Bill Lunney's photography diary.  Blog to everyone else.  :)


Glasgow and Lanarkshire based photography and video

Effects of f-Stop on getting sharp images

During some some property (real estate) shots today I came across something I didn't know.  With the objective of getting images as sharp as possible I had did all the usual prep.  Tripod, remote release etc.  In this case I was using a quality prime lens so the 'sweet spot' associated with zooms wasn't a factor.

Took two images per scene - f11 & f32, manual focus.  The f32 images were, without exception, softer than f11.  I had expected them to be sharper providing there was absolutely no movement during exposure.  Know f32 is a bit extreme but was useful in establishing a reference point.  Both images below are straight out of the camera (Nikon raw).

At first I thought the longer exposure time coupled with a very slight wind may have been a factor.  Still possible but I feel unlikely given I took around six scenes with a three bracket exposure on each.  Every image showed the same 'softness'.

After a little research I found the issue, which I should have known anyway.  I spend a lot of time shooting at small f-stops (2.8 ish) so this doesn't come up.  In a nutshell, smaller aperture (larger f-stop number) equals more distortion of light.  Very small apertures cause increasing light diffraction.  In layman terms, forcing the light through a tiny hole causes light rays to interfere with each other.  At this high an f-stop other factors start to play a part - aperture blade profile, camera shake etc.

As with most things there's a wealth of information on this specific topic, Cambridge in Colour and Brad Sharp's article were two resources I used.

The zoomed images below shows two different scenes, first image f11 second f32.  Images below are 100% JPG exports from Lightroom.  Aware this introduces error but am too lazy to export as TIFF then to PNG.  PNG support in Lightroom is long overdue!

Takeaway here is there's a cut of point on all lenses where higher f-stop does not equal a sharper image.  Some simple tests shooting a series of shots on a tripod aimed at a fine detail subject will show where that is.  As a rule of thumb f11 seems to be sensible.