How To Buy A Lens

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The Cost
The driving influence on the price of a lens is the diameter of the largest element in that lens. This is particularly true of telephoto lenses, less so for lenses around 50mm focal length and then true again for wideangle lenses of retro-focus (inverted telephoto) design. For the latter, in order to clear the mirror of the SLR, the rear element has to be moved away from the focal plane (by an inverted telephoto arrangement) which means that the front element of the lens is proportionally much larger and so the cost model comes back in again (study the graphs!).

In order to get the data onto a single graph (so we could look at it) we have plotted the 'Effective Aperture' of the lens against its street price. The effective aperture is simply the focal length (maximum for a zoom) divided by the maximum f-stop. For telephoto and telephoto zooms, the relationship is more or less linear, ie you get what you pay for – more focal length and more light-gathering power costs more money. As expected the relationship breaks down a little for lenses around 50mm focal length but then starts to sweep up again as larger elements are employed for short focal length inverted telephoto designs.

It is because aperture is the controlling influence on cost that you should spend the greatest time assessing just how much you need. Here is an example: if you are imaging morbid laboratory specimens (ie dead and not moving!) then an enlarging lens with an aperture of f5.6 and some form of bellows attachment is all you would need for imaging at a magnification of 1:1. You would slowly and carefully focus in what will be quite dim conditions, stop down, lock up the mirror and shoot. Things change rapidly if the specimen is a butterfly in the open. Now you have to focus really quickly and the f2.8 macro lens with an automatic aperture comes into its own, possibly with a bit of image stabilisation to help you as well. The difference in cost is many-fold.

                   

Having a small aperture reduces cost but also reduces weight and increases portability. In today's air travel, weight may be critical – Ryanair will be rubbing their grubby little hands in glee as they see you lugging your 400mm F2.8 towards the check-in desk – and you thought you got ripped off for the lens!

Once you have fixed your mind on the lens' requirements you have to start agonising over whether you can afford it. In the UK at the present time you can get an OEM lens for around 70% of its SRP. If you are locked into a camera marque which offers an abundance of lens choice (eg Nikon or Canon) then you are likely to stay with your favourite and keep things in the family. If that is outside of budget you have very little optical compromise to make by moving to the independents (mainly Sigma with a little help from Tamron and Tokina).

On our graph the slope of the line in the telephoto part of the curves represents the average costs compared with the effective aperture. Thus Canon are the most expensive with a line slope of £90 per mm of diameter, then Nikon at £72 per mm of diameter and then Sigma at £50 per mm of diameter. The outliers on the graphs are caused by additional features such as zoom construction, stabilisers or the ability to close focus (although there seems to be little penalty for macro lenses).

Please note that these graphs are intended as indications only, you will have to do your own homework – remember, you are about to invest a load of cash from your business, you need to get it right. Talking of business, it is bad tactics to buy grey imports. The chances are that the low price is obtained by both tax and VAT avoidance. Your accountant and (heaven forbid) a VAT inspector will be asking for your paper work downstream. Should you break a lens the repairers may also need proof of import duty payment (Sigma certainly do). You have been warned!

The Alternative Solutions
Even having considered the choices thus far you still have options. Suppose you need 400mm of focal length (or close to). You can achieve this with the following options from Nikon, for example:

The 200mm plus a 2x converter might be a good deal for someone who can also make good use of a fast 200mm lens (indoor sport?); the 200–400 for somebody for whom flexibility is paramount, but every ounce of light-gathering is not; the most expensive AFS 400mm F2.8 might be essential for the sports photographer shooting under floodlit conditions. On top of all this you have to throw in the effect of the chip size. So much choice and so few sheckles! These deliberations take no account of focusing speed, VR performance, weight or physical size – all are important!

   

What Actually Happens
This is a secret between the photographer and like-minded souls! Often you end up buying the most expensive thing you think you can afford. Mistakes up to 200mm are relatively painless but things get progressively more injurious as you go bigger, either on your wallet or your neck. Never buy anything over £2,000 without trying it out and that includes hiring the brute for as long as it takes to get a true feel of it – what feels good on the shop floor at Calumet feels a whole bunch different after a day at an athletics meeting when you are sun-burned, dehydrated and hypoglycaemic!

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