Photography 101 - Lenses and Focus
The following post is from Australian photographer Neil Creek who is part of the recently launched Fine Art Photoblog, and is participating in Project 365 - a photo a day for a year - on his blog.
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
Bending Light
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.
Fig 1.2.1 A light shone into a glasstank of water bends. Source.
Fig 1.2.2 As light passes into a morerefractive material, it slows and bends.
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.
Lenses
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.
 Fig 1.2.6 |
 Fig 1.2.7 |
Not all lenses are equalIt’s not always the case that focal length equals lens length, as the complex optics in modern lenses can give a “virtual” focal length while keeping the actual lens size small. As a rule of thumb, the focal length isusually pretty close to the actual length of the light path through the lens.
Focusing
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.
Homework
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:
- Project an image with a magnifying glass or a lens from your camera gear and take a photo of it. If you want to get really creative about it, be inspired by this spectacular example seen recently on Strobist.
- Find and photograph examples of light refracting in everyday objects. The clearer the example the better. For example the classic pencil in a glass of water, or maybe play with some large crystals from a jewelery box.
- Shoot some natural lenses. Drops of water can be creatively used as little magnifying glasses to show an inverted image of the scene beyond them. This would be a good exercise for lovers of macro photography.
Resources
- Lenses (optics) on Wikipedia
- Refraction - Ch4 of Optics by Benjamin Crowell.
- Refraction group on Flickr
Next Week
Photography 101 - Lenses, Light and Magnification.
In addition to posting his Project 365 photos to his blog, Neil also runs a monthly photography project. This month’s topic is Iron Chef Photography - The Fork.
Read more posts like 'Photography 101 - Lenses and Focus'
March 19th, 2008 at 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.
March 19th, 2008 at 1:15 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 19th, 2008 at 1:25 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 19th, 2008 at 1:36 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 19th, 2008 at 2:12 am
“I was disapointed at how few of you submitted homework for last weeks lesson. In fact, nobody did!”
:-D
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 19th, 2008 at 2:30 am
It’s a great place to start!
March 19th, 2008 at 3:36 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 “do this” 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 I’ll start doing my homework.
March 19th, 2008 at 4:24 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.
D.
March 19th, 2008 at 4:42 am
Good stuff, Neil. Maybe a little theoretical, but it’s good to know how this stuff all works. Keep the knowledge coming!
March 19th, 2008 at 5:23 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 19th, 2008 at 6:28 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 19th, 2008 at 6:35 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 19th, 2008 at 6:55 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 19th, 2008 at 7:09 am
Very informative!!
kudos! :-)
March 19th, 2008 at 7:09 am
Always cool when what we’re covering in Physics class suddenly shows up in relation to photography. Thanks for the info.
March 19th, 2008 at 9:29 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 – I could be wrong – 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 19th, 2008 at 9:49 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 19th, 2008 at 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…
http://www.flickr.com/photos/visper/sets/72157604153159250/
Neil, thanks a load for these 101’s… I shall be follwing along!
[can I have my gold star now?!…]
sime
March 19th, 2008 at 10:15 am
Just a fast trial with a beer glass full of water and a laser pointer.
March 19th, 2008 at 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 19th, 2008 at 10:31 am
Just how do we submit the homework?
March 19th, 2008 at 10:33 am
Just upload it to your favourite photo-sharing site and post the link here.
March 20th, 2008 at 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 20th, 2008 at 3:45 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.
http://flickr.com/photos/scfd/sets/72157604161670930/
March 20th, 2008 at 4:23 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.
http://picasaweb.google.com/jessicastier/Photography101BendingLight
March 20th, 2008 at 4:25 am
hmmm…. Now I’m wondering if I misunderstood. It wouldn’t be the first time! :)
March 20th, 2008 at 4:46 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 20th, 2008 at 7:19 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 20th, 2008 at 7:52 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 20th, 2008 at 10:05 am
i remember my physics laboratory class
we did an experiment on this. =)
focal lengths & refraction stuff..
thanx for the refresher..
March 20th, 2008 at 1:19 pm
Firemaine,
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!
http://www.flickr.com/photos/67495239@N00/sets/72157604171763547/
I like the theory, I think it is worth learning. Thanks!
Ira
March 21st, 2008 at 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 21st, 2008 at 2:47 pm
Nice tutorials.. I know something about the lens now.. Expect to read more lessons that can be published as a book..
March 21st, 2008 at 9:38 pm
Dear Neil,
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.
Yours,
Char
March 22nd, 2008 at 2:05 am
This is my submission. Very nice lesson. Thanks!
http://farm3.static.flickr.com/2265/2350181494_84f9a4dbce.jpg
March 23rd, 2008 at 6:32 pm
I’m enjoying the lessons so far - I’m getting a much needed theoretical background and look forward to doing the assignments
March 25th, 2008 at 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!
http://www.flickr.com/photos/sandman1973/2332012519/
March 26th, 2008 at 5:38 am
Submission for natural lenses…
http://farm3.static.flickr.com/2333/2350109418_d8d5bc6d1c.jpg
March 28th, 2008 at 6:18 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 28th, 2008 at 1:12 pm
Great stuff, PLEASE keep adding in the theory material so we all develop an understanding of not only what but also why.
Jake
March 29th, 2008 at 4:20 am
Neil,
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 29th, 2008 at 5:23 am
“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.”
I do not understand why that is. I am having trouble grasping this. Could you elaborate a bit?
March 30th, 2008 at 12:32 am
Here’s my belated homework attempt:
http://flickr.com/photos/tsaiek69/2371212634/
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.
Thanks Neil!
March 30th, 2008 at 7:25 pm
Hi Neil,
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 31st, 2008 at 6:45 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.
http://www.glaesers.com/DPS%20Homework.htm
April 4th, 2008 at 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.