Selecting a lens can be a tedious undertaking, with so many different specs and features to compare. Excluding price (a big consideration due to the cost of HD lenses), there are a few essential factors when purchasing a lens: sensitivity, contrast, resolution, color reproduction and geometrical distortion. Given how every single imperfection can be seen in high def, taking all these issues into account is critical to acquiring the best images possible.
Since these factors can get quite complicated, we’ll break them up into three posts. Today, we will highlight sensitivity and geometrical distortion. Subsequent posts will examine resolution, color reproduction, and contrast.
Lens sensitivity is critical because it creates a quieter picture with improved depth of field. A lens’ F rating only tells a user about the size of the iris opening, not how much light hits the sensor. Instead of focusing solely on the F rating, be sure to look at the T rating, which highlights the image transmission. T stop is essentially the F stop combined with the lens transmission losses.
You can lose up to 1/3 stop due to transmission loss, so this is a much more accurate representation of a lens’ true characteristics.
When comparing sensitivity between two or more lenses, it is important to use a camera with fixed level lighting and fixed camera gain. Using those settings, record a video of a gray scale test chart with the F stop opened as wide as possible. This will determine the maximum relative aperture performance of each lens.
The second factor we’ll look at in today’s post is geometrical distortion. To some degree, this occurs in every lens, although today’s lenses do a much better job reducing this distortion than their predecessors from five years back. Due to the internal mechanics and dynamics of a lens, the wider the lens angle, the more difficult it is to reduce geometrical distortion, such as “pincushion” and “barrel” effects.
The best way to determine a lens’ propensity for geometrical distortion is to use the
lens while shooting a studio set showcasing material with intersecting lines. Doors, windows, desks and even picture frames are perfect examples of content with orthogonality that will create geometrical distortion. A difference as small as 1% between lenses will have a big effect on the picture, even to a relatively untrained eye.
Check back here next week to learn about resolution and color reproduction. Or visit www.fujinon.com