Introduction: Changing the size of the x-ray field affects the dose received by the patient, while at the same time influences the amount of scattered radiation that occurs in the patient and reaches the image receptor. When changing the size of the x-ray field, it is also necessary to change the exposure conditions if we want the signal-to-noise ratio in the image to remain the same. In the case of a smaller image field, the mAs must be increased. In order to do so, we can use the factors of changing the exposure conditions when resizing the x-ray field. Purpose: The purpose of this bachelor thesis is to determine how the change in the size of the x-ray field influences the exposure index and the dose received by the patient, as well as to obtain factors of changing the exposure conditions when resizing the x-ray field for the computer radiography system. Methods: Two methods were used to carry out the research. With the first method, we obtained theoretical data by studying diverse domestic and foreign professional literature. The second method of work was the measurement survey where we changed the size of the x-ray field and measured the effect of the change on the exposure index and dose. Using the formula, we calculated the theoretical factors for increasing the exposure conditions. Results: By reducing the x-ray field, the exposure index is lowered, but when the exposure conditions increase by a certain factor, the exposure index increases, thereby increasing the dose. Based on our measurements, the mAs should theoretically be increased by a factor of 1.52 (52 %) when changing the size of the x-ray field from 35 × 43 cm to 24 × 30 cm in order to maintain the same exposure index. When changing the size of the x-ray field from 24 × 30 cm to 18 × 24 cm, the mAs should theoretically be increased by a factor of 1.41 or 41%. Discussion and conclusion: When we want to measure the dose received by the patient, the product between the dose and the surface is more appropriate than the exposure index. By reducing the x-ray field, the surface of the irradiated body decreases, and therefore the dose also decreases. This is also what we found out within the framework of the present thesis when we didn't change the mAs. It was established that under the same conditions of exposure, the dose was reduced by about 50%, with a reduced area of 50%.