Reading the Focus Displays and Adjusting Focus

As the focus is adjusted on a view camera, the image also changes size, and therefore moves slightly as a function of its distance from the optical axis. The human eye-brain follows this movement during focus adjustments, but the digital camera’s CCD remains stationary, sometimes causing confusing focus indications as edges or bright specular high- lights move in and out of the sensor’s view as focus is changed.

C. Focus display for a poorly chosen focus location without distinct edges.

Both of these problems may be mitigated by using the Magniﬁcation slider in the Focus tab panel to change the system’s eﬀective magniﬁcation and viewing area. Increase the magniﬁcation and the sampling area will decrease; decreasing the magniﬁcation has the opposite eﬀect. A higher magniﬁcation allows ﬁner details to be resolved better, and also shrinks the size of the viewing rect- angle to align better with small objects in the set. A lower magniﬁcation covers a longer section of the image, providing more data in low-contrast situations.

Reading the Focus Displays and Adjusting Focus

After the CCD is moved to the desired focusing location, readout and display of the selected channel’s image data begins. The number of pixels output and displayed is ﬁxed at 100; the various available magniﬁcations utilize diﬀerent resolution settings to achieve diﬀerent spans of coverage. The largest sampling area, 4X magniﬁcation, uses one of the lowest resolution settings, which combines many CCD pixels into each output pixel. The highest magniﬁcation and smallest sampling area, 32X, uses one of the highest resolution settings, so few CCD pixels are combined into each output pixel. The CCD is not moving during the focus operation, instead, the same portion of the selected channel is read out and displayed as often as possible, usually faster than ten updates per second.

As each line segment is read out, the selected pixels are ﬁrst displayed as a graph of brightness vs. position in the rectangular box on the right side of the Focus panel. This graph shows each output pix- el’s 8-bit data value (relating to the selected Tone Curve) as its vertical position, with 0 at the bottom and 255 at the top. Best focus across high-contrast edges will be displayed as the most vertical (most abrupt) step changes in brightness (vertical position) (Fig. A); best focus across low-contrast textures will be displayed as the most jagged (most detailed) small changes in brightness (Fig. B).

This line graph can be helpful in establishing approximate focus, and in verifying that the desired image features are indeed being viewed (by correlating relative brightness and tex- ture, etc.), and have adequate variations for measuring focus.