When Are Temperatures Recorded?

[Mike]

Due to a cloudy night I'm capturing darks. I'm using my 60 Da and capturing 900 second frames. I noticed that the first frame, with the camera just turned on, the sensor temperature was 72 degrees F. Over the next 900 seconds the temperature remained reasonably the constant, yet the sensor temperature was recorded at 82 F. The images were captured at the same exposure duration, but there is a 10 degree difference in sensor readings. The third frame was 2 degrees warmer though the following frames have stabilized. This makes me wonder when the temperature is recorded, at the start or the end of the exposure. It seems to me that with a constant ambient temperature the sensor temperature at the end of each exposure of the same duration would be the same. So is the temperature recorded at the start or the end of the exposure? If it's recorded at the start is it possible to change that to record the temperature at the end, or is that a SDK issue?

[photon-noize]

From the experience with my 5D Mark II and Mark III, the sensor temp is recorded at the beginning of the exposure. When taking darks, throw away the first 3-4 darks to allow for stabilization of temperature and dark current noise. 

[astroman133]

Mike,

 

The sensor temperature is extracted from the image EXIF data by BYE. So any issues with the values are not an SDK issue, nor is it a BYE issue. This is purely a camera function. I do not know whether Canon publishes the specs regarding when, during an exposure, the temperature is recorded. It may be easy to discover more about this, however.  With the camera stable at ambient temperature, take a very short exposure, followed by a 900 second exposure, then followed by a very short exposure. Compare the recorded sensor temperatures of the three images.  If the temperatures that were recorded in the first 2 image are essentially the same then you could infer that the temperature for the long exposure was recorded at the start of the exposure. If the temperatures of the last 2 images are essentially the same then it is likely that the temperature of the long exposure was recorded at the end of the exposure.

 

It is normal for the sensor to heat up with use. This will cause the sensor temperature to increase, even while the ambient temperature is decreasing. This is simply because the camera heats up while the shutter is open. There are many factors that cause this behavior, including heat that is generated by the sensor itself, along with the LCD display potentially being on, or the battery discharging. When I shoot darks, I shoot enough that I can discard the first few images that were taken while the temperature is stablizing.

[s3igell]

Remember, that the Sensor Temp reading is NOT a recording of the Ambient Temp outside the Camera, but that of the Internal Temp of the Electronics (not actually the Sensor itself - consensus amongst many tests as Canon won't answer directly).

The Electrical Energy expended to run the Electronics during the Exposure and to Read the Pixels off and convert to RAW Image Data at end of the Exposure, is all converted to Heat within the Mass of those Components.  That Heat has essentially Nowhere To Go, but to Heat Up those Internal Electronics.  The Design of a DSLR is NOT made with too much consideration for Heat Dissipation.  After all they are designed for Daytime Use when Ambient Temps are Higher but Exposures Shorter - there is Less Heat Generated and Less Chance for it to Dissipate.  Especially the Consumer-end Rebel Models have significant amounts of Plastic forming the Camera Body - even less Metal to Conduct Heat - but then who wants a "Hand Warmer" ??

 

So, at the Exposure Durations and Repeat Cycles that we AP Imagers do our Work, it is common to expect the Sensor Temp to Climb even if the Ambient is Dropping.  We only HOPE that the Sensor Temp will Stay within a Reasonable Max Temp so that the amount of Thermal Noise is still Addressible in Post Processing...

 

The GOOD NEWS - and BAD NEWS for certain Essential BYE Features - is that Most of the Heat is Generated during the Sensor Read Operation and not the Exposure Duration.  This is GOOD for Imaging Exposures.  But this is TERRIBLE for the LiveView-based Features where Sensor Read Operations are performed back-to-back in essentially a Video mode.  The LiveView Frame-and-Focus Mode can easily Heat-Soak your Camera such that it needs 30-60min to Cool before usable Long Exposures can be taken...  So, especially in Summer, use your LiveView Effectively but Judiciously!!