What to look for when buying a lens?

Discussion in 'Photography Equipment & Products' started by JeffieLove, Feb 14, 2010.

  1. JeffieLove

    JeffieLove TPF Noob!

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    I am looking at getting a Canon XSi (or maybe just XS since there doesn't seem to be much difference other than megapixels) and I am also looking at lenses... My mom has the camera that is in my sig as my equipment and I've gotten used to it so that is why I have decided on Canon...

    My question is - I wanted to get a high zoom lens for taking pictures of my kids outside and my mom's "birds and squirrels" that she feeds all the time...

    What should I be looking for? I know the lens will say something along the lines of:

    55mm-200mm f/4-5.6... etc..

    I understand what the mm are and what they are used for... I can't quite seem to grasp what aperture and shutter speed really do...

    So, in a good lens, what is a good aperture range?

    Does the lens have it's own shutter speed or is that a camera body thing?

    What do I need to make sure I have in my lens?

    I might also be taking night time photos and I would like to set up a little home studio to do portraits of mine and my friend's kids, my parents, etc...

    Please don't tell me to google this.. I did try and it confused me even more... You all here have a way of putting things into lamens terms for me ;) I am not a complete idiot, but I am a complete NOOB in the photography world ;)
     
  2. skieur

    skieur TPF Noob!

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    Shutterspeed is the speed at which the shutter opens and closes to let in light to the processing chip in order to take the picture. It is expressed in fractions of a second and it is in the body of a camera.

    The aperture is how much the shutter opens to let in light. In shape it is from a very small circle to a circle the size of the lens.

    To "translate" focal lengths such as 25mm, 50mm or 200mm, think of it this way. At approximately 50mm, what you see through the viewfinder is the same size as what you see when you take your eyes away from the viewfinder and look at the scene in front of you. At 25mm, you see a lot more through the viewfinder but everything appears smaller, than when you take your eyes away. At 200mm, you see a lot less in the viewfinder, but like a telescope everything looks larger.

    The f4 - 5.6 in your example means that at 50mm the aperture that lets in the most light is f4. You cannot adjust the aperture to lower than 4 in numbers. At 200mm the aperture that lets in the most light is 5.6. You cannot adjust the lens aperture to lower than 5.6 in numbers.

    In practical terms there are some compromises. A 50mm to 200mm f.4 to 5.6 will be fairly light in weight but it will not be useful in extremely poor lighting conditions such as around sunset or in house with regular lighting from lamps.

    A 50mm to 200m f.2.8 could be used in low lighting conditions but it would be a larger, heavier lens to use and put in your camera bag and it would cost more money.

    I hope this info. is helpful.

    skieur
     
  3. TJ K

    TJ K No longer a newbie, moving up!

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    For a zoom lens a good aperture range is f/2.8. The aperture is the size of the hole inside of the lens. The smaller the number the bigger the hole. So that means if you get a 70-200 f/2.8 throughout the entire range it has a very big hole.

    The bigger the hole the more light can get in in shorter amounts of time allowing faster shutter speeds. Something like f/22 is a very small hole so you will end up using slow shutter speeds even in the middle of the day.

    The f/2.8 lenses are quite expensive though. The 55-200 isn't a bad lens but the aperture is 4.5-5.6 so that isn't a very big hole which means you will have to use a slower shutter speed than at f/2.8.

    I would recommend that you pick up the book understanding exposure it will be very helpful and covers pretty much everything you are asking about.

    Shutter speed is the amount of time the shutter stays to allow light to hit the sensor of the camera. The lens doesn't have anything to do with shutter speed just aperture.

    If you are doing anything with action you will want to use a faster shutter speed somewhere around 1/500 to start. This means the shutter won't be open very long and you will be able to freeze the action rather than getting a motion blurred image. ISO is also something to you're going to want to learn about.

    Also if you plan on doing night shots you're going to want a decent tripod and a wireless/cable release for your camera.

    I hope that helped. If you have any more questions just ask and I will happily help you and you can PM me if you want. GL
    TJ
     
  4. JustAnEngineer

    JustAnEngineer TPF Noob!

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    The $250 EF-S 55-250mm f/4-5.6 IS might be a reasonable starting point for a telephoto zoom lens. The $600 EF 70-200mm f/4L USM is a better lens. The $1800 EF 70-200mm f/2.8L IS USM is better still.

    The "f/4" is a measure of the maximum aperture that the lens allows. In the case of the 55-250mm lens, it can open to f/4 when zoomed to 55mm, but it can only open to f/5.6 when zoomed to 250mm.

    The aperture measures how large the opening is that lets light pass through the lens. It is expressed as a ratio to the lens' focal length (f). An aperture of f/4 means that the aperture is 1/4 as large as the focal length. The standard f-stops go up by the square root of two so that each one lets half as much light through as the one before: f/1.4, f/2.0, f/2.8, f/4.0, f/5.6, f/8, f/11, f/16, f/22, f/32.

    If you have a well-lit scene that is correctly exposed at f/8 and 1/200th of a second, if you change the aperture to f/5.6, you'll only need 1/400th of a second to let the same amount of light reach the sensor. If you change the aperture to f/4, you'll only need 1/800th of a second to let the same amount of light reach the sensor. Lenses with large maximum apertures (low f-stops) are sometimes called "fast" lenses because they can allow shooting with very short exposure times.

    Besides regulating how much light passes through the lens, the size of the aperture affects the depth of field. With a very large aperture (low f-stop), the depth of field is shallow. If you focus on an object that is relatively close to the camera, objects that are far away will be out of focus. This effect is nice for separating your subject from the background by isolating your subject in focus (like the eyes in a portrait) while the out-of-focus background blurs away to a smooth non-distracting blur. If you want the reverse--where you can have near objects in focus while still keeping far away objects in focus, you want to use a small aperture.

    Using a smaller aperture can help mask some of the distortions present in a lens, making the image sharper. The optimum aperture for image sharpness with the cameras that you are considering is around f/8. At very small apertures (f/16 and smaller), diffraction effects become more significant than the lens distortions.


    For indoor portraits without a flash, you would like a lens with a larger aperture than the EF-S 18-55mm f/3.5-5.6 IS kit lens. You could add the $90 EF 50mm f/1.8 to your kit, or you could replace your 18-55mm lens with a better quality larger-aperture alternative like the $450 Tamron Di-II 17-50mm f/2.8, $625 Tamron Di-II 17-50mm f/2.8 VC or the $1000 EF-S 17-55mm f/2.8 IS USM. At f/1.8, the 50mm prime lens can gather ten times as much light as the kit lens can at f/5.6 when zoomed to 50mm.
     
    Last edited: Feb 14, 2010
  5. JeffieLove

    JeffieLove TPF Noob!

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    I understand that shutter speed is how fast the shutter goes... For super hyper children who are moving you would want a faster shutter speed (1/1000 for example)... Versus if you are photographing something still, you can use 1/500, and if you are photographing water or something that is flowing like a waterfall, you will use a slower shutter speed of 1/100... (Is that right?)

    Aperture (how I am understanding it) is basically what causes photos to be either over or underexposed... Right?

    And then the last question from your post: Is the focal length the same as depth of field or are they somehow related?

    So is there like a "rule of thumb" that with a higher aperture you use a lower shutter speed? Or something along those lines? For now I basically just experiment with the settings..

    I am definitely going to pick up that book - as soon as I get paid this week ;)

    ISO is another "lighting" thing right? Like a higher ISO means a brighter picture?

    I am also looking for a tripod (I am trying to do the kit thing on amazon where it comes with a bunch of accessories and stuff...

    What is a wireless release? (Yes I probably could google this, but I'm already here and replying to you! :)

    So the aperture will help with depth of field? A higher aperture (smaller f/#, right?) will give more depth of field where it is needed, whereas a smaller aperture (higher f/#?) will give less depth of field?

    So let me make sure I have this straight... Higher f/# = smaller hole, lower f/# = bigger hole? (God I feel like an idiot! lol)

    I will probably be investing in that $90 lens for my son's 3rd bday party in March :)
     
  6. TJ K

    TJ K No longer a newbie, moving up!

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    A wireless release is something you can use to trigger the shutter without having to press the button. This will ensure there is zero chance of camera shake from your hand touching the camera and also it will allow you to keep the shutter open as long as you want, the camera only lets you do 30seconds without it.

    I wouldn't recommend buying any of those "kits" from online they are often just paired up with a bunch of cheap crap and they sell it for a higher price trying to cheat people and get their money who don't know any better.
    TJ
     
  7. chammer

    chammer TPF Noob!

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    yes, and no. correct in that the aperture affects the depth of field. however, you got the wider aperture and smaller apertures reversed. the wider the aperture (lower f/#) the less depth of field you will have. the smaller the aperture (higher f/#) the more depth of field you will have.

    a good example of both:

    shooting a flower and its stem at f/1.8 may only show a portion of the flower in focus with it tapering off to be extremely blurred by the time it reaches the stem or part way down the stem.

    shooting a landscape at f/16 or so will render everything you see in focus.

    spot on. just remember that one of the key things is that the smaller the hole (higher the f/#) the less light that gets in. the less light that gets in means that the shutter must stay open longer to get the same exposure as one which used a wider (smaller f/#) aperture. so a shot that you did at maybe f/2.8 @ 1/500th may now be 1/10th at f/22 (just an example). :)
     
  8. JustAnEngineer

    JustAnEngineer TPF Noob!

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    ISO is a measure of your sensor (or film)'s sensitivity to light. ISO 200 is twice as sensitive as ISO 100, so it takes half the exposure time. ISO 400 cuts the exposure time in half again. Newer cameras will allow you to set even higher ISO values, but you can begin to see digital sensor noise in your photos when you push to high ISO settings (especially at ISO 3200 and higher).

    If it's a choice between not getting the shot at all because there's not enough light and getting one that has some noise, you may choose to use high ISO settings.
     
  9. KmH

    KmH Helping photographers learn to fish Supporting Member

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    The f# or aperture is a fraction. Just put a 1 where the f is: f/2.8 = 1/2.8

    The 1 represents the focal length of the lens. If you have a 100 mm lens and the aperture is set to f/2.8 the diameter of the aperture (lens opening) is 100 mm/2.8 = 35.7 mm.

    The 100 mm lens set to f/4 has a lens opening (aperture) 100 mm/4 = 25mm.

    If the lens has a 50 mm focal length the diameter of each aperture is different. 50 mm/2.8 = 20 mm and 50 mm/4 = 12.5 mm

    Another important concept to understand is "stops".

    A full stop is a doubleing or a halving of the amount of light the camera is letting in. Since the lens opening is essentially a circle, to let in twice or half as much light, we need to double/halve the area of the aperture, not the diameter of the aperture.

    As it turns out doubling/halving the area is a function of √2 (square root of 2), which in round numbers is 1.4.

    So each full stop is:
    The scale starts at 1 or f/1 but there aren't many lenses that can do f/1.
    • 1 x 1.4 = 1.4 (f/1.4)
    • 1.4 x 1.4 = 1.96 (rounded to f/2)
    • 2 x 1.4 = 2.8 (f/2.8)
    • 2.8 x 1.4 =3.92 (rounded to f/4)
    • 4 x 1.4 = 5.6 (f/5.6)
    • 5.6 x 1.4 =7.84 (rounded to f/8)
    • 8 x 1.4 = 11.2 (rounded to f/11)
    • 11 x 1.4 = 15.4 (rounded to f/16)
    • 16 x 1.4 = 22.4 (rounded to f/22)
    • 22 x 1.4 = 30.8 (rounded to f/32)
    Cameras today usually display 1/3 stops, which give more precise control of exposure, so there are 2 numbers between each of the above:
    f/2.8 → f/3.2 → f/3.5 → f/4 → f/4.5 → f/5 → f/5.6, and so on though , f/3.2 to f/4.5 is also a full stop as is f/3.5 to f/5.

    One of the handy things about the concept of stops, is that it also applies to shutter speeds and ISO settings. Since the area of an opening is not the involved we can just do x2 and don't have to use √2.

    Shutter speeds run: 1/100 → 1/125 → 1/160 → 1/200 → 1/250 → 1/320 → 1/400, etc.

    And ISO follows the same ISO 100 → ISO 125 → ISO 160 → ISO 200 → ISO 250 → ISO 320 → ISO 400, etc.

    Now all you have to do is explore the how the 3 compnents of exposure, aperture, shutter speed and ISO relate to each other.
     
    Last edited: Feb 14, 2010
  10. JustAnEngineer

    JustAnEngineer TPF Noob!

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    The scale can start anywhere. It's possible to have a lens with a larger aperture diameter than focal length (f/0.95, for example), but these lenses are not common.

    √2 = 1.41421356...
    √2 x √2 = 2, by definition. There's no rounding involved. Your math using 1.4 isn't the way that it works.
    √2 ^3 = 2.83, rounded
    √2 ^4 = 4.00
    √2 ^5 = 5.66, rounded
    √2 ^6 = 8.00
    √2 ^7 = 11.31, rounded
    √2 ^8 = 16.00
    √2 ^9 = 22.63, rounded
    √2 ^10 = 32.00

    If you really want to fill in the 1/3-stop increments, they're at 1.12, 1.26, 1.41, 1.59, 1.78, 2.00, 2.24, 2.52, 2.83, 3.17, 3.56, 4.00, 4.49, 5.04, 5.66, 6.35, 7.13, 8.00, 8.98, 10.08, 11.31, 12.70, 14.25, 16.00, 17.96, 20.16, 22.63, 25.40, 28.51 and 32.00. Each one-third stop requires 1.26 times as long an exposure as the one before it would for the same conditions.
     
    Last edited: Feb 14, 2010
  11. skieur

    skieur TPF Noob!

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    :)[/QUOTE]

    Shutter speed and aperture are inter-related in the sense that if you increase the shutterspeed, you need to open the aperture more to let in more light to make up for speed increase of the shutter opening and closing. Put another way 1/1000 sec. at f. 2.8 would give you the same exposure as about 1/500 sec. at f.4. Realize too that the shutterspeed that might be ideal for what you are shooting is limited by the amount of light in the scene.

    With that in mind, related to you examples, shooting children at 1/125 sec to 1/250 sec. would prevent motion blur. Shooting still landscapes etc would be at 1/125 sec. as well, generally speaking, with the possibility of going to as slow as 1/30 sec. toward sunset using a regular lens. With a waterfall, there are different photographic approaches but as a beginner just experiment with shutterspeeds to see what you get and whether you like the results. As you get more experienced, you can 'stretch' these generalizations by using flash, tripod, bracing your camera against a convenient tree, railing etc.

    Aperture and shutterspeed because they work together may cause the image to be over or underexposed but the more common occurence is part of the image being overexposed and detail disappearing in shadows that are too dark. This is where postprocessing on a computer is necessary.

    Depth of field is the "front to back" area that is sharp at a particular focal length and a particular aperture. In general terms the more you go into the telephoto range (higher than 50mm), the smaller/narrower the depth of field. The more you go into the wide angle area (lower than 50mm) the greater the depth of field.

    Aperture also affects depth of field. The smallest/narrowest depth of field is at the widest/largest aperture f.2.8 for many lenses. The largest area of sharpness(depth of field) is at the smallest aperture f.16 or f.22 for many lenses.

    Examples of using depth of field: If you are shooting a portrait of a person outside then it is useful to have the distracting stuff in the background blurred out of the picture using an aperture of f.2.8 and perhaps a focal lenght of 80mm. If you are shooting a landscape with boats or other objects in the foreground, you will want a large area of sharpness right to perhaps the horizon and other side of the lake in the distance. That is where an aperture of f. 11 to f.16 will help along with perhaps a wider angle focal length such as 28mm to 35mm.

    Hope I answered your questions.

    skieur
     
  12. bazooka

    bazooka No longer a newbie, moving up!

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    Concerning lenses, you pretty much get what you pay for. Most cheaper lenses are cheaper for a reason, especially for zoom lenses (any lens that can change it's focal length... like the lens on your Canon is 18-55mm.) Often, at the widest angle (small mm number) and the narrowest angle (large mm number) of a particular less expensive lens, you'll see distortion and abberation, especially around the edges of the picture. You can google images of "lens distortion" or "lens abberation" for examples.

    Before purchasing a lens, it's a good idea to check out some reviews so you know what to expect out of the lens. You made mention of a $90 lens... if you're referring to the f/1.8 50mm, I've read that that's a great lens for it's price. It's cheaply built but gives very sharp photos. I was looking at getting that one, but decided on it's bigger brother, the f/1.4 50mm.
     

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