I know I said this photography stuff would be an occasional feature, but I did promise, this past Monday, to talk about fast lenses and maximum aperture.
So. Without further ado.
First of all, I'm going to publish two photos - one taken with a 50mm lens at its widest aperture (f1.8), and a zoom lens racked out to 47mm and also at its widest aperture (f5.6).
The idea is to show you why the single focal length lens, in this instance, is a better choice for the shot, over the typical 'kit' lens you'll find on offer with a digital camera. It also demonstrates the fallacy in my thinking regarding 'fast lenses' and digital cameras - that thinking being that with a digital SLR in hand, you didn't need to worry about chasing the 'fast glass' in low light and action settings - you just turn up the ISO to handle the shot. That thinking was correct as far as it went, but gave no consideration to the quality of the shot.
This first picture is the 50mm shot.
And the next shot is the zoom lens at 47mm.
Notice how much of the top photo is out of focus - especially, notice how far out of focus the foreground blossoms are, and the tree trunk on the left side of the photo.
Now, compare those same objects in the second shot.
It is much easier to pick out the subject, in the first shot, than it is in the second. The reason is that the subject is isolated - foreground and background objects are thrown completely out of focus - much further out of focus than the same elements in the second shot.
The smaller the f-stop number, the wider open is the lens.
The wider open the lens, the shallower the depth of field.
The shallower the depth of field, the more isolated the subject.
In most instances, the more isolated the subject, the better the shot.
Both of these lenses have fairly good optics - and so the 'bokeh' of each lens is really fairly nice. Next week, a discussion of 'bokeh' - what it is, what it is not, and how to properly use the term.