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Can I this for flats?

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#1 vidrazor

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Posted 25 June 2019 - 01:11 AM

I have a GraphicLite D5500 lightbox , and was wondering if I could use it for flats. It has roughly a 10x10 inch lighting area. I photographed it underexposed to see how the falloff is on it.

 

I also have a 13x18 inch 1/8th inch thick frosted plexiglass sheet I can put in front for further diffusion.

Below are the box itself and with the plexi roughly 15 inches in front, exposed to see falloff. I used 28-75mm lens 75mm @ f/11 to minimize optical falloff.

 

Is this a usable setup to make flats? Thanks.

Attached Thumbnails

  • flats.jpg


#2 oshimitsu

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Posted 25 June 2019 - 01:27 AM

As long as it's a white surface, evenly illuminated and will cover the aperture it should work just fine. I use a spike-a-flat but the fall off is only on the extreme edges 



#3 Alex McConahay

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Posted 25 June 2019 - 09:20 AM

It will probably be usable. Since you have it, it is easy enough to try. 

 

I always worried about perfectly flat illumination of the panel. Then, one day (at NEAF) I was walking by a commercial flat panel, and noticed the fall off at the edges. I asked the rep about that, and he said it really did not matter. That really brought into question my obsession with perfect flatness...….

 

As Oshimitsu says: " I use a spike-a-flat but the fall off is only on the extreme edges." So, maybe it is time to just take some flats with it.

 

Try it. If you get good calibration, it was good enough.

 

Alex



#4 vidrazor

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Posted 25 June 2019 - 10:41 AM

Thanks for your replies. I was also wondering, when exposing for flats, do I shoot for, say, 50% gray (centered light meter), or something lighter or darker? Thanks again.

Edited by vidrazor, 25 June 2019 - 10:44 AM.


#5 Alex McConahay

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Posted 25 June 2019 - 11:08 AM

Yeah, that is the general rule. I would suggest, though, that you may find experimentation handy. My SX35 is supposed to be at about 19K ADU, while my QSI runs at 30K. You need to find what works for you, The important thing is to get the hump in the histogram off the right edge, and ideally, one third-to one half way across at peak. But, as I said, it varies. 

 

Alex


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#6 kathyastro

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Posted 25 June 2019 - 12:03 PM

The bare panel looks like the light falls off towards the edges.  But the Plexiglas diffuser looks good.

 

The important thing about the exposure is to have no clipping at either the black end or the white end.  Having the histogram peak in the centre will usually take care of that.



#7 vidrazor

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Posted 01 July 2019 - 12:01 PM

The bare panel looks like the light falls off towards the edges.  But the Plexiglas diffuser looks good.

 

The important thing about the exposure is to have no clipping at either the black end or the white end.  Having the histogram peak in the centre will usually take care of that.

Well, it turned out when I went to shoot the flats with the camera I was shooting the subs with, I saw these animated nonlinear bands going across and creating uneven lighting, color, and exposure. I switched to another light box that I have and it was much more even, but I was still getting uneven density and color. I then dropped to base ISO and slowed the exposure down to 1/50 to try and even out the exposures and color between the frames, which helped. However I had to overexpose and the flats, and the histogram in mostly to right, but not clipped.

I then processed the frames in Lightroom, adjusted the levels to spread out the data, made the frames monochrome, and output them as tifs. The histogram of both are below (the banding you're seeing on the raws are from uploading compression). Are these usable for flats? Are the unprocessed RAWs good enough as-is, or should I use the tifs, or reshoot differently?

Thanks.

Attached Thumbnails

  • flat test 2a.jpg
  • flat test 2b.jpg


#8 kisstek

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Posted 01 July 2019 - 01:05 PM

One of my concerns with the cheap LED panels with adjustable light levels is they're probably implemented using PWM (pulse width modulation). Ie they blink the LEDs for different lengths of time so that it appears to change the intensity. But if your exposures are really short or their PWM is using a low frequency, then your camera can start to pick up the pulsing, ruining your flats. That's why I went with a fixed output panel and then added enough diffusers to bring the exposure times up a bit.

 

And then there's the other question as to whether the light produced is sufficiently wide band enough to cover the range of one's narrow band filters. I was pleasantly surprised that mine was.




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