DNA absorbs light of certain wavelength so that light of 260 nm passing 1 cm through DNA at 50 ug/ml concentration (in water) has an absorbance of 1.0.
Set your UV spectrometer to use light of 260 nm.
A negative control or blank cuvette (tube) is filled with water or just the buffer solution (such as TE; usually water) that the DNA is in. As the tube has no DNA, it should give an OD reading of 0. The machine however may show the reading to be something else. The machine is therefore 'zeroed' to set the reference at 0.
The sample to be analysed is diluted, usually 50-500 fold (usually 200-300 fold) in water (or buffer). The solution is puured into a similar-sized cuvette as the one used for blank/negative/reference and its OD measured.
The cuvettes used in Ultrospec 2000 have a 'path length' of 1 cm.
Multiplying OD (260 nm absorbance) with 50 ug/ml will give DNA concentration (inside the cuvette) and a further multiplication with the dilution factor (50-500) will give DNA concentration in the sample.
it is OD x 50, 260 Wavelength.
You measure at 260 but the ratios 260/280, 260/230 give you your purity.