CANON ETTL-1: FP-Sync VS. ND-Filter
so listen up and tighten your seatbelts :-)… what’s coming now, is a bit of hard calculating to increase your brain activity… :-) We are going to talk about the FP-”Highspeed”syncronisation (FOCAL PLANE) of your on-camera-flash and the usage of a neutral density filter. Without talking about photos or any example, we are going to look at the hard facts.
For the beginning, we have to make some points clear. We have to do some definition and some explanation to get everyone onto the same level.
Why would you ever use a FP-mode on your camera? You camera starts and stops your exposure time with two curtains. Imagine a theatre and when the show starts, the first curtain rises and when it’s over, the second curtain rises again??? What? Rises again? Why not fall down? Well, to get an evenly exposed image, the part of the sensor which is exposed first, has to be “unexposed” first to. So if the bottom of your sensor gets the first lightbeams as the curtain opens from down to top, it also has to close from down below to the top.
Ok, as we all know, now mechanical part is fast as hell. So how can we get exposure times up to 1/8000 of a second. Very simple. The more you narrow the two curtains to each other, the smaller the space in between gets and the smaller the open “bar” is, the shorter the sensor gets exposed when the curtains move a constant speed.
So, to get this clearer! That “open bar” moves over the sensor and for the time that space is over the sensor, that place gets some light.
And now you know, what the x-sync is. It is the shortest exposure time on your specific camera where the whole sensor is exposed at the same time. If you expose for a shorter time, there will never be a moment when the whole sensor is free! And as we all know as well, flashes do fire only once. ONE MOMENT of flash exposure. Taking that together states, that if you use a exposure time faster than your x-sync, one part of the sensor gets covered by the second curtain and the flash will never reach that part of the sensor. You get that funny black bar over your photo.
What the solution? Very simple, you make your flash GLOW for a longer time. You change your one-time-flash into a short-time-lamp :-)… And that’s what called FP-Sync on your flash! Actually HIGHSPEED is a bit of a lie when speaking about FP-SYNC… The flash doesn’t light up quicker, it’s much longer! MUCH MUCH LONGER!
Ok, and now to the question: WHY DO WE NEED THAT???
Well, just one example out of hundreds. Imagine a very bright summerday. You go out to take photos and you do want to use your flash. The AL is so bright that you come out with using f8 – 1/500s – ISO50. Well, with that exposure time you can’t really use your flash (REASONS JUST EXPLAINED :-)). So you go up with your f-stop. But hey, you do not want to use such a little aperture. You want some depth of field… By opening the aperture, you either get a overexposed image or you just can’t use your flash beacuse of the shorter exp-times.
Ok, solution would be to use FP-SYNC or: ND-filter (neutral density filter). They just take away some light in front of your lense. So every light coming into your lense is reduced by the strength of you chosen filter. A ND2 filter doubles the exposure time, or in other words, it takes away one whole f-stop.
So if you have your perfect settings, just use a ND-filter and open the aperture by the numbers of f-stops you took away with your filter. EASY AS THAT!
NOW, WE CAN FINALLY START WITH THE POST. LETS COMPARE THE POWER OF A FLASH WHEN USING FP-SYNC WITH THE USAGE OF A ND-FILTER! WHAT MAKES MORE SENSE???
Take a look at the display of our test-flash. The 580 EX II with a guide number of 58. We used a 50mm lens, the zoom of the flash was set to auto. Normal ETTLII mode. Have you ever noticed the small numbers on the bottom of the display? It displays a distance in METER!
Now you have to know, that the flash communicates with your camera. The camera tells the flash all the settings for the next photo, and the flash itselfs, nows his guide number. So whenever you press your trigger halfway down, the flash will show you the maximum distance possible to light up a scene.
Very easy calculating. Imaging a flash with GN55 and you are using f11. Then the flash will tell you, that you can fire a maximum of 5m (55/11=5). It even takes ISO-speed into account.
Let’s have a look at what happens when we dial some settings on our camera… let’s take the settings down below! (5D Mark II)
Our flash tells us, that we could fire a MAXIMUM of 6m. You can see the f8 sign on the flashdisplay.
And now, let’s change settings to FP-mode. 1/400s and we then open the aperture up to f5.6!
Although we changed nothing to the intensity of the lighting, we decreased exposure time by 1 fstop, and opened the aperture up 1 fstop, our flash only has enough power to get 3m.
WOW!!! That is half the distance by just 1 fstop more speed and depths of field! That’s a lot of loss!
Just to compare that to the normal mode! Even if you close your aperture down to f11, the flash still gets enough power to get up to…
And that’s the result. By using highspeedmode on your flash, you throw away your power. The strobe needs such a lot of power to increase the time of “flashing” that the power falls way down!
- If you just need a open aperture for depth of field, than the ND-filter is the best thing ever without wasting any power!
- If you need the fast exposure time e.g. to freeze something in bright daylight with your flash, than the FP-SYNC is just great. KEEP IN MIND! You weaken your flash!
- The decrease of power is about two f-stops. That’s like you quarter down you flash…
THERE IS ONE MORE PRETTY INTERESSTING POINT TODAY: When we made the photos for this blogpost, we found something very strange. With using the same settings on our camera, by just setting on FP-mode, we decreased the maximum power. That means, we didn’t even change the exposure time on the camera. The first image was done with the exact same setting on the camera. 1/200s exposure time! So, never leave on the FP-sync just for fun. It costs you power! CONSTANTLY!!!
WE GOT A LITTLE HINT FOR YOU! The flash is a really cool “calculation-machine”… If you use it wisely, you can always have one eye on the maximum range of your flashpower. You always know what is possible or not.
Example: After this post, we all know, that with a 580 EX II in 50mm zoom at f8 I have a range of 6m. If I have my camerasetting fixed at f8 and the flash is 3m away, I have to set the flash to 1/4 power. Without wasting any shots to set up the power. If I do want to use a umbrella I can PREcalculate if there is enough power to do so! YOU GET THE POINT!
See you soon!