DSLR Photography Forum

Hi all,
My Nikon D70 is exhibiting some weird behaviour and I'm wondering whether it's actually normal behaviour and I am indeed the weird part.

When I select "Rear" curtain for my photos, my flash fires a significant fire at the beginning of the exposure in low light conditions. It then fires a far brighter flash at the end of the exposure (as I would hope).

What this produces is a "dual" exposure scenario where the subject of the phtoto can appear opaque through areas of colour and movement.

I was wondering if this is expected?
I think it's not... so I'm wondering what can be done to shut it off?

The initial flash is the pre-flash that the camera uses to set flash exposure. It's always there even with 1st curtain sync, but you just don't notice it because the delay betwen it and the real flash is very short.

Unless something is wrong with your camera, the pre-flash is not causing the "dual exposure." Instead, that effect results from the mix of the ambient exposure and the flash exposure.

This is often used as a creative technique to enhance the appearance of motion. John Hedgecoe's New Book of Photography has a shot of a ballerina twirling in a semi-opaque blur, then finally "frozen" at the end of her spin by the flash. He used a slow shutter and 2nd curtain sync to get the effect. Sounds like what you are describing.

Okay, Thanks slej. I was in my course with my instructor and he did an interesting experiment. I don't have my CF with me, but I'll post the pic tonight or tomorrow.

I thought it might be a pre-flash.. but it's not like a series of little flashes.. it's a burst of light that's about half the power of the second flash. It might still be what you're describing, I was just expecting something else. My instructor said he has never seen anything like it before and doesn't think it should happen.

I like the rear curtain effect, especially the freezing aspect you're describing.

To describe what my instructor did... he leaned to the left when the first flash fired and then leaned to the right when the second one went.

The result was a picture where he was not visible on the left hand side (which is what I would have expected with what you're describing), but was kinda see through on the right hand side which kinda indicated to my untrained eye that the first flash froze the picture and the second one froze more on top of it.

Thinking back on it, this is an inconsistent thought process because if the first flash froze the scene my instructor would have appeared I suppose.

Also, you are probably right because it only occurs in low light conditions.. but I would rather not see it... would you think there is a setting to shut this off?

So did I see this result because my subject was moving. I would have liked the rear-curtain to "freeze him solid" on the right hand side, rather than be slightly opaque.

Does that make sense?

gd Wrote:Also, you are probably right because it only occurs in low light conditions.. but I would rather not see it... would you think there is a setting to shut this off?

So did I see this result because my subject was moving. I would have liked the rear-curtain to "freeze him solid" on the right hand side, rather than be slightly opaque.

Does that make sense?

There should be a setting to turn it off. What camera? On Canons, 1st curtain is the default and 2nd curtain is an option, I think.

You are right - the 2nd flash should have frozen him on the right. I'm wondering if the flash was set for fill exposure rather than full flash exposure? On a Canon if you were in Aperture Priority mode this would be the case - fill flash may not be sufficient to fully freeze the subject.

Yes, that makes sense. Post the example and we can dissect it further, or you can try more experimentation with it.

I floated the suggestion that perhaps that was the red-eye reduction pre-flash, but was quickly denied by gd...

As everyone else is saying -- could be the camera trying to measure exposure as well...

Ah electronics these days -- too smart for us common folk...

Okay.....



During this exposure my instructor stood leaning to the left with the first fire of the flash, then moved to the right, arriving before the main flash.

It's not red-eye, cos a) it's not the red eye setting.. and b) it's only one flash. (the meta data of the pic says RedEyeMode: False...) now that we've put red-eye behind us. Here's my observation....

I would have liked the subject to be solid with the firing of the second flash. Is it because he moved during the exposure to the final spot that he isn't solid? I would have liked the flash to freeze him, regardless of the movement.

the metadata also says:
Flash:
Fired
Strobe light return detected (3)
Auto mode (3)
Flash function present.

so, any thoughts?
is my camera misbehaving.. or is it the cameraman (I would obviously think this is a low probability )

I dunno about your camera... but I see dead people!

My guess would be too much ambient light (look at that overexposed wall) and not enough flash. The camera's doing what it's supposed to do.

okay. mystery solved then

Hi All,
The pre-flash fires before the shutter opens, and therefore has no effect on the picture.

This can be seen by closing one eye and only looking through the viewfinder. Fire a picture with rear curtain flash selected and you will see the pre-flash in the viewfinder -- the mirror is still down showing that the shutter is not yet open.

Also, the pre-flash only fires when TTL is selected as a flash mode. Selecting manual flash mode will eliminate the pre-flash

Astro96