Introduction: Exposure index, that defines the amount of dose on image receptor can be used as mechanism for radiation exposure control with same object. To use the image for diagnostic purposes, it is necessary to adjust the exposure conditions accordingly when increasing or decreasing the image field. When the field size changes from 35 × 43 cm to 24 × 30 cm, the enlargement exposure factor of mAs is 1,25 and when changing to the field 18 × 24 is 1,4. Purpose: The purpose of this bachelor thesis is to determine how the change in the size of the image field affects the EI without changing the exposure conditions. One purpose is also to compare the matching of enlargement factors while imaging with or without use of antiscatter grids. Methods: The measurements were carried out in two parts. For the diploma work we used the descriptive method with a review of domestic and foreign literature. The other method of work was the survey with the help of measurements, where we carried out the procedures, that they led us to the determination of more precise factors of changes in exposure conditions when resizing the image field. The exposure conditions were determined with use of ionization cell while using grid, and electronically without grid. We also upgraded the study by determining the enlargement exposure factors while using the grid (with a ratio 12:1) or without. Results: Enlargement exposure factors were very close to the enlargement factors measured at 85 kV while imaging with grid or without. There are practically no differences between enlargement exposure factors in foot phantom with acceleration voltages of 45, 50 and 55 kV, or the deviations are almost invisible. Discussion and conclusion: The highest values of EI were observed at higher exposures and the lowest in lower exposures, indicating a relative EI and exposure relationship. As a conclusion, it can be summarized that the grids at low voltages are not used, since such small amounts of scattered radiation do not impair the contrast resolution to such an extent that the image is diagnostically inadequate. The purpose of the grids is especially intercepting scattered photons before they reach the image receptor.