Pointless academic question re lenses!

philthejuggler

philthejuggler

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Hiya - this has little or no bearing on photography...but

Why does the image I see when I look through the back of a lens not appear like the image in the viewfinder?

It looks totally out of focus unless I hold it 10inches in front of my face, yet the CCD would only be a short distant behind the rear element if it was mounted on a camera!

Thanks in anticipation

Phil

Oh, and while we're on annoying and pointless random questions - what the hell's the proper name for the moving walkway thing that you walk out of the front of an aeroplane into the terminal through?
 
Don't you want to know why the image is up-side down too?:naughty:
 
Yv said:
its just your lousy eyesight Phil :exit: :p
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Obviously one of my leisure activities is causing progressive eye deficiency!
 
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FourRingCircus said:
Don't you want to know why the image is up-side down too?:naughty:
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I stand on my head to avoid that problem...
 
I tried that but kept spilling my drink.:shake:
 
It's probably something due to the filter in front of the sensor, performs a similar function to the ground glass in your camera, which is the same distance from the end of the lens as the sensor.

The answer to your second question, it's a Travelator isnt it?
 
Your eyes are imaging the back of the lens, which isn't what the bare sensor.

Try using a camera to take a photos of the back of the lens, it should look like how your eyes see it as the camera is like a "eye unit"
 
Photographic lenses are designed to project the focussed image on a spot an inch or so behind the rear element - the eye's lens then gets in the way and tries to focus that image and fails. There are all sorts of clever optics in the mirror box and pentaprism/pentamirror which flip the image right way up and focus it - actually, the image is projected onto a ground glass screen through the mirror then flipped by the odd number of reflections in the penta (Greek for 5) prism/mirror.

And yup, travelator.
 
robhooley167 said:
It's probably something due to the filter in front of the sensor,
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Nah.. the image would still be formed without that.

It's because your eye already has a lens in it... or it wouldn't work :) Your "film" or "sensor" is like your eye's retina.. and the camera lens is like your cornea. If you look through a lens, you are effectively focusing it twice.. once with the camera lens, and once again with your eye's own lens.

For a lens to work, it has to focus parallel rays of light into a focal point, but the image from a lens is already being focused, so we need an eyepiece to magnify that image by correcting the focussed image back into parallel rays that our eyes can refocus.

OZhNg.jpg
 
Yv said:
I have warned you before about getting involved in the late night mass debates. :nono: :nuts:
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I used to be a teacher and once made the mistake of saying 'Lets all have a mass debate about fox hunting' to a group of 13/14 year olds :(

Nod said:
Photographic lenses are designed to project the focussed image on a spot an inch or so behind the rear element - the eye's lens then gets in the way and tries to focus that image and fails. There are all sorts of clever optics in the mirror box and pentaprism/pentamirror which flip the image right way up and focus it - actually, the image is projected onto a ground glass screen through the mirror then flipped by the odd number of reflections in the penta (Greek for 5) prism/mirror.

And yup, travelator.
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That makes lots of sense - now I understand, and thanks for the travelator info!

Pookeyhead said:
Nah.. the image would still be formed without that.

It's because your eye already has a lens in it... or it wouldn't work :) Your "film" or "sensor" is like your eye's retina.. and the camera lens is like your cornea. If you look through a lens, you are effectively focusing it twice.. once with the camera lens, and once again with your eye's own lens.

For a lens to work, it has to focus parallel rays of light into a focal point, but the image from a lens is already being focused, so we need an eyepiece to magnify that image by correcting the focussed image back into parallel rays that our eyes can refocus.

OZhNg.jpg
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The diagram helps a lot - many thanks for the info!

Phil
 
robhooley167 said:
It's probably something due to the filter in front of the sensor, performs a similar function to the ground glass in your camera, which is the same distance from the end of the lens as the sensor.

The answer to your second question, it's a Travelator isnt it?
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Aaaah - travelator!
 
Hold a sheet of white paper 44mm (Canon's lens mount to sensor distance) behind the lens and the image formed will be exactly what falls on the sensor.
 
HoppyUK said:
Hold a sheet of white paper 44mm (Canon's lens mount to sensor distance) behind the lens and the image formed will be exactly what falls on the sensor.
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Fab - that's clear' Thanks

Phil
 
philthejuggler said:
Fab - that's clear' Thanks

Phil
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You'll notice that the image is inverted - handy to know when trying to remove a stubborn bit of dirt from the sensor. Eg, if it appears in the top right of the photo, it will be on the bottom right of the sensor as you look into the camera.
 
HoppyUK said:
You'll notice that the image is inverted - handy to know when trying to remove a stubborn bit of dirt from the sensor. Eg, if it appears in the top right of the photo, it will be on the bottom right of the sensor as you look into the camera.
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**about to get some paper and try that, just for the hell of it**

is it simply flipped though, or completely inverted, thats the bit I can never remember... crap on top right of pic would be bottom left or bottom right of sensor? :thinking:


edit: forget it, course its totally inverted, which IS bottom right, as you look in... DOH!! See, this is why I always get confused
 
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LOL It's confusing! The image is actually inverted and reversed left-to-right, but when you look straight into the camera, that reverses the reversing :D
 
Nod said:
There are all sorts of clever optics in the mirror box and pentaprism/pentamirror which flip the image right way up and focus it -

And yup, travelator.
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surley its upside down ??

sorry Richard ,,,for some reason your reply didnt show until after i posted
 
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Well, yes but IIRC, the image projected on the retina is upside down but a rather clever processor works its magic and makes it appear topside up.
 
what the hell's the proper name for the moving walkway thing that you walk out of the front of an aeroplane into the terminal through?
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I believe it's called a travelator or at least that's what they called the one on the drain (waterloo and city line)

Not sure i'm thinking of the same thing though, this is like an escaltor without the steps and they have them in airport teminal buildings
 
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BTW your eye is exactly like camera lens in the way it projects an image onto your retina - the REAL image is upside down and reversed but your brain has learned to interpret the image by reversing it so you get a view of the world as it is.

A good few years ago a university designed a pair of special glasses for volunteers to wear - these reversed the image of the world in the same two directions.

At first the volunteers were stumbling around as they tried to cope with the world being all wrong, but after a few days the brain did its magic and suddenly the world was normal again.

BUT when they took the special glasses off they once again saw a reversed version of the world - upside down and reversed l/r.

It took a few more days for the brain to re-adjust itself once again.

.
 
I read Phil's original post " what the hell's the proper name for the moving walkway thing that you walk out of the front of an aeroplane into the terminal through?" as meaning the walkway was moving (travelator) rather than the whole shebang moving (jetway)! At my age, I'm generally glad of the travelators whenever I use an airport with them. Saves my short fat hairies and my back a lot of strain. And, I'm lazy!
 
Nod said:
Well, yes but IIRC, the image projected on the retina is upside down but a rather clever processor works its magic and makes it appear topside up.
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Put's on Neil Morrissey "Men behaving badly" voice....

Or..... maybe the world is actually upside down and the brain doesn't have to sort it out?
 
Without pointing or referring to your surroundings, explain left and right to an alien...

I'm now far more aware of which way up is than I was 2 months and a day ago!
 
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