พอดีผมได้อ่านข้อมูลต่างๆเกี่ยวกับการใช้งานกล้อง (ใช้ยังไม่เป็น T_T) เลยไปเจอ บทความเกี่ยวกับ Aperture หรือ A Mode
เขียนโดย Peter Hill ช่างภาพจากแดนจิงโจ้
Introduction
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.”
Whatever.
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.
The Easy Guide To Understanding Aperture
เขียนโดย Peter Hill ช่างภาพจากแดนจิงโจ้
Introduction
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.”
Whatever.
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.