• May 17, 2020

What Affects Image Sharpness?

What Affects Image Sharpness?

What Affects Image Sharpness? 1024 683 David Taylor

Go on admit it. There have been times when you’ve been tempted to buy a lens just because reviews have praised how sharp it is. I know I have. (There are other things I’d admit to, but we’d have to get to know each other a little better first.) If you’ve got pockets that are deep – really deep – you could spend a small fortune on the latest and greatest lenses. But do you know what? That doesn’t necessarily mean that your photos will be any sharper. How sharp a photo is depends on a number of factors…

Getting sharp images often requires ingenuity. This photo was shot in a dark museum and required holding the camera gently but firmly against a glass case.

Shutter speed

If you’re handholding your camera then shutter speed is going to have an influence over the sharpness of your photos. Steadiness naturally varies a lot between individuals, and is also affected by factors such as whether it’s windy or how soft the ground is underfoot. Even breathing has an effect. Trying to hold a camera steady is far more difficult if you’ve just finished a fast walk than it is when you’re fully rested.


It’s much easier to handhold a wide-angle lens and get sharp results than a telephoto lens.

The slower the shutter speed you use, the greater the chance that a photo will be unsharp. The likelihood of camera shake also increases the longer the focal length of lens you use. Finding the right balance between shutter speed and focal length might seem tricky but fortunately there’s a very simple – if rough and ready – rule: set the shutter speed so that it matches or exceeds the focal length of the lens. So, for example, if you’re using a 50mm (or full-frame equivalent) lens you should use a shutter speed of at least 1/50 or faster. Lower than this runs the risk of camera shake. (Image stabilisation, whether lens or sensor-based, does muddy the water a bit, allowing you to use lower shutter speeds than the rule would advise. But it’s still a good rule to bear in mind.)


In low light I regularly use a high ISO to maintain a fast shutter speed. It’s easier to remove any image noise later than rescue an image suffering from camera shake.

There’s no reason for regular tripod users to feel smug at this point though. Tripods are very, very useful tools to keep a camera steady during an exposure. However, they – like any tool – have to be used correctly to obtain the maximum benefit. Set a tripod up on soft ground – sand, for instance – and it can sink during an exposure. Walk around during the exposure and your movements can cause the tripod to move too.

Setting a tripod up correctly is important too. Ideally, keep the use of the centre column to a minimum. And keep the centre column as close to perpendicular as possible. (Seeing a tripod erected at a crazy angle, so that it’s liable to tip over, brings me out in a cold sweat…) If you’re on a slope, open the legs at different lengths to compensate for the slope.

Aperture

The aperture of a lens controls depth of field. As a general rule, lenses are at their very best optically when the aperture is set to a mid-range value – f/5.6 to f/8. Use a smaller aperture than this and you run into the problem of diffraction. Diffraction causes the resolution of the lens to drop resulting in a softer photo than expected. Ironically, it’s common to see photographers stop down to f/22 in order to maximise depth of field, only to create a photo that’s as mushy and mashed potato. If you’re striving to achieve crisp images from front to back, then use one of those mid-range apertures if possible and focus carefully to maximise the available depth of field at that aperture. (Using an app to calculate the hyperfocal distance helps, particularly when shooting landscapes.)

At maximum aperture depth of field is at its lowest. This means that focusing has to be as accurate as possible, particularly when using telephoto lenses. Focusing systems are continually improving and so it’s rare to get focusing errors but they still happen. One common error is back or front focusing – when the lens focuses slight behind or in front of the intended target. For technical reasons this is more likely to happen on a DSLR than a mirrorless camera. DSLRs often have an option to fine-tune focusing – typically found on either the Shooting or Settings menus. Before you go calibrating lenses it’s worth trying the same lens on a different camera if possible. If the same error occurs on another camera then it might just be user error rather than a fault of the camera’s focusing system.


Shoot at maximum aperture and you need to decide what should be precisely in focus. This should always be the centre of interest of the shot.

Optically, a lens is never at its best at maximum aperture. Typically, the centre of a lens is sharper than the corners. It’s only when the lens is stopped down to those mid-range apertures that the corners start to approach the level of sharpness displayed by the centre.

Viewing distance

‘Pixel peeper’ is the slightly disparaging term for someone who obsessively views their images at 100% – or higher – on their monitor. This can be a dangerous obsession. Images at that magnification can look disappointingly soft. But consider this: a 24mp photo seen at 100% on a reasonably decent monitor would be the equivalent of viewing a 75 x 50cm print from roughly half a metre away. Would you look at a print this closely? Probably not. You’d stand further back. In stepping back the print would look sharper. Weirdly, distance increases the apparent sharpness of images. Don’t believe me? Then try viewing the two images below, first from a normal viewing distance and then – if possible – from across the room. I bet both looked equally sharp from over there. So, if a photo doesn’t look pin-sharp at 100% then all may not be lost. Perhaps that news lens was worth it after all…


At a certain distance, both Marilyns will look equally sharp.

The post What Affects Image Sharpness? appeared first on Telephoto.com.

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