Science Buddies: "Ask an Expert"

In the project ' Using Digital Camera to measure skyglow ', while finding the dynamic range


1.How do we consider areas of saturation in the gray level vs shutter speed graph ?

2. What is the purpose of taking double quantities of succesive points (red circles) i.e for comparing what ?

Hi manju1234,

Several other people have asked questions about this project, and several Experts have posted some great responses. Take a look at the following topic threads, and see if they answer your questions. If not, post back with some more details about your problem.

https://www.sciencebuddies.org/science- ... php?t=8326
https://www.sciencebuddies.org/science- ... hp?t=11254
https://www.sciencebuddies.org/science- ... php?t=4533
https://www.sciencebuddies.org/science- ... php?t=9489

This article has some general background on interpreting RGB histograms from your camera, which will be helpful for you to understand:

http://www.sphoto.com/techinfo/histogra ... grams4.htm

Finally, the background to this particular project idea has lots of relevant information:

https://www.sciencebuddies.org/science- ... background

Hi Terik,
Thank you so much for those links, all my previous questions have been answered.
i have come up with new ones though.....

I want to know why we need to use a logarithmic scale to plot shutter speeds ??Is there a specific reason for not using the normal scale ??
Also if we are considering the average pixel graylevel ie at 128( gray level )and noting the shutter speed at this to compare skyglows , then where does the dynamic range come into picture .. are we actually finding it in this process ??

please reply as soon as possible ....my project is due on 29th jan....
thanks again for your help ....

another question
in one of the graphs , i have plotted it doesnt reach till 128 pixel level;
my camera had only shutter spds till 15s so the last reading i got was 98(on y axis ie graylevel).... so how do i find out the skyglow of this particular location ?? should i assume it to be very small ie less skyglow??

Hi manju1234,

I'm glad those topic threads were helpful! Here are some answers to your new questions.

RE: Log scale
The x-axis is on a log scale because the exposure times change logarithmically. Log scales are very helpful when we measure quantities over a very large range of values. In this project, exposure times vary by a factor of 1000. Using a log scale makes it so that each of the data points stands out more clearly. To see what I mean, try plotting the x-axis as a linear scale, and notice the difference between the two graphs.

RE: Dynamic range
The dynamic range tells you how your camera converts photons hitting its detector to a grayscale value (0-255). With a constant aperture and ISO, pictures of the same scene, but different exposure times, will have different numbers of photons hitting the camera's detector. The longer the exposure, the more photons hit the detector, and the higher the pixel value will be (0-255). The shorter the exposure, the fewer photons hit the detector, and the lower the pixel gray value will be. But, at some point the camera's detector will saturate. ABy "saturate", we mean that the camera will keep assigning a pixel value of 255 no matter how many photons hit the detector. So, if your camera spits out a pixel value of 255 after an 8 second exposure, the pixel value would be 255 even for a 8 minute exposure (but your camera's detector might be irreparably damaged, so don't try it!). It also takes a certain number of photons in order for your camera's detector to start assigning a gray value. So, you have to have a certain exposure time for your camera to be able to take a reading other than 0. For the purpose of this project, the dynamic range of your camera is the range of exposure times over which your camera has an average pixel gray value that is >0 and <255.

RE: Graph
I'm not quite sure what your question is. But I think it's OK to say that the calibration wasn't good enough to quantify the skyglow at that location. But, you think it is low because of the low pixel gray value.

Thank you so much !!!
I think I got everything right and ill be able to explain /answer any question my teacher poses about the project and im sure she's gonna like my project .
Thank You so much Mr.Daly and Science buddies

PS. I might bother you if I have any last minute problems...
Thanks again.

You are very welcome! Feel free to post back if you have any other questions pop up.