Quick Flats question

[MG1]

I've just started shooting flats (and associated darks) and was hoping someone could tell me which of the following three sets I should use for processing. I wasn't clear what sort of exposure level I should be aiming for, but figured someone on here would know.

Much thanks

[steppenwolf]

I'd go with the first one.

[jgs001]

Not sure exactly, I rely on the histogram to tell me if my flat is right. Is this with the 5d ? If so, set the camera in Av mode, make sure exposure comp is at 0, that should give you the right level, you're looking at between the halfway and 3/4 bars on the histogram, more towards the 3/4 mark.

[MG1]

These are the histrograms...none of them really get near 3/4, flat 3 is the closest (it's the top one in this histogram list), but makes me think I need to use a much slower shutter speed.

Cheers

[jgs001]

With al's virtual lightbox on my laptop and the screen brightness to max I normally get about 1/10, that's at f/9

[MG1]

So something more like these two...I'm thinking the bottom one (1/20 shutter speed)

btw, the first three were blue because they were shot through the CLS filter as were the associated light frames. That's okay isn't it?

Thanks again

[martin_h]

The colour wont make any difference, I've had all colour flats from pink to blue

[jgs001]

Actually i'd use the slightly faster of those two 1/20 I think it was, the histogram peak looks better positioned. The cls blue is not a problem, just ignore it. I think flats are used in greyscale mode only anyway.

[Demonperformer]

the first three were blue because they were shot through the CLS filter as were the associated light frames. That's okay isn't it?

As I understand it (which isn't a great deal) it is not only okay, but ESSENTIAL that if your lights are shot through a filter, the flats are shot through that same filter as well.

[MG1]

Hi demon....that's what I assumed. It is after all being used to correct the complete optical train.

Thanks for the help guys, you confirmed what I thought but it's just good to talk it through as it's all new to me still.

I'm glad I shot a few different sets of flats and dark flats. It does take that long to do and I can now delete the ones I don't need.

Another little bit of knowledge in the bag then....all building up slowly!

Cheers

[Psychobilly]

Just a thought...

Are you using an EL Panel..?

Theres noticeable banding in those "flats"...

I added a couple of diffusing layers to my flat panel so that it needed longer exposures and this got rid of the banding caused by the Panel driver...

Billy...

[MG1]

Hi Billy,

Yes its an earlsmann panel....I'm glad you spotted those as I thought I was going mad....and that there may be as yet undiagnosed banding issues with the 5D!

I was going to re-build my light box over the next week or so and thought about including a sheet of translucent perspex in front of the panel. Do you think this will work? Currently the panel just sits directly over the end of the OTA.

[Psychobilly]

Mine uses two sheets of opalescent panel one close to the panel the other one a few inches in front... It hangs from the obs wall...

Billy...