Friday 8 August 2008

Sharpening in Photoshop

For further photography-related information check out my compendium of tips.

I recommend you read my previous two posts on sharpening for a deeper understanding of this process: Sharpening using unsharp masks, part 1 and part 2.

Sharpness depends upon the 'crispness' of edges (technically known as acutance) and the resolving power of your camera and lens combination. For an excellent overview with diagrams see Sean McHugh's excellent Cambridge in Colour website. Once you've taken the picture there is no way to alter the resolution of the image but the acutance can be improved using unsharp masking.

An unsharp mask identifies the edges in an image which is then used to mask a higher contrast version of the image overlayed with the original. This increases contrast only in the edges of the image - the dark side is darkened and the light side is lightened - increasing acutance and giving the appearance of increased sharpness (check out the concept of Mach bands to understand why this is so). If an image is over-sharpened, however, artefacts known as halos are observed (areas that are too bright or dark in the image).

In Photoshop, the degree of sharpening using an unsharp mask is controlled by amount, radius and threshold. Amount controls the increase in contrast at the edges, radius controls the scale of detail that is sharpened (the radius relates to the degree of blur added in the unsharp mask - the bigger this is the more detail that is lost) and threshold the existing level of edge contrast that is required before sharpening occurs (in a portrait, a higher threshold will sharpen eyelids and lashes without touching the skin).

An important point to consider is that the best combination of amount and radius to sharpen an image depends upon whether it is destined for screen or print. For screen it's better to have a high amount with a low radius to bring out fine detail, but for print a lower amount with a larger radius to reduce the spread of ink (larger amount means more ink).

Feel free to post comments with any questions you may have.

I'm off to photograph a wedding at St Augustine's Priory in Bilsington, near Ashford, this afternoon. After a very overcast start to the day, the weather is now looking promising - fingers crossed.

Check out my photography here: Kent wedding photographer


Anonymous said...

Your photographs are beautifully clear and crisp and i read your older posts on Unsharp Mask. I wondered if you still use Unsharp Mask to help with such clarity and sharpness and what settings you use (Amount, Radius, Threshold). Thank you.

David said...

Many thanks for your lovely comments.

Yes, I do still use unsharp mask with the following settings:

Amount 100%, Radius 1 pixel, Threshold 0

Bear in mind that all the images you see on the blog have been shot with L lenses and either a 5D Mark III or Mark II - so they're looking pretty crisp and clear even before sharpening!

All the best,