measurement of flash u/w

[f3nikon]

True that light travels in a straight path, however a narrow reflected beam of light from your strobe's array will focus a bright light on a small area, causing a blown out area in the picture. The area on the outer edges of the picture will appear dark, regardless of the lack of obstacles, simply because the lens has wider coverage than the strobe. If you doubt this, try shooting a picture using only a highly focused pencil beam back-up dive light-a really strong one. Then shoot the same picture using an 18watt can light with an adjustable beam. Trust me, there won't be any darker areas in your second shot unless you're doing a wide angle of blue water. I don't claim to be an expert on any of this. There are probably hundreds of photographers on this board with more experience than I have, but I'm not here to get in a pissing contest, I'm just trying to be helpful, based on what small amount of knowledge I have and can pass on.

I think we are talking about two different subjects here...you are talking about the strobes light angle coverage to the overall picture and I am talking about the shadow that is cast on the side of the subject, opposite to the side of the single strobe ie if the strobe was firied from the right side of the subject there will be a shadow on the left side of the subject.

In both cases you would still need two or more strobes for shadow control and also for coverage of the overall picture. And I'll tell you why...

Lets say for example you are tring to light up the entire picture with a single WIDE angle strobe firing from one side of the camera. On wide angle images the actual image that will be in the frame could be about 2 feet across, the strobe's light would have to reach across one side of the frame in order to light up the other side, losing light energy as it travels through the water. By the time the light reaches the far end of the frame you could loose 1 or more fstop of light!

Increasing the strobe's power to correct the low light conditions will not work because now the side closest to the strobe will be blown out. This is why I am recommending two or more strobes and small strobes like your Olympus strobe will work just fine exposing one half of the image and a low cost slave strobe can light up the other half...evenly.

I am not here for a pissing contest as well and your comments on the SP350 were helpful.

 

[pete340]

Most strobes are rated with a guide number. The guide number is the distance (usually in meters) that the strobe will give effective light.

It's more than that, though. With manual settings, you divide the (underwater) guide number by the distance to the subject to get the f-stop. Or you can go the other way, and divide the guide number by the f-stop to get the correct distance to the subject.

For example, if the guide number is 40 and the subject is 5 feet away, use f/8.

 

[Larry C]

Thanks, Pete. I didn't know that.

 

[bobf]

I wrote this explanation for the Guide Number Theory that is found within the Strobe Finder:

The guide number is one way of representing the power of the strobe, watt-seconds being another. The industry standard for determining a guide number top side (Air) is to place an object or target a distance of 10' from the strobe head. Next step is to capture the image of the object using an assortment of aperture openings while the strobe emits a full dump. The aperture which yields the best exposure is then multiplied by 10 (the distance from strobe to subject), resulting in the Guide Number. The ISO chosen is usually 100.

Although an underwater strobe is ineffective at influencing exposures underwater at a distance of ten feet, one could still determine an underwater guide number by following the same formula. Place an object 3 feet from the strobe head. Set strobe for full dump. and fire away, using various apertures. Once again, the aperture that yields the best exposure is multiplied by 3 (in this case, 3 was the Distance from strobe to subject).

Let's assume that an Aperture of f/8 produced the best quality exposure. Multiply three times eight and the resulting number could be therefore considered the underwater guide number.

Example:

GN=A*D

GN = f/8*3 = an underwater guide number of 24.

Of course the clarity of the water would play an important role in the equation. But we can assume that if a strobe manufacturer supplied an underwater guide number, the test was conducted in a filtered swimming pool or crystal clear Caribbean water.

Note: Guide numbers used in the strobe finder are expressed in meters at ISO 100.

 

[f3nikon]

I get this information from three sources.
First, strobe mfg's. specs. Second, I know a few people who have had strobes that they used with their film cameras and transferred to their digital cameras, bought a second strobe of a different brand or model, and discarded the old one because they had two-tone pictures, especially on wide angle shots. Third, put a pic on your RAW conversion or Photoshop program and adjust the color temperature, then look at the difference. It may not be huge, but it is certainly visible.

[/QUOTE]
For some reason, the Seacam Seaflash 350 engineers have taken to the task of offering variable color control output from 4300°-5200*°K

There must have been a reason............

Other strobe temperature samplings:

Ikelite DS 51 5500°K
Ikelite DS 125 4800°K
Inon D-2000 and Z-240 5500°K
S&S YS 110 5400° K
[/QUOTE]

Please refer to the color chart in this website:
http://en.wikipedia.org/wiki/Color_temperature

If you will notice that almost all the strobes that Bob mentioned falls in the 5,000 to 6,000 Kelvin range in the color chart. This is the white light range of the spectrum and is the ideal range because the white lighting should reflect the entire range of colors equally, just like daylight.

If you will also notice that the DS 125 @ 4800K is the only strobe on that his that is bias towards the yellow or warmer end of the spectrum. The other strobes have about the same color temps. @ 5500K… the colors in the chart do NOT change colors until 4500K (yellow) and the 8000K (blue).

I tested my “film” strobes on a digital camera and found NO shift in color tone and the reason why is that my “film” strobe or any other land or U/W strobe falls within that white light range of 5500K! Plus the fact the "digital" color temp standard mirrored that of film which is biased towards daylight.

What were the colors of the different tones in those “two-tone” pictures? Were the tones biased to the color BLUE…like the blue cast U/W? Because this is not the function of the strobe’s color temperature, it more of a function of not getting close enough to the subject or not enough power from the strobe to overcome the strong blue bias U/W.

The point I am trying to make is that all strobes for use on land or sea (some maybe warmer), be it a low cost to over priced strobes…fall within the white light range of the color chart. So the marketing pitch from strobe makers…that digital camera will only work with “digital” strobes because of the color temp. is a bunch of baloney. IMO

Oh and the reason why the DS125 Ikelite and maybe the Seascam strobe are a bit warmer (4300 to 4800) is due to the larger round shaped flash bulb on these strobes. The others have the more common straight tube bulbs of 5500K. Ionizing the gas in the larger tube takes more energy.

 

[f3nikon]

f3nikon,

The preflash produced by the built in flash does not reflect off the subject. The preflash is blocked with Clear Photo System material.

The strobe light that reflects off the subject comes from the external Inon strobe and not the onboard flash. The external strobe is triggered through an Inon fiber optic cable placed in sight of the internal flash. The camera is still capable of reading the reflected light from the external flash that bounces off the intended target. The camera computes this information.

You may learn more about the system by reading the following information:

Optical D Cable:
http://www.inonamerica.com/products.php?product_id=283&pagenum=3&prodcat=4&subcat=1
S-TTL:
http://www.inonamerica.com/content/docs/STTL_D2000.pdf
Clear Photo System:
http://www.inonamerica.com/content/docs/ClearPhotoSystem2F373.pdf

Inon has also developed a system that enhances manual and auto mode control strobe firing. You may read more about it by following the link below, which discusses Advanced Cancel Circuitry:

http://www.inonamerica.com/content/docs/AdvCancelCircuit2F375.pdf

I spoke to an Inon retailer the Inon strobe does NOT work as it is marketed. For one issue, none of their strobes can fire a preflash, so how can they Mimic the preflash of the internal strobe in order to feedback the exposure information to the camera?

"The camera is still capable of reading the reflected light from the external flash that bounces off the intended target. The camera computes this information."
No it cannot this flash is the MAIN flash the shutter is opening at this point the time for the preflash exposure adjustment is long gone! This is why test after test have failed and overexposed the picture.

The Inon can sense a preflash signal coming IN from the camera but cannot fire a preflash OUT on its own, like Nikon SB-800. All the Inon strobe can do is to fire off of the camera's internal flash and output a strobe power that YOU have to set...this is not TTL
(Through The Lens).

There is an automatic feature on the strobe with the photosensor in front of the strobe...still this is not TTL. Many low cost land strobes have this feature as well. IMO I would go for the hardwired Ikelite instead, and if your camera does not have a hotshoe for which to connect your strobe, sell it and get a camera that does. IMO

 

[rjsimp]

I spoke to an Inon retailer the Inon strobe does NOT work as it is marketed. For one issue, none of their strobes can fire a preflash, so how can they Mimic the preflash of the internal strobe in order to feedback the exposure information to the camera?

Interesting, becuase the Z-240 does a VERY distinctive pre-flash when in TTL mode and even in its Auto Mode. You can clearly hear the double pop. Then when you go into manual, there is no double pop, just a single one as you would expect.

Based on what I am seeing, the Z-240 at least DOES fire a pre-flash. So does the YS-90 and YS-110 for that matter (I have been testing with them a lot lately. )

 

[f3nikon]

Interesting, becuase the Z-240 does a VERY distinctive pre-flash when in TTL mode and even in its Auto Mode. You can clearly hear the double pop. Then when you go into manual, there is no double pop, just a single one as you would expect.

Based on what I am seeing, the Z-240 at least DOES fire a pre-flash. So does the YS-90 and YS-110 for that matter (I have been testing with them a lot lately. )

Very interesting indeed, I am just passing along what the Inon dealer told me. I'll check if Inon even mention that the Z-240 can fire or output a preflash, in any of their write ups. Other than firing a preflash the Inon strobe has to be able to accept a power level command from the camera telling it how much power to put out to the flash tube, like the SB-800 or the camera's internal flash, unless they are hardwired together there will be no path for feedback.

Addition:

Could the double pop be the dual flash tubes firing at different times? Because the Inon strobes have two small flash tubes to make up for Ikelite's DS125 larger flash tube.

 

[rjsimp]

Very interesting indeed, I am just passing along what the Inon dealer told me. I'll check if Inon even mention that the Z-240 can fire or output a preflash, in any of their write ups. Other than firing a preflash the Inon strobe has to be able to accept a power level command from the camera telling it how much power to put out to the flash tube, like the SB-800 or the camera's internal flash, unless they are hardwired together there will be no path for feedback.

Well.. I am talking stricly hard wired not optical here. The Z-240 does a pre-flash meter, then it fires its real flash and the camera does control it based on the light metered (as my tests show with my D200, the SP-350, that's another story )

Optical is another story.. so I'm sorry if I jumped in and we're talking optical here since I didn't do any tests that way. I do know they claim to have the S-TTL and the YS-110 has the DS-TTL they both claim give "TTL" like results with optical. It is supposed to work by monitoring when the camera turns of its flash. So for example, when the internal flash fires, the strobe turns on, when the internal flash turns off (because the camera say's it has enough light) then the strobe turns off as well. I have no idea how well this works, but that is what I gathered from the description and documentation.

 

[bobf]

I spoke to an Inon retailer the Inon strobe does NOT work as it is marketed. For one issue, none of their strobes can fire a preflash, so how can they Mimic the preflash of the internal strobe in order to feedback the exposure information to the camera?

All current model Inon strobes, hard wired or optically triggered, can mimic a preflash.

Whatever conclusions you based on the assumption that Inon strobes are incapable of a preflash is flawed.

I'll check if Inon even mention that the Z-240 can fire or output a preflash, in any of their write ups...........

You mean to say that after you make a comment that they can't, then you will research if they can?