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Old Mar 15, 2006, 3:41 PM   #161
E.T
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I could use what's left of week for writing answers if I would go through posts... so I'm going to skip eq. focal length/FOV discussion, reasons behind that should be made clear enough many times.



When it comes to resolving power of objective (we're not talking about sensor now), be it lens or mirror only thing affecting to it is aperture, focal length doesn't have anything to do with that.
Equation for objective's resolving power is 115"/D where D is aperture/diameter in millimeters... and as how much one arc second (1/3600th of degree) is, like one astronomy book I have puts it: "It's angle in which 170cm tall object shows from distance of 351km."
This resolving power limit is caused by thing called diffraction.

Considering astronomy noticeable thing is that 11.5 cm diameter objective is capable to one arc second resolving power which is considered as upper limit caused by atmospheric disturbances in normal "good" conditions.
That's the reason why big telescopes are built to summits of mountains, less/thinner atmosphere disturbing image. Also HST achieved such good accuracy because of lack of atmosphere... now adaptive optics correcting atmosphere's disturbances starts to enable ground telescopes to achieve similar results as HST but normally telescopes with bigger aperture are wanted not because they could resolve smaller targets but because bigger light gathering capability, with modern CCD cameras people can use 12-15cm aperture telescopes to take photos whose details compete with traditional photographs taken using ten times bigger telescope. (phenomenon of hobbyers towards bigger telescopes is called as "aperture fever")


Focal length affects only to how big image objective draws to focal plane, longer focal length objective draws bigger iamge to focal plane. So if resolution of sensor is limiting factor more details can be gotten with longer focal length (makes details in "drawn" image bigger) but as long as sensor's resolution can be increased there's isn't good reason to increase focal length from total resolving power viewpoint because longer focal length means always bigger size. (if we keep optical structure same)

Also longer focal length actually requires larger objective, speed of objective, or "aperture" as it's called erroneously means focal length divided by aperture so f-ratio is way more describing name for it.
No matter what aperture objective or what focal length objective you use "film" placed to focal plane get's exposed similarly if f-ratio's of objectives are same.
Longer focal length objective just means that light gathered by it is dispersed to wider area in focal plane meaning we need to have equally much wider aperture to get equal exposure, this is why F-ratio stays same regardless of focal length as long as EV-value, shutter time and used sensitivity stays same.
As exception considering visual astronomy with eyepiece (or photographing using eyepiece magnification) only thing affecting to required exposure time is aperture (=light gathering power) of objective as long as same total magnification is used.


So as long as sensor isn't capable to resolving smallest detail objective can draw it makes sense to increase resolution of sensor and keep physical size of it and objective same and increase size of objective only when its difraction limit would be limiting factor or sensor's resulution can't be increased. (now that's where things like noise comes into play)


Now do I know you from some other forum, Mark?
I just sense such familiar "flat/hollow Earth" - "glass sky" feeling radiating from you...



Mark47 wrote:
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You keep saying over and over that some how inherently a 300mm lens gathers more light than a 200mm lens.* That is absolutely and totally incorrect because that isn't enough information.* A 300mm f4 lens gathers *exactly* the same amount of light as a 200mm f4 lens.
You are confusing the amount of light captured with the intensity at which you see it. A 400mm lens will be further away from the sensor than a 300mm so the light will be less intense.
And same to you, as long as f-ratio stays same intensity of radiation in focal plane/required exposure time stays same.

Quote:
A small magnifying glass focused close to your hand will burn you, a larger magnifying glass if kept further away will not
That is again about f-ratio which can be used to make sure radiation intensity stays same.


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a 300mm lens has more optical zoom than a 200mm lens, its that simple!
Now how this should be interpreted...
Thing called as zoom is ratio of lens' longest focal length to its shortest focal length.
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Old Mar 15, 2006, 9:31 PM   #162
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Focal length affects only to how big image objective draws to focal plane, longer focal length objective draws bigger iamge to focal plane.

How does it make it bigger? Thats the thing that no one seems to understand or take into consideration, it is conveniently left out of the equation. Focal length does not simply "make things bigger". What it does is increases REACH, so that the FOCAL POINT is closer to the object.

Suppose you are standing 10metres from an object so that the object fills the entireviewfinderusing a 35mm camera with a 200mm lens. We shall say the focal point is 10 metres. You move backwards away from the object so that now it only fills half the viewfinder, so now you are 20metres away, so your focal point isnow 20metres. (this is technically not correct, this is a hypothetical illustration, so you need to understand what I mean by focal point for this purpose, not what it says in the dictionary). Now you are at 20 metres but you want a focal point of ten metres so you can fill the viewfinder/sensor. There are two ways you can do this, you can walk closer to the object, so you are back at the 10m pointor else you can use a longer focal length lens. So you put on a 300mm lens, so that now yourview is the same as the 200mm lens. Although you are actually 20m away, by using the 100mm lens you have "walked" to the 10m focal point. Therefore you are REACHING further with the lens and collecting your light at the 10m POINTeven though you are actually 20m away.

Suppose now you are with the 35mm camera and 300mm lens filling the view. You are 20m away but your focal point is 10 metres. (remember this is hypothetical so dont go off onirrelevant tangents about 50mm lenses)You have another 35mm camera with 200mm lens the same distanceas the 300mmbut as it is only 200mm it only fills half the view, therefore its focal point is 20m. This means that the point you are collecting the light to make your view, is 20m. At ten metres the image fills the view but at 20metres it only half fills the view therefore since you are now at a point where it half fills the view, your focal point has to be 20m. You want the 200mm to have the same view as the 300mm lens, so you use a crop camera. You havent changed the focal length so your focal point is still 20m because you are still 20m away with a 200mm lens. The significance of this, is that at 20m the light is more dispersed as it is not travelling towards you parallel. The further away from the object you are, the less percentage of light will be coming from the source and the more ambient light will make up the total amount of light. It is the same as a slide projector, the further away the screen, the larger the image, the more diluted it is, the less "information" per pixel it carries.

It is the same with sound. The further away from a sound source, the harder it is to hear, and the harder to distinguish individual sounds. Suppose you are listening to two people talking from 2 feet away, you have no problems understanding what they are saying. The further away from them the harder it is to distinguish THE DETAILS of what they are saying. You can hear sound but you cant hear DETAILS, you cant make out every word clearly, so that even though the sound is still REACHING you, not all the sound is reaching you, something has happened to the sound waves along the way that prevents every DETAIL from reaching you.

suppose you are in a building accross from a park and there are two people talking. In between you and them, there is all kinds ofNOISE like traffic, roadworks, birds etc. If youwere to simply record sound from that building and magnify it, would you be able to hear them talking? NO,it wouldntmatter how much you increased the volume, you would NEVER be able to hear them because at the same time you are also increasing the sound of everything else. In order to hear them you need a device to "WALK" you closer to the sound source by FOCUSING the sound from that source.That is what a longer focal length lens does because the further away from an object you are the more "diluted" the light coming from the actual sourceis aslight from other sources mixes in.

Unexplained mysteries!
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Old Mar 16, 2006, 4:10 PM   #163
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Mark47 wrote:
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Suppose now you are with the 35mm camera and 300mm lens filling the view. You are 20m away but your focal point is 10 metres. (remember this is hypothetical so dont go off onirrelevant tangents about 50mm lenses)You have another 35mm camera with 200mm lens the same distanceas the 300mmbut as it is only 200mm it only fills half the view, therefore its focal point is 20m. This means that the point you are collecting the light to make your view, is 20m. At ten metres the image fills the view but at 20metres it only half fills the view therefore since you are now at a point where it half fills the view, your focal point has to be 20m. You want the 200mm to have the same view as the 300mm lens, so you use a crop camera. You havent changed the focal length so your focal point is still 20m because you are still 20m away with a 200mm lens. The significance of this, is that at 20m the light is more dispersed as it is not travelling towards you parallel. The further away from the object you are, the less percentage of light will be coming from the source and the more ambient light will make up the total amount of light. It is the same as a slide projector, the further away the screen, the larger the image, the more diluted it is, the less "information" per pixel it carries.
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I think I may see what you are getting at here. Allow me to re-cast this into an example which is easier for me (and possibly, others) to understand.

Light follows the inverse square law of radiation - intensity decreases by the inverse of the square of the distance (amount of light on a given surface is 1/4 as much if you are twice as far away)

A source of light is the target, so if it fills the frame of a full frame camera at10M, and half the frame at 20M, we are receiving 1/4 as much light, and therefore 1/4 as much information, in the form of pixels.

If we now have two cameras, one which is FF with 300mm lens and 1.5x crop sensor camera with 200mm lens, and place these at the same distance, so the target fills the full frame, the crop camera fram should also be filled, albeit with shorter focal length lens. Lens objective diameters are same, so the total amount of light being gathered is the same. Since the lens circle at focal plane is the same for both lenses, the smaller sensor is only receiving 2/3 the amount of light, and therefore information, as the full frame.

So far, so good.

But wait- the intensity of the light on both sensors needs to be the same-right? If both cameras are set for the same exposure, something isn't quite right. If we assume SS of 1/120s and f/4 for both cameras, then 200mm/4 gives us 50mm diameter and 300mm/4 gives us 75mm diameter. But we said the lens diameters had to be the same, so both lenses were receiving the same amount of light. Change 300mm lens to f/6? Now we have the same amount of light to both sensors, but the FF is dimmer, therefore less information than before. How about if we now change SS to 1/90s? This should give pretty nearly equal exposure. (aperture change would also coincidentally (?) give similar DOF)

Now we have both cameras with the same picture, but the FF having (potentially) more information. It did require longer exposure though. Why? Well, as you have mentioned several times, Mark, a 300mm lens gives more magnification than a 200mm, and as you pont out in your reference to slide projector, more magnification gives a dimmer picture. Something E.T. also pointed out regarding telescope eyepieces.

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Old Mar 16, 2006, 6:22 PM   #164
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I agree there is an unexplained mystery. My argument is that, as far as photography is concerned I don't believe it changes the image in a noticable way. I don't know why, but it doesn't.

Quote:
Now we have both cameras with the same picture, but the FF having (potentially) more information. It did require longer exposure though.

I certainly won't argue that the light is less "intense" but the exposure is the same for the same image.

Maybe someone can correct this situation I put forward. I can't run this test, I don't own cameras with the proper sensors to do it (I do own a 1.3x crop and a 1.6x crop camera so if someone wants me to test those I could), but I believe I'm right:
If I have a FF sensor camera at 12MP with a 300mm f4 lens
and I have a 1.5x crop factor camera at 12MP with a 200mm f4 lens.
assume equivalent optical quality between the lenses.

Then no matter what picture I take, they will both report the same exposure for the same image. One of the images will not look dimmer than the other (to my eye, any ways.) They will both produce the same image. The light would be less intense (based on the forumla given by VTphotog and the description from Mark.) I don't argue this... it fits with my vague memory from physics classes.

Maybe the difference is there, but is very small because of the numbers involved (minimal distances, minimal physical size difference between the sensors)? I don't know. I am not claiming that people are wrong, I'm claiming that the effect isn't seen.... Humm.. here is an idea. Maybe there is a difference but the metering systems of the cameras take it into account.

Maybe the difference between the "theory" we've been taking about and the real world is that the cameras in the real world aren't actually the same.

Eric


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Old Mar 16, 2006, 8:51 PM   #165
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Re-read post. A 300mm f/4 lens has physically larger aperture than 200mm f/4, thus allowing more light.

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Old Mar 16, 2006, 9:25 PM   #166
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Of course it has a larger aperture. That has been my point all along.

Maybe I'm remembering Mark's argument wrong from much earlier in the post. But I recall Mark47 claiming that there was an actual difference in the pictures between having two sensors with the same resolution but different physcal sizes and making up the difference in sensor size with different focal length lenses (while keeping the f-stop the same.)

In other words, he was claiming that what I described in page 6 of this thread would produce different results.

Maybe I'm wrong. I don't have time to go back through the 90 posts (or more, is it 10 per page?) to quote his exact words. No forget that. I'll make the time.

Mark47 wrote:
Quote:
While all the factors are the same, there is one difference between the two set ups. One is using a 300mm lens and the other a 200mm lens. The difference is in the glass. To see this difference it is easiest with my example of two magnifying glasses. The more powerful magnifying glass will bring more light through it, therefore being able to burn you. When something is far away from you, the light is more disperssed, just like the light from a torch therefore you need a more powerful lens to capture more of it.

If you look at a laser beam it seems very parallel, however if you were to shine it at the moon, by the time it gets there, it will be dispersed into a wide area correct? It wont just be the tiny point of light you see right in front of you. So if you were using the light from that laser to expose film, it wouldnt be concentrated enough would it? So you would need a lens to focus the light into the small area of film, just like a magnifying glass will focus light from the sun. So if you took two magnifying glasses one more POWERFUL than the other, it will be like the difference when you focus the light from the sun onto your hand, one will burn you the other wont, so in the case of film, its the same, the more POWERFUL lens will bring in more light so will be able to expose the film better. The more powerful lens will be getting more light from the actual subject and less ambient light, therefore will produce more detail.

A 300mm lens is more POWERFUL than a 200mm lens, so it is able to capture light from farther away from the actual subjectand focus more of it into a smaller area. this is why you need to put the lens further away. You probably wont be able to see any difference with the naked eye.
This was his answer to my suggestion that they would produce the same image back on page 6 of this thread. This makes me believe he didn't believe they would produce the same image.

And then he said:
Mark47 wrote:
Quote:
eric s wrote:
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You keep saying over and over that some how inherently a 300mm lens gathers more light than a 200mm lens. That is absolutely and totally incorrect because that isn't enough information. A 300mm f4 lens gathers *exactly* the same amount of light as a 200mm f4 lens. Period. That is what an f-stop of 4 means, and is independent from focal length. In fact a 400 f2.8 lens gathers *more* light than a 500mm f4 lens. Yes, even though the 400mm lens has less focal length. That is the definition of what f2.8 and f4 mean. An f-stop of 2.8 gathers twice as much light as an f-stop of 4. The 400 f2.8 will weight more than the 500mm f4 because it has a much bigger front element (11.8 lb vs. 8.5lb.) Because a bigger front element (a bigger magnifying glass, like in your example) will gather more light. But that is independent of the focal length of the lens.
You are confusing the amount of light captured with the intensity at which you see it. A 400mm lens will be further away from the sensor than a 300mm so the light will be less intense. A small magnifying glass focused close to your hand will burn you, a larger magnifying glass if kept further away will not, however the larger magnifying glass will clearly have collected more light. the difference is the way that it is focused and the way you see it.
He doesn't seem to understand that with the larger f-stop it gathers more like and therefor the images will be the same.

Eric
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Old Mar 16, 2006, 9:27 PM   #167
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It has nothing to do with the amount of light or the intensity or the exposure. It is the quality of light and the amount of information it conveys. the further away from the subject the more corrupt the information.

You see an object as blue not because it is actually blue but because your mind tells you it is blue. Matter is actually invisible, it has no colour. What you see is an image created by your mind, the same way radar creates a dot on a radar screen, there is no actual dot, it is just a means to identify something. So when you shine a light on something to see it, you are not just illuminating it because it is too dark to see without it, you are creating a magnetic bridge between you and the subject just like sending out sonar or radar. What enables you to see, is that different types of matter absord different wavelenths of light. If it absorbs red light then you will see it as red or if it absorbs blue light you will see it as blue.

So lets say a photon of light hits a blue object. That photon has now had that small change occur, so it carries with it that information. It is a little difficult to explain properly since the current theory of light is so primitve and so very very wrong not to mention completely irational and illogical. But thats the basic idea they are working with. So as the photon of light travels towards you two things happen. One is that light from other sources will interact with it, so it will become corrupted compared to when it was close to the source, and secondly that as it is not travelling parallel with other light, that only some of the light containing the "blue" information will reach you as it will be replaced by light from other sources. Your brain, like a sensor can only process so much information at once, so the more it has to process, the less detail you are going to get that comes from the actual source, as your mind picks out the strongest signatures.

If you look at a tree up close, it is 3 dimensional, it is easy to distinguish individual leaves, and depth. Far away however it is just a 2 dimensional blur. It is more like a painting, where there are no actual leaves or branches painted, but your mind perceives that they exist, something like interplotion, where it makes up what it thinks it should be. If there were a bird in that tree a long way into the distance it would be difficult to identify it, you certainly couldnt see individual feathers, and possibly its not a bird anyway, its hard to tell from that distance, why?

Different matter absorbs light differently, which means that the information from one type of matter or one colour will be more pronounced or stronger than that coming from another source. it is the same with sound, some sound will be easier to hear than other sound, it will carry better. So if you are listening to two people talk from a distance you will still be able to hear the basic sound, however not all the sound will be able to reach you, so you wont be able to distinguish every single word. When you ad other sounds, it makes it even harder to hear what they are saying. The sound waves travelling from the source are getting weaker as they get to you and so they are lost, only the stronger ones remain and your mind fills in the blanks, but at the same time they are being bombarded by sound from other sources, over riding weaker signals, so even less gets to you. Your mind can only process so muchinformation at once, so it can only process the strongest signals, some of which have now come from another source, so it has to distinguish which sound comes from where. It is like astronomers that complain of light pollution, there are so many city lights, it makes it difficult to see the stars.

If you are looking at a bird in a tree, your mind has to process all the information coming from that source. The further away you are from that source, the less information that will get to you, so the less detail you will see, which is why it is difficult to distinguish exactly what you are seeing until you get closer. As the light moves towards you it is currupted and dilated, so the closer you are to the source, the less distance it travels, the more detail you see.

So when you use a 300mm lens you are collecting light closer to the source where it is less corrupted than it would be if you were using a 200mm lens, not because the lens is ten mm longer. You are not focusing on the actual object itself but on the light travelling towards you. It is like light travelling from a star. When you see the star, you are not looking at light that is on the surface of the star, you are looking at light that left the star millions of years ago, so you are not looking at the actual star itself, you are looking at the light that contains the information of what the star is. So when you look at an object, you are not focusing on the actual object itself but the light that has left the object and is traveling towards you. Thefocal point is wherethe "capture point" of the light is, or howfar away from the source you start capturing light as it travels towards you.The closer to the object, the less corrupted the light will be. the focal point of the 300mm will be closer to the object than the 200mm lens, when the two lenses are side by side,so the light you are seeing will be less corupt. So if the capture point or focal point of light for the 300mm lens is 100m, then this means it is capturing all light that is one hundred metres away from the object. The 200mm lens if it is the same distance away as the 300mm, wont capture light until say the 150metre mark, this means that some of the light that is at 100m will not be captured and it has a further 50mm to be corrupted.

When you put a 200mm lens on to a crop camera you are not changing the focal length of the lens to a 300mm, therefore its focal point will still be the same as a 200mm. So if the focal point of the 300mm is 100metres from the object and the 200mm is 150metres from the object, when it is the same distance as the 300mm,the light source from the 200mm will be 50metres further away from the source where it is more corrupt than the 300mm.
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Old Mar 17, 2006, 1:05 AM   #168
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Mark47 wrote:
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So when you use a 300mm lens you are collecting light closer to the source where it is less corrupted than it would be if you were using a 200mm lens, not because the lens is ten mm longer. You are not focusing on the actual object itself but on the light travelling towards you.----- So if the capture point or focal point of light for the 300mm lens is 100m, then this means it is capturing all light that is one hundred metres away from the object. The 200mm lens if it is the same distance away as the 300mm, wont capture light until say the 150metre mark, this means that some of the light that is at 100m will not be captured and it has a further 50mm to be corrupted.
Having an obviously primitive and irrational understanding of light, I fail to see how a lens can capture light that hasn't reached it yet. Simple minded person that I am, I cannot conceive of how a lens can do other than work with light which stikes it.

Perhaps when you develop an explanation that we primitives can understand, we will award you one of those Nobel prizes for physics.

signing off this thread--

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Old Mar 17, 2006, 3:26 AM   #169
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When does a 50mm lens shot look larger and closer on a shot than a 75mm lens shot?

Answer when you compare a Canon 10d too a 5d. These shots were taken together, same distance
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Old Mar 17, 2006, 3:41 AM   #170
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Made a hash of that
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