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Today's e-tip:

How Cameras See - Lenses

Your digital camera doesn't see the way you do. Understanding the differences in the way you and your camera sees is the start of taking great pictures.

How Cameras See - Lenses
Lenses
Your camera’s lens has very different attributes than the human eye. You can never capture exactly what you see. The most obvious difference is that your eyes see in three dimensions - our cameras only can record two dimensions.

In this eTip, we'll explore more of these differences, and how you can use them to your advantage.
Eye vs Lens

Note: The sensor-size of your camera has an effect on the focal length of the lens. If your camera has a sensor that's smaller than a "full frame" sensor, you'll likely see a phrase such as "effective focal length" or "35mm equivalent". This "effective" focal length is the one we'll be referring to in this eTip. For more info see this eTip.

First - a definition - Focal Length
Focal Length is a term that describes the optical "length" of a lens. That sounds complicated, but it really isn't. It's actually quite simple. (note: this definition applies to "prime" lenses (with one focal-length) - we'll explain ZOOM lenses further down)

The technical definition is that the Focal Length of a lens is the distance between the optical center of the lens and the sensor when the lens is focused at infinity.

Technical definitions aside, generally the longer the focal length the longer the lens is physically. There are a few exceptions, but this is generally true. Longer focal lengths also magnify - they have the effect of a telescope.

Here's the non-technical version:
Just imagine you have 2 tubes, both the same diameter: one is 1 inch long and one is 12 inches long. Put each up to your eye and look thru each. You'll have a wider view with the 1 inch tube than you will with the 12 inch tube.

Now, imagine that everything you see in the 1 inch tube fills the sensor in your camera. Now imagine the same thing with the 12 inch tube.

There's obviously more to your camera lenses than hollow tubes! But you can see the similarities. The  "longer" tube or lens has a narrower field of view and when it fills the sensor in your camera with what it sees, it has the effect of not only seeing a narrower view, but also of magnifying what it does see.
Looking thru a tube

The difference between a wide angle and telephoto lens is perspective.

Perspective


Longer focal length lenses (e.g., 300mm) have the effect of magnifying what they see with a narrower field of view - kind of like a telescope, hence, they're called "Telephoto" lenses. You can use a "long" lens when you want to capture a subject that's far away.

Shorter focal lengths (e.g., 24mm) have a wide field of view and are called "Wide-angle" lenses. They provide the opposite effect of magnifying - they actually can make everything look smaller. You use wide-angle lenses when you want to capture a lot of what you see - typically in landscapes and most travel photography.


PRIME lenses cover a SINGLE focal length. A prime lens can be any focal length, but can't "zoom" in. Prime lenses are generally sharper and yield higher-quality images than zoom lenses, but you sacrifice convenience as you may need multiple prime lenses where one zoom may suffice. For example, if you have a 18mm - 200mm zoom lens, you may need 5 prime lenses (e.g., 20mm, 50mm, 85mm, 135mm and 200mm) to give you similar capabilities that you'd get with the one zoom lens.

ZOOM lenses cover a range of focal lengths. A zoom lens may cover a range of 70mm to 200mm, 28mm to 300mm, 24mm to 70mm, etc. Sometimes zoom lenses have a fixed aperture (e.g., 24-70mm f2.8) where the maximum aperture remains the same throughout the zoom range. Other (less expensive) zoom lenses typically have a variable aperture - that is, the aperture varies depending on how the lens is zoomed. These lenses require more light when zoomed towards the longer focal lengths. An example would be a 28-300mm f3.5-5.6 - which would have a maximum aperture of f3.5 when at 28mm, but a maximum aperture of f5.6 when zoomed to 300mm. In between, the maximum aperture would be something in-between f3.5 and f5.6.

Focal Length Modifiers - Tele Extenders or Tele Converters
Some lenses are designed to modify another lens. The most common of these, with DSLRs, is the Tele Extender or Tele Converter (terms are used interchangeably). This device sits between your lens and your camera body. These "multiply" the focal length of a lens. There are 3 common telextender configurations: 1.4x, 1.7x and 2.0x. Each changes the focal length of a lens by a different factor. There are several down-sides to these as well - one of which is that you lose a lot of light with them. They change the effective maximum aperture of your lens by between 1 and 2 f-stops, which can be VERY significant when shooting at longer focal lengths. Here's the effect of each.

Tele ConverterIncrease in Focal LengthLoss of LightEffect with a 100mm lens
1.4x40%1 f-stop140mm
1.7x70%1.5 f-stops170mm
2.0x100%2 f-stops200mm

They also degrade the image to a degree. How much they degrade, depends on the quality of the tele converter and the quality of the lens it's being used on. Unfortunately, there is an extreme degradation when used with zoom lenses. They are really not intended to be used with a zoom lens. They can, however, work reasonably well with most prime lenses. Also, be aware, that not all tele converter lenses will fit on all prime or zoom lenses - check with the manufacturer.
(see our eTip on Teleconverters here)


Lens Types


Macro Lenses
There's another kind of lens attribute that allows a lens to be focused very close. This attribute can be built-into most lenses with a focal length of about 50mm or longer. When a lens has this attribute, you can use it to take pictures of small objects that can fill the image. Most macro lenses allow for an image magnification of 1:1 - which means that the image on your camera's sensor will be the same size as the object - one-to-one reproduction. By being able to get close enough to fill the frame with an object, the resulting image shows the object much larger.

Lenses capable of taking macro images are usually named with either the "Macro" or "Micro" designation, such as the following: Micro-Nikkor 105mm f/2.8G, Canon 100mm f/2.8L Macro, Sony 100mm f/2.8 Macro, etc. The most popular macro lenses are in the 100mm focal-length area, as they allow you to get further away from your subject than a shorter focal length would, plus it provides a little more depth of field than a longer focal length would (but, the depth of field is incredibly small when focusing very close!).

Macro Image
(see our eTip on Macro Photography here)

Depth of Field
For one thing, lenses can only focus on a single plane. If your lens is focused at 10 feet, then only things at 10 feet away will be in focus (see our eTip on Depth of Field for more information). Everything else will be blurred to some degree, based on your aperture. Small apertures (i.e., f16) allow for more of the image to appear to be in focus. Large lens openings (i.e., f4) allow you to blur more of the image. This is a wonderful creative ability that differentiates your lens from your eyes.

This example shows how the aperture affects the depth of field. In both images the lens was focused on the "6" on the ruler. You can see how the two apertures (f4 on the left and f14 on the right) affected how much of the image was in focus.

Click on the image for a larger version.
Depth of Field

When shooting a portrait outdoors, you want to make your subject sharp, but you may also want the foreground and background to be blurred – this will help concentrate the attention of viewer on the subject – not the fore or background. You may want to use a large lens opening for portraits.

When shooting landscapes, you typically want as much as possible to be in focus, thus you may want to use a small lens opening.

This is one way we use the difference between our eyes and a camera lens to our advantage.



Here's an example of an image taken with two different aperture settings: f4 and f14
The image on the right has a nice blurred background, while almost everything on the left is sharp.

(click for a larger image)
Depth of Field Example

(see our eTip on Depth of Field here)


Expansion and Compression
Our eyes can’t "zoom", but it’s likely you own a zoom lens. You may have heard the expression "Standard Lens". Often in 35mm photography, this is often considered to be a 50mm lens. For those technically inclined, a "standard" lens is calculated by measuring the diagonal of the sensor. For a full-frame sensor (24mmx36mm), the diagonal is about 43mm. Thus a "standard lens" would be 43mm, but 50mm is most often used. For an APS-C sensor, the diagonal is about 30mm, thus a 30mm lens would be considered “standard”.

The reason we have the concept of a "Standard" lens, is because a "standard" lens provides a view similar to what we see with our eyes. Of course, lenses NEVER really have anything truly in-common with our eyes, but, this is about as close as you can get.

Why don’t other lens focal lengths provide a view similar to our eyes? It’s because lenses compress or expand what we can see with our eyes. For example, a wide-angle lens (i.e., 24mm) provides an expanded view. It makes the distance from front-to-back seem like more than it really is. This is why you DON’T want to use a wide-angle lens for portraits – it makes faces (especially noses) appear much bigger from front-to-back.

Example of why you don’t want to use a wide-angle lens with a portrait!

The image on the left was taken with a wide-angle lens - you can see how it distorts.
The image on the right was taken with a portrait focal lengh - generally best between 80 and 120mm.
Portrait with a wide angle lens

Telephoto lenses (e.g., > 100mm) compress distance – they make the distance from front-to-back seem like LESS than it really is. Used for a portrait, they’d flatten a face (the ideal focal length for portraits is considered to be about 100mm).

Our eyes don’t expand or compress distances, but our lenses do!

You can use this to your advantage – when taking pictures, try getting closer with a wide-angle focal length vs. getting further away with a longer focal length. You’ll find that you’ll be taking two completely different pictures, even though the subject may be the same!

Here are 5 images of the same subject - a series of arches. The front arch looks to be roughly the same size in all of the images. Each image was taken with a different focal length - when the zoom lens was set to about 230mm, I was quite far away - you can see how the long focal length compressed the arches. When I shot the image at 16mm, I was very close - you can see how that extremely wide focal length expanded the arches. Consciously selecting the focal length can impact an image dramatically.
 
Various focal lengths



Sharpness, color, contrast & aberrations
Different lenses exhibit different degrees of sharpness, color and contrast. Some lenses are very "soft" – they don’t capture very sharp images. These same lenses tend to have poorer color and contrast reproduction. Lenses that can zoom from very wide to telephoto tend to be the poorest at sharpness, color and contrast. We like these lenses, because they eliminate the need to carry more than one lens with us. But, it’s at a price. Unfortunately, NO lens can "do it all" without sacrificing quality. "Prime" lenses (those that don’t zoom – they only have ONE focal length) are typically the best for sharpness, color and contrast. But, they also are the least flexible (no zoom). "Do it all" zoom lenses (e.g., 18-200mm, 28-300mm, etc.) are generally the worst when it comes to sharpness color and contrast. It’s up to you – name your poison!

All lenses have aberrations (often, so do our eyes). These aberrations exhibit themselves as distortion (barrel, pincushion, etc.), color-fringing (chromatic aberration), vignetting or other issues. Some high-end, newer cameras often recognize what lens is attached, and know how to correct for some of these issues. Most modern software also allows you to manually correct distortion or chromatic aberration.

Here's an example of two types of aberrations: Barrel Distortion and Chromatic Aberration.

Barrel Distortion

Chromatic Aberration - is caused by the inability of a lens to focus all colors in the same place. It presents itself with color "fringing", as you can see in this partial image. The inability to focus red and blue in the same place results in purple fringing. Chromatic aberration is generally an issue with lenses, but digital sensors can also have an impact due to "color saturation" issues.
Chromatic Aberation



The good news…

The good news is that you have choices! You can choose to use a zoom or prime lens. You choose your lens focal length – zoomed in or zoomed out; you can choose the aperture (we recommend almost ALWAYS shooting in Aperture Priority (A or Av) mode!), and you can choose to edit your images to help correct aberrations with software. These differences between how we and our cameras see, works to our advantage. We can capture images that are truly unique. Images that represent our creativity.

Imagine how boring it would be if a "paint-brush" artist created images of EXACTLY how a scene looked. Consider yourself an artist when shooting. Understanding how your camera sees is the first step.


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