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09-26-2011, 12:14 AM   #1
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don't understand how to read lenses

Am new to dslr got my first one a few weeks ago the pentax k-r am just getting used to it now but I still don't understand how to read lenses eg f/ I haven't got a clue what this mean please help cheers

Last edited by safc; 09-27-2011 at 07:26 AM.
09-26-2011, 12:18 AM   #2
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That's the aperture number, the smaller the number, the more light the lens sends through (=the larger the aperture). Usually smaller F numbers are desirable, but usually that also means that the lens is more expensive.

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09-26-2011, 12:38 AM   #3
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Cheers for that adam am learning something new every day
09-26-2011, 01:37 AM   #4
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All sorts of stuff may be inscribed on a lens. You may see a number like 49 or 52 or 55 or 55 followed by theta (an O with a slash across it). That is the front thread size in mm, so you know what filters will fit. You might see something like 100/2.8 or 100mm f/2.8 or 100 1:2.8. Those all mean it's a lens with a focal length of 100mm and a maximum aperture of f/2.8. (With very rare exceptions, that f/number will be 11 or less, and the focal length will be 10 or more.)

As Adam said, a smaller f/number means a larger aperture (hole for the light to get through); and a lens of a smaller f/number likely costs more that one with a larger number. Of used 200mm lenses, an f/4 may cost US$15 or less, an f/3.5 may cost US$50 or so, and an f/2.8 may cost US$350 or more.

A zoom lens may have numbers like 70-200mm 1:3.5-5.6. The mm-numbers are the lens' range of focal lengths; the f-numbers are the maximum apertures at those focal length limits. So at the short end, it's a 70/3.5 lens; at the long end, it's 200/5.6, pretty slow and hopefully pretty cheap. A zoom with only one f-number has a fixed maximum aperture; if it's a low number like f/2.8, that lens will probably be pretty good and pretty expensive.

Also on the lens will probably be the country of origin, a serial number, a brand name, maybe a model name, and maybe some letters describing its construction, like APO IF LD DG etc. Don't worry about those now. Once you're familiar with reading a lens, you'll know which of this is important for identifying it, and what isn't. With some third-party brands like Vivitar, the serial number may tell who the actual lensmaker is; with others, it tells us when the lens was made. And with some brands, it tells us absolutely nothing!

Deciphering lens inscriptions can be a dark art. Beware of the lens wizards.

09-26-2011, 04:44 AM - 1 Like   #5
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Here are a couple of free on-line camera simulators that will help you visualize the relationship between ISO - Aperture - Shutter Speed.

The SimCam: Film and Digital Camera Simulator -

Aperture, shutter and ISO value | SLR Camera Simulator

This one even has a couple of camera modes, aperture priority (Av) and shutter priority (Tv)

SLR Camera Simulator | Simulates a digital SLR camera

09-26-2011, 06:01 AM   #6
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QuoteOriginally posted by safc Quote
Am new to dslr got my first one a few weeks ago the pentax k-r am just getting used to it now but I still don't understand how to read lenses eg f/ I haven't got a glue
And for goodness sake, keep it that way! Try to glue off of aperture rings and all moving parts in general when lenses are concerned.

You're welcome.
09-26-2011, 09:23 AM   #7
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I meant clue my spelling not up to much haha
09-26-2011, 07:05 PM   #8
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Just to explain and or clarify a little more. There are bladed that form a hole in the middle (the aperture). They adjust to change the size of the hole. The smaller the number, the larger the hole. 2.8 would be a fairly large hole while 22 would be very small. The bigger the hole, the more light gets in, the brighter the picture. The camera shutter opens for different amounts of time. The longer it opens, the more light gets in (but leave it open too long and you can get blur from camera movement or from a moving object). You balance these settings to get the right picture brightness. Your camera has modes to automatically do this.
The numbers on the lens like 3.5 or 2.8-3.5 are the largest the aperture can open. They can all adjust to a much smaller opening so you really only have to worry about the largest the lens can do. The larger it can open up, the better then lens can operate with less light like indoor or dawn, cloudy etc. The larger the aperture opening, the larger the glass must be which is why fast lenses (lenses are called fast if they have a large aperture opening) can have huge front glass and cost a lot. The smaller the number, the bigger the hole (2.0 is faster/bigger than 3.5).

50mm is approximately what the average human eye can see (it can very quite a bit from person to person). That means that if you look through the view finder, then move the camera aside, things will appear to be about the same size at 50mm looking through the camera as looking without it. smaller numbers than that are wide angle (things will appear smaller but you will get a larger area in the picture). Larger numbers are of course telephoto like a telescope. The higher the number, the larger objects will be but the less area you get in the picture. For instance at a wide angle, you might get the whole beach in the picture. At a more telephoto angle, a person standing on the beach might fill the whole picture. Of course at 50mm, the picture wont be what your eye sees but rather a section cropped out of the area you see. Some people have good enough peripheral vision the can almost see sideways, but your camera can not come close to doing that.

09-26-2011, 07:18 PM   #9
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Best to stop now before LBA kicks in.

I'm only partly kidding.
09-26-2011, 07:32 PM   #10
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I'll add a little something about human eyes: Our visual systems are tricky stuff. What we see is both wide-angle and telephoto, with a dash of memory and fantasy. You'll see much talk about what is 'normal' human vision, but that's tricky too. With both eyes open, we see about as much as a 15mm lens on an APS-C dSLR like your Kr. With just one eye open, we see about as much as a 30mm lens. When we concentrate, focus strongly on something, we see about as much as a 55mm or 85mm lens, depending.

When we talk about 'normal' vision or lenses, that's a moving target -- 'normal' is pretty much what we're used to. A wider field of vision (FOV) seems 'wide'; a narrower FOV seems 'long'. But we may be used to wider or narrower FOV's, so they seem normal. There aren't really any magic numbers here, just focal lengths that we find suitable for various purposes. Landscapes fit well with the focal range of an 18-55mm kit lens. Headshot portraits fit well between 45-105mm, depending on how close you are. Shooting within cramped spaces, an ultrawide or fisheye lens in the 10-20mm range might be right. That kind of stuff...

When we think photographically, we put together the focal length (in mm, millimetres), apertures (in f-stops), camera sensitivity (in ISO), camera-to-subject distance (in feet or meters), to make the picture we want. Every picture is a problem to be solved. We often just pick one or two of those factors and the the camera's computer figure out the rest. We don't need to do the math ourselves, any more than you need to calculate trigonometry functions when you catch a ball that's thrown to you. Select what's important, and wing-it from there.

Last edited by RioRico; 09-26-2011 at 10:52 PM.
09-26-2011, 08:10 PM   #11
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I was debating weather to even say anything about the human eye and what you see but I figured the simple form of a comparison of magnification might help. Honestly, to the original poster, just play with what lens(s) you have to get an idea of what you will get in the photo at different magnifications. As far as what ranges to get next, its a tough question that can sometimes require just trying out some lenses. You can get manual focus lenses on ebay quite cheap to try out different focal ranges for lenses (feeding the LBA, lol).

As far as the human eye, its a very complex fluid dynamic system that can not really even be directly compared to a camera lens. What is optically entering your eye is not even what you are seeing. What is hitting the back of your eye after being altered by the fluid lenses is not what you see either. You are seeing your brains interpretation of multiple images, not a snapshot of one image. I saw a documentary where they implanted sensors in someones eye that was blind due to problems in the front of the eye (can not remember exactly what but it was some sort of degenerative condition). It was something like 14 sensors so they basically put a 14 pixel sensor in the persons eye. You wouldn't think 14 pixels would allow them to see anything. Thing is, the persons brain slowly adapted and they progressively began to se basic images and be able to see. Their brain was able to take multiple images from those 14 sensors and extrapolate a more complex image.
09-27-2011, 10:05 PM   #12
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This has been extremely helpful for me as well, thanks for the great links too.

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