jbcohen
Senior Member
What does it mean when I say I have a 50mm lens?
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50mm
- Thread starter jbcohen
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- sony fe 50mm f1.8
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Labe
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the classic kit lens from the 35mm film days. known to be pretty much the same field of view as the human eye. Makes getting a good looking photo all the harder , until you use it on apsc where its mimicking a telephoto, most of use as our first portrait lens lol.
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jbcohen
Senior Member
That means that a 50mm lens has a d0mm field of view?
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Labe
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50mm is the focal length of the lens which doesn't change (50mm is 50mm). field of view is something different . this alters when your 50mm lens is placed infront of different size sensors .
BrotherBart
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This thread is pretty helpful on the topic.
http://www.dpreview.com/forums/thread/2473862
Some things about focal length and crop sensors. You have to multiple the focal length AND the aperture to get an equivalent full frame including DOF.
That's to say, if you have a 50mm F1.4 lens on a 2x crop factor camera it will behave similarly to a 100mm F2.8 lens on a full frame. Many lens makers are NOT including the aperture with their specs, giving some the impression a 50mm F1.4 lens on a 2x crop factor camera will be the same as a 100mm F1.4 lens on a full frame and that is not the case the Depth of field is also affected.
Also to help understand what you'll see with different focal lengths, lenses work in 1/4ths. That is, take the view you see with your 50mm lens and break the view into 4 equal parts. Each of those parts will be the view of a 100mm lens on your camera. Take that view, break it into fourths, and each view will be what a 200mm lens sees. Do it again, and each will be the view of a 400mm lens.
So the difference in view between 14mm and 28mm is more than the difference between 200mm and 300mm.
Hopefully I didn't confuse you too much...
http://www.dpreview.com/forums/thread/2473862
Some things about focal length and crop sensors. You have to multiple the focal length AND the aperture to get an equivalent full frame including DOF.
That's to say, if you have a 50mm F1.4 lens on a 2x crop factor camera it will behave similarly to a 100mm F2.8 lens on a full frame. Many lens makers are NOT including the aperture with their specs, giving some the impression a 50mm F1.4 lens on a 2x crop factor camera will be the same as a 100mm F1.4 lens on a full frame and that is not the case the Depth of field is also affected.
Also to help understand what you'll see with different focal lengths, lenses work in 1/4ths. That is, take the view you see with your 50mm lens and break the view into 4 equal parts. Each of those parts will be the view of a 100mm lens on your camera. Take that view, break it into fourths, and each view will be what a 200mm lens sees. Do it again, and each will be the view of a 400mm lens.
So the difference in view between 14mm and 28mm is more than the difference between 200mm and 300mm.
Hopefully I didn't confuse you too much...
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Mark S Abeln
Forum Pro
It's a 50 mm focal length.
As a first approximation, think of the lens as being a pinhole, located 50 millimeters from your sensor. Imagine straight rays of light coming from the scene, passing through the pinhole, and hitting the sensor.
From this simple geometric model, you can probably see that longer focal lengths on a given sensor give you narrow angles of view, and that a smaller sensor will also give you a narrower view than a larger sensor at any given focal length.
If you know the size of your sensor, you can make your own lens calculations.
Alex Ethridge
Veteran Member
The above, in my opinion, is the best and most understandable explanation -- provided you know what a pinhole camera is (emphasis on "provided"). The very first cameras were all pinhole cameras.
When I was a kid (human type, not the goat), we used to make pinhole cameras using an oatmeal box. On one end, we put a tiny hole in the center and covered it with black tape. We would go into a light-proof room and inside the removable top, we would tape a piece of unexposed film. Place the top back in place, tape it shut all the way around with black tape.
In those days, oatmeal boxes were light-tight provided the seam and hole were covered. Not so any more.
Then we would go somewhere outdoors on a bright day, mount the oatmeal box in a stable location, remove the tape for a few seconds, put the tape back on, return to the darkroom and develop the film.
Now, here's the focal length. The focal length of the oatmeal-box "lens" is the distance from the pinhole (AKA the zero parallax point) to the film. The focal length of today's more sophisticated glass lenses are measured the same way except it takes a lens designer-engineer to design it so the zero-parallax point is 50mm (or other desired distance) from the film or sensor, more mm is more telephoto, less mm is more wide angle.
A typical oatmeal box might be a 180mm "lens". Cut the box in half and it might be a 90mm "lens". Tape two boxes together and you get a 360mm "lens".
Cameras today are really just highly complicated, very expensive and infinitely more useful improvements on the "oatmeal box" pinhole cameras. :-D
Images of pinhole cameras.
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That is only true when the lens is focused on a subject at an infinite distance, like Orion. Your term "optical center" can be confusing. One interpretation could be the optical center-line, which would make your statement quite wrong. The correct term to use is the "exit pupil", I think.
Alex Ethridge
Veteran Member
True but in order to keep it simple so jbcohen can understand with as much ease as possible, the optical center of the lens runs along the center of the lens barrel from one end to the other.
Again, I'll use the pinhole camera as an illustration to keep this as uncomplicated as possible.
jbcohen, look at the image below. The pinhole is where the light rays all cross, left in the scene becomes right inside where the film or sensor is and top becomes bottom. In other words, laterally and vertically reversed.
The distance from the pinhole to the film plane (or sensor) is the focal length of the "lens". For more clarification, read my post from yesterday.
Note: It is very, very common for people to refer to the zero parallax point as the "nodal" point. The correct term is "no-parallax point" or (I prefer) "zero-parallax point".
That didn't work, because a pinhole doesn't have a FL.
I like to use and get other people to use Barnack. It can answer all the OPs questions, if he will pay attention...
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Leonard Migliore
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Actually, it's the second principal point (the first principal point is the same thing only from the front). The exit pupil can be anywhere from plus to minus infinity.
And it's a somewhat circular definition. The second principal point is the point along the optical axis that is one focal length away from where the lens focuses a plane wavefront. I just let my optical design software find it.
The pinhole analogy makes the most sense to me.
Actually, the optical center-line extends to infinity in both directions. :-0
That diagram helps.Again, I'll use the pinhole camera as an illustration to keep this as uncomplicated as possible.
jbcohen, look at the image below. The pinhole is where the light rays all cross, left in the scene becomes right inside where the film or sensor is and top becomes bottom. In other words, laterally and vertically reversed.
The distance from the pinhole to the film plane (or sensor) is the focal length of the "lens". For more clarification, read my post from yesterday.
Why did you bring up the subject of "parallax"? That will confuse JB. It has no meaning for a pinhole camera. It only is important to folks who are taking image sequences by rotating the camera, then using these image to stitch together a panoramic view. That seems to be off topic? Did I miss something?
However, your preferred term, "no-parallax point" is pseudo technical mumbo-jumbo. The correct, technical term is "entrance pupil". You are correct that many pseudo-technical people who are panoramic experts, use terms like "Nodal Point" and "Principal Point", which are proper technical terms, but not the right ones! Your "zero-parallax point" is just something that was fabricated. Sorry.
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Agreed. But I contend that for beginners, trying to get them to understand what a "principal point" is is daunting. It is an invisible, virtual point in space, the intersection of one of the "principal planes" and the optical axis. It is an imaginary construct. In contrast, I can show them the entrance pupil and the exit pupil. They are visible and real. In actual use, the difference between the exit pupil and the 2nd principal point is academic.
Yes, that is true. If you google "2nd principal point" you find that "definition" rather quickly. It's said to be located 1 FL in front of the image. That's not very helpful when trying to explain what a FL is!
But since the OPs question was about what 50mm meant, that can't be answered or described with a pin-hole analogy. The answer to his question is "focal length" and a pin-hole doesn't have a FL.
Alex Ethridge
Veteran Member
Keeping in mind that the pinhole arguably substitutes for a lens and the zero-parallax point being the important factor in my explanation, the distance from that point to the film plane (or sensor) can be measured in inches or millimeters and therefore is very accurate and very true. Remember, all cameras that are capable of making pictures must have a zero-parallax point located somewhere in front of the film plane (or sensor) and the distance between that point and the sensor is the focal length.
This is an immutable, basic principle whether you call it a pinhole camera, camera obscura, or a $40,000 broadcast camera. If it can make or project a picture, it has a focal length.
The introduction of such a chart introduces complicated, difficult-to-understand elements that from the framing and elementary nature of jbcohen's question, are irrelevant and confusing.
Remember, we are here to help jbcohen and to discuss details not germane to his question among ourselves as though he were not "in the room" is unhelpful and rude.
He did not ask about near- and far-point focus, hyperfocal distance, apertures, blur circles, diffraction spot diameters, entrance pupils or exit pupils. He asked a very elementary question and I think we should stick to answering that question in as simple terms as possible.
I like to use and get other people to use Barnack. It can answer all the OPs questions, if he will pay attention...
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Alex Ethridge
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Okay, you win on that one; but, it does extend from one end to the other of the lens barrel -- and beyond, as you point out.
I will answer as though this is a serious question.That diagram helps.Again, I'll use the pinhole camera as an illustration to keep this as uncomplicated as possible.
jbcohen, look at the image below. The pinhole is where the light rays all cross, left in the scene becomes right inside where the film or sensor is and top becomes bottom. In other words, laterally and vertically reversed.
The distance from the pinhole to the film plane (or sensor) is the focal length of the "lens". For more clarification, read my post from yesterday.
Why did you bring up the subject of "parallax"?
Simple, because it is the name of that point. Trying to keep it simple for jbcohen, I didn't want the confusion of entrance pupil and exit pupil (different things) introduced so I chose "no parallax point", a single phrase that covered both in the case of a pinhole camera.
Read about parallax here
Frankly, I'm puzzled that you had to ask.
If it has no meaning for a pinhole camera, how does the image inside the pinhole camera get laterally and vertically reversed?
I agree, a discussion of panoramas is completely off topic. You should apologize. :-D
Google it. You'll find 800,000 "no parallax point" hits and at site after site, experts define it and, if I'm not mistaken, DPR may be the only site that mentions it in the context of "mumbo-jumbo". :-D
Agreed. I stated this clearly in earlier posts.
Sorry, the difference between "zero" and "no", in this case, is a distinction with no difference. I occasionally come up with my own names for things and in this case, I substituted "zero" for "no". I also refer to APS-C and DX formats as "half-frame" formats and m43 as "quarter-frame". Sorry, I see nothing confusing about that, especially since I usually include clarification when I do it -- as I did in previous posts in this thread.
Now if everyone would be reasonable and just agree with me . . . After all, if everyone would just fully understand things, they would (agree with me). :-D :-D
I like your signature.
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Gerry Winterbourne
Forum Pro
A lens is something that focuses light (which a pinhole doesn't, so the answers that mention pinholes aren't helpful).
Think of an ordinary magnifying glass - you can use it to focus the sun's light to start a fire. To do this you move the glass until the light is a point on the kindling rather than a circle. A given magnifying glass will always focus the sun's light at the same distance: that distance is its focal length. A fatter glass (lens) will focus nearer - a shorter focal length; a thinner lens has a longer focal length.
The sun is infinitely distant; to focus light from something closer the focal length is different. When we describe camera lenses the nominal focal length (like the 50mm on your lens) means that it focuses infinitely distant light 50mm behind the lens.
There's a complication, though: unlike the magnifying glass that's only one piece of glass a lens has several. What matters for defining focal length is the place in the lens that has the same effect as the middle of the single glass. We sometimes call this the optical centre, which is typically somewhere between the front and back pieces of glass.
Note that the optical centre is not the same as the optical axis, which is a line running along the centres of all the pieces of glass and extends in front of the lens and behind it.
To focus the sun both it (the sun) and the focused dot of light are on the optical axis. But for a picture light can also come from all sides of the optical axis. The angle around the optical axis that can get through the lens and be focused on the sensor depends on the focal length. The shorter the focal length the wider the angle. But that's only part of the story - the size of the sensor matters too.
Used on a 36x24mm (FF) sensor a 50mm lens has an angle of view that is considered more or less normal. On a 18x12mm sensor the angle of view is smaller and the angle of view is considered to be moderately telephoto. It is, in fact, the same angle of view that a 100mm lens gives on FF.
So, to summarise: the number marked on a lens is its nominal focal length; the angle of view a lens gives depends on its focal length and the sensor size; we can describe the relative angle of view by reference to how a given focal length works on FF sensors.
Mark S Abeln
Forum Pro
The focal length of a pinhole lens is simply its distance from the sensor. A pinhole is therefore a model of a zoom lens.
It's a basic, fundamental model of what's going on and how lenses work, and it is also a good first approximation for most photography.
The pinhole model defines for us the concept of focal length and indirectly leads to the notion of a crop factor. It also tells us that an image is projected upside down on a sensor. Once one understands this model, along with basic geometry, it becomes immediately apparent how both sensor size as well as focal length changes the angle of view of a lens, which can be very important with interchangeable lenses on various sensors.
Of course it is a simplistic model. But it is fundamental.
Graduating from the pinhole model to the thin-lens approximation, we gain the notion of an aperture, and from this model we can calculate exposure, the focal point, and depth of field based on a circle of confusion. That's three more photographically important variables!
More complex lens models don't give us much more in the way of photographically useful variables, although they explain focus breathing, which isn't present in the two simpler models. More complicated models yet give us barrel distortion, which forces us to refine the definition of focal length and aperture according to small angles of rays of light near the center of the image rather than the overall image size on the sensor. Also, these models introduce qualities of bokeh. Diffraction and chromatic aberrations generally don't fit into a simple geometric model of a lens — since now we graduate from the ray model of light to the wave model — and understanding these becomes more complicated. Once light hits the sensor we need to go from the wave model of light to the quantum model, and things get murkier and yet more complicated and unintuitive.
The perfect is the enemy of the good. I would think that understanding the simple geometric pinhole model of a lens is basic and primary, and if that is not well-understood, additional lens theory may be misleading or at least confusing. How many times has someone posted the idea that a 50 mm lens on a FF camera 'becomes' a 75 mm lens on an APS-C camera? Understanding the pinhole model should clear up this confusion.
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It means that when you are focussed on distant objects the lens is 50mm away from the sensor.
This is easy to show with a simple single-element lens (such as a close-up lens) but all modern lenses are quite thick and that 50mm is calculated from a specific point inside the lens, depending on its optical design.
This is easy to show with a simple single-element lens (such as a close-up lens) but all modern lenses are quite thick and that 50mm is calculated from a specific point inside the lens, depending on its optical design.
jbcohen
Senior Member
Then what does it mean to say I am holding a 50-150 lens. My understanding is that the 50-150 is the focal length and the lens sees things between 50 and 150 degrees to the leens.
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