What the two types have in common
An optical level and a digital level do the same job: they give you a horizontal line of sight and you measure how far below it the staff on the point sits. The difference between the backsight and the foresight reading is the height difference. Both have a compensator that keeps the line of sight horizontal after you have roughly centred the circular bubble. Both are checked the same way, with two staffs and the two-peg test. Up to here everything is the same. With both, the staff is half the instrument. A bent staff, one with a worn foot, or one not held plumb on its bubble gives an error no level can compensate for. The circular bubble on the staff gets checked as often as the one on the instrument.
The difference is in who reads the staff. With an optical level you do, off the millimetre graduations in the eyepiece. With a digital level you still do, but through a camera inside the instrument that recognises a barcode staff and shows you a number on the display.
The optical level: you do the reading
An optical level has no software, no battery for the measurement itself, and nothing to give up in the rain. Set it up, centre it, look. The FOIF DSZ3 is the typical construction level of this kind: erect image, 24x magnification, an air-damped compensator and a button that lets you check the compensator is free. It weighs two kilograms and works from -30°C, which makes it the instrument that always lives in the boot of the van.
Its weak point is the person. On a long run with fifty setups, late in the afternoon, readings slow down and transcription errors in the field book grow. The result depends on how careful you are. For rough construction work, an excavation level, a slab check, it is entirely enough. The DSZ3 lists a standard deviation of up to ±2.0 mm per kilometre of double-run levelling, and that covers the vast majority of construction tasks. A typical job: checking the formation level at the bottom of a foundation excavation before the concrete arrives. Three readings, one sum by hand, and the answer is ready while the mixer is still parking.
There is precision optics too. The FOIF DS03 has a parallel-plate micrometer, 42x magnification and a stated accuracy of ±0.3 mm per kilometre, which puts it in high-order geometric levelling, but with an invar staff and with a person who knows what to do with the micrometer. It is a niche instrument for levelling networks and deformation monitoring, not for every site.
The digital level: the instrument does the reading
With a digital level you aim at the barcode staff, press a button, and after a second or two you have a reading and a horizontal distance. No transcription, no argument about who read what. The instrument keeps the points in memory and the run can be computed on the spot. The FOIF EL302A is the level we show to colleagues switching from optics for the first time: stated electronic accuracy of ±0.7 mm per kilometre, 30x magnification, memory for 3000 points and, what matters in practice, reliable reading in low light. At half past four on a late November afternoon that counts.
The FOIF EL03 Pro is a step up: 50x magnification, a 3.5 inch colour touchscreen and a stated accuracy of 0.3 mm per kilometre with an invar staff or 1.0 mm with an ordinary barcode staff. That is the instrument for a benchmark network, for settlement monitoring, and for jobs where somebody else will check the result.
Where the digital level loses
First, the staff has to be visible clearly enough and for long enough for the camera to recognise it. A branch in front of the staff, strong backlight or a shadow right across the middle of the barcode can defeat the reading, where with an optical level you would read through it. Second, in wind the staff shakes with either type, but the digital one may refuse to read or ask for a repeat, while the eye averages on its own. Third, there is a battery and electronics to think about in the morning. On a site with constant dust, or in the November fogs of the Danube plain, the camera needs clean optics and a clean staff, which means a cloth in your pocket and a wipe every few setups.
None of this is an argument against digital levels. You just need to know that on a windy slope with short staffs you may end up with the optical one, and on a long run along a flat road the digital one will save you an hour and probably one gross error.
Which one for whom
A construction company with several sites where excavations, slabs and pavements get measured is well served with a DSZ3 in every van. A surveying firm doing levelling runs for design, benchmark networks and monitoring will get value from the EL302A as a daily instrument and from the EL03 Pro or DS03 for the precise jobs. Many colleagues keep both types, because the optical one is the backup that depends on nothing. The cost of staffs is part of the sum as well: a digital level needs a barcode staff, precise work needs an invar one, and that is an expense people often forget at the start.
The practical rule: if your readings go into a field book that someone later processes in the office, a digital level pays for itself in time saved and in the absence of transcription errors. If you measure five points on a site and use them on the spot, an optical level is plenty. The full range is in the levels category.
