How To Take Bias Frames on a DSLR?

[AstroManDan]

Hi,

I want to take bias frames on my Canon 550D.

1. Can I take them once and then never have to do them ever again?

2. Should I take a range of them at different ISO settings?

3. Should I take a number and average them?

4. Can I take them with the stock lens on and the lens cap on, or should I set the camera up in a certain way or perhaps leave it on for 30mins to warm up?

In Short, how do I take them?

Thanks

Daniel

[Psychobilly]

Same ISO as the lights that they are going to be used with

lens covered

Shortest possible shutter speed

Take plenty...

Take new ones from time to time... but to be honest I normally take them at the end of each session as they are so quick...

Billy...

[AstroManDan]

Thanks for the reply.

[brianb]

You usually don't need to. DSLRs with CMOS sensors (as opposed to CCD) automatically take and subtract a bias frame every time an exposure is made. If they didn't, the noise would be horrendous.

Almost all current models of DSLR use CMOS sensors; some older Nikons used CCD.

[Psychobilly]

Cheers Brian

Just stretched the Canon Bais frames to breaking point (from 350D, 1000D and 500D) and they are flatter than a flat thing thats been flattened by a very heavy flattener......

Time to dig out the Nikon D50, D200 and KM 5D ones...

Peter...

[AstroManDan]

Brian do you mean the 'Noise Reduction System' where it takes the same length exposure again to remove the noise, or does what you say happen everytime automatically?

[brianb]

do you mean the 'Noise Reduction System' where it takes the same length exposure again to remove the noise

No, that's switchable.

Every time you fire the shutter on a Canon DSLR, after the shutter closes the camera makes another exposure of very short duration (a dark bias frame) and subtracts that from the image. If you have "long exposure noise reduction" enabled, it takes another exposure of the same length as the "proper" one - a "dark frame"and another bias frame, subtracts the second bias from the dark frame then subtracts the result from the de-biased "light" frame - everything except the final result is then forgotten.

As Billy says, if you take a very short exposure with a Canon DSLR & the lens capped, you find there's essentially no data in the raw frame. Using this as a "bias frame" is doing nothing useful for your image, just wasting your time & wearing out the shutter mechanism on the camera.

[AstroManDan]

OK thanks.

But with my dedicated CCD camera, I take it I would need to take Bias frames.

[brianb]

with my dedicated CCD camera, I take it I would need to take Bias frames.

Need - well maybe you'd get away without, if you have one of the cameras with a quiet Sony CCD like the Atik 314L. But it wouldn't do any harm ... if you have a camera using a Kodak CCD you will certainly need full bias, dark & flat frame calibration to get an acceptable noise level, however with this done the results should be equal to or better than that obtained with the Sony CCDs.

Examine your bias frames. If they're "flatter than a flat thing thats been flattened by a very heavy flattener" they're not worth bothering with.

[Psychobilly]

Examine your bias frames. If they're "flatter than a flat thing thats been flattened by a very heavy flattener" they're not worth bothering with.

[astroimpulse]

they are flatter than a flat thing thats been flattened by a very heavy flattener......

Quote of the week...QUOTE OF THE WEEK!!!!

Good info, Brian

[Dave Smith]

Thanks Brian, very useful to know.

Dave

[ollypenrice]

Well, bias frames are useful but are surely contained within your darks already. Don't apply them twice! The only reason a CCD imager would take bias frames would be to use them as darks for his/her flats. I have never applied bias to lights other than via my darks. That applies to both Sony and Kodak chips that I've used. This is very much a majority view, I think.

Olly

[dmahon]

Does this mean that the best way to take DSLR shots is to:

Enable long exposure noise reduction

Take lights (automatic bias and darks)

Take flats (automatic bias and flat darks)

Stack lights and flats only in DSS

[jgs001]

Interesting Brian, that answers some oddities with bias I've seen... not gonna bother with them anymore.

As for using the long exposure NR... you could, but then you lose a lot of possible sky time (50%) and there's not enough of that now to lose any.

[dmahon]

On the odd occasion that we get a clear night, I tend to set everything up and then leave it shooting pictures whilst I go inside and watch TV for several hours. Then I come and take it all down, but set up the camera on the floor taking the dark frames and go watch some more TV whilst cursing it taking so long with the dark frames because I really ought to be going to bed. It might actually work out better for me.

Then again, I can probably get away with only 5-10 dark frames - if I set up the automatic long-exposure setting, I will have to take as many as I take lights.

[jgs001]

Pretty much what I do... and I normally only use about 10 to 12 darks (ought to use more, but at 5 minutes, that's still an hour)

[steppenwolf]

Every time you fire the shutter on a Canon DSLR, after the shutter closes the camera makes another exposure of very short duration (a dark bias frame) and subtracts that from the image.

Well, you lives and you learns - didn't know that, thanks for the info.

[steppenwolf]

and they are flatter than a flat thing thats been flattened by a very heavy flattener......

Just to get this clear, P-Billy, are you saying that these frames are pretty flat then?

[Psychobilly]

ai....

[themos]

You usually don't need to. DSLRs with CMOS sensors (as opposed to CCD) automatically take and subtract a bias frame every time an exposure is made. If they didn't, the noise would be horrendous.

That comes as a big surprise to me. So why do my Canon RAW bias frames show a mean value that is >> than the sigma?

[ashworthacca]

So Flats from a DSLR are already bias subtracted?

You live and learn.....good info Brian, thanks.

[George]

Well, you lives and you learns - didn't know that, thanks for the info.

Neither did I

Ive always used separate bias frames and finding this out might prod me into redoing some old data to see if there is actually a difference.

[dph1nm]

You usually don't need to. DSLRs with CMOS sensors (as opposed to CCD) automatically take and subtract a bias frame every time an exposure is made. If they didn't, the noise would be horrendous.

Hmm - well they do a very bad job of it then, as my bias frames (1/4000 sec in the dark, Canon 1000D) do show structure which is apparent in the light frames.

NigelM

[dph1nm]

OK, so Canon's and other do a CDS (what we used to call double-correlated read) where the chip is read before exposure and this value subtracted at the end. This is to remove any unwanted resiudal charge lurking around. However, I think Canon then add a bias level before writing out the RAW file.

NigelM