also some slight differences in how various Nikon camera/flash combinations will react to the same settings. That is why you don't see much written on the topic.
Tony
 

by photoman4343 on Mon Nov 13, 2017 4:03 pm
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Mike Hagen's book on the Nikon Creative Lighting System is one of the best explanations of how the system works. I have the second edition of this book and refer to it when needed. He also has a lot of helpful things at his web site

https://www.creativelive.com/instructor/mike-hagen?utm_source=google&utm_medium=cpc&utm_campaign=S__NB__INSTRUCTORS&utm_content=INSTRUCTORS__MIKE_HAGEN&gclid=EAIaIQobChMI94Ob7bK81wIVFtNkCh2d7gWVEAAYASAAEgL9zfD_BwE


https://www.amazon.com/Nikon-Creative-Lighting-System-3rd/dp/1937538664/ref=pd_lpo_sbs_14_t_0/134-1782024-1035826?_encoding=UTF8&psc=1&refRID=GNYXNW8BT64JZBEVPJE2

To answer your questions, TTL BL means that the computer in the flash is working together with the computer in the camera. When you set flash exp com and camera exp comp they are additive in the Nikon system. With TTL BL set what you get depends on what both computers are sensing given the light and subject present. The specific amounts of flash output can vary with each subject/lighting situation.

You have more direct control if you set just TTL. When this is set you are more in control of the output of the flash. What is very frustrating is that with some Nikon flashes, TTL means TTL BL. That is why many just use the flash in manual mode and set the flash output needed.

What to set varies too. If i want "fill flash" I usually start at a flash exp comp setting of -1.7. If the subject is darker, maybe -1.3.Just take some test shots and adjust accordingly. The same advice goes if you have TTL BL set.

I use rear curtain synch most of the time. On some flashes and Nikon bodies, there may be no pre flash if you use rear curtain synch. But this will need to be verified by others and by looking at specific manuals.

If you want to use a shutter speed lower than 1/60th you usually will have to set the desired lower limit using a flash Custom Setting. If you are shooting at shutter speeds greater than 1/250 you have to set auto fp high speed sync.

With a better beamer you usually set the flash zoom head to 50mm.

Canon flash and camera exp comp works differently than Nikon's.

Hope this helps.
Joe Smith
 

by kiwijohn on Tue Nov 14, 2017 3:59 am
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Anthony Medici wrote:
I will answer the questions posted though I'm not sure how much that will help since I do not think you really understand what the camera and flash are doing based on the descriptions given above. 

1 & 2. It is not a % difference. The closest approximation for what it is doing is that it is setting the flash exposure compensation to an additional -1 EV. In other words, TTL set to -1 EV flash compensation will be approximately the same as TTL BL set to 0 EV flash compensation. 
3. You are using the term flash sync speed incorrectly. Flash sync speed does not vary. It is the fastest shutter speed that can be used that will allow a full powered flash to record on 100% of the sensor/film. (The shutter is fully open when the flash goes off hence lighting the entire area.) I believe the term you are looking for is the shutter speed for the camera's ambient light exposure. TTL BL doesn't impact the camera ambient exposure. However, since it is trying to be 1 stop below the camera's ambient exposure, it does reduce the amount of light coming from the flash based on the camera settings.
4. All modes in the flash except manual exposure produce pre-flashes.

So I will take this description...
Quote:
Shooting in RAW, I also underexpose by 2-3 stops in-camera and turn the flash system itself down by a further stop.
As you can imagine this results in some pretty dark images, but Nikon NEF's are amazingly tolerant and shadow areas don't seem to suffer greatly when I adjust the exposure back up to where it should be in ACR later.

... and try to describe what is actually happening.

You are setting the camera's ambient exposure to underexpose 2 stops. You don't say if you are doing this with one of the automatic modes or whether you are setting the exposure manually. Assuming you are doing it with one of the automatic settings, that means the camera exposure compensation is set to -2. You are using TTL BL on the flash and using an additional flash compensation of -1.

For the settings above, the camera will set the shutter speed, aperture and ISO as best as it can, within the limitation you've preprogrammed through the various custom settings, to give you an exposure for the ambient light that is 2 stops too dark for the conditions. The flash honors the -2 for the camera, the BL gives the flash an additional -1 compensation and your setting of the flash compensation to an additional -1 means that the flash will light to give the total exposure of -4 stops based on the same metering it used to set the camera. Or the camera is 2 stops too dark and the flash is 4 stops too dark. Of course, the difference between the camera's exposure and the flash's exposure is 2 stops.

So assuming I didn't run into a limitation (shutter speed that can only go so low or so high, ISO that can only go so low or so high, the aperture limitation which can only open so far and the flash which can only go so dim or so bright), the expectation from your settings is that the flash is firing to light 2 stops darker than the ambient light. Of course, that assumes you are using an automatic mode to get your camera's exposure to -2 using in camera compensation. Since the flash uses the camera's compensation setting as well as the flash's compensation setting, there could be a significant difference in how the flash reacts if you are setting the camera's exposure manually and the compensation of the camera is not set to -2.

As a side note... Canon cameras and flashes do NOT react the same way to changes in the setting as Nikon cameras do. And there are also some slight differences in how various Nikon camera/flash combinations will react to the same settings. That is why you don't see much written on the topic.


Thank you Tony, your answer is very comprehensive. Thanks for taking the time.
John


Last edited by kiwijohn on Tue Nov 14, 2017 4:51 am, edited 1 time in total.
 

by kiwijohn on Tue Nov 14, 2017 4:42 am
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Thanks Joe,
Mike Hagan reckons TTL BL is a good mode to use in many situations needing flash rather than classic TTL, although consistency can vary as it is constantly trying to juggle background with subject lighting.
You were saying:
"TTL BL means that the computer in the flash is working together with the computer in the camera. When you set flash exp com and camera exp comp they are additive in the Nikon system. With TTL BL set what you get depends on what both computers are sensing given the light and subject present. The specific amounts of flash output can vary with each subject/lighting situation."
Thats a very good point, and I often forget the sophisticated computing going on behind the scenes, but it also means that I will probably have to do some flash meter tests to find out the difference in output between "classic" TTL and TTL BL for a given situation, and as you say it probably varies constantly depending on the subject.

As you have found, once you have a formula that works, stick to it for your situation!
For myself, I love the subtlety of the system - Not enough light to overwhelm, and just enough to light that fine detail of feathers and eyes - and with the added ethical bonus of not causing the birds to jump out of their skins.

John
 

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