Flash reach, school me please.

I took my granddaughter and a friend to Mammoth Cave today. While in the cave I shot several photos using the onboard flash. It lit the subjects, but that was it. That wasn't a bad thing in itself, but when I tried taking pictures of the cave 'IF' the camera would let me take the picture, I got only close up things. Do I need a stronger flash for that sort of thing? Or am I just doing something really wrong? I got the impression the signal out for the flash wasn't reaching the areas I wanted so the flash wouldn't fire. I was using the kit lense because I wanted to get as much in focus. Also was using the dreaded 'green box'. Thanks for any ideas.

I think the problem was that the cave was dark. Your flash is not powered by hydrogen fusion like the sun is. You have asked it to do an impossible task.

If you want cave pictures, try a 2 minute exposure on a tripod. But don't have anybody move at all during that 2 minutes. You can practice at home with the lights off. What happens?

Of course your right. I was hesitant to even mention it, because there is a real chance I will never be in that situation again. It just peaked my curiosity. Anyone reading this, feel free to pass it by.

Thanks

You might wish to google up "painting with light" which is sometimes used with cave photography.

Hi Mike - Mammoth Cave is quite a place..... only visited a bit of it, but quite impressive.

Part of the problem too is that when the sensor saw the flash bounce back from the subject, it figured that was enough light, and what you wanted illuminated, so it "turned itself off" at that point. You are correct in your assumptions about what the camera was trying to do.

Besides using a tripod, you could also increase the ISO and "drag the shutter" on manual. That is, use a slower shutter speed to allow the minimal ambient light to creep in. If the lens has IS, you might be able to get away with 1/10 sec or so.

Dude's suggestion about painting with light would also work. Some folks will put the camera on a tripod, rock, fence, railing ,etc. and leave the shutter open - Bulb setting - and if they have an external flash, walk around manually firing it at walls... literally painting with light. All of that accumulates on the single exposure.

There is a concept called the inverse square law - A basic law of physics which cannot be overcome with flash power alone.

The intensity of any light source diminishes rapidly with distance. If your flash (or any light source) provides enough light to properly illuminate a subject 10 feet away, then at 20 feet that light will only be 1/4 as strong, or two stops darker.

You can't properly light a relatively close subject, and a surrounding cave, with a single on-camera light source. Take a look at this thread, and the picture of the dixie cups.

Thanks for the info. As always I'm interested in the 'why for' and solution to problems that arise. It's not unusual for me to think about something like this for weeks trying to reason it out. Everything you both have said makes perfect sense. Thank you.

MikeKy55 wrote in post #6216259
I took my granddaughter and a friend to Mammoth Cave today. While in the cave I shot several photos using the onboard flash. It lit the subjects, but that was it.

The Inverse-Square law. If you have a subject 4 feet away and another subject 8 feet away, the subject 8 feet away will only receive 1/4 as much intensity of light from the flash. If you have one subject at 4 feet and another at 16 feet, the one at 16 feet is receiving 1/64 as much light intensity from the flash.

The other issue is that using the built in flash or an external Canon flash mounted on the camera, the camera turns the flash off when it thinks the scene has been properly lit. That means that once it "sees" that the people in the foreground are fully lit the flash turns off and doesn't bother trying to illuminate the background. For all the camera knows you are standing on the edge of a cliff where there is no background. Besides, looking at the numbers above, if you did get enough light from an on-camera flash to light the background then the people would get 64 times more light than they needed so they'd be nothing but white smears (regardless of skin color).

There are 3 ways to get the people and the background properly illuminated:

1. Put the camera on a tripod and use whatever mode you are comfortable with to get a 30 second (or longer) exposure. Turn on the flash and have it take the picture, telling the people to get out of the shot as soon as the flash shuts off. The people will be brightly lit by the flash and their motion out of the way shouldn't be too visible so long as they don't take too long and are not lit too brightly when the flash isn't firing. Keeping them at the edge of the picture will both make it easier for them to get out of the way and probably give a better composition.

2. A flash on camera to light the people and another flash (or set of flashes) to illuminate the background.

3. Take a picture of the people, take another picture of the background, then merge them into a single image when you get back home to your computer.

Mike, this will illustrate the point I was making, as was krb, about adjusting flash for the people and shutter for the backgroud, to get a somewhat balanced exposure between the two. My example below follows krb's suggestion #1.

This was a wedding portrait I did a couple of years ago with my Hasselblad. It was about 5:15 P.M. but quite dark already. The camera was on a tripod, and I'd already taken an incident light meter reading with a hand held meter. I manually set the camera with an aperture of f/8 I believe for good DOF, and then set the shutter for bulb.

I think the exposure was around 20 seconds or so. That would be enough to capture some blue sky and do a nice job of rendering the colored lights; turned out to be right on target. Towards the end of the 20 seconds I manually fired the flash. What I got was essentially two exposures on a single frame of film.

==>

HOSTED PHOTO
please log in to view hosted photos in full size.

Man you guys are scary as it pertains to taking photographs. I have learned much. Thanks!

sapearl, thats a great shot.

Hi Mike - thank you for the kind words, I appreciate.

When I took that photo it was about 24 degrees outside and we were freezing. The bride and groom had been wearing their coats, and I had the best man whip them off at the last second before the exposure.

I was very pleased with the shot and an 11x14 sample is displayed for my clients.

It did take a while to make though....... about 30+ years . The point is that most of this is not learned over night. It takes time, practice and many many mistakes . I have not displayed those here, but they have been numerous and costly over the years. But that's how we learn . - Stu

MikeKy55 wrote in post #6219466
Man you guys are scary as it pertains to taking photographs. I have learned much. Thanks!

sapearl, thats a great shot.

Stu, that's a very cool shot. I don't think I could've stood still for 30 seconds on my wedding day.

And I won't give you any crap about not using tungsten film and a CTO gel on your flash to make the subject lighting match the ambient.

Hi Curtis - thanks .... well, maybe it was only 15 seconds, there was no EXIF on my old cameras back then . And I know you're one for the gels and I do appreciate their value. I suppose I'm just too lazy to use them.

Curtis N wrote in post #6219866
Stu, that's a very cool shot. I don't think I could've stood still for 30 seconds on my wedding day.

And I won't give you any crap about not using tungsten film and a CTO gel on your flash to make the subject lighting match the ambient.

I haven't seen it mentioned so far but your problem may be more than light. I don't believe the 300D will fire if it can't obtain focus....

Dave, that was my experience.