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Why you must sharpen images

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Why you need to sharpen images

Sharpening is a vital step for processing images – why?

Keith looks at some of the reasons he considers sharpening (in all its forms) as a vital tool in editing images for the best quality, whether for web or print.

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Why images from your camera need sharpening

An awful lot of uninformed comment about the need for and desirability of image sharpening clutters up forums.

moonrise at old john

Moonrise – only one part of this image needed -any- sharpening

Myths and misunderstanding spread far and wide, and the temptation to ignore image sharpening, because it’s contentious, is easy to understand.

Well, my own view as a professional photographer and printmaker is that it is an integral part of my work, indeed its understanding and application is up there with colour management as something that can really take you work up a significant level.

There are more practical details in the linked articles.

Why detail is lost

The many stages, from lens focusing an image on a sensor, through to print all involve steps that (in different ways) soften or degrade an image. Note that this applies both to digital and chemical based (film) approaches.

Some of these stages can be compensated for (to varying extents) by processing options and in digital areas, through the application of complex numerical processing to lessen the appearance of some softening.

But surely, if I just get a really sharp lens?

Lens performance is a complex area, in that some forms of deviation from a mathematically perfect optical system may be more apparent than others. Sharp is  one of those freely used terms that similarly requires additional parameters to have any useful meaning ;-)

This variety of ways of ‘fixing’ a lens design may trade off against each other in different ways, and match different tastes in what is important to a user of the lens. 
All practical lens designs have different compromises – the importance of which depend on the photographer and the depth of their pockets.

Once the image is focused on a sensor, there are issues caused by the way that it is sampled by the sensor.

In general, the sampling lowers image quality, although this varies with the nature of the image (detail/contrast/colour)

Then there is the processing (in camera) of data from the sensor, whether the relatively light processing for RAW files of the broader range of processing for JPEG files). This further affects aspects of image quality (which includes aspects of sharpness)

Subsequent image processing can address many of the issues in the chain so far, but with the general observation that once image information is lost it is difficult to usefully recover – I don’t say impossible, given some of the advances in image processing over recent years (one more reason to keep your RAW files).

My lens MTF charts look good…

I’d note that the perception of image detail is not the same thing as any measuring it by some method (test charts for example). Such measurements can help, but are not much more use to most people than choosing the family car by its 0-60 time…

One of my more expensive lenses is the Canon TS-E17mm shift lens – the internal design and MTF chart were not considerations in my purchase. Just what I could do with it.

MTF chart and internal design of TS-E17 lens

MTF chart and internal design of TS-E17 lens

Once you add human perception into the mix, things can rapidly get more complex (this isn’t just a digital issue – think of high acutance developers and edge contrast enhancement in some emulsions)

Lastly if you print the image, then the whole process tends to further degrade image quality, both from the limited tonal range and gamut of printed media and the way that ink droplets are applied (and soak into) paper (aspects of ‘dot gain’).

Keith’s approach to print ‘sharpness’

My own ‘optimum’ approach is to pay attention to the entire chain from light going into the lens, through to print coming out of the printer and the viewing conditions it will be displayed in.

I’ve come across people happy to spend large sums on prime lenses, yet remain unaware of the importance of various types (and amounts) of sharpening in the workflow.

I’m of the opinion that one of the most effective areas to master, if you are making decent sized prints is sharpening and when to apply (and importantly, not apply) it to your images.

One potential tool – Nik Sharpener Pro 3

I’ve used Nik for several years and have written reviews of the last two major releases, that include numerous examples

I’ve been asked about why I use such a tool, rather than the normal tools in Photoshop?

For me it’s about consistency – having a tool that produces a predictable range of results. That and experimentation to see how what might look wrong on the screen (‘over sharpened’) can look really good in a large print.

Sharpening is another one of those areas where what looks best on screen does not necessarily look best as the print.

I make prints for other people to look at, and know that many photographers can be poor judges of what’s best in a print. During my print testing I regularly ask other people to look at prints and tell me what they like/dislike. Sharpness rarely gets a mention until I put a carefully sharpened version next to a straight print.

By ‘carefully sharpened’, I’m including masked sharpening (why sharpen blue sky or strongly out of focus areas for example) and a slight boost in aspects of local contrast enhancement (structure in Nik Sharpener) which can help compensate for some of the reduced tonal range of paper.

Comments converge on things like ‘depth’ or that the print is ‘clearer’. There is also a perception of the print being more detailed. Note that these are real people, and they don’t get loupes out or immediately shove their noses up into prints ;-)

Most people tend to associate ‘over sharpening’ with edge effects, but so note that it can be obtrusive even in broader contrast enhancement. There are examples of this (halos) in two recent articles:

Personally I don’t mind seeing a few sharpening artefacts up close, since I know that I’m not creating a 36″x24″ print for close up study. But, and this is a big but, because I do a lot of print work, I’m attuned to noticing such things, and what I regard as noticeable might be missed by many.  The example of distant snow patches on mountains, I use in the Nik review, would be one example.

In making large prints, sharpening is a critical element of just how the print will look to an audience viewing it on a wall. See my notes on Making large prints from low megapixel images for more.

Only the Moon in the image below required any sharpening whatsoever (‘capture’ or ‘creative’, or ‘output’) The lens was a 1080mm mirror lens, so any sharpening would seriously affect the out of focus parts of the building, and increase noise in what was a flat blue sky (click on image to see a much bigger version)

selective sharpening of moonrise image

Moon rising behind building, only the moon is sharpened.

Many of the sharpening software effects can be replicated with a combination of filtering and layered masks in Photoshop. However I don’t do this every day, so I’d need to remind myself of all the steps every time.  There is nothing wrong in doing it all in PS, I’m just not that much of a PS guru to remember all the techniques ;-)

The important thing from my own POV is that you have to be prepared to experiment and decide for yourself what works and what doesn’t.

A3+ sized PRO-10 printer

Printer testing – some of my standard ‘test’ images

Five A4 test prints of part of a larger image, created with different approaches to sharpening, are not five wasted bits of expensive paper (and ink), they are a guide to getting better, more predictable and consistent results in the future.

The problem is that a significant number of people want a 1->2->3->4->5 absolute series of steps in how to process images (a recipe if you will). Stepping off the path is dangerous, so they miss the opportunity to explore the forest and see what’s over the ridge …and yes, to anyone who knows my dislike of camping, I am aware of the irony of that analogy ;-)

The results?

My own photography stands best on two legs – one the importance of -really- understanding the technology and how it does things to images, and secondly the creative part that decides to capture the image, which bits of the technology matter, and envisions what my final result will be.

All my best work comes from both working together. 

I’ve heard some say that understanding the technology detracts from the art of photography – I no more accept this than think that understanding Rayleigh scattering damages my appreciation of a beautiful sunset.

It’s when the technology becomes an end in itself that you end up with technically excellent pictures that no-one wants to look at a second time.

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