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Hi! Yesterday I was reading about dark frames vs in camera long exposure noise reduction, and something caught my attention. As far as my (so far little but growing) knowledge goes, the best you can do is to take the calibration frames right after the imaging session. This can be a pain in the A, and as I read yesterday, many takes these frames separately, when there is nothing better to do, like on a cloudy afternoon. This is allright, it's a good idea, you can create different master darks and other master calibration frames on different temperatures (room temp, cold, hot etc), and use these when stacking images from your light sessions according to the temperatures the lights frames were capured at. But. As far as I know, my darks should have the exact same settings and focus that my lights have. If I know I use for an example a prime wide angle lens at F2.8 all the time, with ISO 1600 to capture the milky way, that's okay. But what if something changes? What if I use ISO 3200 for some reason? What about the focus (okay, inifinity, but not exactly the same all the time when manual focusing)? What if I use a zoom lens on different focal lenghts? What about the other calibration frames? It's definitely not impossible to be prepared for every scenario, but when you use lenses instead of telescopes, there are more variations. Extra info, if that matters: I'm using a Nikon D5500, which is "ISO invariant". I'm really curious about your replies, as this could greatly improve my image's quality, if It's possible to take calibration frames this way. Thanks in advance! Árpád
A basic guide to Registax shows that you just choose a number of frames (or percentage etc) and click next and move onto the next step. How does Registax know what a good frame is? Does it just use the 1st frame of the video as a reference? I've seen detailed lunar stacking guides where an artificial pre sharpened (in PS/GIMP) is then inserted back into Registax and used as the reference frame for LIMIT. Anyone do this with video stacking? and if so, what's your basic workflow to produce the reference frame?
In an earlier post I examined the noise in single dark frames over a range of exposure times. My conclusion was that the Nikon D7500 was a lower noise camera than the D5300. This was also backed up by an examination of master bias frames that again strongly favoured the D7500. However ... The first image I attempted to produce with the D7500 exhibited very strong streaks in the noise. I had been in the habit of not using dark subtraction with the D5300 as it has very low thermal pattern noise. Accordingly, I again only used bias and flat frame calibration in the workflow that produced the above images. Whilst the streaks are due to patterns in the noise being spread across the image due to errors in the application of dithering during tracking, it did indicate that the D7500 did have significant thermal pattern noise. I found this surprising because the noise in individual frames ( when looked at in isolation ) seems to be completely random. I thought that perhaps my memory had failed me and maybe the D5300 has the same level of pattern noise but my memory was being tricked. That is, all my recent images with the D5300 were taken at lowish air temperatures ( ~5 deg or so ) whereas the image above was captured on warm nights ( low 20s ) and so maybe the D5300 would be just as bad at higher temperatures. To test this I produced bias corrected master darks for both the D7500 and D5300 from images all taken at around 20 deg or just over. The images below have all been stretched using the same screenTransferFunction applied to the Pixinsight histogram tool. The results are striking ... D5300 master dark ( 47 subs, bias corrected ) - red channel: D7500 master dark ( 281 subs, bias corrected ) - red channel: D5300 master dark 100% centre crop - red channel: D7500 master dark 100% centre crop - red channel: And the histograms of the full size images ( red channels ) ... D5300 master dark ( red channel ) histogram: D7500 master dark ( red channel ) histogram: The Pixinsight statistics tool calculates the following: D5300: mean 2.3, standard deviation 9.3 D7500: mean 7.5, standard deviation 20.8 ------------- Analysis: The images and histograms clearly show that the D7500 has higher pattern noise than the D5300. In particular, from the histograms, 0.1% ( 6,286 ) of D5300 pixels are more than 44 ADU whereas, for the D7500 this figure is 27 times as great at 2.7% ( 141,305 pixels ). Furthermore, the master dark for the D5300 was only produced using 47 images -v- 281 for the D7500 so I would expect that this difference would be even higher with more D5300 frames. On the other hand, whilst not shown in the histograms above, my D5300 does have more 'very hot' pixels than the D7500 ( 579 pixels greater than 400 ADU -v- 10 pixels greater than 400 ADU ). However, these hot pixels are very easily removed via dithering during tracking and sigma clipping when integrating. The very large number of warm pixels however are very difficult to remove as dithering just places different warm pixels on top of each other. I went back and examined the 'random' noise seen in the individual D7500 dark frames ... and yes they do look random when seen individually, however, when flicking between a number of frames it is clear that the 'random' pattern is repeated in each frame! Conclusion: My D7500 has very significant thermal pattern noise, albeit randomly distributed in a fixed pattern. ............ Next steps (?) - I could use in dark subtraction during calibration to reduce the impact of pattern noise - however, as my camera is not cooled and the night's temperatures are constantly changing, any master dark will not closely align to the actual thermal pattern noise and as such dark subtraction may help but will not solve the problem - Using in-camera dark subtraction ( Nikon's long exposure noise reduction feature ) would almost completely remove the pattern noise from each frame. However, due to the extra random noise being introduced by subtracting another noisy dark frame from each light frame, as well as the reduction in total light frames by 2, the resultant images will suffer from higher levels of random noise. So whilst this would be an improvement with respect to the pattern noise, it is not a complete solution. - Third option, sell the D7500 and go back to using the D5300 ...
Below is a comparison between single dark frames taken with the Nikon D7500 and D5300 with exposure durations varying from 1 sec to 240 sec ( my usual main light frame exposure ) all at ISO400. Firstly a graph of the standard deviation of the noise in the dark frames versus exposure time: The standard deviation of the noise is a fairly constant 2 ADU less for the D7500 compared to the D5300 ( pretty much the difference in the read noise between the two ) However, the difference is not just in absolute terms but also in the quality of the noise ... Below are the dark frames - ranging from 240 sec exposures at the top to 1 sec at the bottom: D7500 D5300 The D5300 dark frames clearly show the pattern in the read noise ( banding down the bottom ) and also have far more chrominance noise compared to the D7500. At 240 seconds ( the main exposure I have been using ) the difference is starkly different; the D7500 produces images with much lower noise that is significantly more even and random and hence more likely to be reduced during integration.