Sharpness, exposure time, camera shaking
In macrofotography, at a reproduction scale (RS) from 1:2 up to 10:1 or even greater the sharpness of a picture is highly dependent on the focal aperture. The depth of field (DOF) is generally increased by stopping down (e.g. f-stop 32); however, the overall sharpness decreases at a certain setting.
Besides the optimal distance from the object and dependent on the RS the knowledge of the diffraction limit (DL) of the equipment is essential to produce sharp pictures. In literature and internet several tables indicate the optimal aperture, for instance at a RS 1:1 -> f/22 and at RS 5:1 -> f/6.3 (I did not test them and therefore I do not want to publish examples).
In contrast to simple camera/lens systems these tables are useless when bellows, extension tubes and converters are used to increase the RS, due to the increase of the focal length.
The aperture can be calculated: focal length divided by the "effective" aperture diameter.
Comparisons of pictures taken with different f-stops also indicate the best aperture for maximum sharpness.
The optimal f-stop and 1/3 of the related DOF is the base for the distance change in between picture taking for stacking.
The reproduction scale is defined as the ratio of the original size of a motive to its size on the chip-layer. The size of the light sensitive chip does not influence the definition of the RS. If a real 1 cm corresponds to 2 cm on the chip layer, the RS is 2:1.
In the magnification range from 2:1 up to 10:1 or greater the picture detail (or RS) is set with the focus ring or with the bellows etc.. The sharpness is set by adjusting the working distance and using an f-stop at the diffraction limit.
The comparison described below is applied to comparable cameras systems, e.g. system cameras or full-frame (35mm) cameras and its optical lenses; these systems used to expose a 35 mm film.
Full-frame cameras record pictures that are twice the width than pictures taken with cameras having a crop-factor of 2, given that the lens and RS is the same. The full-frame camera is equipped with a larger chip sensor and therefore records at the same optical magnification a larger area in length and width than a camera with a crop factor 2. As an example, if a pencil is photographed full-format (from the left to right side of the sensor) with a crop-camera (factor 2) and thereafter the camera is exchanged to a full-frame system, the resulting picture shows additional areas of half a pencil length on the left and right side and above and below of the pencil.
If these two pictures were developed to e.g. A4 size than, at same RS, the pencil, taken with the crop-camera, would fill the full A4 format, whereas gaps would appear around the pencil on the picture taken with the full-frame camera. Under the same condition on the A4 paper, the pencil would fill half of the length, when taken with the full-frame, compared to the crop camera (factor 2). Are both type of camera equipped with a sensor composed of the same amount of pixels (e.g. 10 MPx) than the pencil is recorded with a twice as good resolution compared to the full-frame camera.
Hence, from this point of view one can draw the conclusion that crop cameras possess advantages regarding the resolution in macrophotography towards full-frame cameras equipped with the same optical layout. Of course, there are several factors that influence the choice of the optimal camera for macro-photography.
In the reproduction scale of macrofotography from 1:1 up to 10:1 or greater the lenses usually resolve less lines than a 10 MPx camera is able to resolve. Pictures published in the internet usually are approximately one-fourth (1000 Px) of the pixel-width of a 10 Mpx camera
Movements of the camera, the lens or the motive, while exposure, are causing blurry pictures.
If a camera is equipped with a chip of 23.6 mm width and a horizontal resolution of 4288 Px, thus one pixel at a RS 1:1 is reproducing a width of 0.0055 mm (5.5 thousandth mm). Hence at RS 1:1, the resolution is decreasing to the half, if the equipment is shaking while exposure with 5.5 thousandth mm. At an RS of 10:1 the picture information smears greater than 10 Px. Therefore, it is obvious that the exposure has to be short. At constant light and an exposure time of 1/8’000 sec, at 1:1, the object is moving with the speed of 158.4 m /h for the width of a pixel, meaning half the resolutuion. At 10:1 it is sufficient if an object is moving with the speed of 15.8 m/ h. The shaking of the camera at the release, on a instable tripod is way faster. Therefore pictures at this RS are likely to blur due to shaking. In the Macro area from 1:1 to 10:1 and greater RS the lenses resolve usually less lines than a 10 Mpx camera is able to resolve and shaking is an additional factor that contributes to blurring.
With the exposure time of 1/8’000 s the availability of strong light is crucial. Neither daylight nor a constant light source is sufficient powerful; flash light is needed. At full power the burning time of the flash is usually 1/500s and by reducing the performance to the lowest possible the flash period is approximately 1/10’000 s down to 1/30’000 s. Shorter exposure times can be achieved by using strobe flashes, however their power is way too weak. Therefore it can be useful to use several flashes with short flash periods (flash power).
An absolutely stiff camera and motive installation combined with a short flash period is crucial to avoid any blurs. A mirror-up function, at least 1 s before exposure and the use of a remote or cable trigger is additionally a must to get clear sharp pictures at these great reproduction scales.