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Old Jun 30, 2010, 5:58 AM   #1
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Default I don't understand fish eye lens!

I have seen 15mm fish eye lens that say they have a 180 field of vision. If this is so, how come you can put a lens hood on them?

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Old Jun 30, 2010, 6:44 AM   #2
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You usually do not put a lens hood (or filters for matter) on them. The lens hoods that are built on, are minimal so that there is something to slide a lens cap onto, and to protect the lens if bumped. Plus its FOV is 180 degrees in the horizontal axis and not the vertical axis.
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Old Jun 30, 2010, 7:34 AM   #3
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The 180 angle of view is from corner to corner. The lens hood prevents flare by blocking light coming in at 180 from the top or the side.
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Old Jun 30, 2010, 9:05 AM   #4
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BTW, a 15mm lens only provides a 180 angle of view on a 'Full Frame' camera body. On an APS-C dSLR, it's just a wide angle lens.
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Old Jun 30, 2010, 11:51 AM   #5
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Hello TCav. When I first got here and was asking about lens changes with the crop factor I thought you told me the only thing that changed was the focal length? Does that not apply in this case because the lens in question is an extreme example?

This is the lens that I have. http://www.mhohner.de/sony-minolta/onelens/af16f28fish When it says "rectangular fisheye", this is due to the size and shape of the sensor correct? If that is the case is there a way to compute the actual field of view and angle changes relating to change in the size of the sensor?

If i am looking at your drawing correctly it seems that the lens is seeing much more than the sensor is capable of capturing. (I dig it when you include the drawings)

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Old Jun 30, 2010, 1:18 PM   #6
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Quote:
Originally Posted by Old Boat Guy View Post
When I first got here and was asking about lens changes with the crop factor I thought you told me the only thing that changed was the focal length? Does that not apply in this case because the lens in question is an extreme example?

This is the lens that I have. http://www.mhohner.de/sony-minolta/onelens/af16f28fish When it says "rectangular fisheye", this is due to the size and shape of the sensor correct? If that is the case is there a way to compute the actual field of view and angle changes relating to change in the size of the sensor?

If i am looking at your drawing correctly it seems that the lens is seeing much more than the sensor is capable of capturing. (I dig it when you include the drawings)
Focal Length is a physical property of a lens, and doesn't change with the camera it's attached to. If the camera has a smaller image sensor, it will record a smaller portion of the image projected by the lens. See 'Focal Length' and 'Crop Factor'

I don't know how I may have misled you, but I'm sorry if I did.

The Sony (Minolta) 16mm f/2.8 Fisheye projects an image with a 180 angle of view onto the focal plane. It's designed for 35mm film SLRs and 'Full Frame' dSLRs. If you use it on an APS-C dSLR, you just get the center portion of the image projected by the lens.
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Last edited by TCav; Jun 30, 2010 at 1:28 PM.
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Old Jun 30, 2010, 11:20 PM   #7
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I had read your article on focal length and crop factor a number of times and think I just missed it somehow. You did not mislead me on anything. I think this one was a case of my trying to absorb so much in a short time and just missing this. When I first read that section and got to the part about bending light I think I stopped there to work out the trig as a mental exercise and then forgot to complete my homework on the remainder of the section.

Thank you for the explanation. I thought I was seeing something funny relating to the angle of view just could not put my finger on what was happening.

Steve

This does bring up another lens question. For the results I am looking for to do the real estate work the diagram above would indicate the need for a 10mm lens (duplicating the results of the 16mm on my film cameras). With that in mind do you have any suggestions which models and brands I should look at (or to avoid)?

Last edited by Old Boat Guy; Jul 1, 2010 at 2:35 AM.
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