Flat frames.

[Calzune]

How do you keep the histogram at 1/3? I cant figure it out.. I use the white tshirt trick with a white ipad screen. Dslr set to Av mode, same ISO as lights but the histogram is always around 1/2. 

I have tried lower the brightness of the screen and folded my tshirt but still no 1/3...

[Adreneline]

22 minutes ago, Calzune said:

How do you keep the histogram at 1/3?

Hi.

I'm no expert but why the need to get to 1/3?

I use a Canon 70D with an ED80, WS-ZS71 and/or a Samyang 135mm and I use BackYardEoS to take all the calibration frames including flats. I set the camera to Av mode with the same ISO and the resulting flats along withe the darks and bias are just fine for calibrating the lights. I use a stretch white tee-shirt on the lens with an iPad as a light source.

Not sure exactly where the flats histogram sits but they work a treat.

Adrian

Edited September 16, 2019 by Adreneline
Clarification

[Star101]

I think, as long as the histogram is left of center, it will be ok for starters.

[red dwalf]

You'll need to lower the exposure time to get a histogram reading to the left, increase it to go to the right.

[ollypenrice]

Half is fine anyway.

Olly

[carastro]

Just to answer your question, though I bow to Olly's opinion on half being OK, personally I prefer left of halfway.

I have owned 2 DSLRs.  The first one (450D) I could always set to AV and 100iso and get the correct length flat.  But I found this did not work with my 2nd DSLR (1100D).  The flat would always be too bright with AV.  

So what I did was change to manual mode and altered the length sub until I got the histogram left of centre.  This varies according to the light as I tend to use the sky for flats, but I recall something like 1/82 of a second or even less, I just had the histogram showing and altered the length exposure until the histogram was in the right place.  You could also dim the light source to achieve this. 

Carole 

Edited September 17, 2019 by carastro

[Nigella Bryant]

High jacking the thread, do light's have to be the same length exposure as the sub or less? I'm assuming less as it would be overexposured. As long as the histogram is slightly to the left or half.

[david_taurus83]

Take the AV settings the camera is using and then switch to manual and lower the exposure to suit.

[ollypenrice]

9 hours ago, Nigella Bryant said:

High jacking the thread, do light's have to be the same length exposure as the sub or less? I'm assuming less as it would be overexposured. As long as the histogram is slightly to the left or half.

I think you may be making an error of terminology here. I'm guessing that by 'lights' you mean flats? In standard imaging parlance your lights are your images of the object, also known as 'subs.' (Strictly speaking, darks, bias and flats are also made using subs combined into a master so it's best to stick to the right terminology.) Your flat subs will be a tiny fraction of the length of your light subs. That's perfectly normal. My own flats tend to be less than a second each and my lights are sometimes half an hour each.

Olly

[Nigella Bryant]

12 minutes ago, ollypenrice said:

I think you may be making an error of terminology here. I'm guessing that by 'lights' you mean flats? In standard imaging parlance your lights are your images of the object, also known as 'subs.' (Strictly speaking, darks, bias and flats are also made using subs combined into a master so it's best to stick to the right terminology.) Your flat subs will be a tiny fraction of the length of your light subs. That's perfectly normal. My own flats tend to be less than a second each and my lights are sometimes half an hour each.

Olly

Sorry Olly, I was brain dead when I wrote that, yes I meant flats (not the kind you live in, lol). 

[dph1nm]

Beware of DSLR histograms! I did some tests with my Canon 1000D and discovered that it only shows 8-bits of data and the right hand end of the display corresponds to 1024. But saturation in RAW on this camera is 4096. So setting flats to 1/3 would result in a very underexposed flat. In fact, even if you hit the right hand end you are only using 1/4 of the available well-depth!

NigelM