I have made up a Flats light box for my ED80 and Canon 20d following some threads on hear and other websites.
Could someone tell me how to calculate the correct shutter speed for flats. I understand it varies with each camera and is around 1/3?? the full well capacity of the camera.
Attached is a jpeg of a flat at 0.035 sec The image looks ok to my untrained eye but the histogram doesn't look correct. I believe it should be a single spike somewhere in the centre. If I reduce the shutter speed I start to get a secondary curve on the right hand end of the histogram.
From what I understand, you want the spike in the histogram to be 1/3 to 1/2 way from the dark end (left end) of the histogram. Can you post examples of the ones you're getting?
I have my "lightbox" solution coming in the mail at the moment, so I haven't actually attempted this myself yet. But what I was going to give a go is to set my DSLR to AV mode, keeping same ISO, and just click and the camera should choose the right speed for the exposure. Chimp the histogram, if it's ok, good. If not, maybe play with exposure compensation until it is.
Leave the camera in manual mode (as that is what you'll be shooting your lights with, as well) and set the ISO to ISO-100. Look through the viewfinder into your light box and adjust the shutter speed so that the exposure/meter falls on the centre tick (which is 0 EV). Take a photo and observe the histogram. Ideally, you want the histogram to peak somewhere between 1/3 to 1/2 way across the display. If it doesn't, then, increase or decrease the shutter speed.
Once you've found a setting that you're happy with, stick to it, and fire off between 15-19 flat lights (make sure you take an odd number of flat lights as you will be median combining them). Then, put the camera body cap on, make sure the viewfinder is covered, and take between 15-19 dark frames which match the same exposure duration as your flat lights.
Hum - any reason to not use Av mode as I was going to? I understand you'd need to take note of what the flats shutter speed was so the flat-darks can match. But just say you weren't taking flat darks, Av mode would work sufficiently?
I've tried both ways and get arround 1/200 or 1/320 sec for a spike thats 1/3 to1/2 halfway along.
HOWEVER ...I've loaded these to images into photoshop and get not one but two spikes and the rgb are not at the same point. I am not sure what this means but I don't think it's good. I don't know of a way to post the histograms, If you can tell me then i'll put them up.
I may need to compare with your new one Troy, when it arrives as I'm at a loss now
If the peaks/curves of the R, G, and B histograms are at different points, it just means that the image is not pure grey, it must have a slight colour cast to it. I imagine you could convert to a greyscale image, or just look at the histogram without the separate channels?
To post the histogram, make sure the histogram window is on top and hit alt-printscreen button to copy it to clipboard, then open new photoshop image and hit ctrl-v to paste it.
My "lightbox" in the mail is one of those elec2go.com.au A4 sheets.
When you've got your camera hooked up to the telescope, does it still work in Av mode? Because, you can't really dial in an aperture value... or, can you?
If it works for you, there's no reason for you to switch to manual.
Geoff, doesn't matter if the image isn't uniformly grey. When you flat divide, the software takes care of all of that for you (greyscale division). I've used flats which have had a slight red and blue cast to them before. No problem, as I don't convert them to CFA in IRIS -- just divide by the raw... RAW.
When you've got your camera hooked up to the telescope, does it still work in Av mode? Because, you can't really dial in an aperture value... or, can you?
Hmm, to be honest I've never tried it. I will soon as I get a chance. But my thinking is that it'll work. I mean, the light meter works to give you the right shutter speed even though it doesn't know what aperture the scope is at when in Manual mode, why would Av be any different?
PS - sorry to partially hijack things, Geoff. Kinda on topic
EDIT:
I just checked. In Av mode, the camera sets the shutter speed to the correct exposure no matter what the aperture is. It just keeps the exposure meter needle on zero (if you haven't set exposure compensation) and adjusts shutter speed to suit whatever light is coming in at that time. Point at something light, you might get say 1/50s. Point at something darker you might get 4 seconds.
Once you trip the shutter, you're locked in on that speed. I pointed at something dark with 4 seconds exposure, then while the shutter was open moved and pointed at something lighter, but the shutter speed remained at 4 seconds so the shot was severely overexposed.
Was this flat taken with your scope?. If it was, theres something wrong, or you dont need flats. Those images are way too flat for any image train ive seen. The whole point of a flat is to cancel dust donuts and other optical deficiencies, if you dont have any, you dont need flats. The exposure time also seems extremely short, too short and you may get shutter speed artifacts. I would say reduce the light output for 1 sec exposures or so.
Thanks for the tip. Yes, it is the telescope I normally use, although I don't think it is focused at it's usual setting as I've recently been swaping in a new field flattener, hence need to remove some vinetting with the Flats box.
Leave the camera in manual mode (as that is what you'll be shooting your lights with, as well) and set the ISO to ISO-100. Look through the viewfinder into your light box and adjust the shutter speed so that the exposure/meter falls on the centre tick (which is 0 EV). Take a photo and observe the histogram. Ideally, you want the histogram to peak somewhere between 1/3 to 1/2 way across the display. If it doesn't, then, increase or decrease the shutter speed.
Humayun,
Just wondering why set the camera to 100iso for the test. I usually take all of my Light frames at iso 800 so should I set the camera at 100 or the same as the light frames?
ISO and exposure times are largely irrelavent (within reason). Too short, and shutter artifacts can intrude, Too long and you need to dark subtract the flat.
The lower the ISO the better noise wise, its a completely seperate image to the main image, the flat settings are not related in any way, unlike a dark . Just set ISO and exposure times to get say a 0.1-2 sec exposure to a third to half of the histogram at the same focus setting of the image. The ambient temp is also irrelavent for such a short exposure. Colour temp of the light scource can affect the histogram of each RGB sub, but if you flat each RGB sub with its own RGB flat (histogram level corrected) then thats OK. A neutral flat light scource makes things easier, but can be hard to achieve. Dusk/dawn flats are the best way, but again hard to capture.
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