Category Archives: Lesson

How do I control macro (and other) depth of field?

On a Flickr discussion the other day, someone asked what the best way was to control and improve the limited depth of field when shooting with a macro lens.

Pollen In case you’re not familiar with the term, macro photography is very close-up photography.  Technically, it’s photography where the size of the image on your sensor is at least as big as real life, but most people use the term more loosely.  I’m not interested in picking technical nits here, so I won’t be picky.

If you’ve ever done any macro photography, you know that getting your whole subject in focus can be tremendously challenging.  When you’re shooting macro, your depth of field is extremely tiny, often measured in millimeters or fractions of millimeters.  Just a little bit of movement from you or your subject and everything goes to out-of-focus hell in a windblown handbasket.  Even when you have a perfectly still subject and your camera rests firmly on a tripod, shooting your subject at the wrong angle will get you in trouble.  For an extreme example, check out the larger version of the grains of pollen on the left.  The camera was at a slightly different angle than the pollen, and you can see the changes in focus at the top and bottom of the photo.  (You can also see some excellent examples of sensor dust if you look closely at the top right and bottom left corners.)

Back to our question:  how do you control depth of field when you’re shooting macro?  My response on Flickr was that you do it in exactly the same ways that you do for any other shooting:

  • choose your aperture wisely
  • manage your camera-to-subject distance
  • choose your focal point wisely
  • when you can, use depth of field preview to check your image before you shoot

We should all understand the first one by now– choose your aperture wisely.  If you’re new to Stop Shooting Auto! and you aren’t completely familiar with aperture and how it works, you should go reread this lesson and the few that follow it.  In short, choosing a wide aperture (a low-numbered f-stop like f/4) will give you a very shallow depth of field.  Choosing a narrow aperture (a high-numbered f-stop like f/16) will get you much more depth of field.

Don’t forget that there’s an online depth of field calculator that you can use.  If you click around on that site you can also find a PC version, an iPhone version, a PalmOS version, and several other tasty tidbits.

People often overlook the camera-to-subject distance when they think about depth of field, but it’s a key parameter.  Try pulling up the DOF calculator and enter the following values:

Camera format:  Canon Digital Rebel, XT, XTi, XS, XSi
Focal length: 100mm
Selected f-stop: f/11
Subject distance:  30cm

Note that 30cm is right about one foot.  Click the calculate button, and you’ll see that your depth of field goes from about 29.9cm to 30.1cm, for a total of .26cm.  That’s about two and a half millimeters, which doesn’t leave much margin for error.

Leave everything the same, but change the subject distance to 60cm.  Now, your depth of field is from 59.4cm to 60.7cm, for a total of 1.29cm. That’s still not a whole lot, but it’s almost five times what you had when you were a foot away.  Try it at more normal focusing distances, like ten feet and twenty feet, and you’ll see that even there, doubling the distance between the camera and the subject gives you about a 4x increase in your depth of field.  In short, if you’re having trouble focusing, move back.

IMG_5835Choosing your focal point can make a huge difference when you’re working with limited depth of field.  It’s often true that your camera can do a better job of focusing than you can, but it can’t choose what to focus on.  If you really care about a certain part of the image being in perfect focus, your best bet is to put the lens into manual focus mode and do it yourself.  This will give you absolute control over where you’re focusing.

In this image, letting the camera focus might well have meant that it chose to focus on a deeper part of the spadix (the sticky-outy part for non-botanists), leaving the tip out of focus.  This might have been an interesting effect, but it wasn’t what I wanted.

Don’t forget the depth of field preview button on your camera.  If you have a shot lined up, and you want to see how much of it will be in focus, press the depth of field preview button.  The viewfinder will get darker, sometimes very dark, but if you have enough light on your subject you can probably squint and get a good idea of how much of the image is in focus.

If you just can’t get enough depth of field, there are a couple of things you can do.  First, you can add more light to the subject.  I often use desk lamps for macro work, since I can aim the light wherever I want it.  Flashes are always an option, though they can be challenging to use.  If you’re outdoors, get your subject out of the shadows, or use a white reflector (even a piece of paper will work) to reflect light onto the subject.

Putting your camera on a tripod will let you use a longer exposure, and therefore a smaller aperture.  This only works for subjects that are stationary, though– don’t try it with moving objects or you’ll wind up with a blur.

And finally, if your camera is parallel to your subject it will be easier to get the image in focus.  If your subject is at an angle to the front of the lens, move the subject and/or the camera to get a better angle.  Look up at the pollen photo for an excellent example of what happens when you get this wrong.

While depth of field issues make macro photographers pull their hair out (and then photograph the strands), it’s challenging in lots of other situations as well.  Learn to control it and you’ll have a powerful tool for taking better photos.

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Histograms: When weird is OK

HazardHistogramIn the last couple of lessons I showed you what a histogram is, and talked about what a good histogram looks like for a typical image. I also showed you a few bad-looking histograms for overexposed and underexposed images.

Sometimes, a strange-looking histogram is actually OK. This happens if the image that you’re capturing has an unusual distribution of levels, such as a lot of very light or very dark areas. It can also occur if you have a very narrow range of colors, or several other things that are out of the ordinary.

Look at the histogram above. It doesn’t look at all like a bell curve, but rather has several fairly well-defined peaks. There might be a little bit of clipping on the left and right, but there might also just be some solid black and solid light-color areas. It’s hard to know for sure from the histogram whether the image is well-exposed or not, but because there’s a good distribution across the range I would guess that it’s probably not too far off.

VegasHistogramAt first glance, this looks like a massively-underexposed image. There’s a huge amount of clipping on the blacks, and what looks like a little bit of clipping of whites as well. On the other hand, it could be an image with fast swaths of black and very dark colors, and a few bright spots.

As luck would have it, both of these images are within shouting distance of being well-exposed.

Hazard Vegas

When we look at the actual images, it’s pretty easy to see why the histograms were weird. The image with spikes has a very distinct color breakdown. I probably could have done a slightly better job with the exposure, but it’s not far off. The image on the right does indeed have vast swaths of black in the form of the night sky and the ground. A few of the highlights are blown out, as we can see at the far right of the histogram, but that’s really hard to avoid when taking pictures of lights against a black background.

What did we learn from this? Well, if I’m any good at all, we’ve learned that you need to consider your subject matter when you’re looking at a histogram and use that to judge what the histogram should look like. I know that sounds impossible, but most of the time it’s actually rather easy to do once you get the hang of it. In fact, here’s one to think about. If you were taking a picture of the full moon at night, what should the histogram look like?

DarkImage

Oh yes. The other day I showed you a histogram with almost all of the pixels left of center and asked what you thought about the exposure. Both Sarah and TJ had excellent things to say about the image, but TJ completely nailed it– “the object… is mostly black.” In fact, it was my black camera bag shot against a dark carpet. There’s a blip at the far right for the embroidery on the logo, but other than that the image is just naturally dark. It was perfectly reasonable to guess that the image was underexposed, but it was sort of a trick question. I’m just mean like that.

Do you feel like you have a good basic understanding of how the histogram works?

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What is a good histogram?

In my last entry, I introduced the histogram and described its basic function. I even showed a couple of examples. If you read the entry you should have a good understanding of what a histogram is, but there’s probably a great big question in your mind right now: “What is a good histogram?”

The short answer is that there’s no such thing.

You don’t like that answer much, do you? Nope, me either, so even though I could save myself a bunch of typing I won’t leave it at that. I don’t want people running around saying that I’m a meanie.

GoodFlowerHistogramThe longer answer is that what a good histogram is depends on the subject of your photo. For “typical” images, those with an average range of light and dark areas, a good histogram will basically look like a bell curve that’s centered around the center of the graph. And look! Here’s one now. Isn’t that convenient?

Looking at that histogram, you might conclude that there’s nothing in the image that’s 100% bright white, and a very tiny amount that’s pure black. The photo has a good range of tones from light to dark, and there are more moderately-light pixels than moderately dark ones. Let’s look at the image and see if we’re right.

GoodFlowerHey, we guessed pretty well. The flower has a range of tones from black to nearly-white, and there are pretty big light-colored areas.

Note that the exposure looks pretty good to our eye and on the histogram. There are no areas where it looks like we’ve cut off dark colors and made them black, or lost information because the pixels have become pure white. If we had, you’d see spikes at the left or right side of the histogram.

In general, that’s the mark of a good histogram for a typical, average photo. You generally want the majority of the pixels to be somewhere in the center of the graph, without a lot of stuff piled up on one side or the other. If you’re shooting something that has a significant amount of pure black or pure white in it, you’ll sometimes get a spike on the left or right side. That’s OK, but you’ll probably be OK if most of the pixels are somewhere near the center.

When I shot this flower, I took a bunch of pictures with the lighting changed slightly, and most of them didn’t turn out as good as this one. Let’s look at a couple of bad ones now.

DarkFlower LightFlower

If you’re like me, you’re probably thinking about the three bears right now– “This one is too dark. This one is too light. And this one is juuuussstttt right.” If you look at the bottom right corner of the images, you can see the histograms for them. (Or click for larger versions of the dark and light ones respectively.) The histogram for the dark flower has the whole graph left of center, and there’s a big pile of pixels right up against the left edge.  That means that all of the colors in the image are dark, and the part against the edge shows that there’s a bunch of solid-black pixels.  That’s not good– it means we cut off a bunch of information.

The too-light flower has almost all of its pixels on the right side of the graph, which usually indicates that the image has been overexposed. However, if you look closely you can see that there’s nothing against the right edge of the graph. That’s a good thing– it means that you can probably use an image editing program like Photoshop to fix the image and get something pretty good. The dark image might be recoverable as well, but it will never be as good because some of the information got lost when the picture was taken.

If you’re taking a picture of something that’s mostly black or very bright, or something with a narrow range of brightnesses, a correct histogram may look strange. In my next lesson I’ll show you some examples of those, and answer the question about the all-on-the-left histogram at the bottom of my last entry.

And here’s something for you to practice: get into the habit of looking at your histograms. Take a few pictures, and review them on the camera. Check your manual to see how to view the histogram, but on my Canons I get it by pressing the “Info” button while viewing an image. The best way to get familiar with it– pretty soon you’ll have a good eye for what’s right and wrong.

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Histogram: What is it?

I confess… I’ve been neglecting Stop Shooting Auto! for a while.  I’ve been traveling constantly for the last couple of months, and that plus a full-time job has kept me quite busy.  In fact, I’m writing this while sitting at SFO while I wait for my delayed flight to take off.

AverageImageHistogram I promised I’d address the histogram next, so let’s do it.

Maybe you’ve seen something that looks like this, either in Photoshop or other image editing software, or on your camera when you reviewed an image.  Maybe you even took a guess at what it is and how it works, and if you did you probably guessed right.  That’s the histogram.

Histogram is a scary, technical-sounding word, but it’s actually a really simple concept.  The histogram is just a graph of how many dark and light pixels there are in your image.  At the far left you’ll see how many dark pixels there are, then moving to the right you’d see count of lighter and lighter pixels until the far right was pure white pixels.

AverageImageIn the histogram above, you can see that there are a very few solid black pixels, lots that are a little bit darker than average, a bunch that are a little lighter than average, and a chunk that are pure white. Let’s look at the image and see what it looks like.

There are a few pure black or very close to black pixels, mostly around the trash can and along the left wall– that’s the tiny hump on the far left.

The pure white pixels are mostly on the sign, in particular the words “San Francisco Marketplace” are pure white, and a tiny bit overexposed.  That’s the vertical bar right along the far right edge.

The dark of the sign, the dark carpet, and the dark part of the wall are the peak on the left. The peak on the right is the lighter part of the walls.

Most histograms that you will see don’t take color into account, only brightness.  That means that a very bright blue will look just like a very bright white.  Some cameras have the ability to do histograms with the colors separate, and of course most postprocessing software can do so as well.

DarkImageHistogramThis one is kind of fun. Without seeing the image, what can you tell me about it?  Obviously you can’t tell me what it’s a picture of, but can you make any guesses about what the image looks like?  Is it light or dark?  Does it have lots of different shades or only a few?  Are there any pure black or pure white areas in the image?

Do you think this image is well-exposed, underexposed, or overexposed?

OK, they’re calling my flight now.  Ciao!

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April Homework Extension

Very few people did the April homework assignment, so I’m extending it until the end of May.  Grab your cameras and go capture something in motion!  There’s a prize, of course– a signed Flower Porn print, and two if enough people participate.

If you’re new to Stop Shooting Auto! or you haven’t done so before, you’ll want to start by going through the exposure lessons in order.  That will give you a gentle, beginner-friendly introduction to the principles of exposure and the way your camera’s controls work.  Don’t be afraid to ask questions and leave comments… I love getting them, and I’m happy to answer as many questions as I can.

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Someone did their homework!

Someone at SFGate.com seems to have done their homework and captured something in motion.   Why haven’t you?

I’ll extend the deadline for a little while, but the challenge is simple– capture something in motion.  Go here for full details of the homework assignment.

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We have a winner!

Tyler's FlowerCongratulations to Tyler, who won the drawing for the March homework assignment. For being favored by my computer’s random number generator, he wins a signed print from my Flower Porn series.

I was impressed with the submissions– you guys really understood what it took to blur your backgrounds. There were adorable dogs, jewelry, walls, lots of flowers, and plenty of other interesting subjects. The bell was a fascinating and slightly nontraditional composition. Of all of the submissions, I think the bouillon was my personal favorite for moodiness and interesting lighting.

Everybody seemed to understand that the right way to do this was to use a very wide aperture, and almost everyone shot in aperture priority mode. Several people mentioned changing the ISO to get a better shutter speed. A couple of you even mentioned moving closer to your subject in order to get a more desirable depth of field– that’s an excellent technique!

If you learned just one thing from this assignment, I hope it was this: visualize the image before you shoot, and then figure out how to use your camera to capture the picture that’s in your mind. Digital cameras make it very easy to just shoot lots and lots of images until you get lucky, but just a little bit of skill and forethought will do much better.

IMG_9209A print of this image will be in the mail to Tyler soon. And get ready, because the April homework is on its way. It might be a little bit more challenging, but it will also allow a lot more creativity. I’m really looking forward to seeing what you guys do with this one, since the March entries were all great.

Thank you all for contributing!

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