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What you need as glass (Part III - Long Prime)

July 11th, 2011

This is part III of a series of what glass you will need. Each part will discuss a certain lens type and its applications.

A long prime for this article is anything from 200mm up. This group can be subdivided further by a section for super-tele longer than 400mm

If we neglect old glass, most of these lenses are quite expensive, they usually start around $4000 and end in the 5-digit range. This indicates that they aim at professionals that cover sports, wildlife (and celebrities sun-bathing). If you are not a professional, but have one of these passions there are two alternatives:

  1. Bite the bullet
  2. Find some cheaper alternative

For alternative one, it is just a question what focal length you need - there is only one choice per focal length (buy Nikkor). The 300mm exists in two variants - the f/4 is very good, the f/2.8 better. A good advice is to use a DX body with a 300/2.8 instead of a (hypothecical) 450/f4 on FX - saves a couple of kilos. If you only have a FX and a 300/2.8 - look how little you have to pay for a D7000 in relation to the price of the current gear (substract the price of the TC14!)

Cheaper

Teleconverters, old glass, mirrors … you get what you pay for. Mirror lenses come in two installments

  1. Dedicated photo lenses (mostly old lenses) give you extreme focal lengths in a compact format
  2. Converted telescopes (T2 mount)

Some are OK with the converted telescopes, but there are a lot of problems with them in terms of IQ and mechanics. All mirror lenses are full manual and have apertures smaller than  f/5.6. This means you have to focus manually, AF confirmation is not reliable to get sharp shots. The characteristic bokeh of mirror lenses require a carefully chosen background. You get a lot of magnification, but it is more to get an image versus getting the image.

Teleconverters (TC)

Useful accessory, instead of buying a 300/4 and a 450/5.6, getting a quality 1.4 TC on the 300/4 saves a lot of money and weight. The mileage may vary, but it is always better and cheaper to buy one quality lens + TC than two second grade tele lenses.

TC should not be used with anything shorter than 200mm (Read your manual!). TCs are basically enlargers that blow up the center of the frame. This means that the resolution will go down by the same factor! If your lens already struggles here a TC will not improve anything, you can as well crop the center, you will gain nothing. But you will loose light: When blowing up two times only a quarter of the light gets used and your 300/2.8 will become a 600/5.6. AF only works down to f/5.6, therefore a TC shall not push the lens beyond this limit A 1.4 TC costs 1 stop, a 1.7 TC 1.5 stops and a 2.0 TC costs 2 stops. This is physics. 2x TCs are always pushing it, but with good glass, stopped down to f/8 can get impossible shots.

Why f/5.6?

DSLRs use (unless when in LiveView) phase detection AF. Basically it means that they use rays entering the borders of the aperture and compute at which focus setting they will coincide. At very small apertures the difference between the borders is too small  to create a signal that allows this computations. It is like with the DOF: If you have a very small aperture, say f/22 and you shoot a tree you can as well turn the focus to infinity without seeing a significant difference in the photograph.

A disadvantage of this method is that it is sensitive for residual aberrations: All camera do AF wide open and if the lens is rather soft wide open it will focus for light that is absent when shooting stopped down.

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