DPP - Digital Image Qualities - Exercise 2 - Highlight Clipping
I thought before I started I'd refresh my memory what full and 1/3 aperture stops. Its not that I don't necessarily know this, its that I sometimes confuse my self.
I've attached a table which should help remind me in the future:
The aperture of a lens is circle which allows the light to enter. The area of this circle, from my school days, is measured as Pi Rsquared where Pi = 22/7 and R = the radius of the circle.
From an aperture perspective the difference of double the amount of light let in by the aperture is 1 full stop. In the exercise I use below I use apertures starting at f/11 increasing to f/4 which is actually 2.92 stops differences, clearly for photography some rounding is used :D
Starting at 1.4, if you double this then there are 2 stops between each e.g.f/1.4; f/ 2.8, f/5.6, f/11, f/22 with a full stop between them at f/2, f/4, f/8, f/16, f/32. There - that was fun!
This is shown in the table above with 1/3 stops
Anyway, back to the exercise. The purpose of this exercise is to find a contrasty scene and to discover the effects of highlight clipping and I would think to understand how increasing exposure reduces the detail captured
I've switched to manual mode on my camera from the Aperture priority I would normally take images at. I've started at f/.11 and played with the shutter speed until I can find the right shutter speed that just starts show blinkies (highlights starting to over expose).
This started at 1/25 when at f/11 and I've then opened up the aperture taking subsequent images at increasing full f/stops of F/8, f/5.6 and f/4. Because I'm in manual mode and not aperture priority increasing the aperture will increase the over exposure for each increasing f/stop
In this set of images:
Bottom Right = f/11
Bottom Left = f/8
Top Right = f/5.6
Top Left = f/4.0
The image bottom right was taken just as the sky started to over expose but the cloud structure and detail is clearly visible. As the aperture increases in the set of images above:
a) the detail of the cloud can be seen to reduce until it has been lost completely
b) the difference between white and light colours reduces until there is no border between the whiter colours
c) On the horizon of the sky where it borders the trees there appears to be a slight purple colour fringing
d) All the colours in the image have started to desaturate
Interestingly the green conifer in the most overexpose image top left is starting to display detail that is not visible image bottom right right. This is the same for the block paving.
Interestingly the image bottom right exposed at the point when my camera just started showing blinkies, when opened in camera in CS6 it doesn't show any blinkies until I increase the exposure 1 & 1/3 stops. I would assume that my raw converter in CS6 to be the more accurate.
I've detailed below the RAW convertor showing the over exposed element for the image top left which has been over exposed 3 full stops from bottom right:
Looking at the borders of the horizon and the fir trees its clear to see where this colour fringing occurs and why - its at thee point where the is significant contrast differences.
Using the raw converter decreasing the exposure 2 & 1/3 stops removes the highlight warning above.
Reducing the exposure 3 full stops, in theory replicating the image bottom right has not been able to recover the detail that is showing in the image bottom right within the sky.
As an experiment I've combined all 4 images into a HDR image using photomatix:
In terms of contrast detail, the image above combining all 4 images now contains the maximum detail present in all 4 images and what is determined, arguably, the best exposure of all key elements e.g. the detail in the fir tree in the least exposed image was not visible, but was in the most over exposed image.
Exercise Learnings:
a) Using blinkies in camera in conjunction with a histogram can help the photograph identify the point where over exposure is starting to happen and perhaps where detail may be lost that can not be recovered.
b) In manual mode decreasing aperture in say 1/3 stops will be one method reducing over exposure though in most instances I personally shoot in aperture mode so I would reduce exposure using say 1/3 stops. This would effectively increase shutter speed by 1/3 of a stop
c) More detail can be recovered from over exposure than under exposure so to err on the side the ideal exposure is just before the camera screen shows the blinkies as seen above detail can be recovered from under expose elements of the image. Again as proven above the camera appears, well mine anyway, that the camera blinkies errs on the side of caution which is a good thing.
d) Combining multiple exposed images can result in a better exposed image as the camera sensor is not as sensitive to be able to capture a wide range of contrast. This is likely the biggest reason why there is some disappointment in photographers not being able to capture the same image contrast as seen by the human eye. However in the HDR image above this can cause some colours to appear different. The car above is actually silver and not the grey shown. This may then lead to other post processing changes to be required subject to the image desired
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