1.4x and 2x teleconverters: are they worth it for spotters?

The Airbus is taxiing at the far end of the apron, your lens is maxed out – and on the display there is a small white speck in a lot of asphalt. That is exactly the moment when many spotters think of a teleconverter. For modest money it promises 40 or even 100 percent more focal length. Whether that pays off depends less on the converter itself than on your lens, your camera and the conditions at the fence. This guide explains what a 1.4x and a 2x converter really cost, when they bring you more than cropping and when you’re better off putting your money into a different lens.

Is a teleconverter worth it? The short answer

A teleconverter is worth it if you own a sharp, fast telephoto lens that is officially approved for the converter and regularly stand at spots where you lack focal length. In that combination, the 1.4x converter is the best investment per euro for many spotters: one stop less light, slightly slower autofocus, but considerably more reach. The 2x converter is a specialist tool for very good light and first-class optics. On a slow superzoom it usually delivers less than you hope – there, cropping is often equivalent.

How a teleconverter works

A teleconverter is a small lens group that you place between camera and lens. At Canon it is called an “extender,” at Nikon a “teleconverter”; the principle is the same everywhere: it magnifies the image your lens produces by a fixed factor. 400 mm becomes 560 mm with a 1.4x converter and 800 mm with a 2x converter.

The catch lies in physics. The converter doesn’t create additional light, it distributes the available light over a larger area. The effective f-number therefore rises by the same factor as the focal length. With 1.4x only about half the light still arrives, with 2x a quarter. Expressed in stops: one stop with 1.4x, two stops with 2x.

Lens	with 1.4x	with 2x
70–200 mm f/2.8	98–280 mm f/4	140–400 mm f/5.6
300 mm f/4	420 mm f/5.6	600 mm f/8
100–400 mm f/5.6 (long end)	560 mm f/8	800 mm f/11
150–600 mm f/6.3 (long end)	840 mm f/9	1,200 mm f/13

The table quickly shows why the starting lens matters so much. A 70–200 f/2.8 with a 1.4x converter is still a very usable 280 mm telephoto at f/4. A 150–600 with a 2x converter, by contrast, ends up at f/13 – in spotting weather in Central Europe often an ISO problem.

Besides the light, the converter also magnifies all of the lens’s weaknesses. Softness, color fringing and soft contrast are blown up along with everything else. That’s why the old rule of thumb applies: A converter makes a good lens longer, but doesn’t make a weak one better.

Compatibility: the first check before buying

Before you think about image quality, settle the basic question: does the converter fit at all? The camera makers’ converters are almost always approved only for a certain selection of telephoto lenses. Many have a protruding front element that reaches deep into the lens. On a lens it isn’t intended for, it can touch the rear element or not lock on at all.

What to look for:

• The manufacturer’s approval list: Canon, Nikon, Sony and others publish which lenses work with their converters. This list is binding, not forum opinions.
• Restrictions on zooms: Some zooms can be used with a converter only in the upper focal length range, because the lens elements would otherwise come too close together at the short end. If the manual says so, stick to it.
• Third parties: Sigma and Tamron offer their own converters, which as a rule work only with their own telephotos intended for them.
• Image stabilizer and camera functions: Depending on the combination, the stabilizer or individual autofocus functions run with limitations when a converter is attached. Your lens’s manual lists the details.

How the big camera systems are set up for telephoto lenses in general is compared in the article Canon, Nikon, Sony, Fujifilm, OM System. For the converter question, though, in the end only the specific approval for your lens counts.

Autofocus: where the converter costs the most

At the airport your subject almost always moves – on takeoff, on landing, even when taxiing. So autofocus is often the more critical point than sharpness.

Two effects come together. First, with less light the camera focuses more slowly and less surely. Second, some camera-lens combinations throttle focus speed as soon as a converter is detected. For the Canon EF extenders, for example, a clear loss of speed is cited, stronger with the 2x model than with the 1.4x model.

With older DSLRs there is also a hard limit: many models focus only up to an effective maximum aperture of f/5.6, better bodies down to f/8 – often only with the center focus point. Mirrorless cameras measure focus directly on the sensor and usually cope with smaller apertures. Whether that still works quickly in backlight or against a hazy sky is another question.

In practice that means:

• Limit the focus range: Many telephotos have a focus limiter. Set it to the far distance, and focus won’t hunt through the whole range.
• Catch it early: Pick up the aircraft early on approach and let focus keep tracking, rather than firing only shortly before touchdown.
• Choose the focus area wisely: A medium-sized area on the fuselage is often more reliable with a converter than the full area. Which modes work well for aircraft is described in the guide Autofocus for aircraft.

Image quality: 1.4x and 2x compared

How much sharpness a converter costs depends heavily on the combination. There are no blanket percentages, but the tendencies are well known.

Criterion	1.4x converter	2x converter
Light loss	1 stop	2 stops
Sharpness loss	mostly slight on good telephotos	clearly visible, depending on the lens
Autofocus	somewhat slower	noticeably slower, unsure in low light
Sensible base	70–200 f/2.8, primes from 300 mm, approved pro zooms	fast primes, very good zooms in sun
Typical use	taxiing movements, landings at some distance	far-off targets, military airfields, details
Sensitive to shimmer	yes	very

A tip from practice: with a converter it often pays to stop down by a third to a full stop. Many lenses become visibly crisper, and the effective aperture shifts only slightly. In low light, however, that collides with short shutter speeds. Which speeds you need for jets and propeller aircraft is in the camera settings for plane spotting.

Converter or cropping?

The most honest competition for the converter is your image editing program. If you shoot a picture without a converter and crop it so that the same framing results as with 1.4x, you keep about half the pixels. With the crop of a 2x converter, a quarter is left.

The decisive question is therefore: does the lens with a converter deliver more real detail than the remaining pixels of the crop can carry? With a very sharp telephoto, good light and still air, the answer is mostly yes. With a soft lens, haze or shimmer, the converter often gets nothing out that the crop doesn’t also show – and you additionally have slower autofocus and higher ISO.

A high-resolution sensor shifts the calculation in favor of cropping. How sensor size and pixel count affect spotting is explained in the comparison APS-C, MFT or full frame. Tips for clean crops and level horizons are under Cropping aircraft photos correctly. And if the higher ISO with a converter brings noise, modern noise reduction with AI denoise helps noticeably.

Converter or longer lens?

The third option is a longer lens. The widespread zooms up to 600 mm in particular have defused the converter question for many spotters: they cover focal lengths without accessories that used to be reachable only with a converter. An overview of this class is given in the comparison of 150-600 mm lenses; the somewhat more compact alternative is covered in the article 100-400 mm telephoto zooms compared.

Roughly speaking: if you already own a 70–200 f/2.8 or a fast prime, a 1.4x converter gets you a lot of reach for little money. If you’re starting from zero and mainly want reach, you’re usually better served by a long zoom. Both variants are interesting used – what to look for when buying is in the guide Buying a used camera and lenses.

When a converter really helps at the spot

Not every spot needs maximum focal length. At observation decks and hills near the runway, a normal telephoto is often entirely sufficient, and with a converter a big jet would quickly get cut off. The converter becomes interesting at spots with a large distance:

• Taxiways and apron in the distance: When aircraft taxi on the far side, the converter brings regional jets and small types into the frame.
• Landings from a distance: At locations where the touchdown point is far away, 1.4x often gives exactly the missing reserve.
• Military airfields: Here public vantage points are often far from the runway and taxiways, and rare guests are meant to fill the frame. That doesn’t apply everywhere, though: at the Teveren spotter hill in Geilenkirchen, the AWACS aircraft come so close, according to the spot description, that medium focal lengths suffice.
• Details: Engine inlets, logos on the vertical stabilizer or the registration of a far-off aircraft.

How much focal length a spot requires in general is best read up on beforehand in the spot descriptions – for example for the Munich Airport visitors’ hill or the Affenfelsen in Hamburg. General focal length recommendations are collected in the guide Lens and focal length for plane spotting.

A quick decision aid

Practice at the fence: how to work with a converter

With a converter the combination gets longer and more conspicuous. Stay on public ground and expect to be approached now and then – how to handle that calmly is described in the article Big telephoto, big suspicion. For transporting long combinations, take a look at Camera backpacks for long telephoto lenses.

Conclusion

A teleconverter is not a magic trick but a trade: focal length for light, autofocus speed and a bit of sharpness. With a fast, approved telephoto, the 1.4x converter is one of the cheapest ways for spotters to get more reach. The 2x converter belongs on first-class optics and in good light. On slow superzooms and in shimmering air, cropping often gives you an equally good result – at no extra cost.