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10-01-2011, 02:00 PM   #1
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The Effect of Lens Flare/Glare on Dynamic Range

I've just looked at a 2008 GoogleTech video of a talk by John McCann of Polaroid:


In it, he says that real-world flare limits DR to a max. of 3 "log units". (To convert from log units to stops, use this formula: log units / 0.3 = stops e.g. 3 log units / 0.3 = 10 stops).

In a flare test his lab performed, in a "minimum flare" setup, he measured 3.5 LU (11.7 stops) for negative film and only 2.8 LU (9.3 stops) for a digital camera, a Nikon Coolpix 900.

The talk was interesting, but I found some statements contentious. He claimed that at a glass-air interface the reflection was 3-5% "coated or uncoated", that lens flare was known about by lens manufacturers but not openly discussed, because it was so bad. He bought up a table on parasitic reflections showing how the number increased dramatically with the number of elements. This contributed to glare and loss of contrast. (Now, if you followed this topic to its logical conclusion, zoom lenses, with their large number of elements, are terrible. You should use a simple prime, with as few elements as possible - leave aberrations uncorrected and fix them afterwards in software.)

I was surprised that no real mention was made about the effectiveness of multi-coatings. I think that the internal handling of glare & reflections is one of major quality differentiators in lenses. Here are some links on coatings and aberrations:

https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/104892-pentax-...ro-bright.html

Coating, anti-reflection and dispersion
coating, anti-reflection and dispersion
Optics: all you need to know about optical aberrations
Optics: all you need to know about optical aberrations
Optics: all you need to know about optical aberrations

https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/48870-super-ta...tml#post470338

Flare control (particularly the scan of a large flare test in the Pop Photo Dec 73 issue: http://www.aohc.it/imart/a07f02.gif)

Dan.


Last edited by dosdan; 10-01-2011 at 04:43 PM.
10-01-2011, 04:15 PM   #2
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He's clearly mistaken...
10-02-2011, 05:24 AM   #3
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I guess he never tried a smc pentax lens.
10-15-2011, 04:25 AM - 2 Likes   #4
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Here's a test of a street lamp to see how much DR I could get without tone curve/contrast/blackpoint manipulation. I was interested in how much effect flare & glare has. In my case, the answer is "a lot".

Pentax K-5 & SMC Pentax-M 50/F1.7 at 1/500s, f/4.5, ISO80 (base sensitivity). 14-bit raw.

Sensel counts for each raw stop (using the RAWSHACK program):
Stop #/Range Red Green Blue Green_2
--------------- ---------- ---------- ---------- ----------
0 [00000-00001] 0 0 0 0
1 [00002-00003] 0 0 0 0
2 [00004-00007] 0 0 0 0
3 [00008-00015] 0 0 0 0
4 [00016-00031] 0 0 0 0
5 [00032-00063] 1,776 1,915 1,809 1,919
6 [00064-00127] 4,060,809 4,060,498 4,060,736 4,060,492
7 [00128-00255] 766 181 719 182
8 [00256-00511] 493 659 552 662
9 [00512-01023] 31 477 52 477
10 [01024-02047] 20 137 23 129
11 [02048-04095] 23 17 24 25
12 [04096-08191] 32 26 35 23
13 [08192-16383] 0 40 0 41

I kept the shutter speed very high (for night-time) to keep the top stop sparsely populated to reduce the possibility of any sensor blooming-induced glare, so the image appears quite dull. (This is mainly a problem with CCD sensors, usually in older video cameras where you'll see phantom vertical lines centred on lights in the frame where the whole column has been affected, but it can effect CMOS slightly too, but very localised. See http://micro.magnet.fsu.edu/primer/digitalimaging/concepts/ccdsatandblooming.html)

There are 9 stops DR in total, but with such a vast amount of darkish stuff at the 8th stop, I think you'd have to call this 8 stops of DR.

Raw processing in RT4 starting with the Neutral profile:

Completely neutral (no NR, EV boost etc)


100% crops (all starting with the Neutral profile in RT4):

Completely neutral


Boosted +5EV (blown highlights). Chroma & luminance noise is now visible, but no pattern noise.


Now with highlight-recovery applied to keep the boosted highlights below the clipping point.


Finally, with NR applied. The glare is quite obvious.



I was hoping to achieve a wider DR, but that was not the case here. I don't have any modern primes (the lens under test is about 30 years old). It's possible the aged inter-element lens cement might increase glare & flare, so a modern prime with modern coatings may do better.

Dan.


Last edited by dosdan; 10-15-2011 at 02:07 PM.
10-15-2011, 06:31 AM   #5
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QuoteOriginally posted by dosdan Quote
9 [00512-01023] 31 477 52 477
10 [01024-02047] 20 137 23 129
11 [02048-04095] 23 17 24 25
12 [04096-08191] 32 26 35 23
Thank you Dan.
10-15-2011, 10:25 AM   #6
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I did watch this talk. Most of the data he was basing the talk on went all the way back to the 60s and 70s when he worked for Polaroid. I do remember the photos of the lens flare and the HDR using film (to later scan it) but I don't remember that he said that they used newer MC type lenses at the time. He was also trying to minimize flare from the target but this also maximize contrast between the lighted subject and the surrounding non-lighted target. This guaranteed that the lens flare would exceed the noise and not be lost in the noise like in most photos.


Over all if you are trying to maximize DR in a HDR photo you should use a lens with less lens elements (like a prime) and try to not have high contrast elements next to each other. I had this problem with the Moon in the dark sky in a HDR photo.


DAZ
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