1. An image processing apparatus, comprising: a camera interface connected to an image pickup part for photographing a measuring object and generating image data, such that a control instruction for controlling the image pickup part is outputted and the image data photographed by the image pickup part is inputted; a display screen output part connected to a display part, for outputting image data for display displayed on the display part; an input part that receives an input from the outside; and a processing part that executes processing, the image processing apparatus having a setting mode for performing setting based on the input received from the input part, and an operation mode for executing processing in accordance with the setting, wherein in the setting mode, the processing part outputs the control instruction for causing the image pickup part to photograph the measuring object through a camera interface, based on a first photographing condition including a plurality of photographing conditions to differentiate a luminance of the inputted image data, receives the input of a plurality of image data corresponding to the plurality of photographing conditions based on the control instruction, acquires the plurality of image data, with luminance of constituent pixels expressed by a first gradation number, calculates a standardized luminance standardized based on a photographing condition corresponding to each image data, regarding the luminance of the pixels constituting each image data, generates composite image data expressed by a second gradation number of which a gradation number of a composite standardized luminance of pixels constituting an image is increased from the first gradation number, by compounding the standardized luminance of each pixel of each image data at a mutually corresponding coordinates position, in accordance with weighting imposed on the standardized luminance previously defined based on the photographing conditions, generates the image data for display by converting the luminance of each pixel as needed such that the second gradation number expressing the luminance of pixels constituting the composite image data becomes a third gradation number, to output this image data for display by the display screen output part, receives designation of an area in the image data for display and sets a designated area, through the input part, and performs processing of setting a photographing condition corresponding to each range of the standardized luminance, such that one or a plurality of photographing conditions corresponding to a range of a composite standardized luminance in the designated area defined by an upper limit value and a lower limit value of the composite standardized luminance of pixels constituting the designated area in the composite image data, are set as a second photographing condition, in accordance with a previously defined correspondence relation, and in the operation mode, when there are a plurality of second photographing conditions set in the setting mode, the processing part outputs the control instruction for causing the image pickup part to photograph the measuring object based on the set plurality of photographing conditions, through the camera interface part, receives the input of the plurality of image data corresponding to the plurality of photographing conditions based on the control instruction, acquires the plurality of image data in which the luminance of the constituent pixels is expressed by the first gradation number, calculates the standardized luminance standardized based on a photographing condition corresponding to each image data, regarding the luminance of the pixels constituting each image data, generates the composite image data, in which the gradation number of the composite standardized luminance of the pixels constituting the image is increased from the first gradation number, by compounding the standardized luminance of each pixel of each image data at a mutually corresponding coordinates position, in accordance with weighting imposed on the standardized luminance previously defined based on the photographing condition, converts luminance gradation regarding the composite standardized luminance of pixels constituting an entire area of the composite image data, by luminance gradation conversion for converting the luminance of each pixel such that a range of the composite standardized luminance in the designated area defined by the upper limit value and the lower limit value of the composite standardized luminance of pixels constituting the designated area in the composite image data, becomes a fourth gradation number, and performs processing of calculating image data to be processed so as to be provided to later image processing, and when the second photographing condition set in the setting mode is one photographing condition, the processing part outputs the control instruction for causing the image pickup part to photograph the measuring object, based on the set one photographing condition, through the camera interface part, receives the input of one of the image data corresponding to the photographing condition based on the control instruction, acquires the image data in which the luminance of the constituent pixels is expressed by the first gradation number, and calculates the standardized luminance standardized based on the photographing condition corresponding to this image data, regarding the luminance of the pixels constituting this image data, converts luminance gradation regarding the luminance of the pixels constituting an entire area of the image data, by luminance gradation conversion for converting the luminance of each pixel such that a range of the standardized luminance in the designated area defined by the upper limit value and the lower limit value of the standardized luminance of pixels constituting the designated area in the image data, becomes the fourth gradation number, and performs processing of calculating image data to be processed so as to be provided to later image processing.