Lens and Aperture Discussion

[Christian]

I've finaly steped up to my first DSLR with a 20D and as my only lens an EF-S 17-85MM f4-5.6 IS USM.
Now I under stand the 17-85MM as it relates to the 1.6 crop factor for the 20D ( or at least I think I do)
However, what is the practical effect of the f4-5.6. I assume this is related to apature values? larger values mean less light/larger DOF?
As I eventually start finding need for other lenses I'm finding I'm not educated enough to know what any specific lense might do for me in comparison with this one.
Also I under stand the IS is the Image Stabilizer does the USM stand for anything significant?

[fish]

Christian,

Congrats on the 20D. The f-numbers do indeed define max aperture. Typically the lower number at the short end of the zoom and the higher number at the long end of the zoom.

Larger value (f-number) means smaller aperture and means less light, more DOF. Smaller value means larger aperture and more light, less DOF.

USM = UltraSonic Motor (silent ring-type focusing motor). As opposed to MM = Micro Motor (gear-drive, louder and slower).

[wxwax]

What Fishmonger said.

If you're thinking about more lenses, your needs would break into two categories: faster, and different focal lengths.

Your lens is f4 at 17mm and f5.6 at 85mm. That's OK, but not the fastest. What do they mean by fastest? And why does it matter?

As Fishfood said, the smaller the f-stop number, the bigger is your aperature. It lets in more light. More light = faster shutter speed. So.... a lens with smaller f-stop numbers is able to give you good shutter speeds even in low light. That's huge.

Good Canon L-glass zooms have a max aperature of f2.8. Not bad. But it comes at a price - add hundreds of dollars for every f-stop you gain in speed. :cry The fastest I think I've seen is f1.

As you note, small f-stop/big aperature also means narrow depth of field. So you don't want to be too close to your subject at f1. Only the front nose hair will be in focus. :lol3 DOF is definitely something to consider when you have a faster lens - ignore it at your peril. DAMHIK :uhoh

The other thing for you to consider when buying another lens is focal length. You have a nice range covered with 17-85. But lots of consumer cameras have built-in lenses that give you more zoom. So your next purchase might be something like a 70-200mm. Or a 100-400.

Careful. Buying lenses can be an addiction. :evil

[fish]

So you don't want to be too close to your subject at f1. Only the front nose hair will be in focus. :lol3 DOF is definitely something to consider when you have a faster lens - ignore it at your peril. DAMHIK :uhoh To build on what waximoto said, the big advantage of that f1 (you gotta be kidding me!) lens, is that you get a huge gain in DOF when you stop it down a couple of clicks, yet you've still got a fast shutter speed. Best of both worlds.

More realistically, the f2.8 lens is one stop faster than the f4, so your chances of getting 1/focal length is better with the faster lens. I might be building too fast, but 1/fl is the rule of thumb for hand holding and still getting a good image. I.e., on a 100mm lens, you want a shutter speed of at least 1/100s. Slow lenses (even those with IS) hamper that rule. So, if you gots the bucks, buy the fastest lenses your budget will allow. Otherwise, be prepared to use a tripod or flash a lot.

:1drink




"fishfood"? :huh

[Christian]

Wow, I knew I would learn something if a simply asked, but I didn't expect this amount of such rich information! Thanks fish, and wxwax for your replies. And I had never heard of the 1/fl rule, I'm going to have to tattoo that on the inside of my eyelids where I keep all my important info...like my name.

I'm sorry for the late reply, it's been a long weekend but I do appreciate the response! Now to start playing the lottery to finance those new lenses!

[cmr164]

To build on what waximoto said, the big advantage of that f1 (you gotta be kidding me!) lens, is that you get a huge gain in DOF when you stop it down a couple of clicks, yet you've still got a fast shutter speed. Best of both worlds.

More realistically, when you get the f2.8 lens, you can stop down to f8 or f11 and still get a 1/focal length for most outdoor shots. I might be building too fast, but 1/fl is the rule of thumb for hand holding and still getting a good image. I.e., on a 100mm lens, you want a shutter speed of at least 1/100s. Slow lenses (even those with IS) hamper that rule. So, if you gots the bucks, buy the fastest lenses your budget will allow. Otherwise, be prepared to use a tripod or flash a lot.

:1drink




"fishfood"? :huh
I know fish knows this but just to clarify his statement. It does not matter if you start with an f2.8 lens or an f4.0 lens, when you stop down to f8.0 they will use the same shutter speed.

[fish]

I know fish knows this but just to clarify his statement. It does not matter if you start with an f2.8 lens or an f4.0 lens, when you stop down to f8.0 they will use the same shutter speed.
Yes, of course you are right, Charles. Not sure what I was thinking, but I went back and corrected it. Thanks for pointing out my error.

[leebase]

f stops are a ration between the opening circle of your lens and the focal length.

Here's how I explain it. Imagine you had a hose filling a bucket with water. When the bucket is full...you have your correct exposure.

If you had a 1" round hose....let's say it takes 10 seconds to fill the bucket. The amount of time is equivalent to your shutter speed.

If you doubled the hose's area...you get twice as much water coming through....so you'd fill your bucket in half the time. If you cut the size of the hose's opening in half, you'd need to let the water run twice as long to fill the bucket.

Your aperature is the size of the hose...and your shutter is the amount of time you let the water flow....and the filled bucket is your optimal exposure (photo lighted just right, not to light, not too dark).

So....let's say you have a situation where your camera is at f5.6, and the shutter speed you are getting is 1/30 at that speed. 1/30 is so slow, that if your subject is moving you'll get blur.

So...you "open up" the aperature "1 stop". Each "stop" is twice the size of the fstop smaller, and half the size of the f-stop larger. Smaller numbers are larger f-stops.

In order: 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32

As you can see, every other f-stop is a double of one, or a double of 1.4

Back to our story...you open up the aperature 1 f-stop. From 5.6 that would be 4. So you have light coming in at twice the rate...and you can reduce the time by half to 1/60 and have the exact same "exposure".

1/60 is still to slow to stop action. So you open up the aperature 1 more f-stop to 2.8 -- but wait...you can't with our lens, and the maximum aperature is f4. But, let's press on for the story.

So now you have f2.8 and a shutter speed of 1/125 which stops a certain amount of action...but your toddler is really moving....so you open up 1 more f-stop to f2.0 and shutter 1/250...and now you have the photo you wanted.

Of course, as you open up the aperature...you narrow the "depth of field"....the amount of the photo that's still "in focus" in front of and behind the plane of focus. For portraits you freuently want a narrow DOF...to have just your subject in focus, with a blurred background.

Now, there are two other things you could do besides opening up the aperature....either because you can't (your lens doesn't support it) or you don't want the narrow dof. You can "add light" via flash....or you can increase the sensitivity of the sensor....by setting the ISO.

The ISO values are also twice as sensitive as the stop before them.

ISO 200 is twice as sensitive as ISO100 and so on.

So in our example...lets say those values were at ISO100. When you got to f4...and still didn't get the speed you wanted. You could have popped up the flash....OR...you could bump up the ISO.

Upping the ISO one stop is equivalent to opening the aperature one stop...EXCEPT...that the ISO doesn't affect depth of field....AND...the higher the ISO, the more noise that's in the photo.

Your job as photographer is to balance these variables in the most pleasing way (in addition to the composition of the photo).

Lee

[bkriete]

Lee-
Thank you so much for the definition and description of f-stops and their effects. Very clear and helpful, and not at all obtuse in the manner of many technical descriptions. Great info on ISOs also. I wish I had the privilege of reading your explanation earlier in my photographic learning. Would have saved me a lot of :dunno and :slosh

[wxwax]

Lee, could you please explain the theory of relativity in similar language? Thanks in advance.




:D

[cmr164]

...
So...you "open up" the aperature "1 stop". Each "stop" is twice the size of the fstop smaller, and half the size of the f-stop larger. Smaller numbers are larger f-stops.

In order: 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32

As you can see, every other f-stop is a double of one, or a double of 1.4
...
Great explanation but I would like to add a clarification for those who can still handle high school math. The difference between each f-stop is the previous f-stop times 1.414... (square root of 2) (Some of you may remember from school days the formula for the area of a circle.) That is why a 1.4 extender changes the f-stop by 1 and 2x extender by 2 stops. Thus the f-stop is the ratio between the width of the opening and the focal length and the width of the opening is related to amount of the light (size of pipe) by powers of 1.414... (because the f-stop relates to diameter but the area relates to radius which is 1/2 diameter)

[leebase]

Lee, could you please explain the theory of relativity in similar language? Thanks in advance.
:D

You are driving 75mph in the left lane. Gramps in front of you is doing 56...and grandma in the right lane is doing 55.

While the motorist fixing their tire on the side of the road sees you going 55 and honking and flashing your lights....YOU see yourself going 1mph in relation to grandma and you give grandpa the bird incase the lights and horns don't communicate that he's going SLOW in the FAST LANE!

That's relativity :):

Lee

[bkriete]

So relativity explains why everyone going faster than you is a moron and everyone going slower is a dumbass?

[leebase]

So relativity explains why everyone going faster than you is a moron and everyone going slower is a dumbass?


:rofl

Lee

[wxwax]

:rofl So relativity explains why everyone going faster than you is a moron and everyone going slower is a dumbass?
:rofl :rofl :rofl :rofl

:thumb Great!

[ashby]

What Fishmonger said.

Good Canon L-glass zooms have a max aperature of f2.8. Not bad. But it comes at a price - add hundreds of dollars for every f-stop you gain in speed. :cry The fastest I think I've seen is f1.:evil

I think Canon had 50mm/f .98 in a 35mm format lens back in the late 60's. It was pretty big news then because f1.0 was supposed to be the theoretical max.

[gus]

apature values? larger values mean less light/larger DOF?

Think of it like the pupil in your eye....at night your pupil opens right to get as much light in as possible to help you see & in bright sun it shrinks right down to only let a small amount of light in.

[luckyrwe]

I think Canon had 50mm/f .98 in a 35mm format lens back in the late 60's. It was pretty big news then because f1.0 was supposed to be the theoretical max.
Point nine FIVE!


http://www.classiccamera.org/foca/canon%207%202.jpg

[wxwax]

Think of it like the pupil in your eye....at night your pupil opens right to get as much light in as possible to help you see & in bright sun it shrinks right down to only let a small amount of light in.
Except when you drink a lot of kava. :evil

[gus]

Except when you drink a lot of kava. :evil
But you will not see the spirits if you dont....





.

[wxwax]

But you will not see the spirits if you dont....





.

With six mangoes a day going into that intestinal tract, I'm surprised you don't hear the spirits.

[fish]

you give grandpa the bird incase the lights and horns don't communicate that he's going SLOW in the FAST LANE!

That's relativity :):


:lol3 well done!

[fish]

Point nine FIVE!


http://www.classiccamera.org/foca/canon%207%202.jpg
Some additional info:

August 1961. This lens is developed as one of the standard lenses for the Canon 7 rangefinder camera. It had the largest aperture in the world for photographic lenses available in the market at the time. It attracts a great deal of attention as a "dream lens", as it is four times faster than the human eye. The lens mount is clip-on, 3-lug bayonet system like the Mirror Box II (cameras have S mount inside bayonet mount). The edge of the rearmost lens element is cut off to avoid friction with interlocking roller of range finder mechanism. It is Gauss type lens with 7 elements in 5 groups.

[fish]

Can you get any faster than an f0.0 lens? (http://www.mudhaus.com/f00.html)

[luckyrwe]

Four times faster than the human eye? So the human eye is about f/3.8?

[Andy]

the hall of wisdom belongs to wxwax as a moderator, but he's probaably too shy to move this thread there so i'm doing it.

thanks to christian, for asking a question many want to ask but dont.
thanks to fish, wax, and leebase for the really thorough explanations!

[luckyrwe]

And thanks to Andy for putting up with the other 1,995 of us! :D

[Eric&Susan]

I think I understand all the numbers and technical jargon but how does it relate to real life situations? What would be good for landscapes? sports? portraits? everyday practical use? Can this be answered or are the questions to general? Obviously there are going to be variables in each situation but is there a general rule of thumb?:scratch

Eric

[wxwax]

I think I understand all the numbers and technical jargon but how does it relate to real life situations? What would be good for landscapes? sports? portraits? everyday practical use? Can this be answered or are the questions to general? Obviously there are going to be variables in each situation but is there a general rule of thumb?:scratch

Eric

Eric, it really does depend on what you're trying to achieve. It helps to visualize what you want the final shot to look like, then figure out how to achieve it.

That being said, typically landscapes are shot with small aperatures/high f-stop numbers so that they have big depth of field. Andy's looking to buy a wide angle lens and expects to use it in the f8 to f11 range. On a wide angle, that's a huge depth of field.

For portraits, you'll notice that the background is often blurry. That helps focus attention on the subject. Portraits tend to be shot with longer lenses, often but not always somewhere between 80mm and 135mm, with a smaller aperature/smaller f-stop number - and a narrow depth of field. That's what allows the subject to be in focus and the background blurry. The danger is that the depth of field is too narrow, and that the subject's entire face isn't in focus (although some people go for that effect on purpose.)

For general walking around, you need to think about the lens you're using, and what constitutes narrow or deep depth of field for that particular lens. I've scrwed up many a time. :cry

[mercphoto]

Can you get any faster than an f0.0 lens? (http://www.mudhaus.com/f00.html)

That was a joke, right?

[fish]

That was a joke, right?

Nope. (http://news.bbc.co.uk/1/hi/sci/tech/841690.stm)

[mercphoto]

Nope. (http://news.bbc.co.uk/1/hi/sci/tech/841690.stm)

[Edit follows: the first paragraph of mine might be in error ;)]
Ok. First of all, nobody yet has successfully sent light, or anything else, faster than the speed of light. Some think it can be done, but there is no evidence of this, or any workable theory of how it might be done.

[Edit: the next two paragraphs however, are still true]
Next, the "speed" of a lens, expressed in terms of the f-stop number, has no bearing on the speed of light. So even if you could increase the speed that which light travels, it would not affect the f-stop of a lens.

Now when it comes to something "faster" than an f/0.0 lens, the answer is no. Actually, you cannot build an f/0.0 lens, as this is a mathematical impossibility. I don't know if you ever noticed, but if you look closely, f-stops are always expressed as fractions, with the value of 1 always as the numerator, and the denominator as the f-stop of the lens. 1:2.8, for example. As we know, as the denominator of a fraction becomes smaller, the value of the number becomes larger. This is why 1:2.8 is a larger aperture than 1:8. Even though 8 is bigger than 2.8, 1/2.8 is bigger than 1/8.

And this is why a lens of f/0 is impossible: you cannot divide anything by zero.

[luckyrwe]

And this is why a lens of f/0 is impossible: you cannot divide anything by zero.
Time to think outside the box. Or the lens, as the case may be. :rofl

[fish]

try to have an open mind, merc...



Faster-than-light speeds in tunneling experiments: an annotated bibliography


One central tenet of special relativity theory is that light speed is the greatest speed at which energy, information, signals etc. can be transmitted. In many physics-related internet newsgroups, claims have appeared that recent tunneling experiments show this assumption to be wrong, and that information can indeed be transmitted by speeds faster than that of light - the most prominent example of "information" being a Mozart symphony, having been transmitted with 4.7 times the speed of light. In this document, I've tried to collect the major references on these faster-than-light (FTL)-experiments. If I find the time, I will develop this into a written introduction on the topic of FTL speeds and tunneling, so far it is merely a (possibly incomplete) collection of references. If anyone has relevant additions/comments, I'd appreciate a mail. (mpoessel@aei.mpg.de)

Most of the references are to the technical literature, presuming that the reader has at least a basic grasp of physics. However, as usual, those articles have abstracts and conclusions, which give an overview of what the article is about. Some references that are in German are omitted here, but can be found in the german version of this page (http://www.aei-potsdam.mpg.de/%7Empoessel/Physik/FTL/nimtz.html).

What's this all about, anyway?

In recent years, some physicists have conducted experiments in which faster-than-light (FTL) speeds were measured. On the other hand, Einstein's theory of special relativity gives light speed as the absolute speed limit for matter and information! If information is transmitted faster, then a host of strange effects can be produced, e.g. for some observers it looks like the information was received even before it was sent (how this comes about should be described in elementary literature on special relativity). This violation of causality is very worrysome, and thus special relativity's demand that neither matter nor information should move faster than light is a pretty fundamental one, not at all comparable to the objections some physicists had about faster-than-sound travel in the first half of this century.

So, has special relativity been disproved, now that FTL speeds have been measured? The first problem with this naive conclusion is that, while in special relativity neither information nor energy are allowed to be transmitted faster than light, but that certain velocities in connection with the phenomena of wave transmission may well excede light speed. For instance, the phase velocity of a wave or the group velocity of a wave packet are not in principle restricted below light speed. The speed connected with wave phenomena that, according to special relativity, must never exceed light speed, is the front velocity of the wave or wave packet, which roughly can be seen as the speed of the first little stirring that tells an observer "Hey, there's a wave coming". Detailled examinations of the differences between the velocities useful to describe waves can be found in the classic book






Brillouin, L. 1960 Wave Propagation and Group Velocity. NY: Academic Press.
Basic information on quantum tunneling can be found in the introductory quantum theory literature. Characteristic of the discussion of the FTL/tunneling experiments is that the experimental results are relatively uncontroversial - it is their interpretation that the debate is about. As far as I can see, right now there is a consensus that in neither of the experiments, FTL-front velocities have been measured, and that thus there is no contradiction to Einstein causality or to special relativity's claim that no front speed can exceed light speed. The discussion how much time a particle needs to tunnel through a barrier has been going on since the thirties and still goes on today, as far as I can tell. This discussion is about "real" tunneling experiments, like the ones a Berkeley group around Raymond Chiao has done, as well as experiments with microwaves in waveguides (that do not involve quantum mechanics) like those of Günter Nimtz et al. An overview of the discussion (including lots of further references) can be found in



Hauge, E.H. & Støvneng 1989, Review of Modern Physics 61, S. 917--936.
The Berkeley group gives a general overview of their research at

http://www.physics.berkeley.edu/research/chiao/research.html
An experiment of theirs, where a single photon tunnelled through a barrier and its tunneling speed (not a signal speed!) was 1.7 times light speed, is described in



Steinberg, A.M., Kwiat, P.G. & R.Y. Chiao 1993: "Measurement of the Single-Photon Tunneling Time" in Physical Review Letter 71, S. 708--711
Articles concerned with the propagation of wave packets that happens FTL and is somewhat complicated by the fact that the waves "borrow" some energy from the medium, but does not violate causality, are

Chiao, R.Y. 1993: "Superluminal (but causal) propagation of wavepackets in transparent media with inverted atomic populations" in Phys. Rev. A 48, B34.
Chiao, R.Y. 1996: "Tachyon-like excitations in inverted two-level media" in Phys. Rev. Lett. 77, 1254.
Aephraim Steinberg, who is a former graduate student of Chiao's, has written two papers especially on the problem of tunneling time, which are available online at

Aephraim M. Steinberg 1995: "Conditional probabilities in quantum theory, and the tunneling time controversy (http://xxx.lanl.gov/abs/quant-ph/9502003)" in Phys. Rev A52, 32-42 (was preprint quant-ph/9502003).
Aephraim M. Steinberg 1995: "How much time does a tunneling particle spend in the barrier region? (http://xxx.lanl.gov/abs/quant-ph/9501015)" in Phys. Rev. Lett. 74, 2405-9 (was preprint quant-ph/9501015).
Some other papers of Chiao's Berkeley group are also online, e.g.

Aephraim M. Steinberg, Raymond Y. Chiao 1995: "Sub-femtosecond determination of transmission delay times for a dielectric mirror (photonic band gap) as a function of angle of incidence (http://xxx.lanl.gov/abs/quant-ph/9501013)" in Phys. Rev. A51, 3525/8 (was Preprint quant-ph/9501013).
Raymond Y. Chiao, Paul G. Kwiat, Aephraim M. Steinberg: " Quantum non-locality in Two-Photon Experiments at Berkeley (http://xxx.lanl.gov/abs/quant-ph/9501016)" (International Workshop on Laser and Quantum Optics, Nathiagali, Pakistan, 9-14 July 1994) in Quantum and Semiclassical Optics 7, 259-78 (was preprint quant-ph/950101).
Earlier experiments by Günter Nimtz of Cologne University (Universität Kön), with whose experiments most of the later newspaper articles are concerned, have been published as

Enders, A. und G. Nimtz 1993, "Evanescent-mode propagation and quantum tunneling" in Phys. Rev. E 48, S. 632-634.
Enders, A. und G. Nimtz 1993, J. Phys. I (France) 3, S. 1089
Nimtz, G. et al. 1994: "Photonic Tunneling Times"in J. Phys. I (France) 4, 565.
A description of the equivalence between these microwave-experiments and quantum mechanical tunneling is described in

Martin, Th. und Landauer, R. 1991: "Time delay of evanescent electromagnetic waves and the analogy to particle tunneling" in Phys. Rev. A 45 , S. 2611-2617.
In reaction to Nimtz' publications, a number of articles appeared which deal with a) why causality is not violated in these experiments, and b) how the results of the experiments come about. These are

Deutch, J.M. und F.E. Low 1993: "Barrier Penetration and Superluminal Velocity" in Ann. Phys. (NY) 228, S. 184-202.
Hass, K. und P. Busch 1994: "Causality of superluminal barrier traversal" in Phys. Lett. A 185, S. 9-13.
Landauer, R. und Th. Martin 1994: "Barrier interaction time in tunneling" in Rev. Mod. Phys. 66, S. 217-228.
Azbel, M. Y. 1994: "Superluminal Velocity, Tunneling Traversal Time and Causality" in Solid State Comm. 91, S. 439-441.
Nimtz's reply and general observations on causality and his experiments can be found in

Heitmann, W. und G. Nimtz 1994: "On causality proofs of superluminal barrier traversal of frequency band limited wave packets" in Phys. Lett. A 196, S. 154-158.
As far as the more recent experiments of Nimtz are concerned, especially the popular tunneling of parts of Mozart's 40th symphony with 4.7fold light speed, I have not been able to find references to a technical article yet. Heitman/Nimtz 1994 (see above) refer to it as "H. Aichmann and G. Nimtz, to be published", I haven't found it in Physics Abstracts (up to July 1996, I think I should look again soon), though.

the problem of tunneling times is also the topic of some articles I've found in the quantum physics (quant-ph) archive, namely



Toralf Gruner, Dirk-Gunnar Welsch: Photon tunneling through absorbing dielectric barriers, (http://xxx.lanl.gov/abs/quant-ph/9606008) Preprint quant-ph/9606008
Andrea Begliuomini, Luciano Bracci: The tunneling time for a wave packet as measured with a physical clock (http://xxx.lanl.gov/abs/quant-ph/9605045) Preprint quant-ph/9605045
M. S. Marinov, Bilha Segev On the concept of the tunneling time (http://xxx.lanl.gov/abs/quant-ph/9603018) Preprint quant-ph/9603018




author/source: Markus Pössel mpoessel@aei.mpg.de

[Andy]

blah blah blah ....

cliff notes, please? 'gus, harry, steve and i would really appreciate that. :deal

[wxwax]

cliff notes, please? 'gus, harry, steve and i would really appreciate that. :deal
The short version goes something like this:

"Can too."

[luckyrwe]

Darn it Fish, I was just gonna say that! You beat me to it! :D

[gus]

The short version goes something like this:

"Can too."
Turn it up !!! ....why was i mentioned 1st ?

[mercphoto]

try to have an open mind, merc...
Excuse me for not being up on all the latest news in physics, Fish. Sheesh! I did edit my own post saying my paragraph about faster-than-light might be wrong. How can that not be open minded? Give me credit there. The rest of my post is still accurate --- f/0.0 is not possible. The math doesn't allow it.

[luckyrwe]

How about f 0.01, technically the same amount of light as 0.0, but infinitely far from it....

[mercphoto]

How about f 0.01, technically the same amount of light as 0.0, but infinitely far from it....

Correct.