PDA

View Full Version : Backscatter *!&!*?*



DevilDiver
12-19-2007, 13:21
I have read allot of post from people wanting to have feedback on their pictures. One issue that is common for everyone beginners and pros is backscatter. Most common this is from using the cameras built in flash but sometime due to conditions or experience level it can be hard to avoid even with external strobes.....
I found this article explaining one technique that will reduce the chances of your photos ending up with those white specks.... Hope it helps!!!! :smiley20:

Strobe Aiming to Prevent Backscatter

Backscatter! All underwater photographers have experienced it. All of us hate it. It's ruined many a picture that would have been fantastic without it!
Here's a technique to help prevent it. Carl Brownd, an award winning Colorado photographer known for his wide angle work, gave a presentation to the Colorado Underwater Photographic Society (http://www.cupsonline.net/) to explain some of his techniques. I've tried to illustrate his technique with these diagrams.
In this diagram, you can see that with the strobes aimed at the subject, the illumination from the strobes intersects at some point between the camera and the subject.
http://www.digitaldiver.net/images/tutor/strobes_1.jpg

The result is that any particulate in that path from the lens to the subject is illuminated, producing what is known as backscatter.
Carl's technique is to actually aim the strobes outward, so that just the edge of the illumination catches the subject. Carl uses what he calls the 'Rule of Thumbs' as a guide. Rest your hand, (relaxed) above the camera with your thumb pointed along the axis of the lens, like so: http://www.digitaldiver.net/images/tutor/strobes_3.jpg

Then point your strobes along the axis of your fingers. Granted, this will vary as different strobes have different angles of coverage, but just use it as a general guideline.
Using this concept, even if there is particulate matter between your lens and your subject, it is not illuminated...
Look Ma....no backscatter!

scubaculture
12-19-2007, 13:30
Great tip, will try it next time I get wet

Mycroft
12-19-2007, 13:45
Another way is to use diffusers on the strobes. Of course when using very short lenses, you have to do this to diffuse the light out to cover the whole area of the photo.

I use a pair of strobes and diffusers myself and don't have a big problem with backscatter.

CompuDude
12-19-2007, 13:46
Interesting rule of thumb re strobe aiming... I'll have to try that sometime.

Puffer Fish
12-19-2007, 14:32
That may work, if you have one seriously powerful strobe, and the water is filled with particles to reflect the light, but in general, all you will get is a very dark, terribly uneven lit image.

The standard "professional" way to prevent backscatter is to have the strobes off angle, sometimes to the point of having them almost at the sides of the object being shoot. That usually takes 2 strobes for even lighting.

I just don't see backscatter as being a major issue.

bversteegh
12-22-2007, 22:48
The important thing to minimize backscatter is to avoid illuminating the water directly in front of the lens - which in effect "magnifies" the size of the particles in the water. If the subject is really close like when shooting 1:1 super macro, backscatter is usually not much of an issue because the water column is so short.

When the subject is at intermediate distances (between maybe 18" and 5 feet) - you need to be more careful - don't point the axis of your strobe directly at the target, or you will get backscatter. I normally shoot dual strobes, and have found pointing the strobes straight forward, or just a few degrees out (if 90 deg is forward and 0 degrees is pointing at the camera, I point them maybe 95 to 100 degrees). I also spread the strobes out (make them wider) when the subject is farther away, but don't change the angle of the strobe relative to the camera. This minimizes the amount of water between the lens and the subject you illuminate.

You need to be very careful if you put the strobes in front of the plane established by the front of the port - significantly increases your odds of lens flare and other nasty things in your pictures.

Gombessa
01-18-2008, 19:29
In the first example picture, the best solution is to adjust the strobes so they intersect right at the subject (or, if possible, get a lot closer so you place the subject at the intersection point).

I don't use underwater strobes, but I wonder if anyone has thought about using highly-focused lights or lasers as "rangefinders" to help you adjust/position.

RoyN
01-18-2008, 19:59
That really helps alot! Thank You!

fisheater
01-19-2008, 10:38
Good tip. I'll give it a try in the near future.

Puffer Fish
01-22-2008, 15:09
The important thing to minimize backscatter is to avoid illuminating the water directly in front of the lens - which in effect "magnifies" the size of the particles in the water. If the subject is really close like when shooting 1:1 super macro, backscatter is usually not much of an issue because the water column is so short.

When the subject is at intermediate distances (between maybe 18" and 5 feet) - you need to be more careful - don't point the axis of your strobe directly at the target, or you will get backscatter. I normally shoot dual strobes, and have found pointing the strobes straight forward, or just a few degrees out (if 90 deg is forward and 0 degrees is pointing at the camera, I point them maybe 95 to 100 degrees). I also spread the strobes out (make them wider) when the subject is farther away, but don't change the angle of the strobe relative to the camera. This minimizes the amount of water between the lens and the subject you illuminate.

You need to be very careful if you put the strobes in front of the plane established by the front of the port - significantly increases your odds of lens flare and other nasty things in your pictures.


You have some excellent examples of that in your pictures.

I would think that the only time this does not work is when using a single strobe, or using one that does not have a fairly wide angle. Over the years, the length of strobe arms, on average, have gotten shorter, and the angle of the strobes has gotton wider for wide angle lens. The combination seems to have made backscatter much more of an issue.

For those shooting with a strobe that has a rectangular flash pattern, and you are not using a wide angle lens, turning the strobe sideways also helps, as with a narrower beam, there is less light hitting particles between the lens and the object.

diver 85
07-09-2008, 23:53
thanks for the info, also a debackscattering 'brush' on Photoshop is helpful........

diverrex
08-10-2008, 00:54
I normally shoot dual strobes, and have found pointing the strobes straight forward, or just a few degrees out (if 90 deg is forward and 0 degrees is pointing at the camera, I point them maybe 95 to 100 degrees). I also spread the strobes out (make them wider) when the subject is farther away, but don't change the angle of the strobe relative to the camera. This minimizes the amount of water between the lens and the subject you illuminate.


This doesn't seem intuitive to many people. I know for myself I first had to plot it out on paper for myself to believe. My two stobes are usually about 40" apart and with the diffuser have a 110 degree spread. For an intermediate distance subject, say 3' away, I can point my strobes "away" from the subject 10-20 degrees out and with the beam spread each will still illuminate the subject.

Aussie
08-18-2008, 07:20
I shoot with dual strobes but their placement comes down to a few things. What lens I am shooting, what I am shooting and sometimes where and when i am shooting.

If I am shooting with my fisheye (Tokina 10-17mm) I have my strobes spread as far out as possible. Most times I have the strobes pointing straight out but often I angle my strobes in towards the lens as I am getting really close to the subject (which often happens when shooting super wide angle). I always use my defuser as it has a wider spread and I like the look of the light it puts out (softer and warmer).

When I shoot macro with a 60mm or 105mm I bring the strobe in as close to the camera as possible and try and angle them at a 45 degree angle towards the lens. There isnt an issue backscatter as the distance between the strobes, camera and subject is normally very close.

Hope this helps,

Mark

coral cowgirl
08-18-2008, 20:04
thanks for the info, also a debackscattering 'brush' on Photoshop is helpful........
____________________________________
Would it be the "dust and scratches" filer you're referring to or another solution?? thx

Aussie
08-19-2008, 06:20
thanks for the info, also a debackscattering 'brush' on Photoshop is helpful........
____________________________________
Would it be the "dust and scratches" filer you're referring to or another solution?? thx

And yes the backscatter removal brush is a very valuable tool. Its also great if you get dust on your sensor.

Mark

mrbheagney
09-04-2008, 18:40
Nice tip, I'll def try it out. I wasn't aware that photoshop had a debackscatter brush. Will check it out immediately. I have been fumbling around with the healing brush.

Aussie
09-04-2008, 22:16
Nice tip, I'll def try it out. I wasn't aware that photoshop had a debackscatter brush. Will check it out immediately. I have been fumbling around with the healing brush.

Its actually called the spot healing brush in PS CS3

Regards Aussie