Canon 7D MkII dark current

[MarkS]

Thanks Fay for letting me borrow your Canon 7D MkII for a while when you were here for the imaging night.

My methodology is to keep the camera at room temperature (19C in this case) in the dark and take a series of 5 minute dark frames.  This is pretty typical of how it would be used for deep sky imaging.

The results are in the graph below:



As you can see, the dark current is slightly lower but not significantly different from other well know Canon cameras.

In Roger Clark's analysis (http://www.clarkvision.com/reviews/evaluation-canon-7dii/ see graph 3) it was being compared to some different Canon cameras with apparently very high dark currents.

I produced another graph of dark current against temperature recorded in the EXIF header:



The dark current axis is logarithmic and this graph is broadly in line with Roger's graph. 

Mark

[Ivor]

I was really excited to read that  my old 400D compared pretty well. I decided to put it to the test tonight only to be provided with an err 99 from the camera, so frustratingly working through the internet solutions in the hope of finding a positive outcome.

[Mike]

How does it compare to your Sony ?

[ApophisAstros]

And my Canon750d modded, does modded have an effect?
Roger

[MarkS]

How does it compare to your Sony ?

Great question!  That is where things get quite fascinating.  At first sight the Sony has much higher dark current when first switched on but it doesn't climb at the same rate during continuous use.:




I wondered if that was because of the larger pixels on the Sony so I produced a new graph where dark current is divided by pixel area, for each camera i.e. a plot of dark current per square micron of sensor:




Look at the remarkable bunching of results at the start of the graph before the sensor begins to warm up because of continuous use.  Now the first point on the graph required two 5 minute frames for the calculation so it represents average dark current over the first 10 minutes.  So it is fair to extrapolate the curves back to a "frame 0".  If you do that then the agreement is extraordinary!  It appears to me that all the sensors I have tested over 10 years, from both Canon and Sony have identical dark current (within experimental error) in terms of electron leakage per unit area of sensor.  Whatever changes have taken place in sensor design over 10 years do not appear to have affected dark current.

Also interesting from the second graph is that some cameras (e.g. Canon 7D MkII amongst others) seem to suffer from heat build up during continuous use far more than others (e.g. Sony A7S).  Maybe modern Canons are simply packed too full of heat producing electronics.  The Sony has an alloy body and has copper plates in contact with the sensor to conduct the heat away to the alloy body - that is how they are built - probably to dissipate heat during video.  The Nikon D7000 also has a Sony sensor - maybe Sony sensors suffer less from heat build up in the first place or maybe the Nikon D7000 is another well built camera - I've never taken one apart.

Roger - modding the camera makes no difference to dark current.

Mark

[Hugh]

Do not understand a thing but in awe of you astrophotographers.  Thank goodness we have you!

Hugh