Bokeh and other optical qualities with extension tubes

Monday, April 3 — St. Richard of Chichester

During the latest macro flower session I dealt with white flowers (Anemonoides trifolia brevidentata) and also tried backlight, two things that can exacerbate optical aberrations. Indeed, from my previous experience with the Samyang 8mm fisheye and a focusing helicoid, I already knew that sometimes they occur in a quite evident way. For what concerns the Nikkor 50mm ƒ/1.8D AF and the Helios 44-2, before today I never looked with care for aberrations, even though I could have bet on their presence.

In any case, the point is not about the presence or absence of those defects, rather about their amount and impact on the final image; and the possibility of reasonably fixing them in post-processing. During this session I took care of all of these issues.

Sony α6000 + Zenit Helios 44-2 58mm ƒ/2 @ 58 mm, 1/800 sec @ ƒ/2, +1.30 EV, ISO 100, focusing helicoid, beanbag.

Anemone con denti brevi (Anemonoides trifolia brevidentata).

Standard shots

In the image below, defects are not much evident. You have to enlarge the blurred borders of the petals to see some colour fringing, but it's nothing that would ruin a photo, according to my personal taste. So, it's to be said that problems aren't due to happen all of the time.

Sony NEX-6 + Nikkor 50mm ƒ/1.8D AF @ 50 mm, 1/200 sec @ ƒ/4, +0.30 EV, ISO 100, focusing helicoid, beanbag.

Fiore di anemone con denti brevi (Anemonoides trifolia brevident

Things are very different with the next photo. In the enlarged crop green fringing is visible at the borders of the rightmost petal of the flower, but some purple and green areas can be seen in multiple parts of the petals, not only in the borders. They are the so-called “bokeh fringing”, occurring behind and in front of the focal plane. In the unfixed original image, bokeh fringing was quite evident also without looking at the enlargement.

Now the question is: is it possible to fix this kind of problems in post-processing? Clarifying that for me “fixable in post-processing” means by the common tools in Adobe Lightroom, because — except for photos that have chances to become really outstanding — I don't have the time to export them to Photoshop and perform complex procedures, the answer is: partially, maybe in an acceptable way depending on the case. In fact the full size image, after the fixes, doesn't show any blatant aberration according to my taste. Indeed the colour fringing tools of Lightroom were ineffective; the fix was done by applying the maximum selective desaturation of the purple and magenta channels, and also applying -100 to the selective hue control for them. It worked because there are no other objects with purple and magenta colours in the photo, otherwise they would have been ruined. So, after a further step, one can say that problems might be fixable, eventually with some creativity.

Sony NEX-6 + Nikkor 50mm ƒ/1.8D AF @ 50 mm, 1/100 sec @ ƒ/4, +0.30 EV, ISO 100, focusing helicoid, beanbag.

Fiore di anemone con denti brevi (Anemonoides trifolia brevident

A more challenging backlight shot

The opening photo also had problems, which are demonstrated by the detail crops below: again purple and green colour fringing at the border of the white petals, but also at the border of leaves; and bokeh fringing inside the petals. In this case an acceptable fix was performed by applying a local brush with a maximum amount of defringing and some desaturation — relatively easy job.

Aberrations apart, for this photo I had to apply more saturation than usual, since with the strong backlight both lenses produced low contrast and pale colours (but not in the same way, see below). As a curious side effect the stamens of the flower looked yellow after a first treatment, which shouldn't be, as they actually are white with some pale-yellow hue: it's a key feature of this flower, that distinguishes it from a similar species (Anemonoides nemorosa) with positively yellow stamens. To compensate for it, the yellow channel needed some selective desaturation.

In the end, this photo required me much longer work in post-prodution than usual, not only to fix aberrations. It's the price to pay for not having a native macro lens, I suppose; but it delivered.


This session provided me with some valuable information. Clearly, the best way to get rid of optical defects in macro photos is to buy a native macro lens, which doesn't require extension tubes and that has been designed to minimise optical aberrations in its normal usage (on the other hand, many native macro lenses aren't completely free from defects, according to reviewers). I'm liking the 50mm focal for macro work, thus Sony's SEL50M28 would be an interesting option. But, as long as I am able to quickly fix the aberrations in post-processing, maybe with some trade-off, I don't feel the need of it. Sure, I can figure out some cases in which post-processing fixes wouldn't be effective and a native macro lens would be required: some more time will tell me how frequent these cases are.

It's also to be said that I like the bokeh of the Nikkor ƒ/1.8D AF and the Helios 44-2, and I'm not sure the SEL50M28 would be the same — also considering it's slower by a full stop. In particular, the opening photo was a very rewarding result for me in this session, resulting in a number of keepers, with some slight composition variants and different approaches in post-processing, with more and less saturation. It's to be noted, anyway, that fixing colour fringing was more or less effective in the different cases, because slight variations of the position of the flower resulted in different amounts of the aberrations. So I learned that with my current tools it's advisable to shot even more photos than usual to maximise the chances that a good quality keeper is achieved. Since shooting digital is practically free, not a big deal.

Sony α6000 + Zenit Helios 44-2 58mm ƒ/2 @ 58 mm, 1/1000 sec @ ƒ/2, +1.30 EV, ISO 100, focusing helicoid, beanbag.

Anemone con denti brevi (Anemonoides trifolia brevidentata).

As a final remark, I was a bit surprised by the difference of performance of the two lenses I used in this specific scenario. The Nikkor lens produced worse results than the Helios, both for the bokeh (less busy and more “painted” with the Helios), but also contrast was inferior (as it can be seen in the loss of details in the leaves, that appear as flat green areas, in the photo below). Honestly this photo wasn't post-processed with the same care of others because it was discarded early, but it's obvious that no post-processing tool can bring tonal details when there aren't in the original, at all.

More photos of the day are available in the diary.

Sony NEX-6 + Nikkor 50mm ƒ/1.8D AF @ 50 mm, 1/640 sec @ ƒ/1.8, +1.70 EV, ISO 100, focusing helicoid, beanbag.

Anemone con denti brevi (Anemonoides trifolia brevidentata).