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DPI: Why You (Probably) Don’t Care

January 14, 2009

I see this question, or some variation on it, over and over again:

“I just got a new camera, and it shoots at 72DPI by default. How do I change it? Won’t 300DPI give me higher-quality images?”

It’s a common question, and a very common misunderstanding about DPI.

First, let’s start with the camera. Your camera’s sensor has some native size, some fixed number of recording points (remember the sensor gnomes?) on its sensor. Each recording point records one pixel of the image. Every image that’s shot with the camera has that size no matter what. Using the Canon Digital Rebel XTi as an example, the sensor is 3888 pixels wide and 2592 pixels tall. If you multiply those numbers together you get 10,077,696, which is why it’s called a ten megapixel (ten million pixel) camera.

Remember that. As long as you shoot at the XTi’s maximum resolution (and by that I don’t mean DPI, I mean L or L+RAW) you’ll get an image that is 3888 pixels wide and 2592 pixels tall. Some cameras let you shoot smaller images, by changing the size from L to M or S. If you do that, you’re shooting at whatever size that particular setting gives you, but it’s still independent of DPI.

So what’s DPI? It stands for Dots Per Inch. For our purposese, a dot is the same as a pixel, so it would be pixels per inch. (There is a technical difference between dots per inch and pixels per inch, but for purposes of this discussion we’ll treat one pixel as being the same as one dot– it’s one unique spot of data in our digital image) DPI is not really a part of the image itself, and doesn’t change anything within the image. Rather, it’s a number that’s stuck onto the side that tells a printer how big it should print out the image– imagine someone handing the file off to be printed, and sticking a post-it note on the file so that the printer will know how big to make the image. If the image is 72DPI, the printer will print out 72 dots in every linear inch. At 300DPI, the printer will print 300 dots in every linear inch.

If you’re working with an image on the computer, and never printing the image, DPI is completely irrelevant. A 3888×2592 image at 72DPI is exactly the same image as a 3888×2592 image at 300DPI or 1DPI, or a million DPI. Really.

The thing about DPI is that you can change it arbitrarily, and it won’t affect the image. As long as it stays 3888 pixels wide and 2592 pixels tall, you can rip off the 72DPI sticky note and change it to any other number, and you still have exactly the same data in your image. That means that you don’t really care what DPI the camera sticks onto the side when you first take the picture– changing it in-camera won’t change the information that you capture, and it’s easy to change it later if you want to print the image out. Plus, if you’re printing the image you probably will want to resize it, crop it, or make other adjustments.

And that, in a nutshell, is why you probably don’t care about DPI. Unless you’re printing the image out, DPI is a completely irrelevant concept, and one you basically just shouldn’t worry about at all.

Next up, I’ll explain how DPI works when you’re printing.

Posted in Discussion, Equipment | 2 Comments »

Slightly Off-Topic: Photographers’ Rights

August 26, 2008

This isn’t really about how to take better pictures, but it’s important enough that I want to share.

Yesterday, while walking through San Francisco’s SOMA neighborhood, I spotted a sign in a bookstore window. The sign reminded me of something, so I pulled out my phone and took a picture of it. Just as I finished taking the picture, one of the store’s employees came up to the window and gestured to indicate that I wasn’t allowed to do that.

I didn’t push the issue, but he happened to be dead wrong. In the US, I have every right to photograph the exterior of a commercial building, so long as I am doing so from public property. If I’m on private property, the owners of the property have every right to ask me to refrain from taking photos, and I must do so.

Every photographer should read the following information on photographers’ rights. It’s even provided as a printable PDF, so that you can keep a copy in your camera bag. In our current social climate, photographers are being hassled more and more in the name of security, and it’s important to know your rights, responsibilities, and restrictions.

Posted in Discussion | Tagged legal, photography, photorgraphers rights, rights | 5 Comments »

Two interesting photos

March 7, 2008

I’m a big fan of the Day in Pictures on SFGate – the photos are generally quite interesting, they’re well-captioned, and the editor likes to play amusing games with proximity.

Today’s set had two photos that are excellent illustrations of the things you can do if you understand exposure. This one shows an elderly man praying in the middle of the road, with cars whizzing by on either side of him. The blur of the cars is worrisome– you’re sure he’s going to get hit at any moment– but it shows his conviction.

The second one shows a landscape with a closeup of poppies in the foreground. The poppies are crisp and sharp, and the background is reasonably sharp as well.

Take a moment to think about how you might shoot those two photos.

Got it? OK, here’s my take.

The first one requires a slow shutter speed in order to capture the motion of the cars. If we look closely, we can see that everything is sharp as far as the eye can see. This suggests that the photographer used a very narrow aperture and a slow shutter speed, and probably a low ISO as well. We don’t know how fast the cars are moving, but let’s guess, oh, 30MPH. We can ask Google and find out that’s 44 feet per second. I’d guess that the shutter speed was somewhere between 1/2 second and 1 second. I’d guess that the photographer also used a neutral density filter (something I haven’t talked about, but it’s basically a piece of grey glass that reduces the light coming into the camera) to get a slower shutter speed.

The poppies were clearly shot at a fairly small aperture, since the background was mostly in focus. The shutter speed is indeterminate, but was probably “whatever it takes”.

Whenever you see an interesting and unusual picture, take a minute to see if you can figure out what the photographer did to get it. This is excellent practice for figuring out your own shots when you’re trying to get a specific effect.

Posted in Discussion, Exposure | 1 Comment »

Why are there spots on my picture?

March 4, 2008

You might notice it in the blue of the sky or the white of a wedding dress. Maybe, as often happens with me, it’s the petals of a flower. Whatever the image, someday you’re going to look closely at a picture and see that it’s full of little spots. These often look like little dark spots ringed by a color that’s slightly lighter than the background, and they show up in the same place on every picture you take.

These spots usually come from dust on your sensor. You won’t see them too often at large apertures, but when you stop the camera down they start to become visible. They particularly stand out in areas that have large expanses of a single light color, but you can often see them in other areas of the image as well.

This flower was taken at f/32, which as we all know is a very tiny f-stop. It’s not a great photo, but because of the small aperture it shows every little piece of dust on the sensor. You can’t really see it at the small size, but if you were to look at the full-resolution image you’d see lots and lots of little bitty spots all over the photo.

On the right is a collection of full-size crops taken from the same image. These spots become highly-visible and quite annoying when the image is enlarged.

There’s an easy way to test to see if you have noise on your sensor. Find a brightly-lit white or light-colored wall, or a piece of plain white paper. Set your camera to manual mode, shutter speed of about two seconds, and the smallest aperture your lens will do. Point the camera at the white surface and take a picture, moving the camera around just a little bit so that the background becomes blurry.

Transfer the image to your computer and look at it full-size. Chances are good that you’ll see at least a few little specks, and if you’ve never cleaned your sensor before it may be pretty bad.

This image originally came from Flickr user djwudi, and I thank him for allowing me to use it. He works in a camera shop, and a customer came in complaining about spots on his photos. This is what the sensor looked like at f/32 before cleaning. I’ve personally never seen one this bad, but dust can start making your pictures ugly long before this point.

If you change lenses, and sometimes even if you don’t, getting dust on your sensor is inevitable. However, there are a few things you can do to reduce the amount of dust that your sensor collects:

Never store the camera without a lens or other protective cap
Minimize the number of times you change lenses, and minimize the amount of time the lens is off the camera
When you change lenses, make sure the body of the camera is pointed down and the camera is turned off
Try not to change lenses in dirty environments

My procedure for changing lenses is to turn the camera off (though sometimes I forget), then take the rear cap off of the new lens and get it ready to go. As soon as it is, I point the camera straight down, remove the lens, and put the new one on as quickly as possible. I then put the rear cap on the old lens and store it in my camera bag.

Do you have dust? Don’t worry! Here are SSA!’s instructions for cleaning your sensor.

Posted in Discussion, Equipment | Tagged camera, dirt, dust, sensor, yuckypoo | 5 Comments »

Shooting Through a Fence

January 27, 2008

In the last lesson, I talked about shooting at the zoo under difficult circumstances and how to break the problem down into manageable pieces. One of the problems that TJ faced was that there was a fence between him and the subject. I promised to take a few sample shots through a fence and show what effects different apertures had on the image.

The following images were all shot with a Canon Digital Rebel XT and the Canon 50mm f/1.8 lens, also known as the nifty fifty. I chose it because I wanted to be able to open up the aperture in order to blur the fence as much as possible. Here are three sets of images that show how the fence looks from different distances at different apertures. I apologize for not finding more interesting subjects, but these were taken during a quick stop at the Port of Oakland when I was late for work.

In the first set, the camera is just a couple of inches from the fence. The camera was in aperture priority mode, and the four shots were taken at f/2, f/4, f/8, and f/16. As you can see, at f/2, the fence blurs so much that it’s essentially invisible. At f/4, you can start to see the fence blurring parts of the image. At f/8 the fence is quite visible, and at f/16 it’s almost sharp.

This time, I’m standing about an arms length away from the fence. At f/2, the fence is just a blurry grid. At f/4 it starts to look like a fence, at f/8 it’s clearly a fence, and at f/16 it’s fairly sharp.

In the final set I’m standing about five feet away from the fence, and also at a slight angle. At f/2 the fence is a blurry mess, at f/4 it starts to sharpen up, and at f/8 and f/16 it’s sharp enough to work as part of the image rather than being a flaw.

Conclusion: if there’s a fence in your way, figure out what you want to do about it and adjust accordingly. If you want to make the fence disappear, get as close to it as you can, and open up your aperture as wide as possible. If you want to use the fence as part of the image, back up a bit and stop down.

Posted in Discussion, Lesson, Question | Tagged canera, lens, photography | 3 Comments »

What do you want to know?

September 19, 2007

I need to go shoot some examples before I write the next couple of lessons. Unfortunately, I’ve been spending painfully long hours in the office this week and haven’t had time to get out and shoot.

In the meantime, what do you want to know? My plan is to fill out the basics of aperture and ISO, and then do… something. What should I write after that? Do you have questions about what I’ve written so far?

Posted in Discussion | 6 Comments »

Stop Shooting Auto!