Explanation

The Images I Take

A large part of my photography is macro and I am frequently faced with the compromise between depth of field and the effects of diffraction when closing the aperture down too far.

For non-macro work (mainly wildlife), I like to make full use of the aperture settings to keep as much of my subject in focus as possible.

The usually involves a compromise; deciding just how much of the subject I want to be in focus while ensuring that the background is satisfactorily blurred.

Why this progam

That is why I wrote this app.  I wanted something when I was out and about which would be a practical help with both depth of field and the effects of diffraction in different lighting conditions.

There are similar apps available, but nothing which quite met my needs or presented the information the way I wanted it.

Diffraction

For macro work in particular, reducing the size of the aperture for more depth of field eventually becomes counter-productive … and you may notice that at smaller apertures your image is less sharp.

This is diffraction at work.

In macro and near-macro work (and with teleconvertors) less light reaches the sensor as your lens magnifies the image and so your effective aperture is smaller than the one you set.   For example, when shooting at ƒ/22, the effective aperture for a 1:1 shot is ƒ/45.

Light spreads out as it passes the edges of the diaphram.  As the aperture is closed down the circumference increases in relation to the area, so a smaller aperture equates, proportionately, to more edges.

As light is squeezed through a smaller and smaller area a greater proportion passes the edges

and more light splays out.

Sharpness Factor

The result is, ultimately, a loss of sharpness: a softer image.

The sharpness factor is displayed, when appropriate, to alert you to a possible softening of your image.

Lighting conditions and the effective aperture are two of the factors that can lead to a softer image.  Another is of course the quality of your lens; I do not factor that in, but I do take account of the size of your camera’s sensor.

Interpreting the sharpness factor depends on how large you want to print or display an image, your eyesight and how critically your prints are going to be examined. 

I use a camera with a full frame sensor and print my images on A3 paper up to about 37cm wide. My own experience is that by the time the factor approaches around 1½ to 2 a print is noticeably softer but may be fine when printed smaller or displayed at a small size on a screen.

At a factor of more than 2½, I find the image unnacceptable in any circumstances when my objective is to have a sharp image.