I shared this post on a recent Instagram story and received a couple of questions from followers asking about the use of converters.
The most obvious benefit of using a teleconverter on a lens is that it multiplies your focal length - duh. This allows you to transform something like a 400mm lens into 560mm (1.4x), 680mm (1.7x) or 800mm (2x). The benefit of this is that you can achieve your ideal composition without having to crop and loose image resolution (pixels). I will often slap on a 2x converter when I am looking for a slightly more abstract and "artistic" composition and even then, find myself wanting a bit more focal length from time to time.
The second benefit relates to cost, lets say you have a 300mm F2.8 lens and a 2x converter which gives you a focal length of 600mm. This combination would be a lot more cost effective than purchasing a 600mm lens. Yes there is a down-side to this, hang tight, I'm getting there.
Staying with our 300mm F2.8 and 2x converter combination, I'm sure you'll agree that this is both a lot lighter and more compact when compared to a 600mm lens. If you apply this to a 400mm F2.8 with a 1.4x and 2x in the bag you have focal lengths of 400mm, 560mm and 800mm, in a VERY compact and lightweight setup. Happy days!
A little known benefit teleconverters is that you can keep the minimum focus length of your lens. This can be handy when you don’t have a macro lens handy and want to get close in on a subject that isn’t far away. It won’t really compare to a dedicated macro lens – but can be handy.
So, with all of these pro's, why would one not want to make use of teleconverters?
The first and most important "downside" to using converters is that the application of any teleconverter to a lens will change the maximum aperture. So, if you have a 300mm F2.8 lens and apply a 1.4 x or 2x converter, you maximum aperture will change from F2.8 to F4.0 and F5.6 respectively. Not bad at all on an F2.8 lens BUT, when applied to a lens with a maximum aperture of F4.5 to start of with we are starting to get into a bit of trouble.
Adding a 1.4x converter to something like the Canon 100-400mm F4.5 to 5.6 lens, this means you will have a maximum aperture of F8. This results in a couple of important changes which you need to be aware of:
- As aperture determines the range of acceptable sharpness or depth of field in an image, you are essentially compromising your ability to "blur" the background and isolate your subject effectively despite having an increased focal length.
- Adding glass in front of glass will always have an impact on the image quality, often resulting in soft images, which aren't tack sharp, particularly when shot "wide-open" at F8. Teleconverters multiply not only the focal length but also any aberrations of the lens you pair it with.
- The camera will ALWAYS use the maximum aperture to achieve focus. Try this quickly, open your eyes as wide as possible. Imagine this is F2.8. Now relax them. This would be the equivalent of F4.5. Now shut your eyes down as much as possible whilst still being able to see. This is F8. Without going into too much technical detail, when you use a converter you are reducing the amount of light which falls onto your cameras AF sensor which in turn makes it more difficult for the AF point to pick up on edges of contrast used to achieve autofocus in a scene. It's like reaching out for your alarm clock first thing in the morning - you end up stumbling a round for a while before finding the dam thing. Taking this even further, all focus points on your camera are not equal with the centre focal point being the most advanced and accurate AF point. Most cameras don't even have a central AF point capable of detecting enough contrast to achieve autofocus at F8.
Shooting at greater focal lengths also means you'll need faster shutter speeds to ensure sharp,crips images. Not a problem given that we can crank ISO but, with a maximum aperture of F8 you'll find yourself shooting at much higher ISO values than you're used to to ensure crisp, sharp images.
If not, here is a recap.
Adding a converter to any lens will reduce the speed at which the camera will be able to achieve (and therefore track) focus, will have a negative impact on image quality (especially when using the maximum aperture) and will decrease your ability to capture soft, creamy backgrounds.
So, we have basically gone from looking at how converters are the best things ever invented to thinking about why one would ever want to use them.
Like everything, there is a time and place to make use of converters.
When do I use converters?
- On Prime Lenses. I'm fortunate enough to shoot on top end bodies and lenses which have a maximum aperture of F2.8. Understanding the changes that take place on these lenses when using 1.4x and 2x converters I am still happy to make use of them BUT I would not want to use a converter on any lens with a maximum aperture of greater than F2.8 (with the 500 and 600mm f4.0 being the only exceptions).
- In good light. The sacrifice of maximum aperture (think back to the squinting exercise) is almost negated to some extent in good light. Shoot at a maximum aperture of F8 in poor light and you'll not only struggle to find autofocus but you'll end up shooting at VERY high ISO values in order to get a shutter speed that will allow you to eliminate camera shake.
- With Static Subjects. Subjects that aren't moving don't require the camera to continuously achieve AF and you can even use manual focus.
- When I'm intentionally trying to capture movement. Autofocus issues aside, when you're trying to capture movement and use slow shutter speeds you'll want to reduce the amount of light entering the camera as much as possible. Think back to the squinting exercise again and a 2x converter will become your best friend in these situations. Not only does it reduce the amount of light allowing you to make use of a slower shutter speed but it also increases your focal length, helping to exaggerate any movement you make.
- When the ideal composition can only be achieved through an increased focal length. These lions were FAR away from us and this is a full frame image captured at 800mm. Had I only been able to shoot at 400mm the image quality of the resultant crop (to eliminate sky and other distracting elements) would been compromised.
I hope you'll find this useful in getting to grips with the pro's and cons of using teleconverters. Knowing where the sacrifices and compromises are made will help you to decide whether its worth your while to add a converter or not.