That forgotten camera, once used to record moments of family history in monochrome, is probably worth less than you might hope. But some antique models certainly can be valuable.

As collections of old cameras and equipment go, we are lucky in Ireland to have the preserved darkroom at Birr Castle where - as Mr Barry Noyce, gardener at the castle, says going into it is "just like going back in time".

It is replete with early photographic paraphernalia including lots of "old bottles of concoctions of whatever she would have used". She being Mary Countess of Ross (1813-1885), an awardwinning photographer and wife of the third Earl of Ross, the man who instigated the building of the famous telescope.

So what are old cameras worth? According to Mr Michael Pritchard, camera specialist at Christie's: "If you were to think of all the cameras that were ever made, there's actually a very small proportion that have any real value to collectors."

There are roughly three types of cameras which interest them. The first comprises cameras from the 1840s to the 1880s - the period of the Birr Castle darkroom. These are the very first, generally mahogany, cameras. "Those we don't see very often. There's a lot in museums now, yet there's very few in private collections," says Mr Pritchard.

The second group which interests collectors are decorative cameras, such as art deco styled cameras with a coloured body. Art deco cameras tend to fetch £80 or £90. Decorative wooden cameras in mahogany and lots of brass from the 1880s through to the 1920s, although mass-produced, tend to be worth "maybe £200 or £300". To the disappointment of many, however, the normal box brownie (a small black box camera) tends to be worth as little as "£1, £2 or £3 because they literally produced thousands of them".

But probably the strongest area of the whole collectable camera field are the very good quality cameras from the 1930s to the late 1960s by companies like Leica and Zeiss. People who collect these tend actually to use them for taking photographs.

Mr Pritchard expects that a lot of readers could have this third category of camera. However, readers might be more likely to have box cameras, which usually have a value of under £5 or £10 or very basic folding bellows cameras. Most of the very standard folding cameras from 1920s or 1930s are only worth about £15 or £20, he says.

It's a good sign if the lens on your camera bears a signature by Grubb. A Dublin maker, his signature could suggest your camera dates from the 1840s to 1880s period. "The value of those sorts of things can range from £500 up to £10,000 or £15,000," says Mr Pritchard.

Stereo cameras, which are of great interest to collectors, take two separate pictures. You look at them through a special viewer and get an impression in 3D. "A good stereo camera from, say, the 1850s would quite easily be, say, £5,000 to £8,000. You don't see many of them, unfortunately," he says.

Disguised cameras, perhaps disguised as books, walking stick canes or pocket watches are "a very strong collecting area in its own right. A basic camera disguised as a book made in 1888 that would sell for around about £1,000," he says.

A black or brown plastic Coronet Midget from the late 1930s - the size of a matchbox - is worth about £80. But a blue one should fetch about £200.

Leica and Zeiss cameras range "from £80". Most Leicas are £200 to £400. Zeiss tends to be a bit less, from £80 up to about £200.