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According to Wikipedia:

In photography and cinematography, a telephoto lens is a specific type of a long-focus lens in which the physical length of the lens is shorter than the focal length. This is achieved by incorporating a special lens group known as a telephoto group that extends the light path to create a long-focus lens in a much shorter overall design. The angle of view and other effects of long-focus lenses are the same for telephoto lenses of the same specified focal length. Long-focal-length lenses are often informally referred to as telephoto lenses although this is technically incorrect: a telephoto lens specifically incorporates the telephoto group.

So telephoto lenses are shorter than their focal length. However, when I look at the longer commercially available prime lenses, there's a very clear relationship between the focal length of the lens and its physical length. The lenses get physically longer, to the point of being very, very long indeed, as their focal lengths get longer.

This raises the following questions in my mind:

  • Are these lenses actually telephoto in the technical sense, or just long-focus?

  • The telephoto group can be used to make a lens shorter than its focal length, but presumably there is a limit to how much shorter? I.e. you can't make it arbitrarily short, right? What is this limit? (Can you use multiple telephoto groups to compound the effect?)

Looking mainly at "full-frame" and smaller format fixed focal length lenses with an angle of view equivalent to an 85mm or greater focal length lens on a full-frame sensor, how likely are these to actually be telephoto in the technical sense (is there any correlation with focal length and/or format size?), and if both technically-telephoto and merely-long-focus lenses are reasonably popular, what are good examples of each?

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Pretty much every commercially available prime lens longer than 70mm for 35mm size or smaller cameras is telephoto in both senses of the wikipedia definition you quote: They are longer in focal length than the diagonal of the sensor they are designed for and they are shorter in physical length than their focal length.

For instance, the Canon 800mm f/5.6 L IS only about 460mm long from the rear to the front optical elements. Add the registration distance from the flange to the sensor and you are at just over 500mm for an 800mm focal length. Include the supplied hood and the tip of the lens housing is still only 650mm in front of the sensor.

The EF 85mm f/1.4 is only 80mm or so long. So is the EF 85mm f/1.8. That makes them both just shorter than their 85mm focal lengths. The EF 100mm f/2 L is about 81mm long, the EF 135mm f/2 is about 120mm long, and the EF 200mm f/2.8 L is roughly 145mm long.

Even though the longer Canon Super Telephoto lenses look totally monstrous in size, they are actually more telephoto in terms of the ratio between their focal length and their physical length (at around 1.33:1) than lenses in the 85-200mm focal length range. Lenses of the same focal length ranges from other manufacturers are similar in size to their Canon counterparts. Canon also makes Diffractive Optics lenses that are even more compact for their focal lengths, but they haven't really caught on because there is a small penalty in terms of image quality compared to the more conventional telephoto lenses. The EF 400mm f/4 DO IS at 244mm long has a focal length to physical length ratio of about 1.66:1.

  • Thanks. I think this is why I was uncertain: for some reason I was expecting the difference to be significantly greater, around the order of halving the length, which it only approaches at the much longer focal lengths (the 800mm and 400mm you cite). 10-20% for the shorter ones is not nothing, but also probably not something I could reliably tell just by eyeballing the lens, "hey, that one's shorter than it should be". – glaebhoerl Apr 24 '14 at 15:24
  • And this has piqued my curiosity, so I took out a measuring tape: my old Pentacon 135/2.8 is 9cm long. Add the 45mm registration distance and you're just about exactly at 135mm. Is it conceivable that this is not a technically-telephoto lens, and/or that not-technically-telephoto lenses are more prevalent among legacy primes? – glaebhoerl Apr 24 '14 at 15:27
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    @glaebhoerl It's highly ulikely to not be a telephot, but you can easily tell - a telephoto lens the aperture looks bigger throught the front element than it does through the rear element, detach the lens and hold it up to the light. – Matt Grum Apr 24 '14 at 20:18
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In the world of 35mm cameras any lens longer than 85mm will be a telephoto design.

Telephoto designs are about more than just reducing the physical length of the lens, you also get a huge weight saving in the rear of the lens compared to a symmetrical design.

In addition to that, most manufacturers' range of super-teles are based around the same 100-150mm telephoto prime, with different teleconverters on the front. A front mounted teleconverter doesn't reduce the maximum aperture (but must be larger than rear-mount). As the focal length increases so does the size of this front mounted TC, thus increasing the barrel length.

It's only in the world of large format technical cameras that you get a choice, i.e. there are telephoto and non telephoto versions available for the same focal length. This is because the optical centre of a telephoto lens is not inline with where it is mounted in a lens-board. For this reason photographers wanting to use movements may opt for a heavier, larger, non telephoto version of the focal length.

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Note that it has to be long focus as well. In other words, a 28mm lens isn't going to be telephoto even if it only takes 20mm to produce the image since it isn't a long focus lens. According to Wikipedia, long focus generally refers to the focal length being longer than the diagonal measure of the sensor.

So in other words, any lens longer than 35mm that takes less than it's focal length to achieve the image is a telephoto lens. Any lens that is a telephoto lens is going to actually be telephoto as the lengths are simply too long not to be, but often standard lenses may also make use of a telephoto group to shrink the lens.

There is a practical limit to how short you can get without having major issues with aberrations and quality loss, but some techniques such as Canon's diffractive optics (DO) help with this.

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Addon: There are exceptions to the statement "any lens longer than 85mm will be a telephoto design".

Early versions of the infamous 500mm f/8 generics are indeed as long as their focal length.

There was a (massive!) non telephoto version of the Enna 240mm f/4.5 (about eight times the practical size of the telephoto version).

Novoflex had some lenses explicitly described as "Fernobjektiv", which is a german term mostly used for non-telephoto long focal lengths.

Some "telephoto zooms" are actually of a physical length exceeding the focal length set, so it would require a closer look whether they are actually telephoto in design.

The Tair 3s 300mm lens is about as long as its focus, and diagrams make it ambiguous whether the rear group is actually negative.

  • Just out of curiosity, which early versions of the 500mm f/8 do you have in mind? I have what I believe to be an early version of the Danubia 500mm f/8 and it's about 35 cm long. I'd be interested to see more info about non-telephoto designs of these lenses, if they exist. – Kahovius Oct 28 at 20:34
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    A google search for Kenlock 500mm shows so many that i won't have to make a pic of my own (and not with it: Optically it is a train wreck IMHO, especialy if compared to eg the Tair! Mine is under constant threat of being repurposed into a mount for a large format lens or similar....But then there is sample variation). – rackandboneman Oct 28 at 21:56

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