How to buy the best digital SLR lens for your camera

Lenses are for looking through, not at, but if the kit lens that came with your camera is not giving you Visit page the best view, it’s probably time to get some new glass. Our guide will give you advice on how to find a good value lens that suits your needs. There’s no current review of this product.

The mount is the connection between the lens and the camera and it’s important to make sure they are compatible. This may not be a problem if you’re buying a lens from the same brand and designed for your camera, but that may not be your best option.

Even within brands some lenses will fit a number of different camera bodies (full frame, FourThirds, APS-C, etc.), but will behave differently. Other lenses are only compatible with certain bodies, even if the mounts match. An APS-C optimised lens can mount on a full frame, for example, but photo quality will suffer. Always check to make sure you’re getting what you expect.

Third-party brands like Sigma, Tamron and Carl Zeiss (to name just a few) produce lenses which can be used with a number of camera brands, and sometimes they can be very good value for money. Just make sure that all the lens and camera functions will work together.

High quality older lenses may fit your camera and do a good job, but auto exposure and focus may not work with some newer bodies. Some manufacturers have compatibility lists online to show which of their lenses will work with their latest bodies. If you’re uncertain, try using the lens you’re considering on a body before buying.

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Falling off at the edges
Photography is a minefield of jargon and never more so than when it comes to optics. You’ll see MTF (modulation transfer function) curves used on many websites – both manufacturers’ and comparison sites – to supposedly give empirical evidence that one lens is better at capturing detail than another. For the most part you should ignore them.

MTF curves are a measure of the lenses’ ability to resolve detail, but the devil is in the detail. If you don’t know and understand the exact parameters of the test, the MTF graph is just lines on a page. Also, if you’re looking at one manufacturer’s graphs against another’s, it’s probable that the lenses were tested under different conditions, so they’re pretty much a waste of time.

To be truly useful MTF measurements need to be done under the same conditions, at the same settings for each lens and ideally on the same camera body. It you want a detailed technical explanation of how MTF curves work try putting “understanding MTF curves” in a search engine.

What else to look for in a digital camera lens
Contrast and compare
If a lens is described as being “high contrast” it means images will look sharp because edges are clearly defined. This generally is a good thing, but don’t confuse it with the lens’s ability to resolve detail. That’s what MTF is for.

Out of shape
Every lens distorts, some distort more than others.

There are three basic types:

Barrel: the middle of the edges of the image bulge outwards.

Chart showing barrel distortion
Pin cushion: where the middle of the edges of the image curve inwards.

Chart showing pin cushion distortion
Wave: a combination of barrel and pin cushion and the hardest to correct.

Chart showing wave distortion
The best way to check this is to take an image with lots of vertical and horizontal lines near the edges. Modern buildings are often useful if you’re not in the habit of carrying a test chart with you. Certain types of lens distortion can have stylistic benefits, however.

Minor distortion is rarely a major issue, but some cheaper lenses can have very ugly waves in them. Most photo editing programs include lens correction tools.

Dark edges
It’s called vignetting when the edge of the image is significantly darker than the middle. Wide angle lenses are the worst culprits, but some telephoto lenses suffer as well.

It’s not always a bad thing. Some images look better if the edges are a little darker, but you can always add this effect later using software.