Does longer focal length result in a darker image?

[IanIan]

From http://en.wikipedia.org/wiki/Focal_length

In photography a longer focal length, or lower optical power, is associated with a larger image magnification of distant objects, or a narrower angle of view. Conversely a shorter focal length, or higher optical power, is associated with a wider angle of view.

I suppose it does seem counter intuitive. Those guys at sporting events with those ultra long barrels on their cameras.. I think those barrels are only that long so that a weak lens can be held a long way from the sensor.

[IanIan]

Cut tube in half (now 150mm) and repeat the viewing. You will have wider FOV

Actually I don't think looking down a tube has any effect on one's FOV. If you look down a tube then you're just looking at the inside of a tube.

[kevinbythebeach]

From http://en.wikipedia.org/wiki/Focal_length



I suppose it does seem counter intuitive. Those guys at sporting events with those ultra long barrels on their cameras.. I think those barrels are only that long so that a weak lens can be held a long way from the sensor.

I think your confusing optical power and FOV.....or you dont understand FOV

whatever your doing right as u read this...turn your head and notice how much side to side view u have...now do it again looking towards the same direction, but looking through a toliet paper tube..
see the difference in what u can see...that is FOV....right? I am not sure whats counter intuitive about it...Maybe your asking the wrong questions?

But here is a cutaway of a lens...

http://www.gatzet.com/canon-ef-28-30...s-diagram.html

PS I just noticed after posting this that Rob posted a similar sugesstion...

Ian.. Do you own a camera?

[IanIan]

Hey,

I think your confusing optical power and FOV.....

Please explain...

So what is the cutaway supposed to demonstrate? Its a zoom lens with image stabilization right? So in that case there's definitely going to be more going on inside than just a weak lens at one end, a sensor at the other and some space in between.

[IanIan]

whatever your doing right as u read this...turn your head and notice how much side to side view u have...now do it again looking towards the same direction, but looking through a toliet paper tube..
see the difference in what u can see...that is FOV....right?

No I don't think it is. You have the same FOV with or without the paper tube. When you look through the paper tube you just see less of the room and more of the tube. If you cut the tube in half then you see more of the room and less of the tube.

Now imagine you're showing me this amazing new lens you've bought and it cost $25000 and is a super wide angle lens and you're telling me that it super sharp from corner to corner. I then pick up a black piece of card and hold it right in front of the lens. Have I just reduced the lens' FOV to zero? Of course not - I haven't effected the lens at all. But you're claiming a similar thing with the paper tube example.

If you put a paper tube on the end of a camera and take a picture, you haven't reduced the FOV, you've just taken a picture of the inside of a tube. To properly reduce the FOV you'd have to take the picture and have the part of the room not obscured by the tube fill the entire area of the resulting photograph.

Ian.. Do you own a camera?

Its on order. Which possibly accounts for my current obesession.

[IanIan]

I recommend going here... http://phet.colorado.edu/sims/geomet...optics_en.html

When its loaded, go straight up to the 'refractive index' slider and move it to the left to weaken the lens. Then watch how the focal point of the image moves towards the right away from the lens (i.e. the camera needs a longer barrel) and the image gets bigger. (i.e. its more zoomed in.) Unfortunately they haven't modelled the image getting dimmer! I've emailed them about this so perhaps they'll write back and convince me that I've got it all wrong.

[JonathanC]

So what if it gets dimmer? No really?

If you use a zoom lens and adjust the focal length by zooming it, or even two different prime lenses, unless the f/ number stays the same then the light falling on the film/sensor will change.

What happens in the real world is that either a) the camera copes with this, or b) you have to, or c) you go manual and neither works for you unless you "get" the f/ number bit.

The long/short focal length versus big/small front lens all works out fine if you use the f/ ratio for the calculations that you or the camera have to do to get the ISO and shutter speed combination correct.

The looking at the inside of a tube bit didn't help here I'm guessing, as it seems easy to assume that the tube was becoming part of the captured image. It wasn't. The idea is to visualise the world through a long tube (long focal length) or a short tube (you guessed already!) assuming that the tube is not part of the image, rather it is the boundary of the final image as captured.

My way of visualising focal length is to imagine a point on the axis of the lens at the focal length of the lens away from the film/sensor plane. If lines are drawn (or pieces of thread are pulled) from the corners of the film/sensor though this point, they encompass the image that would be captured. This is the same, roughly, as working with a pinhole camera where it is obvious what is happening.

[OldCrow]

Back in the old days (1988) when I was shooting with a 4x5 camera, we called this effect "Bellows Extension". When shooting macro shots, we'd have to compensate 1 or 2 stops because the lens was so much farther from the film.

[IanIan]

So what if it gets dimmer? No really?

I've been trying to prove that it does get dimmer and understand why. I looked at the f-stop ratio and thought, hmmm... does the image really get darker as the focal length increases and if so why? All this talk about setting f-stop to compensate and so on is beside the point. Its like me asking why an apple falls down when you let go of it and being told not to worry about it because it doesn't fall if you put a table under it.

You've never wondered why something is the way it is? If you just want to understand photography from a practical viewpoint so that you can take good pictures then good for you. It would appear that that's most people's attitude to this issue who have posted on this thread and with the greatest of respect I think it shows in their understanding.

[kevinbythebeach]

IAN


I meant place the paper tube in front of your eye...not a lens...and your side to side vision is reduced...hence narrow field of view...

[IanIan]

I meant place the paper tube in front of your eye...not a lens...and your side to side vision is reduced...hence narrow field of view...

No your FOV is the same. Your side to side vision can now see the inside of a paper tube.

[Raycer]

Hi,

I've found a couple of places on the internet where people have said that the longer the focal length the longer the light has to travel and as it travels it gets weaker.

I don't find this very convincing because we're talking about very small distances which I don't think account for the f-stop difference between a lense when its zoomed and when its not. Something else must be happening to the light!

Any thoughts?

Cheers, I.

Ahh... here we go again...
YES, its true, what they didn't tell you is the aether inside the lens is much denser compare to the aether outside and it absorbs much more light once the light enters the lens.

Yes, everything on the internet is true. Just have to have an open mind and believe.

come on McQ, can't we have a bit of fun here.

[Bill44]

You've never wondered why something is the way it is? If you just want to understand photography from a practical viewpoint so that you can take good pictures then good for you. It would appear that that's most people's attitude to this issue who have posted on this thread and with the greatest of respect I think it shows in their understanding.

Some of us have been taking photographs longer than you've been alive, and have worked out what is worth worrying about and what is not.

Go get your camera and start taking photos, and stop worrying if the earth is round or flat.

[kevinbythebeach]

Some of us have been taking photographs longer than you've been alive, and have worked out what is worth worrying about and what is not.

Go get your camera and start taking photos, and stop worrying if the earth is round or flat.

Yah no kidding...I am so confused now....lol

I am not even sure what were exactly talking about anymore....

And even if I did.....I doubt it would make me a better photographer...



PS IANIAN........what Camera/ Lenses did you order?

[Colin Southern]

And on that note, I think it's a good time to move on from this thread