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owl eye sensitivity compared to camera ISO?

 
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Kevin Messenger



Joined: 03 Dec 2010
Posts: 3

PostPosted: Fri Dec 03, 2010 9:01 am    Post subject: owl eye sensitivity compared to camera ISO? Reply with quote

Hey everyone. I am so glad I came across this site. My main area is study is in snakes, and more specifically, nocturnal snakes. I have done some studies with the effect of moon phase and found some incredible correlations that were previously only anecdotal. One of the main thoughts behind this avoidance (snakes avoid bright nights) is due to nocturnal predators such as owls.

In my power point presentations that I give whenever I talk about this subject, I usually present the following image:



This picture was taken on a full moon during moon transit (the brightest minute of the night). The ISO was 1600 and the shutter was left open for 1 min.

In my presentations I pose the question, how does this compare to an owl's eye? Is this what they see, can they see better?

Does anyone have any insight into this? I appreciate it!
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owlboy
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Location: Albany, NY, USA

PostPosted: Fri Dec 03, 2010 9:53 am    Post subject: Reply with quote

Your question is a complicated subject, that we have had before on this forum. Check this link: http://www.owlpages.info/forum/viewtopic.php?t=1300&postdays=0&postorder=asc&highlight=blue+rods+eyes&start=15

Owls have exceptional vision and hearing. But, they cannot smell very well. They can hear a mouses heart beating from 30 feet (10m) away. They use both their exceptional hear and sight to hunt for food. Snakes are on their menu. Larger owls, such as, Great-Horned owls even hunt deadly rattlesnakes. They aim for a spot near the head. As soon as they snatch the snake, they will dispatch the head.

We have Timber Rattlesnakes near us. They are very careful to not be in the open where hawks and owls can prey on them. Timber Rattlesnakes are relatively heavy snake. They are as thick as a man's arm.

I would suspect that owls can see as well as the picture. Owls eyes are exceptionally large. They consume a large portion of their head. If I remember correctly, their eyes weight is 50% of the weight of their bones. They have significantly more blue cones (night vision) then humans.

Barn Owls have the best night vision. They can see perfectly well on the darkest nights. But, all owls see very well at night.

Your theory of snakes avoiding nights of bright sun light makes a lot of sense. Owls are most active on nights of good weather, and the brighter the moon, they more likely they will be out hunting. But, on windy nights they may not hunt. That would be an interesting study.
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Kevin Messenger



Joined: 03 Dec 2010
Posts: 3

PostPosted: Fri Dec 03, 2010 11:33 am    Post subject: Reply with quote

Thanks for the response and link. I knew they had great hearing but didn't know it was that extensive. I was also aware that owls tend to perch higher on bright nights and rely mostly on vision, and during low light nights they perch lower and rely more on hearing as opposed to vision.

I have witnessed owl-snake predation on 2 occasions. An owl was perched on top of a cottonmouth one night in the Everglades National Park, and another in the same night tried carrying off a corn snake. Another friend had a pigmy rattlesnake picked up by an owl right in front of his vehicle one night.

I was hoping to see if anyone had a publication or any hard numbers for sensitivity in low light situations. Either in comparisons to film speeds (i.e., 100, 200, 400, 800, 1600, etc etc) or by simple human comparisons (i.e., 2x better nocturnal vision than humans, 5x, 10x, etc etc). I noticed in the link you provided it basically boiled down to "don't try to compare humans and owls" - but I am hoping that is just in response to the exposure to spotlights and such.

The image I posted was taken with ISO 1600 and shutter speed 1 min, which also translates to ISO 25600 and 3.25 seconds. So basically if owls can open their eyes and adjust within a few seconds (which I recall reading about the fast adjustment period) and experience that much clarity, then that would put owl eyes on par with 25,600 ISO. If they could do it instantly, then that would be more like 51,200 ISO.

Whether or not my questions can be answered with hard numbers (and hopefully scientific citations/ publications), I appreciate the help and insights I've already gotten from this forum.

Best,
Kevin
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Kevin Messenger



Joined: 03 Dec 2010
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PostPosted: Fri Dec 03, 2010 11:35 am    Post subject: Reply with quote

just found this on human eyes and the "equivalent ISO:"

"The Sensitivity of the Human Eye (ISO Equivalent)

At low light levels, the human eye integrates up to about 15 seconds (Blackwell, J. Opt. Society America, v 36, p624-643, 1946). The ISO changes with light level by increasing rhodopsin in the retina. This process takes a half hour our so to complete, and that assumes you haven't been exposed to bright sunlight during the day. Assuming you wear sunglasses and dark adapt well, You can see pretty faint stars away from a city. Based on that a reasonable estimate of the dark adapted eye can be done.

In a test exposure I did with a Canon 10D and 5-inch aperture lens, the DSLR can record magnitude 14 stars in 12 seconds at ISO 400. You can see magnitude 14 stars in a few seconds with the same aperture lens. (Clark, R.N., Visual Astronomy of the Deep Sky, Cambridge U. Press and Sky Publishing, 355 pages, Cambridge, 1990.)

So I would estimate the dark adapted eye to be about ISO 800.

Note that at ISO 800 on a 10D, the gain is 2.7 electrons/pixel (reference: http://clarkvision.com/articles/digital.signal.to.noise) which would be similar to the eye being able to see a couple of photons for a detection.

During the day, the eye is much less sensitive, over 600 times less (Middleton, Vision Through the Atmosphere, U. Toronto Press, Toronto, 1958), which would put the ISO equivalent at about 1. "

[additional notes] So, perusing other sites I came across the statement that in low light conditions, tawny owl eyes were approximately 100 times more sensitive than humans. To extrapolate this with the above info, that would mean owls (at least tawny owls) have an ISO equivalent of about 80,000.

In that photograph I took, since shutter speed and ISO and directly inverse of one another you can also extrapolate other values that would produce the same image such as:
1600 ISO/ 1 min shutter
3200 ISO/ 30 sec shutter
6400 ISO/ 15 sec shutter
12,800 ISO/ 7.5 sec shutter
25,600 ISO/ 3.25 sec shutter
51,200 ISO/ 1.6 sec shutter (pretty close to "instant" in my opinion)

This would mean, that at 80,000 ISO, an owl would be able to "develop" this image in less than 1.6 secs. Pretty cool stuff!
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owlboy
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PostPosted: Sat Dec 04, 2010 3:48 am    Post subject: Reply with quote

There has been some research done on this subject, but I can't remember where. They also compared different species of owls. You might want to look around for that info.

Also, did you see both pages of the previous listing. The second page had more info on it.

There is also info on the number of blue cones in owls versus humans, which may help you.
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Ninox1



Joined: 13 Feb 2011
Posts: 38
Location: Sydney, Australia

PostPosted: Sun Feb 13, 2011 6:06 pm    Post subject: Reply with quote

That is an Awesome photograph! I am not sure how sensitive an owls vision is but Australian Ninox owls often hunt in forest and fly through trees rarely injuring themselves so it must be extremely sensitive.

An owls eye (and most nocturnal animals) is packed with low-light sensitive rods and has relatively low levels of cones which are required for colour vision. Consequently I think they would see in something close to monochrome rather than the vivid colours of your image.
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