Understanding the f - stop
courtesy of Peter Hill
Have you ever asked yourself a question similar to one of these?
- What’s with all the f/2.8 and f/1.4 and f/22 stuff people use in their Descriptions? Are they just being numerically anal or are they telling us something useful?
- What’s the freakin’ difference between a 50mm f/1.4 lens and a 50mm f/1.8 lens? Why is one way more expensive than the other?
- What do people mean when they describe a particular lens as being “fast”? Where is it going?
- What do lenses have to do with aperture and f stop? Isn’t that what a camera does?
- What does “stopping down” mean?
- If I’m meant to “stop down” why are the f numbers higher? Shouldn’t they be lower?
- What’s “aperture”?
- Is “aperture” the same thing as f stop?
- How does changing aperture change my photograph?
- I shoot in Auto mode. Why do I need to know about aperture?
If you have, they are all good questions and they are all answered in this Tutorial. Seriously technically-minded readers should, however, stop right now and go and do something else lest they get irate at my basic, and potentially overly-simplistic, explanations.
My aim is merely this: learning about aperture and f stop is like learning to drive a car. Once you “get it” you never forget it. All it takes is an understanding of a few basic principles, so in my book it doesn’t matter how they are expressed.
And if you reach an understanding of aperture and how you can control it, it is my firm view you will take your photography to the next level.
By the way, aperture and f stop are indeed references to the same thing. Aperture is the mechanism and f stop is the measure of engagement of the mechanism.
Seeing the light
Photography is not about selecting buttons and switches on a highly sophisticated piece of electronic and optical equipment.
Photography is about light.
Without light, there is no photograph. End of story. Light controls what you see in a photograph and how you see it. And because natural light changes constantly, we need to be able to control its intensity as it passes through the lens and into the camera as we take a photograph.
That is what aperture is. It’s one mechanism by which we control the amount of light hitting the camera’s sensor (or film). Other mechanisms, and I think there are only 2, are filters and shutter speed. (Filters are advanced stuff (see for example this), but shutter speed is not, so I discuss also the relationship between speed and aperture in this Tutorial.)
Controlling the light is important because shooting with too much light and you have an over-exposed image, and not enough light gives you an under-exposed image. We’ve all taken those, haven’t we!?
Of course, your modern DSLR or Point & Shoot or Digital Compact, being a sophisticated and precise piece of equipment, can compensate for light automatically, should you choose to let it. But here’s the rub: the camera has the artistic intelligence level of a mud brick, so it will make compensation adjustments purely as a numerical function and without regard to composition or depth of field.
Case in point: True story. I once met a photographer (not my girlfriend) who shoots weddings as a sort of pro-hobby to help out her brother’s wedding video business. With no prior experience, she was given a brand new (full-frame) Canon 5D DSLR with an EF 24-105mm f/4 lens and told to leave it on Auto and just concentrate on composition. This she duly did, and was generally taking ok shots. But she was also confused. Every time she focused on a bride’s face in a group shot, the whole group came out focused instead of just the bride. She wasn’t very happy with the equipment as a result, and thought her brother had wasted $5000. It took some patience and some explaining about the role of aperture and f stop in taking a photograph because the expectation held by this Y Gen lady was that today’s technology should do all that stuff. But bottom line was that if she had shot in Manual mode and set the aperture to f/4 or had shot in AV mode with aperture set at f/4 she would have achieved her aim of selective focus. I demonstrate and explain this later in this Tutorial.
How does the aperture control light?
Ever seen a science fiction movie where the scene is inside some fantastic spacecraft? The actor walks through a circular doorway and hits a button on the doorway wall and these sliding steel blades suddenly come out from all around the inside of the rim of the doorway and curl inwards towards the centre of the doorway until the space is completely closed up? (with that lovely steel-grinding sound effect!)
Well, that ain’t science fiction because such a scene replicates the aperture mechanism of a camera lens (without the noise), as can be seen from this cross-section of the aperture mechanism of a Canon EF 50mm f1.8 II lens:
Notice the blades in the centre circle? Count ‘em, there are 5. And they are actually called “aperture blades”. They move in sync in and out depending on the aperture being set (either by you or by the camera). Notice also that the inside edges of the blades are not round and that the space allowing the light to go through the lens is thus not circular but instead pentagonal. So, whilst the Canon EF 50mm f1.8 II lens will produce exceptionally sharp images at the right aperture, the effect of having 5 non-rounded aperture blades is poor quality bokeh for out of focus (OOF) areas of the image, ie points of light in OOF areas will be pentagonal in shape, not circular. But hey, I can buy this lens brand new for $135, so it doesn’t really matter that much.
By comparison, you’d want a macro lens to give you better quality OOF results. This comes at a price. For example, the $1200 Canon EF 100mm f2.8L IS Macro Lens has 9 round aperture blades, thus ensuring perfectly round points of light. (Just something to look for when considering that new lens.)
Anyway, back to the photo of the aperture mechanism. Any ideas of what the aperture setting (f stop) is? I’d say it’s reasonably wide open.
Compare it with this chart, which shows the relative width of aperture according to f stop number. It is the standard “full-stop” f-number scale:
Not to scale
As you can see from the above, an f stop of f/1.4 is really wide. It is letting a lot more light into the lens and ultimately onto the sensor than, say, f/8. Thus:
The lower the f stop number, the wider, or bigger, the aperture.
So, if someone tells you to “stop down”, they are referring to decreasing the size of the aperture, which is achieved by increasing the f stop number. If I could wave a wand and get everyone to remember just one thing from this Tutorial, that principle would be it. Remember it and you have 95% “got it”.
Aperture and shutter speed
Given a low f stop number means a wider aperture which in turn means more light being allowed through the lens, changing the aperture for a particular evenly-exposed shot means you need to shoot faster to compensate (or increase the ISO). This is because shooting faster reduces the amount of light going through the lens. This is the relationship between aperture and shutter speed. And because both are based on logarithms and standardised, there is a direct link to how much compensation is necessary.
For example, and ignoring ISO for the moment, if you have an evenly exposed photograph shot hand-held at f/8 and 1/30th of a second, you could instead re-shoot it at a faster speed to get better definition and focus by also changing the aperture to, say, f/5.6.
Most modern digital cameras use a one-third f-stop increment scale, ie
1.0, 1.1, 1.2, 1.4, 1.6, 1.8, 2.0, 202, 2.5, 2.8, 3.2, 3.5, 4.0, 4.5, 5.0, 5.6, 6.3, 7.1, 8.0, 9.0, 10.0, 11.0, 13.0, 14.0, 16.0, 18.0, 20.0, and 22.0.
So, changing the aperture from f/8 to f/5.6 equates to 1 full stop, and thus you need to increase the shutter speed by the same increment to maintain the even exposure. The numeric expression of the relationship between aperture and shutter speed has been described elsewhere as
“The aperture is proportional to the square root of accepted light, and thus inversely proportional to the square root of required exposure time, such that an aperture of f/2 allows for exposure times one quarter that of f/4.”
Maximum and minimum apertures
All lenses have a maximum and a minimum aperture at which they can be used. These limits are known and clearly communicated by the lens manufacturer because each lens is different and thus the aperture range does affect a buyer’s choice.
So, if a lens has a stated aperture range of f/2.8 to f/22 it means the smallest aperture at which the lens can be shot is f/22 (the minimum) and the largest aperture is f/2.8 (the maximum). That’s a fairly typical range, as is f/4 to f/32.
When you see someone describe their lens as, say, the Canon EF 50mm f/1.4, they are telling you the image was shot with a lens with a maximum possible aperture of f/1.4. It doesn’t mean they shot the image at that aperture, but it is a way of specifying which particular lens was used.
But what about zoom lenses and this type of description? Canon EF 70-300mm f/4-5.6 Zoom Lens. What that means is that when shooting with this lens at a 70mm focal length, the maximum aperture possible is f/4, whereas at 300mm the maximum possible aperture is f/5.6. This compares with the Canon EF 24-70mm f/2.8L Zoom Lens, which means that the maximum possible aperture is f/2.8 regardless of the focal length.
As a general proposition, it is also possible that Auto Focus is more precise the larger the maximum aperture. Canon, for example, certainly make this claim in relation to their EF lenses.
What’s a fast lens?
It is generally accepted that an aperture of f/2.8 is pretty wide. Since the wider the aperture, the more light, and the more shutter speed required to compensate, a lens with a maximum aperture of f/2.8 or more is described as “fast”.
Such lenses are desirable because:
- Better “stop-action” is achieved with faster shutter speeds, especially hand-held shots.
- Lowly or poorly lit shots are made easier to achieve without noise from ramping up the ISO to compensate, and/or without a flash, and/or without a tripod.
And those are two reasons why people drool and salivate over such lenses as the Canon EF 24mm f/1.4L II, the Canon EF 50mm f/1.2L and the Canon 85mm f/1.2L II lens. The fact that these particular lenses are way out of our price range is also why people look at alternatives, such as the Canon EF 50mm f/1.4 and, say, the Sigma 50mm f/1.4 – much cheaper yet still fast.
There is also the Canon 50mm f/1.0L lens which has attained mythical status because it is no longer sold new. It can be obtained from between $3000 and $4500 second-hand and was, until recently, the fastest interchangeable SLR lens in the universe. That status is now held by the Leica Noctilux 50mm f/0.95 lens – released in 2008 and a steal at $11,000.
But wait, there’s more
There’s one other reason why a “fast” lens is desirable. Bokeh. The wider the aperture, the narrower the depth of field.
This can be simply demonstrated by the following two photographs. The first was taken at f/32, the second at f/5.6:
You get the picture.
Varying your aperture can also be a very effective way of shooting portraits. For example, this is an image of my youngest son:
It was shot hand-held with a Canon EOS 5D Mark II and a Sigma 50mm f/1.4 EX DG HSM lens, using available light indoors. The aperture I used was f/1.4. For a portrait like this, such a wide aperture allows so much light in through the lens that perfect focus is only achieved for a very narrow vertical focal plane, in this case Dylan’s nose, ie at the front of his face. Every other feature of his face is blurred because none of it was on the same vertical focal plane as his nose (thank goodness!). In hindsight, I should have tried harder to focus on his eyes, but I had only recently acquired the lens and was basically experimenting.
The next shot was taken with the same camera and lens, under similar lighting conditions, but with an aperture of f/1.8. It is a casual shot of Linda, who has been a very good friend of mine for 30 years. She has just retired from 35 years of primary school teaching. Notice how on this occasion, the vertical focal plane is not as narrower at f/1.8 as it was with Dylan at f/1.4. It’s not much of a difference but it is enough of a difference to affect the outcome of a portrait shot.
Now compare those shots with the following portrait, Leila, shot by Tatiana with a Canon 5D and a Canon 85mm f/1.8 Lens at f/2:
Notice that Tatiana has been very selective in her use of a slightly wider aperture and the angle of the profile to get Leila’s eye and cheek in focus.
All 3 of the above shots were taken in the Manual shooting mode. That is, the aperture was deliberately chosen. Would the camera have chosen the same apertures had we been shooting in Auto? Absolutely not. (See also the discussion below on shooting in Aperture Priority mode.)
All 3 were also all shot from roughly the same distance from the subjects, but you can (hopefully) imagine that the farther away the subject, the narrower the aperture needs to be. For example, the next shot was taken by my good friend Crispin with his new camera, the Canon 1D Mark IV, and a Canon EF 70-200mm f/2.8L IS Zoom Lens + 2.0 x converter, at a focal length of 400mm (!). He used an aperture of f/5.6 but even so, there is a drop-off of focus on Avril’s face.
Crispin understands aperture very well. I know this because his photography stands out for his control of DOF through the aperture he selects and given the lens he selects for the shot. For example, he understands the creative effect a wide aperture can have on his DOF:
An interesting thing about that last shot, taken by Crispin at f/9, is the broad DOF. In fact, setting an aperture of smaller than f/9 on the 5D Mark II, ie f/11, f/16 and so on, will not achieve any greater DOF. This is due to the Diffraction Principle, which applies to DSLRs with a large amount of pixels available to the sensor. In the case of the full frame 5D Mark II, 21.6 megapixels are competing for the narrow light created by such small apertures, so much so that definition is not improved. And this is why I would only shoot at f/11 and down further when using my own 5D Mark II to prolong a long exposure. If that camera has a downside, the application of the Diffraction Principle is it.
I’m a strong advocate of not shooting in Auto mode (where the camera makes all the decisions on settings) unless circumstances dictate quick, responsive, shooting. I shoot almost exclusively in Manual mode (where I make all the decisions), but I also shoot in Aperture Priority mode when using my film SLR, the Canon EOS-3. What is Aperture Priority?
Turning your camera’s Mode Dial to AV (which stands for Aperture value) means that you set the aperture you want and the camera compensates by making the decisions on shutter speed. This is a particularly good way of ensuring you achieve the DOF you want from your image. (Just remember that, as with Shutter Priority mode (TV) and Auto mode, the camera will compensate for even exposure, whereas in some cases you may not be looking for that level of light control. That’s why I shoot in Manual.)
Related to this, many of the newer digital cameras now offer DOF preview, which means that once you have composed and focused your shot, you can press a button on the camera with a finger on your left hand, whilst scrolling through the various f stops with a finger on your right hand, and see the changes to DOF through the viewfinder as you scroll through the apertures. Cool.
This story would not be complete unless I quickly mention Group f/64. One hundred years ago, Pictorialist photography was the thing. Grainy, soft focus, dark-toned images, created in the darkroom by a variety of manual techniques, was the height of photographic fashion. Much like the opportunities provided by today’s Photoshop with its textures and layers, back then such creativity boomed in the hands of the motivated. Another explanation I’ve heard of the birth of Pictorialism was that it was pursued to demonstrate the legitimacy of photography as a valid form of art, in part to overcome the overwhelming contemporary perceptions of photography stemming from Kodak enabling the masses to literally point and shoot with the mass-produced Box Brownie.
Group f/64 was a conclave of photographers who railed against pictorialism, among the founders being Ansel Adams and Edward Weston. Their aim was to promote the cause of photography as it was shot. And they shot as their eyes saw, with perfect focus throughout the frame – foreground, background, and everywhere in between. Their cameras were large format and f/64 was the smallest aperture possible to give that clarity, especially in landscapes. We digital camera users can do nought but dream about using f/64. Either that or dream about picking up a few long lost Ansel Adams negatives in a garage sale. Now wouldn’t that make your day?
Well, I’ve looked at the questions at the start of the Tutorial and I think I’ve answered them. I hope the answers are able to facilitate anyone’s understanding.