Wednesday, 10 February 2010

Using EF Extenders

There are plenty of comments in the press that claim that the use of lens extenders is a waste of time or, at the very least, that they significant loss of quality. I wanted to see what all the fuss was about so, seeing as I was about to head off to Tanzania to shoot African game, decided to bite the bullet so to speak and try a 1.4X and a 2X lens EF lens extender for myself. (Click the photos to see full frame)

One negative feature is undeniable: use a 1.4X and you lose a full f-stop of light. Use a 2X extender and you lose two f-stops. So, before you rush to buy one of these quite expensive accessories, consider this loss of light, and therefore the possible implications to the shutter speed (and therefore to camera shake). You should also note that, with Canon products anyway, these accessories are designed specifically for 'fast' lenses. They don't work on 'regular' glass, only on lenses with fast f4 or wider apertures.

Initial tests were done using a Canon EF100-400 f4.5-5.6L IS USM lens. The manual for this product states that this works OK with the EF1.4X and EF2X extenders, but only with manual focusing. This is because lenses need light to 'see' to focus, at least enough to drive a minimum aperture of f5.6. As this is a variable aperture lens, once the 1.4X converter is fitted, the focal length becomes 140-560mm and the variable maximum apertures change, from f4.5-5.6, to f6.3 - 8, not fast enough for accurate auto focusing. With the EF2X Extender the scenario changes again, boosting the focal length to a massive 200-800mm with a scary maximum aperture range of f9 - 11.
If you have trouble seeing to focus manually, you'll have a hard time, even with the 1.4X converter. With practice, and with the help of a tripod, you'll be able to get some pretty good, sharp results.

Shooting Tips: Lens magnification varies between APS sensor cameras with a 1.6X magnification and full frame same sensors. Also the more you magnify the image, the more image problems you might see. Especially with camera shake and focus errors. To ensure sharp, clear results, always choose a shutter speed that's at least double the focal length. This means that using an APS sensor camera, you should be shooting at 1/1600s at full. And if the subject is moving fast, choose an even faster shutter speed. Because you generally can't open the aperture further to get a faster shutter speed, raise the ISO to suit.

Second lens I tried was the EF70-200mm f2.8L USM, the standard pro Canon workhorse. Both extenders worked perfectly with this faster lens. AF response is slightly compromised but, you'd be hard-pressed to quantify by how much. It's still very snappy. The viewfinder also goes a touch darker, but again, it's a negligible change.

In Use
Because of the slight
ly negative press I've seen on these extenders, I was expecting average results. I won't say using them is easy, especially if you have to focus manually, but the results appear excellent. What exactly does 'excellent' mean?

Aside from a heavier and more cumbersome lens, my chief concern was image sharpness. Sharpness is not the tack-sharp look you get with the straight EF70-200mm lens but the results are pretty good - and of course, a bit of Unsharp Masking puts most of that sharpness back into the file. I was most impressed with what I got. You will find a loss in quality if focus is not accurate and if the shutter speeds are not fast. This probably has as much to do with overall quality loss as the added 'glass'.

On closer inspection, the 2X extender does introduce some flare and some colour fringing, especially towards the edges of the sharper parts of the frame (i.e. just at the edges of the sharpest part of the focused subject). In some examples it looks a bit like the diffusion you'd get if there was a slight smear of grease on the outer lens coating.

Conclusion: Considering that you get two or more lens combinations for the price of an inexpensive ($600) extender, I think that the slight quality loss seen in the examples is eminently bearable.

Extender Negatives:
Slight softening of image
Some colour fringing along edges of focused subject
Bigger, physically longer lens
Added weight to your regular lens

Extender Positives:

Significantly cheaper option to buying a bigger magnification lens
Significantly lighter
Physically smaller than the lens it replaces
Although some loss of sharpness is recorded, much can be reinstated using an Unsharp Mask

Consider this: an EF800mm lens costs in excess of $11,000. You work it out! (Please note that all pictures here are presented full-frame, shot using a 2X Extender and are not retouched)

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