Saturday, September 25, 2010

Nikon wireless flash basics

When working with multiple groups using Nikon's CLS system it helps to know the basics of how it meters .
First we need to understand the limitations of the system .
The camera , or SU800 -whichever you use to command the remote flashes , doesn't know how many flashes are in each group . You could put 10 flashes in group A and the commander system you use wouldn't even know there was one there until it requested a pre-flash - Once it 'sees' the pre-flash reflected off the subject it knows there is at least one flash in group A and not much more .
This means that the flash output in each group is equal from all flashes in that group whether it's half power from one flash , 1/4 power from two flashes or 1/8th power from 4 flashes .
One situation where this is useful to know is when when you have a flash behind the subject making it difficult for the metering system to work correctly .
Disregarding the under-exposure due to the white subject in this example , here is an image taken with one flash from the front left of the subject ..

The system has metered the pre-flash reading using the 'focus point diamond' in much the same way that Nikon's 3D matrix metering measures the ambient in normal photography . It 'maps out' an area , in this case the pre-flash reading , and determines that that is the subject and adjusts flash output accordingly .

Now see what happens when we move the one flash to an angle behind the subject  ....

Nikon TTL-BL , which is the metering mode that wireless CLS uses [Though it doesn't include distance info in its calculations with wireless CLS] unless you are in spot metering mode , is programmed to measure overly bright areas and disregard them from the equation classifying them as highly reflective surfaces . Understandably this is good for the system but also means that if you are using one flash behind the subject you will have very bright highlights as the system is not seeing the light in the direct line of the flash itself , rather it is getting readings from an angle and can only do so much in metering it correctly .
It rejects the very bright spots and meters for the reasonably exposed areas .....

Now we put another flash in the front left , also in group A and see what happens ....

The system has measured the pre-flash that came from both flashes at the same time , without knowing there were two of them , disregarded the bright spot which it deems to be a highly reflective surface - which it is at that angle , and requested a flash output that  gives it the 'correct' exposure on the rest of the metered area .
This gives a more accurate result with less blown highlights - it would be almost impossible not to have a small bright circle from the rear flash considering the angle of the flash and reflectivity of the subject's surface .

What would happen if they were in separate groups ? Well the system would meter them independently and add their combined exposures on any common areas where both contribute and calculate an output to prevent over-exposure of any of the areas under the focus point diamond
Like this ....

The system now knows that there is more than one flash because when it requested a pre-flash , in sequence from each group , it got one from group A and one from group B . The reading it got from group B was mostly disregarded due to the bright reflection that it detected resulting in an excessive exposure on one side and a 'correct' exposure in the front of the image using the readings from A and the small amount from B on that area , combined .

What I get from these results is a clue as to controlling a light from behind  the subject- by placing it in the same group as the front light and adjusting distance from the subject to vary its brightness , since it will be the same output as the front light , we can be confident that it will never be brighter than the front light - since they fire at an equal output .
The more complicated option is to use exposure compensation for a separate group or use manual settings for that group .

Monday, September 20, 2010

Moving the focus point can affect flash output !

Update: This issue has been fixed with the newer bodies as I found while doing the same test with the D7200, the difference being that with the D7200 the meter changes once the flash is turned on unlike the D90. This is the result I got in June 2016. The D7200 did not do this.

First off : This is not a big cause for concern when using TTL-BL or wireless CLS , it is just an exercise in understanding how TTL-BL 'thinks' and very hard to duplicate in a test scene let alone have the possibility of it happening during normal photography :) .
I decided to 'correct' an incorrect blog on the subject .
I'll just emphasize here that the flash does not meter off the selected focus point , the change has nothing to do with the way the flash system meters .
This will also only happen in TTL-BL mode and with Nikon's CLS wireless flash which also meters in TTL-BL mode . The possibility of it happening falls within a very small window of the metering system - when the meter detects under-exposure causing the flash system to compensate by increasing output , or conversely by decreasing output if it detects that the exposure is suddenly correct after it showing as being under-exposed .
The possibility of this happening when a focus point is moved also depends heavily on the metering mode selected [ it must be matrix metering ] and how matrix metering works in that particular body - because it varies between models .
I used the Nikon D90 in these tests , I've tested the matrix metering system before and understand [ to a degree] how it 'thinks' .
In many of the Nikon's [ not too sure about Canon ] matrix metering is strongly weighted toward the selected focus point and the 'focus plane' that it sits on , and it adjusts exposure accordingly . 
This means that if you are photographing a landscape and move the focus point from bright sky to shadow area "in the same frame" your exposure could vary by as much as two stops without even moving the camera .

Now it has been suggested that the changes seen in flash output are not due to the metering but rather due to the change in distance info from the lens which affect the output of TTL-BL flash [ not wireless which doesn't use the distance info ] .
This will cause changes as well but not the type I am demonstrating here . I have selected a scene in which the darker object which causes the an increase in output , is actually closer than the lighter object proving that the change in this case is not related to distance info from the lens .

Here is my test scene , the back of a camera closer to the camera with a white multi-plug adapter further back . 

Theoretically  when I change the focus point from the white object to the black object closer to the camera the flash exposure should decrease as the lens registers a closer focus point .

First I take a picture with the focus point moved onto the white object :

Then I move it onto the black object closer to the camera .... the flash exposure increases !?!

Why did that happen ?
Well TTL-BL and wireless CLS 'watch your meter" and adjust output accordingly .
Here's the back of the screen in the first shot : 
The camera's meter was happy with the exposure with the focus point on the white object - that 'focus plane' decided that the camera's settings were correct for the scene so the camera told the flash to back off accordingly .

After taking the picture I simply moved the focus point onto the black object closer to the camera , which should have backed off the flash according to the distance info , but when the 'focus plane' shifted and the white object was no longer a dominant part of matrix metering's equation it decided that the image was under-exposed and the meter moved to "-1"
No camera settings were changed , the focus point was simply moved .

The flash output was increased because matrix metering decided that the scene , for that focus point , was under-exposed and so it decided to increase the flash output accordingly !
So this proves that regardless of the changes caused by the distance information from the lens , there are isolated instances where flash output can be affected by moving the focus point in the same scene with no other changes to the camera's settings :) .