Exposure loss from grid modifier.

[Skiver101]

I cobbled together this grid modifier, and it seems to be adequate; considering what it is.
I haven't shot any 4x5 FP4+ sheets with it yet. I decided to try to find it's working parameters first before using any film.
According to 'what I have been able to find online', the straws' 2'' length and 3/8'' diameter should return a 25'' diameter hot-spot with a 50'' diameter circle to falloff - although I cannot say for certain at what distance this relates to - I will assume 10 foot.

** My question is - ...If there is any exposure difference to be expected when metering a bare flash; as compared to metering the centre throw from the same flash; with the hot-spot grid attached ? **

Intuitively, I would say no, the intensity of light at the centre of the hot-spot should be the same as the centre of the bare flash's throw. However, when I set up and tested this; as shown in the pics attached, using the chair back as a target spot (2 foot distance); I found I was getting a one to two stop difference in the readings.
I am assuming that the rough-and-ready dimensionalities of my cobbled together creation have resulted in a hot-spot that is being cast slightly off centre, and that although being careful not to, I am metering a section of the inner falloff circle. Also, there is a 2cm gap at the rear of the modifier for attachment to the flash front, and only 1cm of that is holding the grid in place; meaning that there could be light being lost in the 1cm gap between the flash element and the back of the straw 'block'...but a one to two stop difference ??
I realise that in practical use this will probably NOT make THAT much of a difference to any shots; but the journey has made me somewhat curious as to the science behind the mechanisms at play. I have no previous experience of grids or modifiers of any sort, so I don't know how 'properly made' modifiers would/should perform in regard to light loss and exposure settings.

A seemingly long-winded and involved scenario, so I've included a few pics to make myself more easily understood.

JP

 

[markbarendt]

Two things going on.

One is that anything you put in front of the light source will to some extent limit the output of the light source. So the flash can't reach as far or as strongly as without. If you are calculating the output mathematically, good luck.

Second is external metering. External metering corrects (within limits) the loss of power issue. If the flash unit is using A-mode (speaking in Nikon here) the flash unit itself has a sensor that 'sees' the light reflected back from the subject and uses that to adjust flash output. If your flash does similar it will self correct in that mode, albeit with a shorter range than without your modifier. Similarly in manual mode a hand held meter reading will indicate properly.

 

[wiltw]

A grid works by blocking light rays which are not coming out within the somewhat narrow cone angle permitted by the grid spacing and depth. So a grid will just about always reduce the number of photons gettting from the light source to the subject.

 

[Skiver101]

Sorry guys, I should have mentioned that I am using a Vivitar 283 flash unit (manual), and a Minolta Spotmeter F.
I'm only measuring the exposure reflection in f-stop difference - all other parameters being equal.
From what I can tell, you are both saying that exposure will be affected regardless of the grid's build quality, and that therefore I will have to compensate...i.e. the difference in my readings are good.
Maybe the grid-to-subject alignment is better than I at first feared.
That's encouraging; as it justifies the time and effort I put in to the construction of the grid.

 

[markbarendt]

Yes, the light output will be affected.

If your meter is automatically adjusting, that's not just a Nikon thing, then the unit will try to compensate for the reduced output by extending the flash duration and your external meter may not see any difference.

If your strobe is in manual mode your external meter may or may not show the difference. The external meter may not display precisely enough to distinguish, in essence a 1/3-stop difference might get hidden because of the rounding used to generate the displayed reading, it is more likely though that the meter would return two readings 1/3-stop apart; the displayed info on most meters is limited (the meter itself can do better internally). If this is the case though the modifiers effect can be ignored in a practical sense.

Side note. Flash duration is the normal way strobes adjust output volume, the output luminance is not adjusted. A low output flash from your strobe might only last 1/20,000th of a second, a max output flash 1/1,000th. Notice that in both cases the flash is much shorter than normal shutter speeds for this type of work. Also flash output is affected by battery charge and capacitor state.

 

[M Carter]

All the handheld meters I have measure flash in 1/10th a stop increments - I don't have any antique/analog stuff though.

I'll say with grids though - definitely some light loss. And with your DIY straws grid - it looks like some of those straws could be squished or bent. As far as hot spots or non-centered falloff, that may be a limitation of DIY and plastic.

I've seen aftermarket grids for strobes out there, you might get more accuracy with those? (I've only used metal grids in large sizes, for pack/head reflectors or fresnels, though i do have a snoot with a snap-in grid for extreme aiming accuracy).

 

[markbarendt]

Yeah it depends on how the user has set the meter to read too. The idea is the same, the precision displayed is limited.

 

[John Koehrer]

An additional SWAG is that part of the loss is because the inside of tubes are painted black and absorbing light