Deal 8: Here it is: The most requested deal of 2014!
Welcome to the second lesson in Photography 101 – A Basic Course on the Camera. In this series, we cover all the basics of camera design and use. We talk about the ‘exposure triangle': shutter speed, aperture and ISO. We talk about focus, depth of field and sharpness, as well as how lenses work, what focal lengths mean and how they put light on the sensor. We also look at the camera itself, how it works, what all the options mean and how they affect your photos.
This week’s lesson is Lenses and Focus
Last week we discussed how we can use a tiny hole to direct light so that it forms an image. All that a pinhole camera does is excludes all the light that doesn’t make an image. As we learned, however, the problem with that technique, is that it results in very dim images. As photographers we want bright images, and although that may seem obvious, we’ll discuss why in detail in a later lesson. Fortunately, there is a better way to do it.
As we touched on briefly in Lesson 1, light is a form of energy that can be bent. Bending light is called refraction. What happens when light is refracted is that it actually slows down. It’s a common misconception that light always travels at the same speed. In fact, the speed of the light depends on the type of material that it is travelling through. The really useful thing about refraction is that it can bend the path of light.
I don’t want to get into the mysterious “dual nature of light”, but remember that light can be seen as a series of waves. Line after line of these waves make up light, similar to waves hitting a beach.
Imagine we have a fishtank of water and a torch. For the sake of simplicity lets also imagine that we can see the beam clearly in the air and water. When you shine the torch at the surface of the water at an angle, from the side of the tank, you can see that the beam has been bent, see Fig 1.2.1. The many wavefronts of the light are aligned perpendicular with its direction of travel. When the wavefronts encounter the water, one part of the front hits it before the rest. The part that has entered the water and slows down, while the rest of the wave is still travelling at the same speed. The effect of this is to bend the beam. See Fig 1.2.2.
Okay that’s enough physics for now. Lets talk optics.
This bending of light can be very useful! Lets say we wanted to concentrate all the light from a wide beam onto a narrow point. If we can direct each beam of light by bending it slightly – a little right for the light in the left side of the beam, a little left for the light in the right side of the beam – then we should be able to focus the light. This is exactly what a lens does.
There are two main factors that determine how much a lens bends the light. The refractive index of the material, which is how much it slows down the beam, and the angle of incidence. The angle of incidence (or incident angle) is how far from perpendicular the light beam is when it passes through the surface. The greater the angle, the more the bending. This is why wide-angle lenses, that need to bend the light a long way, have such a bulging appearence.
Fig 1.2.3 How much the light beam is bent depends on the angle at which it hits the lens (all other things being equal). Light passing through the very centre of the lens is unaffected, while those at the edge are bent the most. This is why lenses are curved.
Fig 1.2.4 Different shaped lenses focus the light at different distances. This is the focal length of that lens.
A simple experiment
Click for larger version
Fig 1.2.5 An everyday magnifying glass can create an image. In a darkened room, set up a candle, a magnifying glass and a sheet of paper as a screen. With the magnifying glass squared up with the cangle and the screen, slide the glass and screen backwards and forwards until you bring an image of the candle into focus. Just as with the pinhole camera, the image projected by the lens us upside down. Notice that the shadow of the glass is dark except for the candle, even though the magnifying glass is see-through. This is because all of the light that passed through the glass has been focused into the image.
So far, we’ve been imagining a perfect beam of light hitting a refractive surface. In this beam all the light is parallel. Parallel light passed through a lens will always converge on the same point. The distance from the surface of the lens to the focus point is called the focal length and is measured in milimeters. Most lenses are described by their focal length. Zoom lenses have a range of focal lengths, a feat which is accomplished by using a complex series of lenses which can be moved relative to each other. The mm number translates into a real distance, from the front of your lens to the chip of your camera. In that way you can tell that a 400mm telephoto lens will be much longer than a 24mm wide-angle, without even looking at the lens.
If an object is close to a lens, even several hundred meters away, its reflected light entering the lens isn’t perfectly parallel. The closer the object to the lens, the less parallel, and the more the lens must be moved in order to keep focused. This change is much more noticable when objects are very close to the camera, and is one of the reasons why the depth of field in macro photos is so small – a point we will return to in a future lesson.
Fig 1.2.6 The closer an object is to a lens, the more its focus point moves, and so the more the lens must be moved to compensate.
In order to keep the image of a close object sharp, the lens must be moved relative to the screen (or camera sensor). This process is called focusing. When you are focused on an object at a certain distance, then objects which are closer or more distant than that will not be in focus. The situation can be helped somewhat, by reducing the size of the lens, just like we did with the pinhole camera, to restrict the variety of angles of light entering the lens. But we again are faced with the loss of brightness as a result.
We’ve hinted at the main reasons to use a lens: to make an image brighter and to make it bigger (or smaller!). Next week we’ll take what we have learned about lenses and see how we can use that to understand the concepts of focal length and f-ratios, and how they translate into maginification and image brightness.
I was disapointed at how few of you submitted homework for last weeks lesson. In fact, nobody did! Peter Emmett deserves some extra credit however, for his DSLR body cap pinhole camera photo taken coincidentally the weekend before the first lesson. This week’s lesson is challenging for setting homework, so I’d like to encourage you to experiment and think of how you can apply what you have learned here. Here’s some suggestions:
Photography 101 – Lenses, Light and Magnification.
February 20, 2013 12:59 pm
It's probably just me, but one part that wasn't explained in a way that sunk in is why changing the size of the aperture only affects brightness, and not the field of view? I would have expected that if I'm looking through a hole, and that hole is made bigger/smaller, then I'd be able to see more/less of the world through it. But obviously I know from using my camera that isn't the case and only the focal length affects the field of view....
December 6, 2012 03:06 am
Amazing website and lessons. You are clearly one of the best in your field and picked the right content and sequence to present to the beginner. Very happy I found your site. Thank you.
Don't be discouraged by those that want to skip the theoretical stuff. This is very important stuff to understand for anyone that considers themselves passionate about photograpy.
May 5, 2012 12:14 am
Thanks for these lessons. They are well put together and clearly laid out. I have a rebel XTi, had it for a few years now, and now that I've decided to make a serious hobby of this I've been scouring the web for good lessons and I'll say that yours, so far, works for me. Again, thanks for putting this site together. look forward to completing all the lessons in a few days.
December 30, 2011 12:38 pm
I LOVE YOUR LESSONS!! THANK YOU so very much! I needed to know the why behind the how and your lessons are by far the best, most organized gatherings of necessary information that I have found online. I feel blessed to have stumbled upon them. Just what I needed. :)
December 24, 2011 09:05 am
Anas I believe the shift in focus is acheived through a slight change to the focal length. I tested this with a 50mm prime mounted on a tripod with focus set to infinity and the closest point. The 2 images produced markedly different fields of view as shown in this blog post on how focus works
Neil, thanks for these lessons. I'm really enjoying getting into the theory behind photography.
November 8, 2010 11:16 am
I have always loved photography, but am just now making a decision to dive deeper into it. I recently got a Nikon Nikkormat EL - camera w/ numerous lenses, from my grandfather. I am very knowledgable in the digital technology so, utilizing w/ 35 mm film is going to be an adjustment. Can you recommend a good source to understand this camera & all of it's functions & what everything means???
April 22, 2010 11:04 pm
Hi.... Thanks for the tutorial.
I appreciate theoretical approach such as this, in fact i long for more in depth technical explainations especially the mathematical side of it.
I have a question.
When we want to capture an image with a camera, we pick a lens, say a 50mm prime lens. It has a fixed focal length of 50mm.
Focal Length is the distance between the "middle" of the lens and the focal point.
according to your animation, as subject to lens distance change, we have to move the lens forward/backward. It also shows that the distance between the focal point to the middle of the lens have changed, wouldnt this means that the focal length also has changed?
as with photographic lens, the focal length is fixed, but we can still adjust the focus to selectively focus near or distant subject.
can you please relate this more with the photographic lens.
October 6, 2009 03:11 am
Thank you very much!
I´ve been photographing "by ear" and I think now it´s time for me to learn it properly, So I really dig your classes so far. Thank you!!!
August 17, 2009 07:08 am
I'd just like to say thank you very much for explaining the elements of camera obscura, it is something I've never quite got my head around until now - its simple once you know!
July 7, 2009 01:10 am
I also found this extremely helpful! I have been wondering about the "WHY's" for the past few months that I've studied photography on my own ( I am a newbie), and no one has explained it as well as you have. Maybe it is not the best for someone who just got a camera and is looking for a quick formula (which doesn't exist!) ; but it is PERFECT for those of us who learn this way and want to have a clear picture of what a camera is and does before attempting to take pictures in a skillful manner.
June 30, 2009 05:43 am
I am at my wit's end so it would be a great help if you answered my query...I have a 16 mm lens for a CCTV camera. The image plane is 6.3mm (H) and 5.4mm(V). Due to constraints, I can place the lens at maximum the focal distance of 16mm. The object I want fitted into that image plane is approximately 35mm x 15mm or a bit larger. Is it possible to get a sharp image of this object?
Its not a pinhole camera..the lens has an extra angle of 19 deg.
Thanks for any help offered
April 25, 2009 02:59 pm
These lessons are great! I agree with the benefits of discussing theory and physics. I've been reading lots of information about how to understand all of this stuff and your lessons are the first that I can actually grasp, understand how photographs are created and therefore am retaining the knowledge so I can use it in the field.
I'm reading through all of the lessons now and plan on trying some of the homework assignments.
March 7, 2009 05:33 am
Hi, I'm new to your tutorials and am loving them. I teach 11 year olds and have already used the pinhole camera to explain light paths, I now want to set up your magnifying glass experiment. I have been into photography for a year now and understand a lot more than I did at the beginning - your tutorials are strenthening my foundations and giving me the will to improve. Thank you.
February 17, 2009 12:56 am
I'd like to second Luc Piche's thoughts... it would be nice if there was a "next" link between tutorials to help navigate. If one was on the first 101 tutorial, one wouldn't even know there was another part. There are no links between 101 and 101.2, nor is there a listing on the 101 page. Just a recommendation... but thanks for the great work. We all appreciate it, I'm sure.
February 4, 2009 04:36 am
hi neil.. i am new to your site here and i am enjoying every bit of these tutorials..and yes i think there
are useful if one wants to take photography to a higher level in time to come. indeed learnng the
fundamentals and theories is just what i needed. i am glad i found them here and yes i will be submitting
the assignment soon as i can. but i hope u dont mind cos i am midway into your class and i miss the datelines.... : )
January 31, 2009 01:40 pm
sorry - here's the link
January 31, 2009 01:39 pm
check out this "magic" lens - intuitively, i would say this is impossible, it must be a camera trick! do you have an explanation why it behaves the way it does? thanks
January 4, 2009 12:44 pm
I love the lessons and the science behind the photography. I am having trouble navigating to each of the new lessons. Is there an index page or a page with all the links to the different lessons?
October 30, 2008 02:02 pm
I agree. This has really broadened my knowledge base. Thanks for the incredible information.
October 20, 2008 06:04 am
Thanks! This helped me to write my paper about the physics of photography!!! :]
June 6, 2008 08:12 pm
Hi Niel I have to admit I am not into theory much however I agree with what you say about it being a necessity to reach peak performance. Thus the reason I am following this thread, and I thank you for putting a difficult topic into an understandable format. Great work keep it up! I do not understand how anyone could complain about such great work! There are heaps of places to find pictures and basic photography techniques but you are providing much more.
April 4, 2008 12:16 am
Just another vote for your approach of starting with the theoretical aspects before getting to the practical aspects...
I know all the basic optical physics having studied it all to university level, but I've never once considered it when taking photographs... it's nice to see you tie it into photography.
No F1 racing driver would be any good without understanding at least a reasonable amount about the mechanics of the car, and I think the same applies to photography.
March 31, 2008 06:45 am
I am a newbie to all things technical about cameras. It took me a long time to convert to digital because I prefer quality over convenience. I recently upgraded from a Canon point and shoot to a Canon Rebel XTi, so I am pretty clueless as to what f/stops, etc are for or what they mean. I will admit I am a little overwhelmed by the theory, but it is interesting to know.
With that said, this is my attempt at the homework. I know it is a couple of weeks late, but I just found these great articles. Thanks for your time, I hope to learn a lot!
My first attempt (flower with Eddie Bauer) was in complete darkness, but I had trouble getting a clear shot without the flash. The second was in the daytime, but with the side light blocked. I think my main problem may have been the fact that my flashlight sucks! The one with the maglite (firemaine's) was so bright and clear. It is reflected through my 75-300mm lens.
March 30, 2008 07:25 pm
Thank you for your excellent tutorials. As a "senior citizen" just getting into digiatl photography, I find your explanations, including the technical aspects, well-focused (!!) and essential to grasping the practicalities.
Your effort and time given is much appreciated.
March 30, 2008 12:32 am
Here's my belated homework attempt:
I do find the theories n basics very valuable and really appreciates Neil's approach. His recommended link to the "Dollar Egg" is a direct example of how such pictures are achieved thru the understanding n application of such basics.
March 29, 2008 05:23 am
"If an object is close to a lens, even several hundred meters away, its reflected light entering the lens isnâ€™t perfectly parallel. The closer the object to the lens, the less parallel, and the more the lens must be moved in order to keep focused."
I do not understand why that is. I am having trouble grasping this. Could you elaborate a bit?
March 29, 2008 04:20 am
LOVE the tutorial. Thank you so much! I've been a VERY amateur hobbiest for a couple of years now and just last year bought my first DSLR. Technique tutorials are a dime a dozen. Today is the first day that I really understand the relationship between my lense's focal length, aperture, and the image that I am producing.
This is SO much better than "wider apertures will decrease the depth of field and give you a blurry background. the end."
March 28, 2008 01:12 pm
Great stuff, PLEASE keep adding in the theory material so we all develop an understanding of not only what but also why.
March 28, 2008 06:18 am
Here's my homework for lesson 2, I went for the easy option this week with the pencil in the bowl of water. Take a look and see what you think, it does illustrate the concept quite well. Lesson 2 Homework
March 26, 2008 05:38 am
Submission for natural lenses...
March 25, 2008 11:02 pm
OK, I took this a few weeks before, so it can't be counted as "homework". But it's an example of a natrual lens, and something to do on a rainy day!
March 23, 2008 06:32 pm
I'm enjoying the lessons so far - I'm getting a much needed theoretical background and look forward to doing the assignments
March 22, 2008 02:05 am
This is my submission. Very nice lesson. Thanks!
March 21, 2008 09:38 pm
Thanks for your lessons! I'm sorry to have missed the first one but greatly enjoyed this one. One problem I'm having is figuring out where to submit the homework.
March 21, 2008 02:47 pm
Nice tutorials.. I know something about the lens now.. Expect to read more lessons that can be published as a book..
March 21, 2008 12:16 am
Thanks for the good breakdown of how refractions applies to focusing and capturing light. I am very much in favor of sprinkling in these theoretical foundations along with the practical lessons.
March 20, 2008 01:19 pm
I like the use of the mirror to direct the light onto the subject.
Here is my submission: I used an old zoom lens set to 90mm. Notice the focal length with the ruler of ~3.5in ~= 90mm!
I like the theory, I think it is worth learning. Thanks!
March 20, 2008 10:05 am
i remember my physics laboratory class
we did an experiment on this. =)
focal lengths & refraction stuff..
thanx for the refresher..
March 20, 2008 07:52 am
Just wanted to let you know that I just started reading your wonderful blog very recently.
The reason I'm not participating in the assignments right now is because I took one of the very first pieces of advice I read here, and sent my camera off to have THE SENSOR cleaned. It (along with the rest of the internal components of the camera) needed it badly! I didn't know why my pictures were turning out darker, uneven, and with spots - as soon as I read that post that you all kindly put up, I realized what the problem was!
Thank you SO much for all the wonderful information, articles, ideas, and help you put up here. I can't wait to start participating in the assignments, but til then I'm so happy to go back and read what you've posted in the past. Love to you all, take care and God Bless
March 20, 2008 07:19 am
Thanks so much for these tutorials! It's great to know how my camera works so I can use it properly.
As for the homework, I'm currently traveling, but I'll keep my eyes open for examples of refracting light.
March 20, 2008 04:46 am
I'm finding your coverage of the subject extremely helpful! I've been searching to find the basics presented like this...It saves hours of looking through texts and websites and trying to sort it out myself. I find your examples clear and relevant and very useful in developing my understanding of photography. Thank you so much for your efforts! Please keep up the good work!
March 20, 2008 04:25 am
hmmm.... Now I'm wondering if I misunderstood. It wouldn't be the first time! :)
March 20, 2008 04:23 am
I love these lessons. It is so important to understand the theory and physics. They are the building blocks.
I did my bending light project with our fish bowl and some daffodils in water. I don't have any other lenses so I had to try it with something else - water.
March 20, 2008 03:45 am
OK, Here goes. Thought that I would try one of the assignments. First let me say that by doing this I learned a lot and found out just how much I did not know.
I used an old 50mm 1:1.8 nikon series E lens a mirror and a flashlight.
The mirror reflected the light onto the object and through the lens onto a piece of paper.
I also shined the flashlight directly onto the object.
The results can be seen here.
March 20, 2008 12:41 am
interesting stuff indeed...thanks for your time Neil. Gonna have to run home and try and find a magnifying glass and do some homework =D
March 19, 2008 10:33 am
Just upload it to your favourite photo-sharing site and post the link here.
March 19, 2008 10:31 am
Just how do we submit the homework?
March 19, 2008 10:15 am
Nicely done sime! You do indeed deserve a gold star :) That's a clear demonstration of projecting an image using a lens. Did you gain any particular insights from the exercise?
Kristarella: It's a good suggestion. Darren is planning to set me up with an author account on the site soon, so I'll be able to go back and update the posts. I'm planning to use the intro posts as a table of contents to link to each lesson as its added. I'll also put next and previous links at the bottom of the lessons. This should help with the navigation.
March 19, 2008 10:09 am
Right, I just burnt my hand on my darn ghetto lamp, but, I thought i'd try turn my 100mm macro into a projector - as per the homework! The results can be seen here...
Neil, thanks a load for these 101's... I shall be follwing along!
[can I have my gold star now?!...]
March 19, 2008 09:49 am
I built a pinhole camera out of a box. I did not have a dark room so I tried polaroid film. I had found a website that seemed to have success with this film. I did not. All I got was streaked images. I did enjoy tring. It made me think about light and its effect on film.
It was conceptual. I like our classes thus far. I would like to remind the fellow photographers who have years of experience to please remember when they first started out.
I am thankful that this website exists. I enjoy everyone's comments. I finally feel I have a source of instruction.
March 19, 2008 09:29 am
Great post Neil! Well explained and great diagrams. I like learning about the theory. There's so much to photography that it's difficult to know where to start. I have already started with practicals like aperture, shutter speed, ISO, DoF etc. I'm very happy to put some theory back in!
I was tempted to do the homework last week, but apart from being busy I don't think my camera shoots when there's no lens attached. I have a Nikon D50. The not shooting thing is from memory â€“ I could be wrong â€“ anyone know?
I'll try to do some of the homework this week though - bending light sounds like fun :)
Also, might I suggest putting a link in each new article to all the previous articles? It was surprisingly difficult (or maybe just lengthy) for me to get back to the pin-hole post just now.
March 19, 2008 07:09 am
Always cool when what we're covering in Physics class suddenly shows up in relation to photography. Thanks for the info.
March 19, 2008 07:09 am
March 19, 2008 06:55 am
Another really interesting lesson, it's been nearly 20 years since I was in a physics lesson so it's good to be reminded. I love the fact we are learning the fundamentals behind photography, it will give me a better understanding of how my equipment works, and even why it's there in the first place. Keep them coming Neil!
In regards to the homework it took me a little longer than I thought to get my camera obscura to work! But I will be posting about my efforts tomorrow...
March 19, 2008 06:35 am
I'm enjoying your lessons so far and am learning the basics I should have learned a long time ago. I look forward to learning more.
Thanks for all the hard work!
( now maybe I can repay you with some hard work off my own and get to work on your assignment)
March 19, 2008 06:28 am
another brilliant tutorial - thanks for this Neil. I personally am enjoying a more theoretical coverage of the topic - I guess we're all wired differently and this is appealing to some of us who might have been thirsting for a more theoretical approach than other tutorials on this site. Keep up the great work.
March 19, 2008 05:23 am
Thank you very much. The lesson is great. As you said, theoretical background is important if one is willing to master photography.
Thanks again :)
March 19, 2008 04:42 am
Good stuff, Neil. Maybe a little theoretical, but it's good to know how this stuff all works. Keep the knowledge coming!
March 19, 2008 04:24 am
I really like your course so far, as I find it interesting to understand what is behind the whole concept. Just now it became clear to me what the focal length on the lenses mean. All I knew before was tele = large number, but I had no idea what that number relates to.
Regarding the exercises, your explanations are great and I am not so much an experimenter, more of an observer, so I elected to skip your exercise. I might go out to find some natural lenses, though.
Keep the style, I am sure it will all fall into place further down the path.
March 19, 2008 03:36 am
I respectfully disagree with the theoretical and negative comments. There are LOTS of web sites to read if all you want is someone telling you â€œdo thisâ€ for focus and depth of field.
I find your explanations concise, well presented and extremely relevant to the subject.
Keep up the good work and Iâ€™ll start doing my homework.
March 19, 2008 02:30 am
It's a great place to start!
March 19, 2008 02:12 am
"I was disapointed at how few of you submitted homework for last weeks lesson. In fact, nobody did!"
Well, maybe people just want to learn more photography, and less physics. Like, this week's lesson - lenses and bending light. Ok, but that's 10th grade physics. Using focus, depth of field in photography, on the other hand, is not, and isn't that what people really want to read about?
March 19, 2008 01:36 am
Well, I for one am glad to see the theoreticals.
I've read too many photography texts that hinted at this stuff but tried so hard not to get bogged down with the science of it all that it never actually managed to make it make sense to me. I don't do well when someone just tells me what to do. If I'm going to remember it, I have to learn the WHY.
That said, I'm very short on time these days and won't be submitting "homework." But rest assured that I'm filing these away for a later date when I can work through it all.
March 19, 2008 01:25 am
I understand your concern, and I was expecting that this may be a problem for some, however, I strongly believe that an understanding of the theoretical fundamentals is essential to becoming a great photographer. If it's really going to become second nature, then you need to understand why things happen as they do.
If you understand refraction, then you will know how to deal with lens flare, chromatic aberation and hyperfocal ranges. If you understand the pinhole camera, then you will know the importance of your aperture setting on your focus. I could go on.
Honestly, you can take great photos without knowing why any of this works, however if you find yourself in an unfamiliar situation, knowing the theoreticals means that you can take a good guess at how to deal with it. If you're "playing by ear", so to speak, all you can do is stab in the dark and hope it will work out.
Learning by trial and error is an inefficient way to go about it. Learning from fundamental principles is more rewarding and ultimately more useful.
The early lessons may seem very theoretical and only vaguely relevent, but these points will be referred to time and time again in the future when the lessons become more practical.
My aplogies if this is all too dry for you, but if you stick with it, I think you will find it rewarding in the end.
March 19, 2008 01:15 am
I thought the lessons would be a little less theoretical. I struggle to read through them and the homework are not very interesting. That's just my opinion. I do need the basics as I'm just starting to learn about photography but wow: refraction??
March 19, 2008 12:40 am
I hope you all enjoy this week's lesson. I slipped up and submitted the post to DPS with a missing link. I intended to include a reference to an explanation of the optics diagrams I use. I'm hoping Darren will be able to update the post when he gets the chance.
In the meantime, please feel free to read the guide here: Reading Optics Diagrams.
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