Capturing Trails

[ketan]

Hi,

A question - I can capture light trails on a road by keeping exposure open. However, if I do the same in a shopping mall or street to capture busyness, people almost disappear from frame. Like a car, why cant i capture trail of person..I am missing some fundamental, not sure what..

Thanks

Ketan

 

[rlemert]

When you're capturing the light trails from the cars on the road, you are capturing direct light sources. The car's head/tail lights are generating their own light. People do not generate their own light - they can only reflect light from other sources.

If you were to have all the people in your pictures carry a lantern with them while they were walking, you'd get light trails from them, too. You probably still wouldn't see the people, but you'd have trails.

 

[ketan]

Thanks.

Well I hear what you say and take it as a rule. Not able to understand the physics behind it.

Can i say that if a car with lamps on passes on road then with shutter speed of 15 sec i will see a complete trail - start to end, whereas same experiment is repeated in daytime (lights off) the car will almost disappear or will have a small hazy streak.

 

[Solarflare]

Can i say that if a car with lamps on passes on road then with shutter speed of 15 sec i will see a complete trail - start to end, whereas same experiment is repeated in daytime (lights off) the car will almost disappear or will have a small hazy streak.

Yes.

If you check your car light pictures, very likely you cant see much of the cars there, either.

 

[TheBiles]

You can use a rear-curtain flash to clearly capture people after a long exposure for light trails.

Sent from my Transformer Prime TF201

 

[Joshonator]

It's pretty simple. Lights are a lot brighter than anything else. Lets say you took a 1/20s picture of a person in the dark and a 1/20s exposure of a lamp in the same room with the exact same camera settings. The light source is going to be way brighter and the person will be underexposed. Likewise if you take a 20s exposure of a car or something, if the car is moving the light and car only stay in that exact position for a very short time (lets say 1/20s) and then they move, so while the light only takes that much time to properly expose, the car would be underexposed. The car would have to stay still for longer to reach the same brightness as the light, but since they are attached it's not going to happen.

The other problem with a car or a person is that the background is usually just as bright as the subject so if you bumped up the ISO and opened up the aperture too much then the whole picture would be overexposed. That's why long exposed shots of people usually have a black background with a bright light shone at the person so that they are much brighter than the background.