Monday, November 3, 2008

On Perfect Exposure

I wrote a short article to just explain what exposure is, how it works and how it relates to ISO, Aperture and Shutter Speed, "Apertures, Shutter Speeds, ISO and Exposures Oh My!" and in the following article "A Quality Affair" I went on describing how exposure is vital to creating a high quality image.

Perhaps I didn't explain well, or perhaps it is one of those 'understood' things that I didn't mention, but someone made the statements "I always found the exposure as a very subjective aspect in photography. Theoretically I don't believe in the concept of a good exposure. We can talk about a correct exposure from a technical point of view but since the exposure is so subjective, what you might think is correct for a particular subject could be entirely different for someone else". This led me to review and think about what I stated in those articles.

Here I will make a statement; "Perfect Exposure is NOT subjective." It can be precisely measured with scientific methods. The reader is probably referring to two possible options here when he refers to "exposure being subjective".

The first being that the tone that the final images takes is subjective. Tone, here referring to how light or how dark the image is supposed to look. A good example is a twilight shot where perhaps the final image should mostly look dark. A perfectly exposed version of this would not come out 18% gray. Properly exposed for that specific scene the image would come out looking dark as it did in real life. This is not subjective.

The second option is that he may be referring to "Artistic" judgement. Perhaps an artist may want to take on an artistic trait that always has their images slightly over exposed. The key here is that it is over exposed. The image may even look better over exposed. But, if he is showing the image as is was shot, then it is 'properly exposed'.

How can I say that? Lets go back to our night scene where perhaps we have a lone smoker waiting for a bus under a street light at night. We want to capture the scene that includes the dark street with a few light stands and want to capture the fine details of the cigarette smoke. What exposure are we to use? Using the cameras built in average meter will not do it for us. Perhaps we decide that the smokers face should be exposed so that it is clear and well defined. Using a spot meter we get a base exposure and then we take five shots as follows; -2EV, -1EV, 0EV, +1EV and +2EV.

We now have five images that are all perfectly exposed! But again, wait before stating "How can you say that". Each image is exposed perfectly to show what the camera has captured. When viewing the five images the photographer can choose the image that best captures what the scene looked like. Conversely, the hidden 'artist' in the photographer may decide that the slightly darker image best shows the mood of the image he captured. The -1EV image is perfectly exposed for the image he selected.

Here the photographer had a choice between two perfectly exposed images, one darker than the other lighter one, but both were perfectly exposed.

So what am I talking about then? A non-perfectly exposed image would have been the 0EV image that was darkened digitally so that it looked like the -1EV image. That process of manipulating the image and changing it will cause exposure and color clipping that will cause you to loose valuable details and will introduce more noise.

Now, artistically and photographically an image can just look badly over or underexposed as for what the human brain thinks it shouls be seeing. Getting that right is partly artistically based but mostly it is an aquired and learned skill.

So, when I speak about correct or perfect exposure, it is based on the photographer capturing an image perfectly as the photographer or artist wants it. If we took the -1EV image and the 0EV image that was manipulated to look like the -1EV image and had them both analyzed, the properly exposed -1EV image would be sharper with better tonal range and less noise than the manipulated 0EV image. This would not be subjective.

© 2008 Francois Cleroux

(Version 1.11 - October 2008)

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I value thoughtful comments and suggestions. If you like or dislike this post, please let me know. If you have any ideas or suggestion, comments or corrections (I do make mistakes) please also let me know. Thanks.

- Francois Cleroux