Lens Reviews often provide the most accurate and comparable idea of how sharp lenses are since they tend to provide standardised data on lens characteristics. Specific distortion percentages, measurements on the exact EV loss due to vignetting and sharpness ‘blur’ units make it easy to get an idea of how the lens is likely to perform and how it compares to other lenses. The only problem is that sometimes there is variation on performance between different samples of the same lens and it’s necessary to know how to test your lens by yourself without necessarily resorting to shooting MTF charts and complex measurements. This is particularly useful if you are suspicious of the shots your camera is taking.

The most important characteristic to measure is sharpness and this article addresses the three checks you should perform to check sharpness performance. When performing these tests, you should keep your ISO setting at between 100-200 so as not to introduce image noise due to high ISO that may confuse the results of your test.

In evaluating the tests it is important to note that you may discover that your lens does produce noticeable shifts in sharpness. To some extent it’s a case of you pay for what you get. It’s not reasonable to expect pin-point sharpness from a cheaply manufactured lens. One expects to get better sharpness from a more expensive lens that is marketed as being a professional choice but bear in mind that there is not a direct relationship between sharpness expectations and expense. Razor-sharp super-wide zooms are far more difficult and expensive to manufacture than smaller zooms or fixed focal lenses. Also, while another lens may be sharper and cheaper, it may produce more distortion or chromatic abberation. Being able to evaluate the level of sharpness of your lens though is of value. You can always go online and compare other people’s tests of the same lens to evaluate whether you’ve got a bad model. You’ll also understand the limitation of your lens and what setting you should adopt under given circumstances to produce your desired result.

Edge Sharpness

The centre of your picture will always be sharper than the edges of the frame. Choose a middle aperture like f5.6 or f8 on a distant subject that you can focus on, preferably at infinity. If it’s a zoom, set your focal length to mid-way of the range that it covers. Make sure there are objects with a similar amount of detail in the top, right, left, bottom and center areas of the frame. A landscape or city-scape are ideal subjects. It’s important that the objects in all these areas of the frame are at a similar enough distance to all be in-focus.

Compare the edges of your frame against one another and the centre point to determine:

• the overall loss of sharpness from centre point to edge
• whether any edge is particularly less sharp than the others – this could point to a manufacturing defect or at the very least make you aware of the impact it will have on your pictures – something you can plan against.

Centre vs Edge Sharpness on the Canon 70-200mm f2.8 L IS USM

On top is a reduced size image of the full frame taken by the camera. The red squares show the areas that were enlarged to 100%. These areas, taken at the edges of the frame most likely to show any edge focus detrioration are compared to the enlarged area of the centre of the frame (the area that is most in focus). This lens performs superbly as no difference in focus is discernable from centre to edge. What is also of significance is that edge-to-edge brightness is fantastically consistent with no perceptible vignetting. (to the right) Area cropped from the centre of the frame enlarged to 100%. While it seems just slightly off pin-prick sharp, it cannot be described in any way as soft.
(above) Area cropped from the top left of the frame enlarged to 100%. No discernable increase in softness.	(above) Area cropped from the top right of the frame enlarged to 100%. No discernable increase in softness.
(above) Area cropped from the bottom left of the frame enlarged to 100%. No discernable increase in softness.	(above) Area cropped from the bottom right of the frame enlarged to 100%.No discernable increase in softness.

Loss of sharpness at different apertures

At the largest apertures – f1.8, f2, f2.8 f4, depth of field is very limited and focus will be difficult to evaluate since only objects within a small range of your focus point will be sharply in-focus. At the small apertures like f16, f22, f32, your lens will start to experience a loss of sharpness due to diffraction.

How diffraction affects sharpness

Light passing through the centre of the aperture continues straight on its path to the sensor. The diaphgram blades that define the size of the aperture affect the light passing through them by dispersing the light that hits them – effectively bending it outwards past the aperture hole. At large apertures this has less of an effect on image quality because the amount of light passing through the centre compared to that passing at the edge is relatively minute. At smaller apertures, the amount of light passing at the edge is proportionately more, causing focus to be affected more at the smaller end of the aperture range. Normal expectations of acceptable sharpness at the smaller aperture range are f22 on full frame sensors and f16 on APS-C size sensors.

Images demonstrating the aperture test

	(to the left) Cropped area from a photo shot at f2.8 enlarged to 100%
	(to the left) Cropped area from a photo shot at f8 enlarged to 100% Increased depth of field makes this shot appear very slighly more in-focus.
	(to the left) Cropped area from a photo shot at f16 enlarged to 100%. Slight degradation of the image perceptible compared to the f8 shot.

Loss of sharpness at different focal lengths

Obviously this consideration only applies to zoom lenses. A zoom lens does not produce a consistent level of sharpness across the entire focal length range. Normally the variation is quite minimal, but it is more likely to be softer near the start of the focal length range and at the end of the focal length range. Eg a 18-200mm lens is likely to be slightly softer closer to the 18mm mark and the 200mm mark than in the middle of the zoom range. Compare results at the start, middle and end of the zoom range holding your aperture in the middle of it’s range.

Images demonstrating the focal length test

	(to the left) Cropped area from a photo shot at 70mm enlarged to 100%. Settings were ISO 100, shutter speed 1/800, f5.6
	(to the left) Cropped area from a photo shot at 135mm enlarged to 100%. Settings were ISO 100, shutter speed 1/800, f5.6
	(to the left) Cropped area from a photo shot at 200mm enlarged to 100%. Settings were ISO 100, shutter speed 1/800, f5.6