Jimmie's Guide to Photography

Camera Terms De-mystified

Camera and lens terms are somewhat confusing, and also have a way of being used to mean more than one thing, which makes them even more so.
For example:

Lenses are specified as "50 mm", "135 mm", and the like. What is that a measure of?

What exactly is this semi-universal term "f-stop"? Some articles refer to f-stops in reference to lens opening, so that an opening of f/5.6 is called "one stop down" from f/4, but other articles might talk about "f-stop" in shutter speed, as for example, a speed of 1/60 is "one stop down" from 1/30, and still others might say that ISO 200 is "one stop down" ISO 100.

You will see a photograph labeled 1/125 @ f/4, and you will see a lens labeled 1:4. Does the "4" mean the same thing?

What makes a lens "fast" vs "slow"?

So some clarification is in order.

First item: Lens "mm" numbers such as "50mm", "70-200mm" and the like refer to the "focal length".

For "simple" one glass element, lenses: Focal Length is the distance between the lens to the camera sensor (or film plane on film cameras). So for example, the lens element of a simple "50mm" lens would be 50 mm (1.96 in) from the sensor. (There are 25.4 mm per inch.)
 For lenses with multiple glass elements, Focal Length is the distance from the place where the light converges inside the lens to the camera sensor.
 Some lenses are fixed (often called "prime"). Others are adjustable, allowing you to "zoom" in or out. So for example the author's 15-85 mm lens will zoom from 15mm (wide-angle) to 85mm (low telephoto).

So, to simplify it can be said that the "mm" number essentially just indicates whether the lens is a "wide-angle", "nominal", or "telephoto" lens. The pictures below show the differences that you would typically see for various lenses (or zoom settings) of the three classes. Note that the image at the top displays the scene at the "nominal" focal length, or "standard" lens, meaning this is what you would see with the naked eye (not counting peripheral vision). A lens zoomed to approximately 31 mm would yield this image, for most digital cameras. For "Full frame" digital cameras and film cameras, the "nominal" focal length/standard lens is 50mm.
 Focal lengths BELOW this are considered "wide-angle". Among other things, the perspective of a scene is altered by the lens. Distances are expanded; The scene appears further away than it really is.
 Focal lengths ABOVE this are considered "telephoto". The perspective is compressed, distances are shortened; The scene appears closer than it really is.

Second item: f-stops as applying to all three: lens aperture, shutter speed, and ISO:

In photography, "f/N" is the "f-number" or "f-stop", as in "f/2.8", "f/4", and the like. It applies to the lens opening, or aperture. Canon cameras label this as "Aperture Value" (AV).
 But the terms "one f-stop down" or "one f-stop up" refers to lens aperture, shutter speed, and ISO. This meaning of "f-stop" is "how much light is admitted to, or processed by, the camera." It's called a "stop" probably as a holdover from film cameras, when the lens opening and the shutter speeds had detents, or "stops" (images on right).
 Each "stop" represents that the amount of light is reduced by half. So f/4 admits half as much light as f/2.8 for lenses (higher numbers mean smaller openings). And a shutter speed of 1/60 sec. admits half as much light as 1/30. And finally, an ISO setting of 200 makes the camera sensor half as sensitive to light as ISO 100.
 Here is a list of the "f-stops" (The strange numbers for aperture have to do with the square root of 2. The shutter speeds involve dividing the previous number by 2, but some of the numbers got rounded, such as 1/15 instead of 1/16):

Aperture: f/1, f1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32

Shutter (in seconds): 1, 1/2, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000

ISO setting: 100, 200, 400, 800, 1600, 3200, 6400

Third Item: The difference between "f/4" as applies to a camera or photograph and "1:4" as applies to a lens.

As described above, "f-numbers" like "f/4" apply mainly to the lens opening, or aperture. Yet the label on the front of lenses is usually "1:N", such as 1:2.8, 1:4, 1:5.6 for fixed, and some zoom, lenses. For most zoom lenses a range of numbers are given, such as 1:3.5-5.6. The first number applies to the lowest zoom factor and the second applies to the highest zoom factor. This means the lens gets effectively "darker" when fully zoomed out. In this case it's not that the opening is smaller. It is that the light has to travel further and interact with more pieces of glass. This has the same effect as a smaller opening.
 The front label is the only place you will see "1:N". The lens itself is still referred to as "f/N", such as f/2.8, f/4, f/5.6, as in "This is an f/2.8 lens", or "That's an f/4 lens." Some articles may even refer to a "2.8" lens or a "4" lens.
 For lenses, then, this number - "2.8", "4", "5.6", and so forth - can be thought of as the light-gathering ability of the lens when it is wide open. Smaller numbers mean the lens lets in more light. Many things determine how much light the lens is capable of delivering. Mainly it is the lens diameter. But lens quality, and even lens simplicity (fewer glass sections) can affect light-delivering ability.
 The number is actually an aperture ratio rather than an "f-stop". This ratio represents (focal length) divided by (lens diameter). For example, a simple "prime" 50mm f2.8 lens will have a lens diameter of approx 17.8mm. Why? Because 50 / 17.8 = 2.8, the "aperture ratio". For zoom lenses, the maximum zoom is used to get the ratio. An example is the 2 offerings of 70-200mm lenses that Canon offers.
 They are "f/4" and "f2.8", and are shown on the left.. Their maximum zoom factor is 200mm.
 The 70-200mm "f/4" lens has a glass diameter (at the front) of about 50mm. So the "aperture ratio" is 200/50, or 4.
 The 70-200mm "f/2.8" lens has a glass diameter (at the front) of about 71mm. So the "aperture ratio" is 200/71, or 2.8.

So for lenses, the number 2.8, 4, and the like could be considered the "maximum aperture ratio". If we change the word order to "Aperture Ratio Maximum", we could create an acronym "ARM". Why not? It's an acronym-driven world we live in. . . Thus for example a a "2.8 ARM" lens gives a maximum opening, a maximum light-gathering-ability, of "f/2.8" when it is wide open. Remember, the lens also has an internal iris which will close that opening when you select "f/4", "f/5.6" and the like on the camera. This is the "f-stop", or "stop-down" part.

Fourth item:"fast" vs "slow" lenses

Lenses with a lower "Aperture Ratio Maximum" are said to be "fast". Those with higher numbers are said to be "slow". In this sense the number might be thought of as a "lens rating". Lenses with lower numbers will let in more light, allowing you to use a faster shutter speed (Hence "fast" or "slow"!)
 What determines the achievement of the lower number?

It's usually because the lens is of higher quality, better ground glass, and the like.

Or it can simply be that the lens has a bigger diameter. See the image above, showing the two Canon 70-200 lenses. Both are of essentially the same quality, but the "f/2.8" has a bigger diameter (and more weight, too!)

The lens can be simpler and with fewer glass elements. Some of the big telephoto lenses have 15-20 pieces of lens glass!
The image on the right (top) shows two simple Yashica film lenses. The smaller one came with the camera, and is a 50mm f/2 (!) The larger one is a 135mm f/2.8.
The image on the right (bottom) shows a current Canon 50mm f/1.8 (!)
These are all very "fast" lenses despite being much smaller (and having a much smaller diameter) than, say, the 2.8 Canon 70-200mm lens shown above.

So, what is the real takeaway from all this! What is the biggest tip of all?

The "focal length" will help you decide on "wide-angle", "prime", or "telephoto", based on your picture-taking preferences. The Maximum Aperture Ratio can help you decide if you need a bit "faster" lens or not.

But the biggest "tip" of all is this:
 In almost all cases, you will have a much better chance of taking clear, properly focused photographs if you ensure that your lens is never "wide open"!
 Why is that?
 Two reasons. The first is that the most accurate portion of the lens is the center, and not the edges. This is an analogue to our eyes. For those of us whose vision is not as clear as it used to be, squinting always yields a clearer scene, no matter what sort of glasses or eye correction we have. It's even better if one looks though a pinhole in a piece of cardboard. While it is true that a high quality lens will have accuracy even when the lens is wide open, we have the second reason: Closing down the lens a bit yields a wider "depth of field", and thus much more chance of having the subject in focus.

Depth of Field:
If you focus on a given object, the "Depth of Field" is how far in front of and how far behind that object is also in focus. Older film camera lenses showed that to you! See the image on the right.
The lens is focused on 20 feet. (20 is right above the line with the diamond.) If you have the lens "stopped down" to f/22 - a very small opening - objects from about 16 feet to 30 feet will be in focus. If you have the lens stopped down to f/8, you have a range of about 18 to 22 feet. If you have it "wide open", your range is likely 20 feet plus or minus a few inches!

NOTE: Most of the time, in the "P" (Program) mode, and most of the "pre-set" modes as well, digital cameras will do their best to have the lens wide open! For example, the author tried the "Sports" automatic setting on his camera for a few shots during the summer 2015 Air Show. This resulted in very fast shutter speeds such as 1/2000, but the lens was wide open each time. The result? Almost all of the shots were "soft" and not in focus.
 This is the reason for many of the "suggestions" discussed below ("Stopping down", "Changing the ISO"). Generally you want to override this behavior and keep your lens stopped down one or two stops. Wide open may or may not be good for a distant landscape scene with a tripod, or in very low light, but if you are shooting anything that MOVES, you simply don't have time to focus carefully. Better to be stopped down to get that extra "Depth of Field" margin.

Finally, what's the advantage of the "faster" lens - the lower "Aperture Ratio Maximum"? It is that when you are 1 or 2 "f-stops" down from the maximum, the lens can admit more light. A "2.8 ARM" lens stopped down 2 stops is at f/5.6, whereas a "5.6 ARM" lens stopped down 2 stops is at f/11!
 As the images below show, the "faster", "2 ARM" lens provides a bigger opening than the "3.5 ARM" lens. The opening is also smaller when both are stopped down one notch.