[Recovered Thread: 85008]

[Mark47]

JimC wrote:

Your analogies may zero sense at all to me.

The brightness of a lens is not determined by focal length, and often you'll have shorter focal length lenses that are brighter.

Don't confuse the diameter of a lens, and how much glass it's got with brightness. If you have a lens with larger available apertures for a given focal length, sure it's going to be larger. If you've got a lens with smaller available apertures for a given focal length, it's going to be smaller.

As far as how much light is being gathered, a larger format lens needs to spread the light over a larger surface area (the film or sensor). A smaller format lens designed for a smaller sensor or film size doesn't need to be as large to give the sensor just as much light, because the surface area of the sensor is smaller.

The confusion comes in because many lenses that can be used on DSLR models were originally designed for 35mm cameras. So, people assume that the angle of view they see is because of a crop. It's not, it's because of the focal length combined with the size of the sensor or film.

An 8mm lens on a camera with a tiny 1/1.8" sensor that has f/2.8 available gathers just as much light for the sensor, with the same angle of view that you'd have using a 39mm lens with f/2.8 available on a 35mm camera. The lens for the 35mm camera is larger. That doesn't necessarily mean it's better or brighter, as far as how much light is going to the sensor.
]

you cant be trying too hard they are not that complicated, you are thinking like a photographer rather than scientifically. I am correct, can you blow up a image taken of Mars using a 300mm lens, so that you can make it out? NO It doesnt matter how much you blow it up you will never be able to see any detail because there is not enough light coming from the source. In order for you to take a photograph of mars you need OPTICAL magnification, not digital. The bigger the lens the more optical magnification it provides.

[Mark47]

Ah, thanks for summing it up. WRONG. As long as were not trying to do things with light that are not physically possible (which were not, with whats on the market) you CAN get a detailed picture of mars using a 40 terrapixel sensor and you favorite 50mm lens. Why wouldnt you? Are you saying that the lens is unable to resolve more than X megapixels? True, but it seems that X is more than 10mp with the most common SLR lenses, and more yet with quality lenses.




WRONG.

[Mark47]

you would never get any detail. the further something is away from you, the more the light will disapate and distort before it gets to you and the only way to correct that is through optical magnification, you cant do it digitally.

Thats some great theory, but it DOES NOT APPLY to the cameras on the market today. We are well within these limits, sorry. The difference from 200mm to 300mm is indescribably small.


Try shining a laser to mars and keep it straight. The fact is that itdoes exist, so why argue with facts on a subject you know nothing about?

[JimC]

In most cases, you don't see those limitations (lens outresolving sensors).

When you do see those limitations, then you go to a larger format (and that goes for film or digital).

It's not uncommon to see a consumer digicam with 8 Megapixels with a lens that has an actual length that's much shorter, resolve more detail compared to a 6MP DSLR model with a much longer focal length to get the same angle of view.

You have to take sensor size into consideration for apparent magnification, brightness and detail comparisons.

You have the same thing with film.

If you go to a medium format model, you'll need a longer actual focal length lens to get the same angle of view that you'd have with that focal length on a 35mm model.


Does the medium format model outresolve the 35mm model, when the 35mm model has a much smaller surface area and can use a shorter actual focal length for the same angle of view? It depends on the film being used and the quality of the lenses.

With film, you'll usually see more difference between larger and smaller formats. But, with Digital, you can have more photosites in a smaller area. The primary limitation is noise levels as ISO speeds are increased with smaller sensor sizes, not the focal lengths of the lenses used on them.

[Nagasaki]

This is a photography forum.

Try taking a photograph of a distant object with a 1.5 crop DSLR and a FF DSLR with 200 and 300 lenses respectively would you be able to tell the difference. I don't believe you would. The lenses are both capable of resolving sufficient detail for the sensor and both give the same field of view.

[GWHayduke]

Thats some great theory, but it DOES NOT APPLY to the cameras on the market today. We are well within these limits, sorry. The difference from 200mm to 300mm is indescribably small.

I have to come to Mark 47's defense here: What Mark was getting at is that by using a 200mm lens (with X angle of view) to capture details of Mars, you tend to get a greater amount of stray light interfering with the light from Mars. If you recall Young's double slit experiment (from basic modern physics), even a point source of light creates fringe patterns of interference. As you decrease the angleof view (i.e.swap to a longer focal length on the same sized sensor) theinterference fringes become dramatically reduced in size, allowing the sensor toabsorb more lightdirectly from Mars and suffer less distortion due to stray light. You can knock this explanationall you want, but this is a basic principal ofphysics that can be demonstrated and proven with ease.

-Blake

I don't know who's defense I'm coming to, but whoever pulled the quote above isthe personthis explanation is meant for.

[Tom LaPrise]

Mark47, you're arguing with people who know a lot more about lasers, lenses, sensors, and film than you do, and with every post you make, you prove it.

"A 300mm lens will take in more light from a smaller area at a greater distance than a 200mm lens. Its not taking in more light but more light from a smaller area."

More light from a smaller area? A smaller area means LESS light. That's why shorter focal-length (higher magnification) eyepieces give a dimmer view than longer focal-length eyepieces in telescopes.

"Light travels in waves and expands outwards, to correct that you need a more powerful lens."

Light travels in packets (photons) and travels in straight lines unless refracted by a lens, dust or water vapor in the atmosphere, or a gravity field, and lenses can't make all the light (or ANY more of the light) from a source reach the lens.
"The light from celestial objects travels through the vacuum of space relatively unhindered. As this light passes through our atmosphere, it is refracted by various amounts and in different directions, depending upon the density of the air it encounters."
http://www.astrophys-assist.com/educ...ble/hubble.htm

"the further something is away from you, the more the light will disapate and distort before it gets to you and the only way to correct that is through optical magnification"

You can't correct distortion by magnifying. You're still the same distance away. And you can't un-dissipate light with a lens--the lens can only "see" with the light that reaches it, and if that light is distorted or dissipated, that's what you get. Lenses don't pull light from objects; they receive and focus light, and that's all they do.

"In order for you to take a photograph of mars you need OPTICAL magnification"

YOU need to understand what optical magnification IS before you try arguing about it. Read my example about telescopes and eyepieces.

[tmoreau]

Lets say 300mm lens will reduce a Ø0.5" beam to Ø.10" for the film to record (1:5 magnification). The 24x36mm frame will be printed on 4x6" photo paper, so the Ø0.10" spot of light will appear on the print at Ø.42"

Now lets say a 200mm lens will reduce the same Ø0.5" beam to Ø0.06". (1:8 magnification, apx.). Now your APS-C sensor size of 16x24 image is printed on 4x6" photo paper, the spot of light is Ø0.42"

The original light (your input) is the same, your 4x6" print (the output) is the same, so WHAT exactly does the magnification have to do with this?

Are you honestly convinced that the difference in lens performance is measurable in the real world, between these examples? That this difference in light defraction and the number of photons collected is measurably different? The 40 terrapixel mars example most likely IS beyond these limitations, but a 300mm lens paired with a 10mp sensor is certainly not. You cannot apply something from one extreme end of the scale, to the other, while completely ignoring the tolerances involved.

[Mark47]

Mark47, you're arguing with people who know a lot more about lasers, lenses, sensors, and film than you do, and with every post you make, you prove it.




Being able to quote something from a book doesnt mean you understand it, and I have seen nothing so far to prove otherwise, so stop being smarmy. Using a few technical terms doesnt equate to understanding either.

http://au.search.yahoo.com/search?p=...aves&meta=

Take your pick

[Mark47]

Rising Above

Simply stated, the primary function of a telescope is to collect light. The larger the telescope, the more light it can collect. The more light collected, the fainter and more distant the objects that can be observed.



Get it yet?