Tuesday, October 19, 2010

Turbo flash

This afternoon I decided to put the new "mutant" flash through its paces to see what it could do .
I fired it about 15-20 times all together without giving it much time to cool down .
I took a few shots to compare it to the SB800 on full power .Ignore the background getting brighter , it was raining and the sun was coming out again . Concentrate on the foreground and shadows .
I used the D50 because it can sync flash at any speed .

I had the "super-flash" on half power which was half the capacitors in the original flash plus the 3500uf cap I added .

Then I moved up the driveway to really put it to the test ....

I then moved to the top of the driveway . And tried at 1/3200th sec .

I would imagine the flash tube will last as it doesn't appear to have suffered any side effects .
"Full power " is when the other half of the capacitor bank inside the flash handle is also connected.

Saturday, October 2, 2010

Extending the SU800's "angle"

For some this may be common knowledge but today I was doing some experiments with the SU800 outdoors .
I had the SB800 on a stand behind me in remote mode firing at 1/128th power and kept changing the angle of the camera until it would no longer fire .
When they were both pointing toward that fence it fired every time even with the flash behind the camera - maybe it was bouncing the signal off the fence ?
Anyway I reached an angle when the SB800 would no longer fire ...

You kind of wonder why Nikon would make the trigger system facing directly forward which is the last place anyone would have their lighting .
Then I added something to the equation , without moving anything , which made it fire again ....

I had tried simply placing my finger in front of the SU800 and that worked , the signal bounced off my finger and triggered the flash behind the camera .
I just added this little white object in front of the SU800 which bounced the signal backwards and made the flash fire every time .

I see possibilities here , for the SU800 and SB800 , I think I could design something that attaches to each of them to send and receive signals from any angle - or at least increase their reliability in some situations .

I did some tests to see what range the SU800 has in bright sunlight with nothing to bounce off . I measured it out as 19 large steps .

When I turned around it would no longer trigger , understandably , but I found that placing a wide angle mirror in front of the SU800 would trigger the flash but the range was now down to 7 large steps .

Not many people will ever need that range especially using flash outdoors but it's good to know what the limits are .


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 to 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 :) .

Monday, April 26, 2010

The SB26 vs SB800 "SU-4" comparison

I recently did a few tests with my newly acquired SB26 and the SB800 wireless flash modes using direct flash as a trigger on my other blog .

Saturday, April 3, 2010

TTL/BL increase in exposure with diffuser dome .

The manual suggests using the diffuser dome at 60 degrees for bounce flash .I have found that generally TTL/BL and wireless CLS under-exposes with bounce flash with white objects .
I noticed recently in some tests using the diffuser dome [ which I seldom use ] that the exposure seems to increase when it is used .
With plain TTL flash there seems to be a slight increase in exposure when the diffuser dome is on , possibly something to do with reflectivity and the way the metering pattern of TTL works .

With TTL/BL though the change is a lot more pronounced .

There's a slight increase in exposure when the wide angle diffuser is pulled out and the flash head pulls back to wide angle inside .

Then there is another slight increase when the diffuser dome is fitted .

Now when you fit the diffuser dome there is a small button under the flash head that gets pressed in . Watch what happens when I simply press that button ...... the flash head zooms back and BAM !

To get the same increase in exposure with direct flash I had to dial in an extra two stops of compensation .
So there is something in the TTL/BL program that responds to that button , besides the fact that the flash head zooms back to its widest setting it also seems to increase its output excessively disregarding the pre-flash reading .
Another possible concept is that when it expects a wide flash output and more diffused light perhaps the program that detects reflections and disregards them from the pre-flash reading starts to allow for wider areas of reflection resulting in over-exposure as it starts reading more from the darker areas . [ these are all just theories for now ] .

Now in the past I have noted that TTL/BL under-exposes quite badly with white objects and bounce flash but this increase in output seems to counter-act that under-exposure if you remember to either use the diffuser dome or the wide angle diffuser - both of which increase the output .
The manual for the SB800 [ page 101 ] states that best results are achieved when the flash head is at 60 degrees with the diffuser dome on [ depending on the situation of course ] , perhaps they have calculated their over and under exposure and the amount of light the dome throws forward at this setting  ?
Either way I tried the same subject with bounce flash ......
And of course we have the usual over-reaction to whites that TTL/BL displays with bounce flash .

Then we fit the diffuser dome .... much better !

And once again either pulling out the wide diffuser or simply pressing the button under the flash head that the diffuser dome switches still gives us a favourable increase in exposure ...

So my thoughts for now are that if you use bounce flash then either have the wide diffuser out , or the diffuser dome , or be prepared to dial in positive flash exposure compensation with 'normal ' bounce flash  .

I was also requested on a forum to post a picture of the room .....

Update : some tests with bounce flash and distance info ...

When you use plain bounce flash with no modifiers the TTL/BL program "almost" ignores the distance info - we'll call it a 95% pre-flash reading and a 5% distance reading calculation just to make it easier to explain .

The pre-flash reading identifies the white ball as the subject and responds by aiming for a neutral gray output .

Changing the focus and obviously the distance information from the lens has a very small effect on the exposure .

As soon as the diffuser dome is added or the wide diffuser is pulled out [ Just in case the program assumes that the bounce card will be used I left it extended ]  the distance info affects the output to a larger degree and this shows in the change in focus [ and the resulting change in distance info ] .

It's starting to look more like a 50/50 equation now with regard to input from the pre-flash reading and the distance info .

The histograms are almost exactly the same with the wide diffuser and bounce card .

This explains the 'superior' output with bounce flash when the diffuser dome is used but doesn't really explain why the output increases with direct flash .

I also did some tests with the SU800 and one flash in wireless mode .
There was no change with focus distance showing that distance info doesn't seem to be any part of the equation with wireless flash .
There does still seem to be that slightly better look and exposure with the diffuser dome though .....