Mysterious hairline shadows

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johan
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Post by johan »

Yes - both light and dark fringes, it's very annoying =).

Some sensible ideas here, I'll keep plodding and try them out, see if it makes any difference. Will report back if I find out what's going on.

Thanks again
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lauriek
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Post by lauriek »

Is the screen exactly perpendicular to the lens?

johan
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Post by johan »

No - I vary it across shots, a screen at an angle gives you darker tones. This shows when it's perpendicular, leaned back and leaned to the left. But I do use the screen in landscape mode so I'll try rotating and see what happens.
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Chris S.
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Post by Chris S. »

lauriek wrote:Is the screen exactly perpendicular to the lens?
johan wrote:No - I vary it across shots, a screen at an angle gives you darker tones.
I suspect this may be a key question and answer. The brightness of many LCD panels varies markedly depending on the viewer's position (something that those of us with SIPS panels, which exhibit much less of the effect, may tend to forget). We can think of it as if the light coming from such a screen's pixels having only a small set of angles from which it can fully be seen. The effect, when using the screen as a light source, might be very much like using a small softbox with a grid attached--and one of the reasons that people use grids on softboxes is to get effects such as the one you are seeing.

Of course, you are not intending to use the field monitor panel as a backlight--only as a background. But could it be also acting as a bit of backlight, and a rather directional one at that? Seems distinctly possible, and easily testable.

As a quick test, if you shut off your subject lighting and shoot with the field monitor background only, do you see some edge lighting? If so, does it vary as you change the angle of the monitor?

If not, what happens if you keep the monitor turned off and use just your subject lighting? Any chance that your subject lighting is reflecting off the monitor and acting as an uneven backlight on your subject?

If the former is the case, I wonder if the problem can be diminished if add a thin layer of diffusion material between your subject and this panel. If the latter, can you shield your subject light from falling on the monitor? (Or does it help to move the monitor farther away?) If none of the above, we get to keep scratching our heads. :)

--Chris

rjlittlefield
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Post by rjlittlefield »

I'm betting on some issue with the polarization. Try putting a single layer of white facial tissue over the monitor. I'm away from my equipment right now so I can't confirm, but if I remember correctly a layer of paper will scramble the polarization quite well.

--Rik

lauriek
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Post by lauriek »

I think you need to try the rotation test with the screen perpendicular to the lens, then if that doesn't affect the placement of the dark/light halos then try the tilting both ways without any rotation and again see if that moves things around...

johan
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Post by johan »

In the end, this I think has turned out to have little to do with the rotation of the screen but quite a lot to do with the direction and reflection. Having fiddled and muddled my way through a series of tests, I can now almost eliminate it in theory, though how it'll work out in practice I don't know yet. Here is my little checklist:
  • My microshadows occur, I think, partially because transmitted light from the screen falls onto a subject, and partially because of flash light creeping in where it shouldn't
  • Using screen tilt or swivel to set a darker tone seems to add more microshadows. My (limited) experience suggests that when the colour and the zone on the photo matches the screen output the effect is minimised. If it's out, either under or over, you seem to get more of this shadow. So 1s perpendicular, nice (1s seems to match ISO and F of test setup, elnikkor 50@5.6), 3s with screen perpendicular = yuck, 1s with screen at 45 deg = yuck
  • Minimising the reflection of this transmitted light inside the diffuser seems to decrease the effect (I swapped diffuser to a non internally shiny one)
  • Some colour combinations show it more than others (in the others it blends in)
  • Diffuser which doesn't let flash light seep into it through the front (ie no space between diffuser and lens) shows less of the effect
  • Flashes/lightsource pointing forward onto subject rather than relatively perpendicular show less of the effect
  • Broader gives less effect than thinner. Ie flashes over polystyrene cup work better if wide dimension is perpendicular to lens rather than parallel
So no real definite answers but some measure of progress. Strange though as I still don't understand what physical plane this shadow is actually falling on.
My extreme-macro.co.uk site, a learning site. Your comments and input there would be gratefully appreciated.

DQE
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Post by DQE »

I just had another idea, and hope it hasn't already been covered in this thread:

Is it possible that the LCD panel has a shiny (specular reflecting) surface? If so, perhaps it can sometimes reflect a lot of light, sort of like a mirror. This might depend very sensitively on the angle of the LCD panel vs the position of both the camera and the flash light source. If the LCD panel were like my iPad, for example, this theory might be a possibility since my iPad has a very smooth, highly reflective surface. Simply adding a diffusing sheet to the LCD panel might get rid of this effect, if it is in fact an issue. Some LCD screens are diffusely reflective, having a matte surface layer for their exit surface.

Again, just a thought, not a proven theory, hoping that it helps in some way.
-Phil

"Diffraction never sleeps"

johan
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Post by johan »

I'm still investigating this, but I've narrowed it down to something known as flash ghosting. The cause is ambient light and flash pops being in conflict.
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rjlittlefield
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Post by rjlittlefield »

That explanation makes perfect sense. The "shadow" is actually the subject seen as a silhouette against the bright monitor during the long exposure for backlighting, and something has moved a little bit between then and when the flash goes off to illuminate the front of the subject. Given that the problem occurs even on the wasp antenna, which is pretty rigid, I'm suspecting camera movement. Have you tried second curtain sync for the flash?

--Rik

johan
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Post by johan »

That's exactly what I'm trying to get working, but I can't seem to figure out a way to use a rear sync manual 1/64th power setting on a 1s manual exposure in Pentaxland. argh.
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Rylee Isitt
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Post by Rylee Isitt »

Yup, I agree. I've seen this effect in my own shots as well. When you "drag the shutter" during flash photography, you freeze the subject with the flash, but if the subject was otherwise silhouetted against a brighter background, the remaining exposure will capture these strange shadows if there is any sort of movement at all. I've seen the problem in photos of people at sunsets, where the photographer has used their camera flash, and I experienced this problem when taking indoor portraits using the dragging shutter technique. I've also had similar issues with handheld shots at low magnification, where I dragged the shutter to take in ambient light.

Image

See the shadow beside the katydid's antenna? Same issue. This was shot outdoors against a banana leaf, so it's not your LCD's fault. It's the general mixing of ambient light with flash that can cause the problem.

The fix is to have the entire scene illuminated with flash, not to use flash at all, to prevent all movement (good luck), or to accept the flaw. Your images look amazing, and I think you're fixating on a problem that few people will care about!

However, using a colored card for your background and illuminating it with a flash unit in sync with the rest of your system will likely completely fix the problem. The other option is to take your little monitor apart, remove the backlight, white backing, and lightguide, and pop a diffused flash through it from behind in sync with your whole flash system. Uh, don't do this if the monitor is expensive.

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