Today is officially the first day of summer and so to mark the occassion I decided to post this photograph taken last year in the Iveagh Gardens in Dublin city centre on a summer’s morning as I walked from the Luas to my office which was then on Harcourt Street.
This photograph was taken the same day as yesterday’s shot in St. Stephen’s Green shopping centre (which I took on my way home from work) and so here again I was experimenting with my new camera. The photograph was taken in aperture-priority mode, and the idea was to see how well the camera handled the exposure without much input from me.
To isolate a couple of the flowers from others in the foreground and background, I chose the widest aperture I could with the lens I was using, and I let the camera decide the shutter speed. In fact, in this case I let the camera decide the ISO also, as I was using the D300′s ability to set the ISO to give a good hand holding shutter speed. This is a pretty useful feature, most often for low light situations where you want the shutter speed to stay high enough to keep hand-held shots sharp, and you are willing to accept some noise due to a higher ISO to achieve that. In this case, there was lots of sunlight, so the camera sensibly chose the ISO that its sensor is optimized for.
When a camera chooses the appropriate shutter speed for a given aperture and ISO, what it is doing is trying to get the scene to be 18% grey. This 18% is the magic number for exposure and is half way between 0% (black) and 100% (white) on a logarithmic scale. So the 18% represents a mid-grey. The idea is that a general scene will look best if it is exposed so that the light in the scene averages out to this mid grey luminance. If the camera can put the exposure at the mid point between black and white, then for many scenes, if there are blacks in the scene and whites in the scene, they should be represented as black and white in the final image. Things get complicated if the scene is predominately black or white though – for instance if you take a photograph of a black card with a camera set to an automatic exposure mode, the camera will set the exposure so the card appears as a mid-grey. Similarly for a white card. So a photographer has to compensate for these scenarios, or a camera has to be more clever about its exposure.
I’ll explain this further with some relevant images in future, but for this image I was giving the camera an easy test as green grass is actually very close to that 18% grey in terms of the light that reflects off it. So I could let the camera do its thing and ended up with a good exposure without any further input from me.
The result was an image that needed very little tinkering in post processing – I didn’t touch the exposure and just bumped the contrast a tiny bit. I like the colour in the final image, and the isolation of the flowers on the right from the background thanks to that narrow depth of field.
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