Trying to understand Optical Lenses

Peatstack

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Me again Hi,
Now remember this is a beginners forum so here goes. Why is it when you look through an optical lens things look smaller and further away than with the naked eye? until you zoom in. Its like looking through the wrong end of a telescope.

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Tony S

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Because it's built to show a wider field of view. Normal vision is considered to be comparable to a 50mm focal length. In simple terms anything shorter than that is wider and anything longer is narrower (making it appear closer) than your field of vision without a lens.
 

Gavjenks

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Me again Hi,
Now remember this is a beginners forum so here goes. Why is it when you look through an optical lens things look smaller and further away than with the naked eye? until you zoom in. Its like looking through the wrong end of a telescope.

Peatstack.
I'm not sure what you mean. What kind of lens are you talking about? What you say is not true of all lenses. Also, what do you mean by "until you zoom in?"

And where are you holding your eye versus where is the object that is appearing smaller? As you change the distance from your eye to the lens and from the lens to the object, depending on the focal length of the lens, different things will happen. You can make it appear smaller OR larger, right side up OR upside down, etc.

I can't really explain what you're seeing without more info.
 
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Peatstack

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Well Gav, I am looking through the viewfinder eye piece at minimum (18mm) zoom and I think TonyS has explained it. It gives a wider angle and so makes things look smaller than with normal vision. By zooming in I mean slowly increasing the zoom up to 55mm. Things start to look more normal. Hope I'm being a bit clearer.
 

Gavjenks

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Ah, well yes, wide angle lenses show... well, a wide angle. The wider the angle, the more stuff is crammed into the scene, and thus the smaller it all has to be.

In terms of how wide of an angle is wider than human vision? That's a much more complicated question. Saying the eye has a 50mm focal length is, well, wrong, and it's overly simplistic to assign a single value at all anyway (Although 50mm is often said to be a "normal" focal length for reasons I have never understood).

Factors to consider:

1) The human eye has a variable strength lens. As muscles pull on the lens it changes its refractive strength. This is not really very equivalent to anything in a camera lens, per se.
2) The actual physical distance from your lens to your retina is not constant, because your retina curves, making it fundamentally unlike a camera in many basic ways.
3) If you insist on taking a distance measurement, the straight line distance from the lens to the back of the retina is about 1 inch, 24-25mm. Although again, since the lens can change shape, this shouldn't be taken as very meaningful. My Canon camera has a distance from the lens to the sensor of 45mm. That doesn't mean all my lenses have a 45mm focal length. The meaning of distance from a lens changes with both refractive strength and the distance of the object you desire to focus on.
4) The angle of view covered by a single eyeball is about 150 degrees, which would be the equivalent of a full frame camera lens of something like 10-12mm. The angle covered by your binocular vision is significantly larger than this, closer to 180 degrees, which would require a 0mm rectilinear lens, and can thus only be matched with a single camera lens using distorting fisheyes, or a pinhole punched into infinitely thin material.
5) The eye does not have a uniform distribution of photoreceptors (Most of your vision happens at the fovea), nor does it have a particularly rectilinear projection.
6) You can't actually see an entire scene in focus and detail both at once like your camera sensor can, so you are constantly darting your eyes aroudn to take in a scene, This has a huge effect on your practical, real life effective angle of view and "focal length" equivalent, since you are covering a larger area in reality than the eyeball structure would imply. You may eve turn your head to take in a scene, providing potentially up to 360 degrees of visual angle, etc.
7) The eye has a much "smaller sensor" than a full frame camera does. Your retina is about 20%-ish the size of a full frame DSLR sensor, which changes all the millimeter equivalents and blah blah.
8) Your brain does all kinds of crazy **** to the image after it hits your retina. Much MUCH more than your camera does to images after it hits your sensor. Half of what you see is more or less not there at all, and 100% of it is filtered/doctored at least a fair amount.
9) You have to take into account the distance you're going to be viewing the print from if you're talking about a print or an image on a computer. If an image has the same angle as your eye, for instance (let's say 10mm, and assume you are blind in one eye and the other is paralyzed and can't move, to make it comparable to a camera), yet you make an 8x10 print of it and look at it from 2 feet away, then it's going to look oddly wide, because you're used to only seeing about an 85-90mm lens' visual angle in that SUB PORTION of your vision. If you look at it from half an inch away, it won't seem odd anymore in terms of perspective. So what will look like a "normal focal length photo" actually depends on the size of the print, the viewing distance, AND the focal length.

or if you're looking into a viewfinder, you have to take into account the portion of your normal visual field that the image in the viewfinder takes up. It is NOT 100%!!! When you look into a viewfinder, you see an image plus a whole bunch of black nothingness around it. The blackness is space that you would normally be seeing stuff in.

Thus, to figure out what the "normal" focal length is, you need to figure out what angle of view your eye would normally see within that portion of its angle of view. One easy way to do this is to hold your camera in portrait mode, look through the viewfinder with one eye, look at the object with your other eye straight up, and zoom until the two images match in size.

When I do this, I usually get about 70mm on my lens before the two images are exactly equal, on a full frame DSLR. Usually.



However, that number will change based on the size and shape of the viewfinder, it might change if I'm tired or drunk, and it doesn't represent the way you normally look at a scene (darting your eyes around), and it doesn't take into account the area that your OTHER eye would normally add to your vision (you can only look through a viewfinder with one eye, but you look for things to photograph with TWO eyes), etc. etc.
 
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Gavjenks

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TL;DR:

If you want to fully understand any of this, you probably would need to get a doctoral degree in cognitive/perceptual psychology. And then continue studying it for another couple decades after that.

And none of it is terribly relevant to being a good photographer. I'd say you shouldn't worry about it, unless it just interests you in and of itself.
 
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Peatstack

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Thanks Gavjenks, Wow theres a lot of info to take in there but I think I get your drift.
Thank you too gsgary but every body does the stones. Every time we have visitors it's up to the Calanish Standing stones for photos, All the photo galleries are full of the Stones. I like to get out on the moors and around remote lochs to try to get something different. I must admit though every time I go to the Stones the light seems to be different. Thats one thing on this Island each day is different because of the light on rocks, moors and lochs. The wild life is really great. I'll stop.
 
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gsgary

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Thanks Gavjenks, Wow theres a lot of info to take in there but I think I get your drift.
Thank you too gsgary but every body does the stones. Every time we have visitors it's up to the Calanish Standing stones for photos, All the photo galleries are full of the Stones. I like to get out on the moors and around remote lochs to try to get something different. I must admit though every time I go to the Stones the light seems to be different. Thats one thing on this Island each day is different because of the light on rocks, moors and lochs. The wild life is really great. I'll stop.

Study the stone shots then go up and shoot them in a different way it would be a good exercise, when i take out new members of our club its fun to see how every one shoots things differently
 

Helen B

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Well Gav, I am looking through the viewfinder eye piece at minimum (18mm) zoom and I think TonyS has explained it. It gives a wider angle and so makes things look smaller than with normal vision. By zooming in I mean slowly increasing the zoom up to 55mm. Things start to look more normal. Hope I'm being a bit clearer.

There is quite a simple explanation for that, and it has nothing to do with human vision, oddly enough. It is simply the combination of the lens' angle of view and the viewfinder's magnification. There will be one combination of lens angle of view (or lens focal length) and viewfinder magnification (usually fixed) that results in 1x overall magnification - so the view through the viewfinder looks like the actual view. Nothing to do with the properties of the eye at all, in fact.

edit: The focal length at which this 1x magnification happens need not be the 'normal' focal length for the format.
 
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Peatstack

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Thanks Helen B,
I guess what I am looking for is an exact copy of what i see in my minds eye if you like. It seems to me that unless your intention is to create a work of contemporary art, that images just remind you of what you have seen, not copy what you actually see. Can you set your camera to take what you see or is that the holy grail of cameras.A perfect visual copy of real life.
 

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Real life is 3-D. A photograph is 2-D, though a stereo-graph can create the illusion of 3-D.

I doubt technology will ever be able to make a perfect visual copy of real life.
 
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EDL

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Ah, well yes, wide angle lenses show... well, a wide angle. The wider the angle, the more stuff is crammed into the scene, and thus the smaller it all has to be.

In terms of how wide of an angle is wider than human vision? That's a much more complicated question. Saying the eye has a 50mm focal length is, well, wrong, and it's overly simplistic to assign a single value at all anyway (Although 50mm is often said to be a "normal" focal length for reasons I have never understood).

I think the first link KMH provided explains this well enough:

"Each eye individually has anywhere from a 120-200° angle of view, depending on how strictly one defines objects as being "seen." Similarly, the dual eye overlap region is around 130° — or nearly as wide as a fisheye lens. However, for evolutionary reasons our extreme peripheral vision is only useful for sensing motion and large-scale objects (such as a lion pouncing from your side). Furthermore, such a wide angle would appear highly distorted and unnatural if it were captured by a camera.

Our central angle of view — around 40-60° — is what most impacts our perception. Subjectively, this would correspond with the angle over which you could recall objects without moving your eyes. Incidentally, this is close to a 50 mm "normal" focal length lens on a full frame camera (43 mm to be precise), or a 27 mm focal length on a camera with a 1.6X crop factor. Although this doesn't reproduce the full angle of view at which we see, it does correspond well with what we perceive as having the best trade-off between different types of distortion"
 

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