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Blooming and Glow


david strachan

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I'm having a bit of trouble understanding if these terms are synonymous.  And exactly what causes these phenomena in lenses...and sensors (?).

The effects can be attractive, or not, to various photographers...usually depending on the subjects.

 

I suppose veiling flare comes in here too. But that tends to be across the whole subject, rather than blooming or glow.

 

Perhaps there are some experts out there who can help me understand.

 

cheers..

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I don't know of any formal definitions, but since nobody else has offered their interpretations--

 

I think it worthwhile to differentiate flare, glare, bloom, and glow. This is how I understand those words in the narrow context of photography:

 

{tl;dr: flare results in spots, glare results in reduced global contrast, bloom spreads exposure from bright sources, and glow spreads all light near the focus plane.}

 

Flare refers to distinct circles or blobs that fall along a line that includes a bright light source, usually point, and the lens axis. I believe flare is caused by internal reflections among lens elements, but can also come from reflections off bits of the lens or the sensor itself. Simple reflections with uncoated lenses lead to big spots that simply have more exposure than other areas. Simple reflections with coated lenses usually develop color. Complex reflections develop into blobs, and vignetting caused by lens shades or the simple limitation of sizes of elements can crop off parts of simple or complex blobs. Reflections off non-optical bits of the camera or lens are almost always unwanted. Artistically, I consider flare to be problematic unless the intention is to draw the viewer's attention to the act of photography itself.

 

Glare is an elevation of shadow exposure across the frame. I believe glare is also caused by internal reflections, but behaves in such a way that there is no clear cut-off between flare spots and the rest of the image; in other words, the flare spots are larger than the image circle, or diffuse reflections come from the lens' non-optical internal bits. I suspect "veiling flare" is a subset of "glare," but in practice are essentially the same. The difference between flare and glare is in whether the effect is readily identifiable as a visual artifact or not, which is caused by what is adding the technical imperfection in the first place (such as which elements are reflecting that light). Functionally, glare lowers global contrast, which facilitates the capture of high dynamic range scenes but compresses the information captured in low dynamic range scenes.

 

Bloom is an elevation of exposure in proximity to bright light sources in the scene. Bloom looks like a sunstar without the distinct lines (which in turn, I believe, come from reflections off the edges of aperture blades). I believe bloom is caused by imperfections in the lens elements that catch and refract light in a fan rather than toward a point, which is usually below the threshold of perceptibility with scenes of normal dynamic range. I tend to see blooming in uncoated lenses, and I believe that this is not due to the anti-reflective nature of the coating but rather the coating acting as a high precision finish on the elements the deters fanning of bright light; I'm not very confident in this, however, so am likely wrong. :-) In practice, to my eye, blooming does a very good job at telling the viewer that "this 100% saturated white point is brighter than 100% implies," and draws things like lightbulbs as glowing rather than just hard white blobs. I rather like bloom, but it is a specialty effect.

 

Glow is a spreading of image points near the focus plane; unlike flare, glare, or bloom, glow is still apparent even in low dynamic range scenes. Glow is primarily visible in the transition from in-focus to slightly out-of-focus. I understand glow as the result of spherical abberation, which is visible in pretty much every fast lens, especially (but not limited to) those with no aspheric surfaces. The thing I like in particular about glow from Leica lenses it that they tend to be very well color corrected, meaning that the subject just appears to be softly radiant, while comparable lenses without such good color correction make the subject appear to glow with a colored, comic-radioactive tinge. Glow does inherently reduce contrast at high resolution, though, while the other artifacts can maintain high contrast at high resolution; which is "better" depends of course on the photographer's intent.

 

Hope this helps,

Jon

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Thanks Jon.  A very thorough explanation which all makes entirely good sense.

It's interesting you see all these "defects" due to lens design, including use of coatings, rather than "oversaturating/overpowering" of the sensor (or film).

 

I'm very happy with your explanations, and thank you for the effort.  I'll reread this again to make sure I haven't missed any subtleties.

I knew I'd have to wait for the right person to answer my question.

 

all best...

Edited by david strachan
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Thanks Jon.  A very thorough explanation which all makes entirely good sense.

It's interesting you see all these "defects" due to lens design, including use of coatings, rather than "oversaturating/overpowering" of the sensor (or film).

 

I'm very happy with your explanations, and thank you for the effort.  I'll reread this again to make sure I haven't missed any subtleties.

I knew I'd have to wait for the right person to answer my question.

 

all best...

 

 

Cheers, mate. [...if I may attempt to imitate an Australianism that I'm sure I don't properly understand... I recall one of my presidents famously doing the same with two fingers in the intended shape of a peace gesture...]

 

All of these things are lens issues, except for where digital sensors exacerbated what film already did (in reflecting more light back into the lens). I'd love to be able to show you samples with different Leica lenses, but I'm unfortunately not in the position to have a full archive of tools to work with. But, lacking that, if there are any faults in my clarity which you would be willing to point to, I'd be happy to try to clarify them. I'd likely learn from that, too.

 

All the best,

Jon

 

PS: if I don't respond promptly, that just means my attention was otherwise engaged. Send me a direct message and I'll be more likely to catch up. :-)

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