1) True Focal Length

What is the true focal length of a lens? This one is extremely important to understand. Focal length is an optical attribute of a lens, which has nothing to do with the camera or the type of sensor it uses. The true focal length of a lens is typically what manufacturer says it is on the lens. For example, the Nikon 50mm f/1.4G lens (below) has a true focal length of 50mm, irrespective of what camera you use it on.

Nikon 50mm f/1.4G AF-S

2) Field of View

The “field of view”, which is sometimes called “angle of view” is simply what your lens together with the camera can see and capture from left to right, to top to bottom. If you are shooting with a DSLR camera, the field of view is typically what you see inside the viewfinder. Some DSLR cameras, have less than 100% viewfinder coverage, which means that what you see inside the viewfinder is actually less in size than what the final image will be. For example, if you shoot with the Nikon D90 DSLR that has 96% viewfinder coverage, what you see inside the viewfinder is going to be about 4% less than what the camera actually captures. Hence, the actual field of view is always what the camera captures, not necessarily what you see inside the viewfinder.

Here is an example of differences in field of view between 70 and 400mm:

The top-left 70mm image looks almost “wide”, while the 400mm image shows a much greater magnification with a much narrower field of view.

Lens manufacturers often publish the term “angle of view” or “maximum angle of view” in lens specifications, because they define the field of view in degrees. For example, the Nikon 24mm f/1.4G lens has a maximum angle of view of 84°, while Nikon 300mm f/2.8G telephoto lens has a maximum angle of view of only 8°10′ when used on film or full-frame cameras. Take a look at the following illustration:

300mm vs 24mm AoV

As you can see, 84 degrees is very wide when compared to 8 degrees. That’s why you can fit a lot of the scene when shooting with a 24mm lens, while a 300mm lens allows you to capture a narrower, but much more magnified portion of the scene.

3) Equivalent Focal Length

Let’s now move on to the term “equivalent focal length”, which like I stated in the beginning, is a term that many photographers misunderstand. The word “equivalent” is typically in relation to 35mm film. You see, back in the 35mm film days, the focal length of the lens was always whatever the lens said on the label. With the invention of digital SLRs, the camera sensor (the device that captures images) is often much smaller than the 35mm film, primarily because of high cost. This reduction in size of the sensor results in cutting of the image corners, the process that photographers call “cropping”. The interesting thing, is that the image is actually not cut by the sensor or the camera – parts of the image are simply ignored. Take a look at the following illustration (red arrows represent light entering the camera):

35mm Film/Sensor Camera (Left) vs Cropped Sensor Camera (Right)

As you can see from the above illustrations, the 35mm film/sensor cameras capture a large area of the lens, while the smaller sensors (also known as “cropped sensors”) capture mostly the center. Note how the light enters the camera chamber in exactly the same way in both illustrations, but the smaller sensor is only able to capture a certain portion of it, while the rest of the light falls outside of the sensor. The term “cropped sensor” can be confusing, since “cropping” an image is often associated with cutting it. Once again, in this case, there is no cutting – the light rays from the edges of the lens just overshoot and do not make it to the sensor.

Manufacturers knew about this “overshooting” process when they designed smaller sensors, so they started producing lenses specifically designed for cropped sensor cameras to make them cheaper. Nikon calls them “DX”, while Canon calls them “EF-S”. Basically, the lens itself passes through a smaller image circle and by the time it gets to the sensor, not much of the circle is actually wasted. Think of it as the right part of the above illustration, except the circle is much smaller. Obviously, lenses like these do not function as they should on full-frame/35mm cameras – only half of the scene will actually make it to the sensor. Nikon full-frame cameras are programmed to recognize DX lenses and will automatically decrease the image resolution, while the Canon EF-S lenses will not function on full-frame cameras at all.

How do two cameras with different sensor sizes have the same image resolution? For example, both full-frame Nikon D700 and cropped sensor Nikon D300s have 12.1 Megapixels while having different size sensors. This is because the Nikon D300s camera has much smaller pixels (and hence, higher pixel density) compared to Nikon D700 – that’s how 12.1 million pixels are able to fit on a smaller sensor. What this essentially means, is that the smaller sensors with smaller pixels enlarge the center area of the lens more in this case. If a lens is not of very high quality and is not able to resolve fine details, the images might appear less sharp on cropped sensors.

Let’s now get back to the term “equivalent focal length”. I’m sure you have seen manufacturers claim something like “The 28-300mm lens has an angle of view equivalent to a focal length of 42-450mm in 35mm format”, which is a correct way of saying it. Others may say something like “the lens focal length is equivalent to 42-450mm on DX sensor”, which is an incorrect way of saying it. As I have shown above, in relation to the camera sensor, the focal length of the lens never changes – only the field of view/angle of view does. Saying something like “my 28-300mm lens on my Nikon D90 is like a 42-450mm lens” is incorrect for this reason.

Where do these larger numbers such as 42-450mm come from? Let’s now look into the crop factor and how these “equivalent” numbers are actually computed.

4) The Crop Factor

By now you understand what “equivalent focal length” truly stands for and how the smaller sensors ignore the larger circle area. Let’s now talk about the crop factor – the term that manufacturers and photographers often use to describe camera sensors and to calculate the “equivalent focal length”. You might have heard people say something like “Nikon D90 camera has a 1.5x crop factor” or “Canon 60D has a 1.6x crop factor”. The term “crop factor” came up after smaller sensors were invented to make it easier for people to understand how much narrower the angle of view gets when a lens is used on a camera with a small sensor. Manufacturers had to somehow explain how an image on a smaller sensor camera looks enlarged or “zoomed in” compared to 35mm film.

If you take the sensor area of a full-frame sensor or 35mm film and compare it to a cropped sensor, you will be surprised to see that the former is at least twice larger than the latter. For example, the Nikon full-frame cameras approximately have a sensor size of 36mm x 24mm which gives us a surface area of 864. Cropped-sensor cameras like the Nikon D90, on the other hand, have an approximate sensor size of 24mm x 16mm, which is around 384 in surface area – a whopping 2.3 times smaller compared to Nikon D3s! But when it comes to focal lengths, you do not use the surface area of the lens. The crop ratio is computed by taking the diagonal of the full-frame sensor, divided by the diagonal of the cropped sensor.

Now you will have to remember some math. Remember how to compute the diagonal? Here is the formula in case you forgot it: √(X² + Y²). The full frame camera has a diagonal of 43.26 (square root of 1296+576), while the cropped sensor cameras have an approximate diagonal of 28.84 (square root of 576 + 256). If you take 43.26 and divide it by 28.84, you get 1.5 – the ratio of the full-frame sensor diagonal to the cropped sensor diagonal (these numbers are rounded – the actual ratio is a little bit higher, around 1.52).

What do you do with this ratio? You multiply it to get the “equivalent focal length”. For example, the Nikon 24mm f/1.4G lens has an angle of view equivalent to approximately 36mm when mounted on a cropped sensor camera like Nikon D90. What this means, is that if you took a 24mm lens and mounted it on a cropped sensor camera, then took a 36mm lens and mounted it on a full-frame camera, you would get about the same view. If you put it the other way, to have the same angle of view as the 24mm mounted on a full-frame camera, you would need a 16mm lens on a cropped sensor camera. For example, if you were standing from one spot and could fit a house in your frame using a 24mm lens on a full-frame/35mm camera, to be able to fit that same house on a cropped sensor camera, you would need to have a much wider lens with a focal length of 16mm.

Hope this clears up the true definition of the above terms for those who do not understand them well. If you have any questions or comments, please post them in the comments section below.

The above images are not cropped in post-production and represent equivalent focal lengths relative to 35mm. The longest field of view of the 1200mm shot was captured with the Nikon 200-400mm f/4.0 + TC-20E III TC @ 400mm (800mm effective) on a DX body, which is equivalent to 1200mm. The shortest focal length was captured with the Nikon 70-200mm f/2.8G VR II at 70mm.

Let me share a few tips on how you could obtain maximum bokeh from your camera setup.

1) Use a large aperture

Bokeh is not created by the camera – it is your lens and its optics that are responsible for rendering the out-of-focus areas. Therefore, the first thing you should do is set your lens aperture to its lowest value, also known as “maximum aperture”. You can do this by changing your camera mode to “Aperture Priority” and setting the “f” number to the lowest value your camera will permit. On Nikon DSLR cameras, this is typically done by rotating the front dial towards the left (counter-clockwise).

What is the effect of lowering the lens aperture? It basically decreases the depth of field (which is the area that appears sharp relative to the background) to a very small or “shallow” area.

2) Minimize the distance between yourself and the subject

The closer you stand to your subject, the blurrier the background will get. This happens because when an object is very close, the lens will focus closer and the depth of field will be the smallest. It works the same way with our eyes – try to extend your index finger close to an object two feet away from you, then focus your eyes on your finger and start moving it towards your eyes. You will notice that as you get closer to your eyes, the object behind your finger will get blurrier and blurrier every time. Lenses work exactly the same way, which is why subject distance plays a big role in rendering of the bokeh.

3) Increase the distance between your subject and the background

If the subject you are photographing is very close to a busy background, the bokeh will definitely suffer. Remember, depth of field is not just a hard line after which everything is supposed to be completely out of focus – it gradually transforms from sharp to out of focus, as can be clearly seen in the below image. Therefore, in order to get a pleasant-looking bokeh, you should try to put your subject away from close background objects. For example, if you are taking a portrait of a girl that is standing very close to a tree branch with leaves, those leaves might not look completely out of focus. If the girl moved closer to you and thus increased the distance between herself and the tree branch, the leaves would look more “out-of-focus”.

As you can see in the above image, the nearest leaves on the tree look sharp and in focus, while the ones a little behind on the left-hand side look somewhat blurry. In comparison, the leaves from the other trees further away look completely out of focus.

4) Use longer focal lengths

Given that the distance between the camera and the subject remains the same, increasing the focal length of the lens decreases the depth of field. So, if you have a zoom lens, you should zoom in to the maximum focal length your lens allows to separate the subject from the background even more. This also means that if you zoom out and use the lens at its shortest focal length, the depth of field will increase, which is desirable for landscape and architectural photography.

For example, if you have a 70-300mm zoom lens, shooting at 300mm focal length will isolate the subject the most (which is what you want for the best-looking bokeh), while shooting at 70mm will bring more objects in the background to focus.

5) Use a long lens

Since increasing the focal length means decreasing the depth of field, the longer your lens is, the better the bokeh you will get. This is not necessarily always true, because the rendering of out-of-focus areas also heavily depends on the optics of the lens. For example, both Nikon 18-200mm and Nikon 70-200mm have the same long focal lengths (200mm). However, the Nikon 70-200mm has much better optics than the 18-200, which is why it has exceptionally beautiful bokeh when compared to the 18-200 bokeh. So, when I say “use a long lens”, I mean “use a high quality quality long lens” :)

6) Use a fast lens

And last, but not least, use the fastest lens you have, since aperture impacts the depth of field. The best lenses for beautiful bokeh are portrait lenses such as Nikon 50mm f/1.4, Nikon 85mm f/1.4 and Nikon 70-200mm f/2.8 that have large maximum apertures and highly optimized optics for portraiture. The cheaper alternatives such as Nikon 50mm f/1.8 and Nikon 85mm f/1.8 also produce great bokeh.

Back in the film days, photographers were forced to carry a pen and a notepad with them to record important information such as shutter speed, aperture and date. They would then use this information in the lab, going through one picture at a time, hoping that what they wrote actually corresponds to the right image. It was a very painful process, especially for newbies that wanted to understand what they did wrong when an image didn’t come out right. Nowadays, every modern digital camera has the capability to record this information, along with many other camera settings, right into the photographs. These settings can then be later used to organize photographs, perform searches and provide vital information to photographers about the way a particular photograph was captured. This stored data is called “EXIF Data” and it is comprised of a range of settings such as ISO speed, shutter speed, aperture, white balance, camera model and make, date and time, lens type, focal length and much more.

Being able to read such data can be of great importance not only for beginners, but also for other photographers who want to find out what settings and tools were used to create a particular photograph. Unfortunately though, the only web-friendly (in terms of size) file format that can handle EXIF is JPEG, which means that you wouldn’t be able to read EXIF data from other image formats such as GIF/PNG and also from websites that use Adobe Flash or other similar products. In addition, some photographers choose to strip EXIF Data from their images to protect their image and their style, while others do it to save website traffic (yes, EXIF does add up to the size of the file). Those, who leave EXIF Data in their images either have no idea that they even have it, or they intentionally leave it like I do – for others to see and possibly learn.

If you look at my Landscape Gallery, for example, you will notice that most of the images contain EXIF data (on the bottom side of each photo there is a section called “Photo Properties”). But what do you do when a website does not provide EXIF information to public like I do? Don’t worry, because there are multiple quick and easy ways to read the EXIF Data from images.

Method #1: Firefox Addon “Exif Viewer”

Install Firefox browser if you do not already have it. Once installed, get the Exif Viewer addon by clicking the “Add to Firefox” button. After the addon is installed and Firefox is restarted, you will be able to instantly view the EXIF Data by just right-clicking on an image and selecting “View Image Exif Data”.

Go ahead and try it on this image:

Once you click on “View Image Exif Data”, you should see a new window that looks like this:

Use the scroll bar on the right hand side of the screen to go up and down the page and see more EXIF details. As you can see from the above screenshot, the EXIF information indicates that I used a Nikon D700, with ISO sensitivity of 200, an aperture of f/14 and a 5 second shutter speed to photograph the above fireworks. The original date/time field indicates 2009-07-04, which means that those are Independence Day fireworks :) Further down, there is a long list of other settings that I used in my camera at the time of taking the picture.

One thing to keep in mind though – thumbnail images typically do not contain any EXIF Data. So, whenever you see a clickable thumbnail image, do not select “View Image Exif Data”, but rather select “View Link Exif Data”. That way, the EXIF Data is taken from the original file the thumbnail is linked to.

Try it on this image:

If you see an error that says “Unable to extract some or all of the Exif data”, it means that the JPEG file you are looking at contains no EXIF information.

Method #2: Save the file and use a photo viewer

If for whatever reason you do not want to install Firefox or the Exif Viewer addon, you can also use this method to read EXIF Data. It is much slower than method #1, because it requires you to save the JPEG file on your PC, then go to Windows file properties or use a third party photo viewer that is capable of displaying EXIF Data embedded in JPEG files.

Save the first photo on this page on your PC, then right click on the file and go to “Properties”. Click on the “Details” tab and you should see the following:

Although the most important EXIF information is provided, just keep in mind that this method will only display some of the data. But if you are only after aperture, shutter speed and ISO, this would be more than adequate for the job…

The shots were taken indoors because it was too cold outside :)