Image size and DOF
As a little follow up to my piece highlighting issues raised elsewhere on the internet about depth of field (DOF) I thought I would add something about the consequence of reproducing the same image at different sizes. With each post (at least so far) I have added a picture, sometimes to illustrate a point, and more often just as a form of decoration. These images are put up quite small (400 pixels on the largest side) and are resized from much larger images and then sharpened - as I would with any image.
In some cases, reducing the size does things to the image other than simply make it smaller. Several of the images are modified to make them "work" at this size. In particular it has an impact on depth of field. Two images that I previously put up serve as an example of this. The one of droplets on a web (part of the piece on "Chasing pixels") is a crop from a larger image. Had I shown the whole image reduced in size it would not have made the droplets to the left and right hand side look as soft and out of focus. The whole reason that I took the image at an angle rather than straight on was to play with selective focus. This didn't really show up when the image was reduced, but cropping the image and only reducing the size a bit kept more of the feel that I wanted, although cropping bits off still reduced the effect to some degree as the bits at the edge were even softer.
The other example is the image that accompanied the previous article on this topic (DOF - Does it need fixing?). The full size image has relatively shallow depth of field. For example the bird in the distance is noticeably soft in the full size image, yet looks quite sharp in the smaller one. I have reproduced part of the image here at the sort of size at which it might be printed, so that you can compare.
In fact the only time depth of field is not affected by size of reproduction is when it extends from front to back in an image even at full enlargement. Just something else to think about.
The image at the top of this piece was taken in Norway in 2005