**This post was originally written in October 2006, but to this day seems to attract a lot of attention from google searches. So to start off a retrospective series of posts, here is james circa 2006:
Ever wondered what this magical DOF people spoke about was, How about Bokeh…? lets start with the easy one…um
DOF as is commonly used on the interweb is yet another highly descriptive TLA (Three Letter Acronym) is stands for Depth of field and refers to the distance between the first point in focus and the last point in focus for a given picture. In what could become this photoblogger’s catch cry, lets have a look at this photo, (No explanations yet just have a look at it…)
If you look carefully at the photo photo above there is only one small region in focus, the front of the leaf is very blurry and the background is also blurry, however the bit in the middle, the subject, is clear and we can see all the detail. In simple terms, the depth of field arises due to the inability of the lens to focus light from everywhere in the photo onto the razor thin photographic plane. Whilst framing this photo I focussed on the point where the two fronds meet and form a U shaped crux. The lens can then only focus the light from a certain distance either side of that point. This means that everywhere else in the photo the light coming from a single point is focused to slightly different places on the ‘film’ plane and hence the area is out of focus. This creates a nice blurry effect called Bokeh.
Now we can get around having such a shallow depth of field as it is simply a function of the focal length distance to subject and the aperture. Stopping down, or increasing the aperture (by decreasing the entrance pupil diameter) will result in a wider (deeper) Depth of field. This of course has flow on effects with respect to shutter speed etc…
In what he may have later reflected on as a somewhat silly time of life, Ansell Adams was a founding member of the f64 group. A ‘gaggle’ of landscape photographers who only shot at f64. This was achieved using a pinhole type lens on bellows with a large format film camera, the resulting exposure times were measured in minutes and hours and everything in the photo was in focus, the depth of field was approximatley infinite.
However, Depth of field is a tool and one that is used very commonly to isolate your subject matter…
In the photo above i have isolated the heart of the dandelion’s head by having a very shallow depth of field. The result is almost a crossectional view of the dandilion. Looking carefully you can see that the front and rear of the dandilion are effectivley out of focus, thus the depth of field is around 1cm , compare that to the photo below with a depth of field around 3cm and the effect is markedly different.
Not to play favourites but the first photo is mine…
So have a play, it’s the only way to learn.
