How big of softbox with my speedlight???

Wilt wrote in post #15289579
Admittedly with single diffusion softbox, there is a hot pattern + visible, with double diffusion the hot pattern disappears.

Just 'for grins' I'll put my bare bulb Dynalite 4040 into the same softbox with only single diffusion, and shoot the softbox for comparison, and also measure evenness of illumination using incident meter at center and four sides, both with Metz 45CL and with Dynalite bare bulb head.

I just repeated the test with the same softbox with double diffusion panel removed, with a Dynalite 4040 bare tube head in place. Interestingly, there is a + pattern visible even with this head, so that phenomenon is not related to a flash lens distribution vs. bare tube configuration.

Wilt wrote in post #15295668
I just repeated the test with the same softbox with double diffusion panel removed, with a Dynalite 4040 bare tube head in place. Interestingly, there is a + pattern visible even with this head, so that phenomenon is not related to a flash lens distribution vs. bare tube configuration.

And just to have the 45CL flash head image to conveniently compare to the above bare tube pattern...

As for intensity uniformity across the single diffusion softbox, I measured in center, then 6" in from each edge of the softbox...
With the 4040 bare tube head, I measured sides down -0.5EV from center, and the top/bottom down -0.3EV from center
With the 45CL and 24mm wide angle setting, I measured sides down -1.2EV from center and the top/bottom down -0.7EV from center

So the bare tube head is much more uniform than the speedlight lens, even without the double diffusion panel in place.

Wilt. Just a thought. I could be totally wrong but here's a stab in the dark. Since we see the same pattern with two entirely different shapes of flash tubes, perhaps the reason we are seeing them this way (or... the camera sees them this way) is due to the way the shutter curtains move, or the given lens and aperature used creating an almost star shaped light pattern.

Wilt wrote in post #15295668
I just repeated the test with the same softbox with double diffusion panel removed, with a Dynalite 4040 bare tube head in place. Interestingly, there is a + pattern visible even with this head, so that phenomenon is not related to a flash lens distribution vs. bare tube configuration.



And just to have the 45CL flash head image to conveniently compare to the above bare tube pattern...


As for intensity uniformity across the single diffusion softbox, I measured in center, then 6" in from each edge of the softbox...
With the 4040 bare tube head, I measured sides down -0.5EV from center, and the top/bottom down -0.3EV from center
With the 45CL and 24mm wide angle setting, I measured sides down -1.2EV from center and the top/bottom down -0.7EV from center

So the bare tube head is much more uniform than the speedlight lens, even without the double diffusion panel in place.

Wilt, the difficulty with any light source inside a soft box when the light source is facing the diffusion fabric is that there will be a center bias to the light since an appreciable amount of the total light is coming right at the fabric. In my experience the best way to counter that, other than an indirect light source pointed at the back of the modifier, is to put some kind of deflector in place. That's one reason I like the Buff and other soft boxes that use an umbrella mechanism, it offers a support for mounting a deflector. Fotodiox EZ-Pro soft boxes come with a 10" plastic deflector that slides over the umbrella center post so it can be "tuned" to the light source and modifier.

The test I did was with a Westcott style indirect soft box.

McNalley's logic for using the stofen is apparently based on the same desire to minimize the center bias.

As for your X pattern, is it being caused by the weave of the fabric?

Aressem wrote in post #15295718
Wilt. Just a thought. I could be totally wrong but here's a stab in the dark. Since we see the same pattern with two entirely different shapes of flash tubes, perhaps the reason we are seeing them this way (or... the camera sees them this way) is due to the way the shutter curtains move, or the given lens and aperature used creating an almost star shaped light pattern.

Since the flash fires once the first curtain is fully open, and the second curtain has not started to close, I would say that shutter motion does not have a bearing. As for the lens, the Tamron lens has 7 aperture blades so at f/32 the star pattern one might expect has 14 spokes, not four. Methinks that this old Photoflex softbox had an issue with the weave pattern, that someone mentioned.

David,
I ordinarily would not use a softbox without double diffusion, so this test was a bit of an invalid situation for me. But knowing that lighting amateurs often buy a budget box, I decided to eliminate any advantage provided by secondary diffusion, in helping a speedlight fill the box. Particularly if the box is huge and the flash lens coverage insufficient. Worse case conditions being tested.
We can certainly measure better uniformity from bare tube, even one facing forward like most budget boxes. The apparent wisdom of McNally's use of a Stofen does appear to have merit.