Tuesday, January 26, 2010

Realsynch flash

I'll state from the beginning : This will only work with a Nikon D40 , D50 or D70/S since they do not have a focal plane shutter , instead the sensor fires electronically which gives it the advantage of capturing the whole flash in one go as opposed to a slit moving across the frame as with Focal Plane [FP] shuters .

First off two samples to whet your appetite comparing what I call "realsynch" to the high-FP trick mode that modern cameras use in bright sunshine with high shutter speeds .
This is the best a Nikon D90 and SB800 can do at a distance of 5 metres ....
taken indoors at night .


And this is what "realsynch" does ....


I used a modified SB24 flash for my "realsynch" flash . Because the cameras mentioned fire electronically with their sensors they have a 1/500th flash synch speed meaning that since the SB800 , for example , fires at full power for around 1/1000th of a second the camera will be able to capture that entire flash in 1/500th of a second without any overlap in the timing of the electronics .
With a D90 this time is only 1/200th of a second and after that it changes to FP mode where the shutter slides across the frame in a slit - then it has to resort to a weak continuous light so the whole frame receives the same amount of light .... more on that later .

First we have to make sure the D40/50/70/S we are using doesn't know there is a flash connected and there are two options , either slide a piece of paper under the two back pins as you slide the flash into the hotshoe or do what I did - open the base of the flash and cut the two wires to the back two pins -BUT : be careful of high voltages , you could get zapped !
Now if you have an SB600/800/900 the paper trick is the safest if you are not 100% sure of what you are doing , also you don't want to damage a newer flash .
And this is a picture I took a while back to show the flash pins .

The centre pin gets earthed to the 'ground' contact on the side of the hotshoe when the camera wants it to fire . The single pin at the top is the 'camera to flash' communication pin .
The two at the bottom are for the flash to communicate with the camera so I disconnect them so the camera doesn't know the flash is connected and can't default to maximum synch speed .
In any case it doesn't speak the same language as the newer cameras so I have found that with my D40 and D50 I can fit an older flash without bypassing or blocking any of the pins and it still works at any speed , however I can't guarantee that there won't be any mis-communication between the camera and flash so I generally use my SB24 with the wires cut ....
I leave the one at the top connected because it is useful in that you can switch the SB24 to "standby " mode and when the camera is switched on that pin wakes the flash up - so the flash knows a camera is connected but the camera doesn't know a flash is connected !
Of course this means you have to input the aperture , iso and zoom of the head manually - but you can still leave the flash in 'auto' mode and it will use its sensor to control the flash output accordingly - based on the settings you have told it to use .
The advantage ? Well you can see that from the two pictures at the top !
If we go by the 'sunny 16' rule : Bright sunlight at F16 gives you iso 200 and 1/200th max for the D90 . The SB800 tells me I have 2.7 metres working distance at this setting but when I open the aperture - F8 1/800th , F4 1/3200th ... the flash has to go to high fp mode and tells me I have 1.4 metres working distance .
Now the SB24 : F16 on the D50 allows me 1/500th sec at iso 500 , full power - which gives me 4 metres . f8 iso 200 1/800th full power is cutting it a bit fine to prevent overlap but as we drop in flash power it fires much faster . At 1/4 power it fires at 1/2700th sec and at 1/8 it fires at 1/5500th sec so at F4 1/3200th and iso 200 I can leave it on 1/4 and still catch a high percentage of the flash and have 4 metres working distance .
These distances are at 24mm wide angle on the zoom head and increase [ on both flashes ] as I zoom in and the power used would be enough to light a subject in total darkness as shown by the first two pictures . Of course outside in bright sunlight we would not need all that power and could use the flash as fill only , bounce it off a reflector , or close the aperture without changing the settings on the flash thereby fooling the flash to think it doesn't need to fire so strong [ increasing the iso setting on the flash will also reduce its power ] .
Here are two more pictures illustrating the power you have outside in bright sunlight .
Just remember one thing though , you have so much power available you may need to make sure you don't get too close to your subject/s since the flash may not be able to fire weak enough !


Now the 'rough' maths . Due to the inverse square law when you are twice as far from the subject - the light from the flash going forward in a 'rectangular' pattern means that the length and breadth of that 'rectangle' double and twice the length times twice the breadth = 4 times the area . So if you are getting twice the working distance it actually means you have 4 times the flash power ! The results may not agree 100% with the maths but they look close enough to prove that there is a huge advantage in 'realsynch' compared to high-fp mode .
So if the SB800 tells me I have 1.4 metres working distance and the SB24 tells me I have 4 metres then 4/1.4 = 2.85 X the working distance . 2.85 squared gives just over 8X the power !
That's 3 stops more powerful than a D90 and SB800 combination achieved with a lowly D40 and SB24 flash !
After doing some tests in manual mode without flash to get the same histograms as the first pictures [ not 100% but close ] it appears I am getting 2 1/3 to 2 2/3 stops more power than high fp mode .
I just did some calculations on the difference between normal flash and High fp on the SB800 , hopefully they are correct ;
When I set my camera manually to iso 200 , F16 and 1/200th sec the flash tells me I have 0.6-2.7 metres working distance available . As I take it to F8 1/800th or F4 1/3200th it goes into high-fp mode and tells me I have 0.6-1.4 metres . Now 2.7m/1.4m gives us 1.928X the distance . 1.928 squared tells us we have 3.72X the flash power when we are not in high-fp mode .
1/3.72 = 0.2688 which is the power we have in fp mode compared to normal flash mode .
That's less than 27% of normal flash power with a D90 and SB800 flash , when in high-fp mode .
At iso 100 and 35mm the SB800 has a GN of 38m and the SB24 a GN of 36m . By my calculations the SB24 has less than 90% of the power of a SB800 .
But when we start comparing "realsynch" to high fp mode the SB24/D40 combination has almost 8X more power at higher shutter speeds due to the faster flash synch speed !

Today I did some tests to try to turn "day into night " .
First a picture without flash .

Under expose the background by increasing the shutter speed ...

And with the D40 , [35mm 1.8 lens] and SB 24 we still have enough power at 4 meters to make the picture look like a bad 'direct flash' shot at night !



FlashMike said...

Excellent article Desmond. I am off to play with my flash & experiment. Thanks Mate!

Desmond Downs said...

No problem :) I hope Nikon don't give up on electronic shutter cameras in future .

Anonymous said...

I have a Sigma EF-500ST Flash. We do not operate via radio triggers. I've heard that does not work because it can not be full manual setting. What can he bridge to work?

Desmond Downs said...

@Anonymous - I'm not too sure about the Sigma but if it's Nikon compatible then slide a piece of paper under the back two pins of the flash.