Focal Length

Delta lists many lenses, at many focal lengths – ranging from 7-700mm. But not all focal lengths are viable for all applications.

First, there’s a broad correlation between focal length and image circle: the 43mm diameter light-puddle needed to illuminate a full-frame sensor at infinity focus is usually not created by enlarger lenses – and hardly ever by projector lenses – shorter than 43mm. Notable tested exceptions include the Palar 40/2.8 and Hoya Super EL 40/3.5. But at working distances over 1m, most 50mm enlarger lenses have obvious vignetting at wide apertures – at least.

At the other end of the scale, lenses of focal lengths greater than 150mm will have their image circles truncated by the inner surface of a tube of M42 helicals or extension tubes. Therefore, most lenses of this focal length range should be adapted into M56 or larger helicals and stepped down to M42 at the camera mount.

On average, shorter lenses are optimised for higher magnification, and therefore resolve better. However, lenses of 35-60mm focal length rarely deliver a consistently sharp Zone 3 at all working distances: 50mm enlarger lenses, for instance, struggle to render aberration-free corners at distance: a task they perform brilliantly at working distances under 1m. Barely-there field curvature on a stand inevitably escalates into problematic field curvature at longer distances. Only very well corrected short-range lenses are useable without compromise outside their preferred working distance.

For a number of reasons, therefore, there is something of a sweet spot with regard to focal lengths: not too short that FFD and image circles constrict the range of focus, or movements – and not too long so that resolution suffers because of excessive air gaps or optical complexity. For a general purpose, short-range taking lens I would suggest this sweet spot exists between 60-90mm. Shorter lenses are compromised by their small image circles; longer enlarger and projector lenses (with a number of exceptions) tend not to be as sharp. Delta’s tests therefore prioritise this range.

When adapting enlarger lenses to digital cameras you hit hard limits based on flange-focal distance requirements. Mirrorless cameras made it possible to obtain infinity focus with 50mm enlarger lenses combined with slim adaptors, but not with tilt movements. But not all 80mm lenses, for instance, have the same F-Fd. In practice, adapting a Meogon 80mm requires a shorter helical than the Nikkor 80mm f5.6. Most 50-135mm enlarger lenses cover 35mm with plenty of space for movements, but when allowing space for a tilt mechanism and a focusing helicoid, around 80mm is a minimum when movements and infinity focus are required – although there are exceptional wide-angle projector and enlarger lenses that purposefully throw a large image circle despite a short focal length. These are noted in the archive. When infinity focus is not required (ie, for very short working distance and/or high magnification), many lenses with a nominal sub-43mm image circle can practically cover a full-frame sensor, especially when stopped down – which naturally increases coverage, and illumination levels of the outer circle.