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Photography Basics
superglue
Registered User regular
Registered User regular
Basics: The tools you'll need to take better pictures, be they professional or just candid shots, I hope I can do something to help you out.
Glossary: (if more than one applicable definition, the bold one is that will more likely be used to critique)
Aperture: the opening on a lens through which light passes.
Depth of Field: the area of acceptable focus.
Exposure: the total amount of light received by a photosensitive surface or an area of such a surface, expressed as the product of the degree of illumination and the period of illumination.
Film Speed (ASA or ISO): This is how quickly your medium (film or digital) can absorb light. The higher the number, the faster it will absorb.
Common Speeds: 100, 200, 400, 800.
High Speeds: 800, 1600, 3200
You may also find oddly numbered ones, 320, 640, 1280, but the same principle applies.
Focal Length: the amount of magnification (or more commonly, Zoom) *this is determined by the distance of the rear glass of the lens from the focal plane. not basic, but it's a nifty tidbit to know.
Point and Shoot: refers to camers that do not require aperture or shutter speed settings.
Shutter Speed: the length of time a shutter is opened to expose film.
Single Lens Reflex (SLR): cameras using mirrors to reflect an image to the eye, commonly allow more control over an image using aperture and shutter speed.
Tripod: a tool with 3 legs commonly used with a camera to ensure steady shots. Often used for shots where the shutter is open for extended periods of time.
Your Camera:
Using a Nikon D50 as an example -
Most newer cameras, SLR or not will have the Mode Setting Dial and all cameras will have a shutter release (when you tell strangers how to use your camera they point and go "Is this the button I press?" and you say "yes.")
Focal Length is measured in mm. In this image, it has been set to 35.
Shutter speed is measured in seconds. Or rather, fractions of a second.
This is set to 1/4000th of a second. This is what is referred to as a fast shutter speed. If the speed were set for 1 second or longer, the number would be followed by quotation marks. I.E. 5" would be 5 seconds, where as 5 would be 1/5th of a second.
Aperture - Common apertures are 4, 5.6, 8, 11, 16, 22. Numbers may be lower or higher. 4 is usually wide open (allows more light), while 22 is nearly closed (allows less light).
The Mode setting Button: Auto is automatic. P is program. S is Shutter Priority. A is Aperture Priority. M is manual.
I'll touch on these later.
Glossary: (if more than one applicable definition, the bold one is that will more likely be used to critique)
Aperture: the opening on a lens through which light passes.
Depth of Field: the area of acceptable focus.
Exposure: the total amount of light received by a photosensitive surface or an area of such a surface, expressed as the product of the degree of illumination and the period of illumination.
Film Speed (ASA or ISO): This is how quickly your medium (film or digital) can absorb light. The higher the number, the faster it will absorb.
Common Speeds: 100, 200, 400, 800.
High Speeds: 800, 1600, 3200
You may also find oddly numbered ones, 320, 640, 1280, but the same principle applies.
Focal Length: the amount of magnification (or more commonly, Zoom) *this is determined by the distance of the rear glass of the lens from the focal plane. not basic, but it's a nifty tidbit to know.
Point and Shoot: refers to camers that do not require aperture or shutter speed settings.
Shutter Speed: the length of time a shutter is opened to expose film.
Single Lens Reflex (SLR): cameras using mirrors to reflect an image to the eye, commonly allow more control over an image using aperture and shutter speed.
Tripod: a tool with 3 legs commonly used with a camera to ensure steady shots. Often used for shots where the shutter is open for extended periods of time.
Your Camera:
Using a Nikon D50 as an example -
Most newer cameras, SLR or not will have the Mode Setting Dial and all cameras will have a shutter release (when you tell strangers how to use your camera they point and go "Is this the button I press?" and you say "yes.")
Focal Length is measured in mm. In this image, it has been set to 35.
Shutter speed is measured in seconds. Or rather, fractions of a second.
This is set to 1/4000th of a second. This is what is referred to as a fast shutter speed. If the speed were set for 1 second or longer, the number would be followed by quotation marks. I.E. 5" would be 5 seconds, where as 5 would be 1/5th of a second.
Aperture - Common apertures are 4, 5.6, 8, 11, 16, 22. Numbers may be lower or higher. 4 is usually wide open (allows more light), while 22 is nearly closed (allows less light).
The Mode setting Button: Auto is automatic. P is program. S is Shutter Priority. A is Aperture Priority. M is manual.
I'll touch on these later.
superglue on
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Depth of Field: Depth of Field is determined by 3 things
a. Distance from subject
b. Focal Length
c. Aperture setting.
Excuse my ascii art.
Example A - Distance from subject
Camera is focused on 1. This is the closest object. All objects, 2 3 and 4 will be out of focus.
Camera is focused on 2. This is the second Nearest object. Objects 1 and 4 are out of focus. See how as you get farther away the DoF becomes larger?
Camera is focused on 3. Now, 2 3 and 4 are in focus.
Finally, Camera is focused on 4. 3, 4 and everything behind 4 are in focus.
Example B - Focal Length.
Camera is set so that 1 is a 3/4 shot (waist up). The subject is close, so you don't need to zoom in as much. The Asteriks show the area that would be in focus.
Camera is set so that 2 is also in a 3/4 shot, but 2 is much farther away. Zooming in a lot, the asteriks show what will be in focus. Not much, is it?
Example C - Aperture Setting.
Camera A has an aperture setting of 4. The Depth of Field is smaller.
Camera B has an aperture setting of 22. The Depth of Field is larger.
All these things contribute to your depth of field. For examples wikipedia has a large resource.
http://en.wikipedia.org/wiki/Depth_of_field
Also, if you have a Palm Pilot, there's a nifty FREE tool you can get pCam for your palm at
http://www.davideubank.com/
Shutter Speed - Controlling your motion blur
No Ascii here. But I do have this nifty GIF from wikipedia.
If you notice, in this picture, it's a static shot of the waterfall, just taken at different shutter speeds. Look down in the right hand corner.
You notice the numbers?
At 1 second, the water looks silky and smooth, almost painted in.
At 1/30 it has a slightly different effect, showing movement, but not being one continuous plane.
At 1/200 it looks like the water has stopped in mid air. Pretty cool.
At 1/800 you can see every little droplet of water that's coming over those rocks.
So, summary? To stop action, use faster shutter speeds, 1/250, 1/500 or faster.
To blur it, use slower shutter speeds, 1/3, 1 second, or slower.
Experiment and see what you like best.
Wikipedia again comes through here.
http://en.wikipedia.org/wiki/Shutter_speed
Composition - Rule of thirds
Well it not really a rule, it's more of guidelines.
No, ACTUAL guidelines.
Okay, imagine you're looking though your viewfinder. Split your frame up into 9 equal squares like this. You want to try and put your main focus on these lines. If it's people you're taking pictures of, you want to try and keep their eyes at the top third. This is called headspace. If their facing to the left, try putting their eyes in the right third. This is called leadroom.
Of course, this is just a suggestion, but in many cases it makes a better picture.
http://en.wikipedia.org/wiki/Rule_of_thirds
Lly suggested I add this link for other composition rules. It's very well put together, I don't think I could explain it any better myself.
http://www.colorpilot.com/comp_rules.html
The diagonal rule is very simple, but can do a lot for your image, and I've definately used the diagonal golden section rule.
I've also been reminded to touch on MINIMUM FOCAL DISTANCE. All lenses have a distance at which they will not focus. If you're too close to your subject, the picture will not come sharp. On many lenses, it's 2 feet. On longer lenses it's about 5 or more. Macro lenses can get in very close, I.E. under 1 foot. Take this into consideration when you're taking your pictures and buying lenses. Sometimes a small step back will allow you to get focused.
If there's anything anyone else would like to add, please let me know, and I'll do my best to add it.
Sheri Baldwin Photography | Facebook | Twitter | Etsy Shop | BUY ME STUFF (updated for 2014!)
Edit: to be honest, is any thread REALLY necessary?
Great advice, superglue; very clear and easy to read and good use of example photos!
This is an excellent idea. As I move closer to being able to buy my first SLR, I'll need to know this stuff.
As it is, the few times that all of this has been explained to me I haven't been paying attention for one reason or another. >.>
I can't remember all of these, and can't go into depth on any of them, but perhaps a seasoned or learned photographer might speak to the other elements of composition, including (but not limited to):
Movement
Lines
...dammit, I was really hoping to remember a couple more. I read in a book and then in a magazine about like 7 conceptual elements that should be considered and could be used to frame a better photo.
Is this post useless, or did I say enough to trigger someone's memory who then might be able to expand on these ideas?
My Website | My "photo-a-day" 2010
http://www.colorpilot.com/comp_rules.html
It's pretty much the same for photography and any kind of art.
Dear satan I wish for this or maybe some of this....oh and I'm a medium or a large.
My Portfolio Site
How does the Rule of Thirds relate to Phi (1:1.618...) which is the golden mean?
EDIT: I found this site: http://www.ukphotographics.co.uk/magazine/technique/pt_200511.cfm
Apparently cutting a picture vertically so that you have a square leaves a golden rectangle (what size picture is not dicussed). This remaining rectangle is approximately one-third the width of the original picture.
:-/
My Website | My "photo-a-day" 2010
Good topic!
I certainly plan on it, I was going to use my old camera as an example (entirely manual, so it's a perfect example) But with my computer down it's a little more difficult to custom tailor something like this.
You can read this as a rough guide, and then we can discuss and belabor the points and i don't know. i'll edit it and add in whatever anyone else what's to say about it.
Buying a camera (or anything for that matter) is like buying a car. There are tons of cars for tons of different things that cost all different amounts of money. The question you have to ask yourself is: what do i need?
For example, a Lamborghini is "better" than a BMW sedan. A BMW sedan is "better" than a Honda Civic. But do you really need a Lamborghini or BMW to drive around in? Lamborghinis are high performance machines, BMW are luxurious, but most people can get by in a Honda Civic. Same thing applies to cameras.
Most of us don't have money to buy cars. In my case I don't have money to buy cameras, so we usually don't check to see why a BMW is better than a Civic, and in my case, I usually don't check why one camera is better than another.
But since my roomate asked me to help her get a camera, I did some reading. (I really like all this stuff, reading specs and applying them to situations . :mouth14: )
So lets compare four cameras of varying levels:
Canon Powershot SD20
Olympus Camedia c7070
Canon 20D
Canon 1DS Mark II
PIXEL COUNT
The first thing is always pixel count (because people will usually look for highest pixel count first):
Canon SD20
5.0 million pixels
Olympus c7070
7.1 million pixels
Candon 20D
8.25 million pixels
Canon 1DS Mark II
16.7 million pixels
The question you have to ask yourself is: what is going to be the final output of the image? computer monitor, 3x5, 5x7, 8x10, larger?
Higher resolutions become pretty irrelevant on the internet, since pictures must be scaled down and a 72dpi resolution is used (meaning pixels are erased), Any of these cameras will produce nice pictures for online viewing because all exceed the minimum amount of information needed for average display on the internet.
Here's some information:
By knowing this, we can see that the SD20 would be good for printing up to 5x7 inch pictures, which is fine for giving pictures to friends for framing on their nightstand. While both the c7070 and the 20D can both produce 8x10 inch pictures, which is more suitable for hanging on a wall.
The 20D is just shy of printing out 8x12 inch images, which is considered standard for portfolio pieces since an uncropped 35mm negative is closer to a 8x12 than 8x10.
(why is 35mm negative standard? who knows. it's the same for movies, eventhough movies are shot on different sizes and 35mm is cropped to different ratios just like photographs)
The Mark II can produce 10x14 inch and 12x16 inch pictures. Sometimes pictures just look better bigger, and people stick big ones in their portfolio to impress.
So now you can answer your question: am i putting these images only online? am i printing it out for friends and photo albums? do i want to hang them? am i builing a professional portfolio?
Here is a caveat though: although pixels are lost when scaling down, they are usually averaged. Meaning that the more pixels you have, the more information there is to be averaged and the better picture quality/image. So a 20D could produce a better 8x10 image than the Olympus, but an even better camera (like the Canon 1DS Mark II) can produce a better 8x12 image than the 20D.
BIT DEPTH
Since image/pixel information is vital, the more information the better, which leads us to the next specification, color depth:
Canon SD20
24-bt
Olympus c7070
24-bit
Candon 20D
24-bit/36-bit RAW
Canon 1DS Mark II
36-bit
This has to do with how well color is rendered. The more bits, the more information can be stored for each pixel, the truer the colors.
The question now is: how much detail do i need?
Again, the internet doesn't matter in this case, since color resolutions can be changed by each individual user on their desktop. Some computers are limited in their color rendition by their video card. Meaning that even if you have a 24-bit image, someone online may only be viewing it in 16-bit color rendition, so they're not seeing all the detail anyway. This is especially true for web browsing.
This question does come in handy when you are taking pictures of very complicated color schemes. Let's say a forest in the autumn or a very complicated fabric design. Again cameras are still quite a bit short from the colors we can see in real life, but higher bit depth helps in recreating what you see. One thing to consider is that most of the places we see and visit aren't that vibrant. A 36-bit color depth isn't necessarily needed if a lot of the colors are repetitive or the lighting is very even.
The first three cameras are all the same, although the 20D does have 36-bit RAW. More information means more storage space. And so you could be spending a lot of money on extra memory cards, or even portable harddrives. If you're taking a lot of pictures in some far off place where you can't regularly copy and erase your memory cards, you're going to be investing in quite a few memory cards so you don't run out of media when the really interesting stuff starts happening.
Here's an example: my friend takes pictures for local concerts, big bands, little bands, doesn't matter. He gets hired either by the record company, or the event center, or magazines to cover the events. He told me he takes an average of 500 pictures per show. A 1G memory card will hold 80 uncompressed pictures, meaning he needs about 7 of these cards. At almost 100 dollars each, that's a $700 expense added to the camera purchase price. But to him it's just considered a business expense, and his photos will pay for it over time.
This also has to do with taste. A lot of people can't tell the difference between 24-bit or 36-bit images just the same as a lot of people can't tell the difference between 44.1K CD audio and 48K digital audio. But some can. So some will want the higher resolution images, just like audiophiles will always want the higher resolution audio signal. Some companies will want a lot of images at average quality (think catalogues), while som company will want a lot of images at very high quality (think BMW or Nike brochure) It all depends on you and your purposes.
So ask yourself: what am i taking pictures of? do i need such a high color resolution? do i need to use up large amounds of memory? is my audience even going to tell the difference?
FILE FORMAT
All this color depth is nice, but again it takes up space. Also, the file format in which it is saved has to be able to support that color depth and resolution, so we go to our next specification, file format:
Canon SD20
JPEG (SuperFine, Fine, Normal)
Olympus c7070
RAW,TIFF,JPEG (SHQ, HQ, SQ1, SQ2)
Candon 20D
JPEG (Fine, Normal),RAW (12-bit),RAW + JPEG
Canon 1DS Mark II
RAW, JPEG, RAW+JPEG, (JPEG quality can be set from 1 (min. quality, max. compression) to 10 (max. quality, min. compression)
We are all familiar with JPEG, and to most people, especially with snapshots that is tan acceptable file format. But JPEG does compress the image, meaning it gets rid of some of the information. This creates smaller files, but the quality isn't the best and it can present a problem when trying to blow up the image.
On some of these cameras the resolution of the JPEG can be changed. This comes in handy if you have a limited amount of space on your memory card, but you know what pictures you are taking. For example if it's for a website just showing the local soccer game, a low resolution, medium compressed image will be good since it will allow for more pictures to be saved and the end output will be online anyway.
RAW is an uncompressed format used to edit pictures, it keeps all the information, and all the resolution, hence bigger file size. These are great to edit your photos with, but they do away with a lot of the features of your camera. Meaning that it ignores white balance, exposure, and effects added in camera, although it will record the settings within the file for future reference. If you plan to record in RAW mode, you may not need a camera with a lot of extra effects and adjustments, since all adjustments can be made later, and RAW mode ignores your settings anyway.
RAW+JPEG means it will record one RAW file that ignores the settings, and one JPEG file with the settings applied. This is good if you want to review the settings you had on your picture and then re-edit the RAW file.
TIFF is another uncompressed format, used mostly in printing. TIFF will be recorded with the settings put by the camera, so once the settings are made, the image is recorded uncompressed with those settings.
Again, you have to ask yourself: am i editing my pictures a lot afterwards? Am I doing all my adjustments in real life and photographing that? Do I need large uncompressed pictures? or are high quality JPEGs good enough?
Cameras like the Mark II take this a step further by adding 10 levels of compression to your JPEG, so you can maximize your memory card space and customize it to the photographic situation exactly.
LENSES (SIMPLIFIED VERSION)
What's all this color and memory and everything else matter if you don't have a nice lens to capture it all? So here we go:
Canon SD20
fixed 6.4mm (39mm equivalent)
Olympus c7070
fixed 5.7mm - 22.9mm (27mm - 110mm equivalent),Olympus Wide Lens (8 lenses in 7 group)
Candon 20D
Canon EF and EF-S lenses , Focal Length Multiplier 1.6x
Canon 1DS Mark II
Canon EF Mount , Focal Length Multiplier 1.0x
Okay, so lenses need a whole explanation unto themselves which I'll try to type up, but for now we'll stick with the basics.
There's two kinds of lenses: fixed, which means they cannot be removed from the body, and mounted which means they can be removed.
Fixed lenses include disposables, snapshot cameras, toy cameras, most lomo cameras, etc. If it is a 35mm fixed lens camera, it will say the focal length of the lens somewhere near the front of the camera. If they are a digital camera or a APS camera (remember those? i loved them), they will have a number, and next to it, a number in parenthesis. Above the word equivalent was written in parentheses, but most of the times it's not written on the lens in that way. This takes a little explaining:
Again, a 35mm film frame is considered standard. Most digitals have a small chip, much smaller than a 35mm frame. APS cameras use a 25mm film frame. Physically the optics and lengths of the lenses for these cameras have to be smaller in order to focus/zoom/etc. The number in the parentheses apply to what the focal length equivalent on a 35mm camera would be.
So physically, the SD20 has a 6.4mm lens because of the small chip that records the image, but in relative to a 35mm film frame it would be as if you were using a 39mm lens. This is so you can calculate the depth of field, field of view, and distortion of the image of your SD20 compared to 35mm. (More on that in another thread)
The important thing to notice is that a 39mm lens is a wide angle lens with a deep depth of field. So the SD20 6.4mm lens takes pictures that include a lot of the subject in the frame, and distorts the image the closer the camera is held to the subject. This is good for enclosed spaces where there isn't a lot of room. But it gives you a lot of depth of field, meaning that blurring out the background with this camera is near impossible. So the SD20 would be a good camera to take snapshots in an enclosed space (a car, the corner of your room, held at arms length for a self portrait). It's a snapshot camera meant to be used quickly and on the go, so the wide angle lens ensures that you get most if not all of the subject in the frame. The excess area can be cropped later.
The Olympus model includes a fixed zoom lens. The range is given, plus it's 35mm equivalent, so you can determine how far in you can zoom into a subject. Once you get better at measuring distances from the camera to the subject, you can figure out what one focal length will look like compared to another without even having to look through the viewfinder. This comes in handy because when you're choosing your lenses/camera, you can do it in your head instead of having to test everything out before hand. Also, it comes in handy for figuring out depth of field, field of view, and lens distortion.
There is anothe parenthetical that reads " 8 lenses in 7 groups". Every lens comes with this information, no matter who makes it. In this case it is included with the camera information because the lens cannot be removed.
To help clarify this, I'll use this picture:
The yellowish part of the figure is the lens body, the white part is the glass elements in the lens that help it focus/zoom etc. It is by moving these pieces of glass backwards or forward that a any lens is able to zoom or focus. Each piece of glass is a lens in itself, much like the lenses in your glasses. Each one slightly affects the image of the picture you are taking. The more glass, the more the image is affected.
The affect can be either good or bad. One affect of this lens is that it allows you to focus by moving one of the lenses back and forth. Another affect is that it allows you to zoom. There are some bad affects as well, but that's for another thread.
Each lens has it's job to do with the light entering the lens body. Sometimes, a lens can't do it's job on it's own, so it is paired with another lens, as you can see in the middle of the diagram, there are two pieces of glass pressed up right next to one another, so that one fits inside the other. Those two pieces of glass perform one job, together. Each piece of glass performing a job is called a group. A group can have a single piece of glass or multiple pieces of glass. In the diagram there are two pieces joined together, so that's one group; and the rest of the lenses do their job on their own, so they are their own separate groups. This is why the lens is said to have 7 groups. There are 7 different things happening to the light entering the lens. But the entire mechanism is made up of 8 pieces of glass, hence, "8 lenses in 7 groups"
Sometimes the lenses are referred to as elements, so another way it may be displayed is "8 elements in 7 groups"
A deeper understanding of the lens group will help you in deciding what kind of fixed-lens camera you would like, or what kind of mountable lens body would suit you. This is why the information is given with fixed lens camera bodies.
Mounted lenses come in all different kinds of mounts, different camera bodies use different mounts, some overlap some don't. Certain lens companies use certain mounts, some overlap, .some don't. There's D-mounts, C-mounts, P-mounts, and Bayonet mounts (i'll try to find pictures later for the lens thread). Different lenses require different mounts because of the stress they put on the camera, as well as the degree of precision that the lens must be aligned with the camera body.
Certain companies. like Nikon and Canon make their own lenses with their own mounts. This is because Canon lenses have electronics in them that help them autofocus, and so they need a special coupler in order to talk to the camera body. Putting other lenses on the camera could damage the coupler, so make sure the lens you are using is made to be used with your specific camera.
Canon calls their specific mount an EF mount, and their lenses EF lenses. Older canon cameras (non electronic ones) do not use EF mounts, they use one of the others.
Mountable lenses come in two varities: Prime and Zoom. Prime are fixed focal length lenses. You cannot zoom, and they will vary in size and weight depending on their function. They usually have less glass in them, and produce sharper pictures. This is why professionals perfer mounted cameras.
Zoom lenses mounted on a camera work exactly the same as a fixed-lens zoom. The glass moves back and forth to zoom in and out. They have a lot of glass in them to produce this effect, and the more glass, the more the light is affected.
The 20D has a Focal Length Multiplier. This has to do with the physics of the smaller chip in comparison to a 35mm camera. I believe that using a 35mm lens such as a 100mm would would actually work as using a 160mm lens on that camera. Again, that affects your depth of field, field of view, etc.
So now you ask yourself: what kind of pictures am i going to take? snapshots of the city life and my friends (like terry richardson)? pictures with deep focus? pictures with distortion? am i really going to buy other lenses for my camera? am i going to use primes?
These are important questions, because a fixed zoom lens camera (film or digital) may be the answer for you eventhough you may think you need a interchangeable lens camera.
On a more esoteric note, sometimes the fixed lens of a camera has the look you prefer moreso than a mounted-lens camera. Lots of professionals have fixed lens cameras that they use, from different companies, different lenses, different features, just to get the look that they want.
Another thing is that certain cameras can be modified to accept different mounts. So if you like a certain kind of lens that a company makes, but they don't make it for the camera body you like, you can take in the camera body and they can modify the mount. This can be expensive though, but it is an option.
Some photographers also have lenses screwed onto the camera body, so that the lens can not be removed. This is to keep dust out of the camera or to keep the lenses from falling out.
So do you need a customizable camera? or would a fixed lens do you fine? are you going to be spending lots of more money on lenses, or do you just need a decent range of zoom to cover your subjects?
FOCUSING SYSTEM
Speaking of focusing, the next step would be determining what kind of focusing system is suited to your needs. Here is what they offer:
Canon SD20
TTL autofocus, Focus Modes, 9-point AiAF, 1-point AF (Fixed to center), AF-assist Beam (On/Off)
Olympus c7070
Dual AF (CCD: Contrast detection/Passive Sensor: Phase detection), Focus Modes AF (iESP, Spot), Fulltime AF, Predictive AF, AF illuminator, Manual Focus
Candon 20D
TTL-CT-SIR with a CMOS Sensor (TTL CrossType secondary image registration) with 9 AF Points and AF-assist (Intermittent firing of built-in flash), Focus Modes, One-Shot AF, Predictive Al Servo AF, AI Focus AF, Manual focus (MF)
Canon 1DS Mark II
TTL-AREA-SIR with AF-dedicated CMOS Sensor with 45 AF points (Area AF)
Focus Modes, One-Shot AF (ONE SHOT), AI Servo AF (AI SERVO), Manual Focusing (MF), AF Point Selection (Automatic selection, manual selection, home position)
People get intimidated by this because of all the technical talk, but that's because engineers work hard on developing "smarteR" AI and so they like to show it off. For most people, you can ignore all the little technical stuff. Some photographers need very specific focusing modes for their job, and so that's when all this technical stuff comes into play.
TTL means "Through the Lens" meaning that the focusing system evaluates the image coming in through the lens to focus. There are some cameras (film cameras) that don't do this, they have a separate lens that views the subject to focus and another that actually takes the picture (you may have seen them, they're pretty big cameras with two lenses mounted on them like this:
Those cameras have reasons for that focusing, and maybe I'll get into that in the lens thread, for now, most people like to focus on the image coming in through the lens that is taking the picture, so that is how most cameras work, TTL focusing. The rest of the acroynyms is stuff you don't really need to worry about.
The next important thing about the focusing mechanism is how many points of focus it has to evaluate the Auto Focus. Camera makers build their AF systems on what they think the camera would most likely be used for.
There are two systems of focusing: Active Focusing and Passive Focusing
Active focusing is starting to be used less and less, but it is used widely in consumer cameras. The camera emits a sound signal or a infrared signal that goes towards the subject, bounces offf, and then comes back towards the camera. By measuring the time delay of the signal and echo, it can determine how far away the subject is and then set the lens to that distance. This works better when your subject is in the middle of the frame, and there aren't too many things around them. Having a lot of subjects, and standing a bit far away throws off the measurements.
The advantage of an active focus system is that it can focus in almost complete darkness (since sound/infrared waves are being used), and then use the flash to get the proper exposure.
Passive focusing systems are mostly what we use today, they analyze the picture coming in through the lens at the camera and then determine the focus. These are more accurate since they can evaluate different areas of the frame at any distance, and can focus on either a part of the frame or average out all the readings to get as much in focus as possible.
The one thing about these systems is that they require light, and so many have a light that comes on to help focusing, which may get you attention you don't want or be an annoyance to your subject.
The way passive systems work is by splitting the frame into small areas, or points, similar to this:
This is an 11-point system, if you disregard the two little rectangles to the side, it becomes a 9-point system. The way the autofocus generally works is that it looks at the 9 points, and then evaluates which of the points are most important, and those points take the priority in focusing.
This is important because if your composition is strange or complicated, the AF may get confused and not know which of the points to focus on. But if your subject takes up the left hand side of the frame, the 3 focus points on the left would recognize the subject and your AF will try to get those 3 left hand points into focus. The camera usually does this on it's own, but there are models made that allow you to tell the camera which focus point to pay attention to.
The contrast reads the contrast in those points. Blurry pictures have less contrast than sharp pictures, so when there is a sharp contrast between the pixels(for digital) or reflected light (for film) it knows that it is in focus.
The one thing about these systems is that the camera may not understand that you want the subject in the foreground blurry while having the background in sharp focus, even if the foreground subject takes up most of the frame. If the camera doesn't allow you to select the focus point, you have to focus manually to get the right picture.
Higher end cameras have more complicated AF systems because they need to be more flexible. We'll break it down camera by camera:
The SD20 has these modes (i am quoting to break up the page):
The Olympus has these modes:
The Canon 20D has these modes:
The 1DS Mark II has these modes (this is where the professional stuff starts getting crazy):
ISO
ha. that's where i ended all of that. uh. maybe i'll keep going, but there wasn't much more to add other than ISO, shutter speed, chip sensitivity.
but i don't have the time for that just now. oh and i think the 20D DOES have point selection. i forget whether it was the 20D or the D70, but i wrote all this before people started to let me play with their higher end equipment.
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