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Archive 2013 · Beauty Dish and ND

  
 
leethecam
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p.1 #1 · p.1 #1 · Beauty Dish and ND


Here's a peculiar thing...

I'm using the new Bowens White Beauty Dish. Similar in profile to perhaps the Profoto, with a deepish bowl and a tightening curve towards the edges, (a bit like a big salad bowl rather than the gentler curves of some 3rd party dishes.)

I've been calibrating my light meters and getting consistent and accurate results now - all at a range of powers and f-stops etc.

Strange thing is when I use an ND sheet on the dish. My ND is cut to cover the front of the dish, rather than go around just the tube and is a 3 stop reduction.

Ideally I like to think I'm getting the same characteristics of the dish - but just 3 stops less lower, (so I can run the flash harder at shallow DoF or get those crazy F1.2's...)

However when I meter the light with it's new ND sheet on and do a test shot, the exposure of the shot is about 1/2 stop or 2/3 underexposed. Essentially the new metered reading and the subsequent shot with the camera do not now agree. Meter readings and camera shots did without the ND.

Checked with two meters and get the same results so it's not a strange fault.

I should clarify that I am doing a fresh meter reading with and without ND and of course resetting the camera appropriately each time according to those readings.

I can only assume the ND is causing an issue with how the light behaves from the dish - although as I'm metering at the same point as the 12.5% card I'm using as an exposure guide it seems strange.

When I pop the diffuser cap on I get no issues.

It is weird that without an ND there is an agreement between meter and camera, but with ND on the dish there is a disagreement...!



Nov 08, 2013 at 06:08 AM
Dave Jr
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p.1 #2 · p.1 #2 · Beauty Dish and ND


Perhaps the ND sheet is actually more than 3 stops?


Nov 08, 2013 at 12:00 PM
Gregg Heckler
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p.1 #3 · p.1 #3 · Beauty Dish and ND


Take a picture of a wall or seamless with and without the ND. I would guess the ND is effecting how the light is dispersed or like others say it's not really 3 stops.


Nov 08, 2013 at 12:45 PM
leethecam
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p.1 #4 · p.1 #4 · Beauty Dish and ND


I doubt very much that the ND is truly exactly 3 stops. It doesn't really matter as the meter should be reading an appropriate level that the camera should agree with.

It's this discrepancy that is bugging me.

Camera and meter should agree at all times when nothing moves. If I change the power of the light I can get a meter / camera match every time. Same should be true if any ND is placed in front.

Thoughts are that the ND is changing the characteristics of the BD and perhaps the dome is picking up slightly different reflected light off the grey floor when the ND is in front of the BD, (the rest of the studio is black).

When I have time (and patience) I might try tests on 3D things and portraits to see if the error is translated to the "real world."



Nov 08, 2013 at 03:13 PM
RustyBug
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p.1 #5 · p.1 #5 · Beauty Dish and ND


Dome vs. flat @ incident metering?
Flash only or flash + ambient?

Seems a bit odd indeed.
+1 @ should match, light falling on meter is the same light falling on subject.
+1 @ ND changing pattern of BD, but that should still yield ^

Curious if you were to reflective spot meter a gray card (rather than incident) with the ND if the outcomes would remain consistent, or if they too would reveal the discrepancy. Off the top, I'm thinking that the dome is giving you an exaggerated reading. I'd retract my dome or change to flat incident reading or to spot metering off gray card to see if how they play with the ND.

Gotta be a reason ... just not sure what it is, but I think it'll be dome related.



Nov 08, 2013 at 03:40 PM
leethecam
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p.1 #6 · p.1 #6 · Beauty Dish and ND


Yes I think you are right. I think it is dome related too.

When shooting at full power I've had a relatively low ambient light on but with ND tests and low power I've had the lights off.

I'll have to wait until I get a chance to retest but in the next few days I'll see and post back here.

Again, thanks all.



Nov 08, 2013 at 08:39 PM





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