Beginner Dynamic Range

[Daisy_]

For university, we had to work out the dynamic range on a Canon 7D at different ISOs. I understand it all up until the last point after you put the photo in photoshop to find the pixel value. I don't understand how to count the stops between the lightest and darkest. Because obviously they're all between 0-255 as you can't be darker or lighter but then that would mean each ISO has the exact same dynamic range. And if it was done on multiple cameras, surely it would mean every camera has the same dynamic range. So is it objective? Do I count the stops of the ones I can see detail from the naked eye and dismiss the photos where I can't make out anything without editing the photo? I have put the steps of the experiment below.


Step 1: Stick a piece of white paper on the wall. Take a photo of the paper on ISO 100 and F5.6 on automatic mode so it will work out shutter speed itself. Make sure the paper has an even distribution of light and there is no shadow.


Step 2: Write down the shutter speed. Turn camera onto manual mode. Keep ISO 100 and F5.6 but half the shutter speed. For example, if the shutter speed came out at 1/10 in automatic mode, use shutter speed 1/20 in manual mode. Repeat this, halving the shutter speed each time. Continue until pictures have been taken 7 ½ stops below the original. Example; if it started at 1/100, it should finish at 1/1250 (1/1250 is the closest stop to 1/1280).


Step 3: Now take a photo of the paper doubling the shutter speed from the original, in this case it is 1/10. Do this only 5 times. So if it started at 1/10, it should finish at 2”.


Step 4: Repeat steps 1-3 doubling the ISO before step 1 each time.


Step 5: Put photos into Photoshop. Using the Rectangular Marquee Tool, make a small square in the centre of each photo, about 10% of the size of the whole photo. Go to Image>Adjustments>Hue/Saturation. Slide saturation to the left as far as it will go to completely desaturate the selected square. This makes each pixel have equal R, G and B values. This is because colour information removed therefore, there is only a single number to record as the pixel value.


Step 6: Go to Filter>Blur>Average to average the selected pixels together. Use the Info Palette on the right side of the screen to measure the pixel value inside the selected area.


Step 7: Write down the pixel value along with the ISO and shutter speed.


Step 8: Do this for each photo.

 

[Faldrax]

I'd need to do a test to be sure, but I suspect the idea is to find the point at which the average value is 0 or 255.

So if, for example, you find at ISO 100 you get 0 (black) at 1/125, and 255 (white) at 1s, you would have ~ 7 stops (You should have more range than this!)
Up the ISO to 1/800
If the dynamic range were unchanged, you would expect 0 at 1/1000 and 255 at 1/8s
But if you actually got 0 at 1/800 and 255 at 1/10s your dynamic range has dropped to 6 1/3 stops

 

[ah5168]

You have to process each set of images at the same ISO, as you process the under exposed images you will one where the pixel value is 0 (black) that is effectively the end of the dynamic range. Repeat the process for the over exposed images until you get one that reaches 255 (white) and then count which will give you the other end. Work out the from you notes the number of stops between the 2 points as the ISO increase the dynamic range will fall if you plot it as a curve you should get something like this copied from DXOmark.

 

[holty]

im glad im not doing a degree
you probably won't use a lot of what you are learning on the course
so good luck with the studying

 

[MatBin]

It isn't about using what you actually learnt but using the process etc in other situations and expanding your research abilities.
As to the op - no idea, sorry.