Most camera systems have at least two teleconverters (“TC”) – a 1.4x and a 2x power (new camera systems such as Canon R and Nikon Z are yet to develop these).
These little devices are designed to sit between the camera and the lens and contain optical lenses which magnify your image according to which strength you have.
These are also called tele-extenders – not to be confused with a macro extension tubes which have a different function – that of allowing more magnification by allowing you to focus more closely.
Benefits of teleconverters
These are light, small and relatively inexpensive (although high quality ones may set you back $500-$1000 – but this may still be cheaper than buying a more powerful lens and certainly lighter and more compact than carrying two lenses).
A 1.4x teleconverter will give you a bit more “zoom” effect, so your 200mm lens effectively becomes 1.4 x 200 = 280mm in focal length.
A 2.0x teleconverter will give you a lot more “zoom” effect, so your 200mm lens effectively becomes 2 x 200 = 400mm in focal length.
This increased magnification can also be very handy for close up macrophotography work where magnification may be important, for instance the Olympus 300mm f/4 lens when used with the Olympus MC-20 2x TC can give almost 1:1 macro in full frame terms at its close focus of just under 1.5m which is great for shooting insects without scaring them too much!
It can prevent dust entering your sensor in adverse conditions if you have two telephoto lenses which you interchange but leave the teleconverter in place.
Some teleconverters are designed so that they can be “stacked” to multiply the effect but this also multiplies the cons. Most modern ones can’t be stacked as they are designed to be used only with certain lenses and they have a protuberant inner component which prevents stacking being possible.
Cons of teleconverters
As I repeatedly state on my blog posts, EVERYTHING in photography is a compromise, in this case you get extra magnification with minimal weight and size, but there are MANY downsides to this.
- your aperture is reduced
- a 1.4x TC reduced your aperture by 1.4x (ie. 1 f stop) so that your f/2.8 lens becomes f/4
- a 2x TC reduced your aperture by 2x (ie. 2 f stops) so that your f/2.8 lens becomes f/5.6
- this mans that you may need to increase your ISO (and lose image quality) and / or, slow down your shutter speed (and increase camera shake or subject movement blur)
- your image quality is reduced
- adding extra optical elements is almost certain to degrade your image quality although the high performance modern TCs keep this to a minimum (perhaps a reduction of 5-15% with prime lenses and 15-25% with zoom lenses), but you still may in effect lose half a stop or so of sharpness, requiring you to stop your aperture down to obtain optimum sharpness at the expense of ISO or shutter speed.
- TC’s are prone to increasing distortion, coma, astigmatism, spherical aberration and chromatic aberration, especially when mated with complex zoom lenses
- TCs lower contrast due to adding reflective surfaces.
- if your aperture is f/8 or smaller, diffraction issues may further reduce sharpness.
- your autofocus speed is likely to be reduced, especially in low light
- the reduction in light transmission means the AF sensors will have a more difficult time, especially in low light levels
- many dSLR PDAF cross-points either cease to function or lose their cross-point capability at f/5.6 or smaller meaning you may have to resort to only using the centre point.
- you may actually lose AF capability
- some cameras (especially dSLRs) are not able to AF if the wide open aperture is smaller than f/8 and some will only be able to AF using the centre point at f/8 – this issue has largely been eradicated with mirrorless cameras
- its another element that may cause failure
- the extra element may cause failure of weathersealing, failure of electronic communication between camera and lens, and extra wobble which may contribute to the above as well as causing optical misalignment issues.
- you may only be able to use it with certain lenses
- most modern TC’s can only be used with certain telephoto lenses – usually the expensive “pro” lenses
- it may alter the biomechanical ergonomics
- the extra distance from camera to lens may make a heavy lens feel even more heavy due to the physics of levers.
What are the alternatives?
Essentially you only have two alternatives if you need the extra magnification.
Crop your image:
Cropping your image has MANY benefits over TCs – you get to have your normal AF capability, your normal ISO and shutter speed to optimise image quality (no point using a TC to get extra magnification if the image is blurred from longer shutter speed, you couldn’t lock focus or the higher ISO impacted image quality too much).
To gain the same effect as a 1.4x TC, you will lose half of your pixels so that your 20 megapixel image becomes 10 megapixels – still plenty for most purposes. This is my preferred approach.
To gain the same effect as a 2x TC, you need to lose 75% of your pixels so that your 20 megapixel image becomes 5 megapixels – perhaps enough for some purposes – but you would probably better having a 20mp 2x cropped sensor camera in the first place instead of carrying a heavy, expensive full frame lens around and only using 25% of its image capabilities and those 25% of pixels are probably not going to be as sharp as a dedicated Micro Four Thirds lens which is optimised for such cropping.
Buy a more powerful lens:
This may actually be more cost effective than an expensive 2x teleconverter and provide at least as good a result even if the lens is not a “pro” lens.
A good example is when you try to mate a very good zoom lens such as the Olympus 40-150mm f/2.8 with an excellent 2x TC and you get similar results as a more powerful but lower aperture zoom lens such as the Panasonic 100-300mm f/4-5.6II lens and you may get better AF in the process. You may find that you will prefer to have both lenses rather than buy the 2x TC.
Unfortunately, buying a more powerful lens may not be possible – if you are already using the biggest, most expensive lens you can afford – then using a TC or cropping is the only options you have left, but if your big lens is a prime lens with wide aperture then you may still get superb results with a TC specifically designed to be mated with that lens.
Since writing this post, I discovered a great YouTube discussion by Steve Perry based on his experiences with Nikon dSLRs and super-telephotos.
He concludes that using a 20mp 1.5x cropped sensor dSLR will give better results than a 20mp full frame dSLR with a 1.4x TC when using the same lens wide open. The TC not only degrades sharpness by 5-15%, but you lose contrast, lose any ISO advantage of full frame due to the loss of 1 stop aperture, and you may lose most of your PDAF AF cross-points at f/5.6 making your AF more problematic. If you try to get sharper images by stopping down then you run into image degradation by high ISO and potentially diffraction aberrations.
There is no logical reason to assume that the above does not also extend to using a 20mp Micro Four Thirds camera vs a 20mp full frame camera with a 2x TC and a similar focal length and aperture lens.