Subject in focus, with light trails.

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iamtehzen

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Hannah
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I feel like this is a very basic question, but here we go.
I've seen in a lot of wedding sets recently shots where there are people or whatever subject in focus, but then all around the main subject is light trails. Like these random pictures found on Flickr here and here. How do you get the dancing crowd sharp, and the rest of the shot all motion-blur-light-trails?
Thanks:)
 
Hi,

Although I'm very much an amateur I recognise that kind of effect from a few videos I've seen on youtube. I think the answer is using the rear setting for flash and using a long exposure time.

The theory is that you expose the camera for the background, as it's dark lighting you'll have to really slow down the shutter speed. This leaves you with the motion blur effect around the subject.

Then by changing the flash to 'rear' the flash would only trigger just before the second curtain closes leaving whatever subject your're shooting relatively sharp.

As I said I'm am amateur, there's plenty of experience in this forum to correct me if I'm wrong :-)
 
Thanks! I'm an extreme amateur, so still trying to learn about exposure, and how to expose 'for' certain areas of the image, so this is helpful! I just couldn't figure out at all how this look would be achievable.:thinking:
 
Simply turning on rear curtain sync isn't going to magically make light trails start appearing.
Actually, for simple light trails like this I would say rear curtain sync would put you at a disadvantage.

All you need for light trails is a long shutter speed, and to freeze motion of the main subject use a flash.

For more detail, a guide I wrote might be useful for you. It's aimed directly at nightclub photography, but it uses exactly the same method of balancing camera exposure with flash exposure.

http://davidwalkerphotographyblog.wordpress.com/2012/01/04/a-guide-to-nightclub-photography/
 
It's a technique known as slow-sync flash - basically a long expsoure so that the ambient light burns through, then the flash freezes the subject.

Rear-curtain or second-curtain sync is a bit of a red herring. It often makes no difference (may or may not have been used in the two pics you've linked) but is best avoided unless you know you really need the effect*.

The additional trick in the two linked pics is the that lens has been zoomed during the exposure to create those radiating streaky blurs of light.

* Second-curtain sync fires the flash at the end of a longer exposure, instead of at the start. It basically means that with a moving subject the ambient blur appears behind the subject instead of in front. It's irrelevant in the linked pics.

The problems with second-curtain are that it's very hard to time the moment the flash fires, so if you're after a capturing a fleeting gesture, chances are you'll miss it. Also, when using auto-TTL flash, the pre-flash goes out first before the exposure starts, and then there's a delay before the main flash. Some subjects move just after the pre-flash, thinking the picture has been taken, so the shot is ruined.
 
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