October 15, 2016 at 4:10 pm #12480
In what conditions should First Curtain and second Curtain Flash be used? How does each setting effect the results? I have never used Flash either as fill in or as a tool in the capture of difficult light conditions.
October 16, 2016 at 4:08 am #12482
- This topic was modified 2 years, 1 month ago by wbaxter.
This is easier to understand if we start from first principles (so forgive me if this sounds like teaching grandmas to duck eggs!). When you use flash, there are always two exposures happening: one from the ambient light and one from the flash. The ambient light exposure is controlled by the aperture and the shutter speed; the flash exposure is controlled only by the aperture. (This is because the flash is a burst of light that for practical purposes is instantaneous, which “freezes” the scene at that moment.) In other words, when you use a flash, you get what you can think of as two overlaid exposures: a frozen, instantaneous flash exposure, mixed with a shutter-speed-dependent ambient exposure. Both are made at once, and both light sources add to the exposure.
So let’s say you photograph a dancer with flash. And let’s say you use a slower shutter speed, so that the ambient light contributes to the picture by illuminating the background. But as the dancer is moving while the shutter remains openand exposing the background, the dancer’s movement will be visible in the picture as blur. But at the moment the flash goes off, the dancer will be frozen, and exposed clearly and sharply.
So exactly when the flash goes off during the slower, ambient exposure, will affect where the clear picture of the dancer is placed in her blurred movement. If the flash goes off at the start of the ambient exposure, the blurred movement will be captured in front of her in the picture; if it goes off at the end, the blurred movement will be behind her.
So if you synchronise the flash to the front curtain of the shutter, it goes off at the start of the exposure, as the front curtain opens and reveals the sensor to the incoming light; and then the ambient light adds its contribution to the picture while the shutter remains open. But if you synchronise the flash to the rear curtain, it goes off at the end of the exposure, after the ambient light has already made its image on the sensor.
Which you use will depend on how you want the picture to look: a clear dancer who then seems to become a fading blur across the frame, or one who shimmers across the frame to a sharp final pose. If you’re photographing a moving motor bike, do you want the motion blur in front of it or behind it?
It’s an artistic choice as much as anything.
Of course, in many situations where you want to include some ambient light exposure – for instance, an environmental portrait where you want to see the person’s workplace in the background – nothing changes or moves during the exposure, so whether you use front-curtain or rear-curtain sync will make no difference to the picture.
Hope this helps!October 16, 2016 at 8:31 am #12484
Very well put Ian especially as it was posted at 4am! Using flash as a fill-in light source is quite a skill as you don’t want the flash to to overwhelm the subject. You don’t really want the viewer to tell that flash has been used. Some people just set the flash to under exposure by 1 stop to fill in the shadows . This can help in bright sun light where you have dark shadows and bright highlights.You could also set the flash manually set to a low power to fill in the shows. Then you know it’s giving a consistent exposure. However, in the case the exposure will depend on how far the flash is from the subject.October 16, 2016 at 8:40 am #12485
Very well put Ian especially as it was posted at 4am!
Haha! Thanks, Peter – but I should reveal I’m in Dubai, so it was actually after 7am here!October 16, 2016 at 10:39 am #12486
Roger had some examples on Thursday night, which is perhaps why the question is being asked. It could be fun to play with on action shots, as Roger pointed out. There was a good article in EOS magazine a while back which will help to illustrate Ian’s very clear explanation, I’ll see if I can find it.
There were a number of practical points which came up during Roger’s talk which are worth investigating.October 16, 2016 at 1:56 pm #12489
One of the best known and most highly regarded experts and trainers on use of small-light flash is David Hobby aka “Strobist”. His introductory and more advanced courses are here:October 16, 2016 at 5:44 pm #12492
Thank you Ian for the as ever lucid explanation of complicated subject. This seems to be the opening of a door onto a completely different way of taking and depicting photographic subjects, and one I may follow up.
Thank you all for the informationOctober 20, 2016 at 4:03 pm #12496
I have located the article I referred to. If anyone would like to read it, email me and I’ll arrange that.
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