Sunday, 8 May 2011

Project 1: dynamic range

Camera manufacturers don't like to divulge the dynamic range of their cameras therefore I must endevour to discover it for myself. Dynamic range of the sensor is the difference between the darkest shadows and the brightest highlights that it can capture in one scene without losing detail. If detail is lost this is known as clipping. Within RGB solid black measures 0,0,0 and solid white 255,255,255.

Handy table to remember the f-stop going to print this out and take it with me!


Sometimes I think the written instructions are actually more confusing than the task an LSA and having to write or make information very basic I do double take and think why didn't it just say X? Calculating the DR in DPP was the task I put off for ages, now realising it wasn't that hard to do I wasn't so fussed about this one.
To start- I had a plain white piece of card in daylight. ISO set at its lowest,image size smallest.Used Custom WB and Manual exposure.Filled the frame with the card, adjusted shutter speed for average exposure.Noted the setting. Next I calculated a setting that was 6 f-stops darker by altering the aperture, however due to the lens I was using I found I couldn't change enough stops so increased/decreased exposure by using shutter speed instead. I also had a minor problem with the lighting, being a cloudy day I had to wait for the sun to come back out from behind the clouds to continue enabling me to have consistent lighting conditions. Then it was windy and my card kept falling over!

Apart from that this was fairly straight forward to complete; the mid point metered to F8 @ 250 however, even adjusting my shutter speed I couldn't change down more than 4 stops;  progression was f8 @ 1/500, 1/1000/ 1/2000 and finally 1/4000. In the opposite direction I had no difficulty in the shutter speed f8 @ 1/125, 1/60, 1/30, 1/15,1/8,1/4.

The images were then opened in Photoshop and out of curiosity I took RGB values before converting to greyscale. The black point read (8.8.8) the mid range (110,114,125) which suggests the metering wasn't spot on as mid grey should be as near to (128,128,128) as possible, and the white point did hit (255,255,255)

The ascending scale was the created and here is the resulting chart.

Dynamic Range Canon 400D

Histogram showing tonal peaks

The image was cropped to stop at the black and white points. Once converted the (8,8,8) read as (3,3,3) which I though was quite strange? The mid point (115,115,115) which seems to have averaged out the original readings, and the white point remained the same (255,255,255)

Counting the steps including the cut off points (9,9,9) and (255,255.255) - I didn't count the (3,3,3) due to the fact anything below (5,5,5) would be black with no viewable detail, it would appear that my camera has an 8 stop range which is quite normal. The histogram also appears to reflect this. When testing the dynamic range of my camera for DPP the scene results gave a dynamic range of 7. This test is a little more complex and gives a range of 8. However due to the fact that the lowest point shifted when converted, it may possibly fall between 8/9 and the following website found that the 400D produced a consistent dynamic range of 8.4 with ISO 100-800 and 7.8 at ISO1600.

This reassures me that I carried out this task correctly (even if my metering was a bit off) and my camera is acting in the way it should. It was also good to note that my mid-point is midway on the chart so should avoid burnt-out highlights and blocked-up shadows. Armed with this information I should be able to judge how much information I can caputure in a given scene, especially one with high contrast.

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