fuzzy focus simulation
Toronto. Modern smart phone and digital camera buffs only see fuzzy focus when they move or try using dim lighting or are just too close to the subject. No need to understand photography these days to get correct lighting. Focus and framing and white balance need more skills.
When photography began, a lot of skill was needed. The portrait artist also had to select the equipment, the location, advertise, take the images, develop them, and present them to the sitter for payment. Considerable skill and learning was necessary for success! In those early days, light sensitive media was only blue and violet sensitive as these were the stronger wave lengths. Worse, early lenses were not always colour corrected so the more visible red and green colours focussed at a different plane than the blues and violets.
A practitioner, focussing carefully on his ground glass, would cause the blue light sensitive coating to be out of focus giving a blurry image (fuzzy focus) unless he corrected his focus for the effect. As sensitive media was improved, first to orthochromatic and later panchromatic, and lenses became colour corrected, visual and actinic or chemical focus merged.
This effect is nicely described in the Large Format Forum on actinic focus. A member using the name “Doremus Scudder” said this about actinic focus back on October 10, 20212: “… First, you only have to worry about “actinic focus” being different from the visual focus if you are using blue and/or UV sensitive materials. Modern panchromatic films are sensitive to the visual spectrum and, therefore, visual focus = actinic focus with these materials. And this is regardless of the lens being used.
“With things like collodion and other blue-sensitive emulsions, the problem still only becomes significant if you have a lens that is not well-corrected for chromatic abberation, i.e., one where the blue end of the spectrum focuses at a significantly different place than red and green. With some vintage lenses and meniscus single-element lenses, this is an issue with these materials. In the past, when lenses were less-well corrected and photographic materials were only blue-sensitive, correcting focus to compensate for this was more important than it is today. If you use modern lenses and pan film, there’s no issue at all.
“Even orthochromatic materials are sensitive to green, where the human eye is very sensitive. If you’re using a meniscus lens with ortho material, you might see a bit of focus discrepancy, but this could easily be remedied by focusing through a cyan filter, so that your eye is only seeing the same wavelengths as the film sees.
‘There is the problem of overall out-of-focus/softness in images made with lenses that haven’t been well color-corrected. When used with panchromatic film, there ends up being a range of focus; the blue end focusing in one plane and the red end in another, with everything else in between. With no filter and a multicolored subject, you’ll have a range of focuses, even with one subject. Imagine something magenta, with a mix of red and blue, and it’s edges focusing at two different planes and being rendered at two different sizes. This produces a halo typical of chromatic abberation. In this case, a yellow filter, which filters out the blue end, would improve overall sharpness, simply by eliminating one extreme and leaving only red and green, which focus more closely to one another.
“With older (or poor-quality) enlarging lenses and graded papers, which are only sensitive to blue light, there could be a discrepancy between actinic and visual focus. The usual remedy for this was to focus through a blue filter. A blue focusing filter was supplied with some grain magnifiers for just this purpose. Nowadays, enlarging lenses are generally well color-corrected, this is no longer a real issue, especially since most papers these days are VC and sensitive to green as well as blue.
“Keep in mind as well that the vast majority of photographic lenses pass very little UV. Glass is a poor transmitter of UV; lens cements as well. Plus, modern coatings do even more to limit the UV portion of a lens’ transmission spectrum. Lenses made of special materials (like silica and calcium fluoride) instead of glass, and even plastic, are needed for UV photography. So, even if you have materials that are strongly UV sensitive along with being blue sensitive, it’s only the visible blue (and maybe the near UV) that needs to be compensated for. If you, indeed, have a set-up with blue sensitive materials and a non-color-corrected lens, then applying a focus adjustment would be one way to compensate for the discrepancy. Using a blue filter would be another.”
