How Autofocus Works, a really simple explanation

In my last article, I posted this photo and asked you to identify what’s wrong with it:

IMG_3750

As it turns out, this is actually a really easy question to answer– the background of the photo is in focus, but the bear isn’t. Now, I’ll be the first to admit that teddy bears are supposed to be fuzzy, but this isn’t the kind of fuzziness that most people have in mind. The more interesting question here is why the camera got the wrong thing in focus. In order to do that, we have to learn a little bit about how autofocus works.

Before we do that, let’s try a little mini-exercise. Pick up your camera. Look through the viewfinder, point it at any interesting subject and press the shutter button halfway. You’ll probably see a display with a bunch of black boxes and some red ones. In this image I’ve colored the black focus points green so that you can see them better. Your camera might only have a few little boxes or it might have lots and lots of them. Those little boxes are called autofocus points:

EyeFocus_focuspoints

So how does autofocus work? It’s a complicated, highly-technical thing, but here’s the simplified one-paragraph version that will tell you most of what you need to know.

When you use autofocus to take a picture, the camera looks at each of the autofocus points and looks for areas of sharp contrast. The camera then changes the focus on the lens just a little bit and checks those points again to see if the contrast is better or worse. It keeps doing this, adjusting the focus a tiny bit each time, until it thinks it has the best focus. Once it does, it takes the picture. There’s a whole lot of black magic going on behind the scenes to make this happen, but that’s the really simple version.

The camera might choose to use all of the focus points if it can. More often, it will use one or a few of them to find the subject. As you’re looking through the viewfinder, the camera will light up an individual focus point if it’s happy with the focus in that area. If your camera is guessing wrong you can boss it around and tell it exactly which points to use, but that’s a subject for a different lesson.

So that’s it– when you autofocus, the camera looks at the areas in the little black boxes and then changes the focus on the lens until it’s happy with the results. That’s not so hard, is it?

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When Autofocus Goes Bad

[It’s been a while since I updated Stop Shooting Auto! Too long, in fact. It’s time for me to get back to work, starting now.]

Thus far I’ve written a lot about the things that go into exposure– aperture, shutter speed, ISO. By now you should know a lot about how these things work, what your camera does automatically, and ways that you can be smarter than your camera. If you’re not comfortable with these things yet, go back through the exposure lessons and brush up on the things you’re missing. In particular, at this point you want to have a good handle on aperture and depth of field.

Exposure isn’t the only thing that your camera does automatically though. In almost all circumstances, your camera will also automatically focus for you. This is often a good and wonderful thing, since it makes your life easier, but sometimes the camera gets autofocus wrong. Let’s look at an example.

IMG_3750

This evening I decided to set up the Stop Shooting Auto! Bear Portrait Studio and take some shots. Shooting bears is great for practicing your photographic skills because they’re eminently patient with you and won’t squirm around, fall asleep, or wander off to grab a beer from the ‘fridge.

The setup for the bear portrait studio is pretty straightforward. There’s an extra super ugly backdrop, which in this case is a really tacky sheet hanging from a backdrop stand. In front of that is a bear sitting in a chair. I have a few cheap incandescent lights set up, with two pointed at the backdrop and one on the bear. Finally, the camera is mounted on a tripod. For this example, the camera’s sensor is just under eight feet away from the backdrop. The bear’s eyes are 51 inches from the backdrop. The bear has quite the schnozz on him, as the tip of his nose is four inches in front of his eyes.

(My ethical standards require me to make the following disclaimer: Using a zebra print sheet for a backdrop is a profoundly bad idea, and you shouldn’t do that. Doing so may cause permanent damage to your retinas, or your psyche. Sometimes I think my purpose in life is to serve as a warning for others.)

I’m shooting with a Canon 5D Mark II, and a 50mm f/1.8 lens at f/4.

So, with this setup I took a portrait. Something went horribly wrong, though– the image sucks. Can you guess what the problem was? Click on the image if you want to see larger versions of it.

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Filed under Autofocus, Introduction, Lesson

What lens should I use to take a picture of the moon?

A couple of years ago I wrote How to take a picture of the moon.   It was, as you might guess, a lesson on how to take photos of the moon with your dSLR.  You may all now compliment me on my creative and clever naming scheme– I spent hours wordsmithing that one.  It’s my masterpiece.

In it I wrote “The first thing you want is the longest telephoto lens you can get, so that the moon looks like something other than a white dot. 200mm is about the shortest, 300mm is better, and 500+ is even better than that.”  That’s a true statement, but it lacks any substantiating evidence.  As you know, I’m a huge fan of experimentation and examples.  You can probably guess that a longer lens is better for shooting a far-away object, but how much better is it?

A few months ago, my friends at the absolutely fabulous equipment rental company BorrowLenses.com had a crazy sale on super telephoto lens rentals, so I did something I’ve wanted to do for a long time– I rented an obscenely large lens.  Specifically, I picked up the Sigma 800mm f/5.6 super-telephoto lens.

If you’ve never seen an 800mm lens, you should.  This beast is almost two feet long, about six inches across at the front element, and weighs in at a whopping 13 pounds.  It comes in its own well-padded metal carrying case and a sherpa to carry it, and has its own gravitational field.  OK, I made up the part about the sherpa, but it should come with one.  Maybe my trusty Gitzo tripod needs a name?

In any case, the moon.  I timed my rental so that it coincided with the next full moon.  I had an evil plan!  My plan was to steal a bunch of photons from the moon so that I could show you, dear reader, what the moon looks like when captured with various focal lengths of lens.

In order to do this, I packed the following gear:

My plan was simple: I would set up the tripod and shoot the moon with various combinations of lenses and teleconverters to show the image size of the moon with each combination.  For example, I could use the Canon lens at 200mm, and then add a 1.4X teleconverter to show what it looks like at 280mm.  Then I could do it with the 2X teleconverter to show 400mm, and finally with the two teleconverters stacked to show 560mm.  Stacking teleconverters really isn’t a good idea, since it causes your images to get all soft and cuddly, but it’s certainly OK just for demonstrating what the focal length looks like.

When you look at these images, bear in mind that they’re taken with a full-frame camera.  If you have a cropped-sensor dSLR, as most people do, imagine that at portion of the outer edge of each image has been cut off.  This Wikipedia image has a pretty good comparison of sensor sizes.  Look at the APS-C for Canon or Nikon, and that’s about how much you would cut off the edge of my images to get them to look like yours.  All of these images have a full-size version available if you’re logged into Flickr.

200mm 200mm
280mm 280mm = 200mm +1.4X teleconverter
400mm 400mm = 200mm +2X teleconverter
560mm 560mm = 200mm + 2X teleconverter + 1.4X teleconverter
800mm 800mm
1120mm 1120mm = 800mm + 1.4X teleconverter
1600mm 1600mm = 800mm + 2X teleconverter
2240mm 2240mm = 800mm + 2X teleconverter + 1.4X teleconverter

As you can see, longer lenses bring the moon in closer. Even 2240mm doesn’t quite fill the sensor on a full-frame dSLR, although about 1600mm would do so on a cropped-sensor camera.

That doesn’t tell the whole story, though– you don’t have to fill the whole frame in order to get a decent photo of the moon.  If you click through and look at the full-size image taken at 400mm, you’ll see that it has a fair bit of detail in it.  Still, when it comes to shooting the moon, longer is better.  This is one case where size really does matter.

If you decide you want to rent a long lens and try it yourself, I heartily recommend BorrowLenses.  The Sigma that I rented is pretty expensive unless you have other uses for it, but I heartily recommend the Canon 400mm f/5.6L for shooting the moon.  It’s quite reasonably priced for the focal length, and it has very good image quality.  Add a 1.4X teleconverter to it if you need additional length.  It’s also a lot easier to wrangle than the beast.

There are a couple of other things to know.  You really want to use a tripod, or something else to stabilize your camera, when you’re working with long lenses.  This will keep your images from having ugly camera shake.  The downside of this is that the moon moves pretty rapidly through the sky, so if it’s been a couple of minutes since you aimed the camera you’ll probably need to look through the viewfinder again and see if the moon is still in the frame.    Also, working with long lenses can be its own challenge.  One technique is to rest your hand on top of the lens, just above where it sits on the tripod.   This article explains it, and has a great illustration too.

Now what are you waiting for?  Go shoot the moon, and share your images with us.

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The Joys of Equipment Rental

I’m a gearhead, and always have been.  It’s an engineer thing, I guess, but photographic equipment is inherently interesting to me.   I like playing with different kinds of equipment, and experimenting with new techniques.  I also find myself wanting for specialized equipment from time to time, such as a very long telephoto lens.

A few weeks ago I decided that I was curious about tilt-shift photography.  It’s a fairly esoteric type of photography that’s used for architectural photos, landscapes, and special focusing effects.  Tilt-shift lenses are expensive, though– the cheapest Canon, the TS-E 24mm f/3.5L, runs around $1200 mail-order.  That’s a lot of money for something I probably wouldn’t use very often, and it’s way more than I’m willing to spend on my curiosity.

What’s a budget-constrained gearhead to do?

For about $50, I could rent the TS-E 24mm for a week, and I did.  None of the photos that I took with it were terribly interesting, but that wasn’t my goal.  I really just wanted to play around with it and get a better understanding of how tilt-shift photography worked.  Buying a lens for that purpose would have been cost-prohibitive, but renting was an affordable alternative.

Renting equipment is a great way to try something out before you buy it.  Do you really want that specialized macro lens, or a fisheye?  Rent it and try it out  before you plunk down a lot of cash.  Lenses are the most common things to rent, but you can also get camera bodies, lighting equipment, tripods, and more.  I discovered my favorite tripod by accident, when I rented it for a weekend.

I especially like rental for long telephoto lenses, since I don’t need one very often and they’re terribly expensive.  My next rental will be the Sigma 800mm f/5.6 super-telephoto lens.  I have it reserved for pickup next week, and I’ve timed the rental so that it occurs during the next full moon.  If all goes well, I’ll be updating “How to take a picture of the moon” with some new examples.  Wish me luck!

So maybe you want to rent equipment yourself.  How do you do it?

Most big cities will have at least one camera shop that rents equipment.  Check your local yellow pages, call around, or search the web to see what you can find.  The advantage of this is that you can often pick something up immediately.  Supporting your local camera shop is also worthwhile.  The downside is that local rentals have a tendency to be a bit on the expensive side.

Another option is one of the internet-based rental houses.  Several of them have sprung up in the last couple of years, and I’ve personally done business with a few of them.  They have their inventories online, and ship equipment to you when you rent it.  You keep the packing materials, and at the end of the rental period you repackage the gear, put the return shipping label on the package, and send it back.  Doing this takes longer than renting locally, but you often get a better selection of equipment than you can get locally, and the prices are better.

As I said, there are quite a few different internet-based rental companies.    All of the ones that I’ve done business with have been very good, but I’m particularly enamored of BorrowLenses.com.  They have an excellent selection, their prices are competitive, and I’ve gotten spectacular service from them.  Additionally, they are local to the San Francisco bay area, which is a huge advantage for me– I can order things online, then swing by their shop and pick it up, thus saving myself shipping charges.  I even enjoy their Twitter feed, which contains interesting information for photographers as well as occasional sales.

Rental is a great option for short-term needs, trying something out before buying it, or just playing around.  I highly recommend it as an alternative to buying.

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SFGate photographers’ outtakes

SFGate has some excellent photographers working for them.  While I’ve seen the occasional clunker of a photo on their pages, many of the shots are extremely good– well-seen and well-captured, and they often tell a story with few or no words required.

I just came across a couple dozen outtakes of the year from them.  Many of the photos are quite good, and some are just short of being good.  The thing I particularly enjoyed about them, though, and the reason I’m posting this here, is that each of the photos has camera and exposure information in the caption.  Several of them also have the photographer’s thoughts about why they took the shot the way they did.

Many of them were shot at high- or very-high ISOs, including one at ISO 4000.  The photographer was using a Canon 5D Mark II for the shot, which performs brilliantly at higher ISOs.  It’s also my camera of choice these days.

Study the images and think about why the photographer used the settings they did.  I think you’ll find them quite educational.

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Holiday gifts for the dSLR owner, 2009 edition

Last year I published a list of holiday gifts for your favorite camera owner.  I still love all of those recommendations, but here’s a new list for 2009.

The GorillaPod is an incredibly innovative portable tripod.  It’s small enough to fit into a camera bag or backpack, so your favorite photographer can always have it handy.  The articulating legs allow it to be used as a tabletop tripod, or to wrap around the nearest tree branch, street sign, or whatever else is handy.   While it’s not the only tripod a photographer could ever need, it’s incredibly brilliant at what it does.

There are several models of GorillaPod to choose from.  Be sure to choose one that’s right for the camera it’s being used with.  Many of the smaller GorillaPods are designed for small, lightweight point & shoot cameras, and they won’t do a very good job of holding up a dSLR.  The SLR model is excellent if your photographer friend uses fairly small lenses, but step up to the SLR Zoom model (pictured above)  if they use long lenses, or if you aren’t sure.  A little bit of sturdiness won’t hurt.

In the age of the high-tech digital camera arms race it may seem strange for me to recommend a cheap film camera, but hear me out.

The Holga 120N is exactly that– a cheap Chinese-made plastic film camera.  It’s low-tech.  It leaks light like crazy.  It vignettes, and the cheap plastic lens often distorts like nobody’s business.

And that’s what makes it fun!  Sometimes it’s nice to take a break from the exquisitely flawless photography of the digital age and do something messy and imperfect.  The Holga is the classic camera for that. If you don’t believe me, check out the images in this Washington Post article, including an award-winning photo of Al Gore on the campaign trail.

The basic camera above is a great place to get started.  Amazon also sells a Lomographic Holga Starter Kit, which includes a Holga with built-in colorflash, a book, and a roll of film.

The book Light, Science, and Magic is a little bit more advanced than I would usually recommend here, but I’m doing it anyway. It’s a fantastic book!

Photography is all about capturing light, and Light, Science, and Magic is the best I’ve ever encountered about photographic lighting.  There is plenty of material that is accessible to a beginner, but also enough depth that the book will grow with them over time.   This isn’t just one of the best photography books I’ve ever read, it’s one of the best books on any subject.  Really!

If your photographer friend is still trying to remember that they have to take the lens cap off before they shoot, or most of their pictures come out blurry, this book will be a bit too advanced for them.  However, if they’ve mastered the basics of their camera and seem interested in learning to take better photos, this could be just the book they need.

And finally, one thing photographers like to do is show off their photos.  There are so many digital photo frames on the market that I can’t even begin to recommend a specific one.  You can find some that are keychain-sized, and others that are 17 inches or more.

Happy holidays from Stop Shooting Auto!

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What is Exposure Compensation?

I have new kittens!

I’m not normally one of those people who takes hundreds and hundreds of pet photos, but I’m not completely immune either.  This evening I turned around and saw them sitting on top of the cat tree looking utterly adorable.  I also happened to have my 5D Mark II sitting on my desk, with a 580EX flash and a Sto-Fen Omni-Bounce diffuser attached.

If you’ve ever been around kittens, you know that they’re not terribly prone to sitting still for very long.  I slapped the camera into P mode, turned the flash on, and started shooting.  After a few shots, I looked at one of the images:

We're cute and underexposed

Yuck! Underexposed. I’d put the camera into P mode because I was in too much of a hurry to think about the proper settings for flash (which I almost never use), but I’d forgotten that the diffuser I was using steals light. You know how frosted glass doesn’t let as much light through as clear glass? The same thing happens with the translucent plastic of the flash diffuser. It does a great job of softening the harsh light of a flash, but it also means that your images will be underexposed if you don’t do something about it.

The kittens were staying put, so I had time to shoot a few more, but I wanted to get a better exposure. I knew from experience that the diffuser reduces light by about one stop, so I used the exposure compensation setting on the camera to overexpose the image by one stop. Actually, I screwed up and set it for one and a third stop of overexposure rather than one stop, but I didn’t notice that until later. The results, while not perfect, were appreciably better:

Hi!  We're cute!

If you look at the full-size version of the image, you can see that there’s actual definition in the black fur.  The white fur is a little bit blown out, but the net result is better than the first one.  I could fix both of these to some extent in Photoshop, but it’s better to get it right in the camera.  I haven’t adjusted the exposure of either of these, though I did crop them and sharpen the second one a bit.

So what is this exposure compensation thing?

Well, we already know that in most modes, including program, aperture priority, and shutter priority, the camera uses its meter to evaluate the image and then picks an exposure based on it.  In manual mode, you do this yourself, though you usually use the meter as a guide.  The camera does this by looking at the average brightness of the image and then picking an exposure that will best capture it.  When using automatic flash, the camera also tells the flash unit how bright it should be.  This often works very well, but in my case I knew something that the camera didn’t– there was a diffuser on the flash.

Exposure compensation tells the camera that it should overexpose or underexpose an image by a certain amount.  In my case, what I said to the camera was, “Hey, camera.  Make your best guess about the exposure, and then add one stop to it.”  I did this by changing the exposure compensation setting to +1 rather than 0.  Doing that allowed the camera to correct for the diffuser, and get a better-exposed image.

The exposure compensation on your camera looks something like this:

-2..1..0..1..+2
       ^

What you’re seeing is a representation of the camera’s meter, and an indicator underneath it showing what the compensation should be.  When the arrow is under the zero, the camera will pick the exposure it thinks is right.  However, when it looks like this:

-2..1..0..1..+2
          ^

the camera will add one stop to the exposure it thinks is right, resulting in a brighter image.  If the arrow was under the 1 on the left, the camera would subtract one stop and the result would be a darker image.

Exposure compensation is useful if you know that the camera is going to predictably underexpose or overexpose an image.  This might happen if you’re shooting with a very light background like snow, or a very dark background like a dark theater.  There are lots of other cases where it can happen as well.

Oh!  One other thing.  Sometimes people discover that their camera is suddenly making all of their pictures too dark or too bright. They wonder what’s wrong, and think their camera is broken.  What has usually happened is that they’ve accidentally set the exposure compensation, and the camera is doing exactly what it was told to.  Also, don’t forget to set it back to zero when you’re done, or your next shoot will have lots of over- or underexposed image.

I hope this helps explain the basics of exposure compensation.

And yes, yes.  I really did admit to using P mode on my dSLR.  As I said above, I didn’t want to think about the right settings for the flash.  I was in a hurry, because I wanted to catch the kittens before they moved.  Plus, I wasn’t really aiming for super high-quality images– in essence, I used four thousand dollars worth of equipment as to grab a quick snapshot.   P is only bad when it’s used for everything, or used without forethought or understanding of its weaknesses.

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