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Old Oct 19, 2004, 8:37 AM   #1
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I'm new to the D-SLR world, coming from a Sony DSC-P72. I have the Nikon D70, awesome camera. I want to learn more about photography but first I need to make sure my foundation is rock solid hense the really really basic questions below.

Aperture: That's the size of the hole (like the iris of your eye) right but why is the number, the f-stop the inverse, smaller number means larger hole?
What does this number mean, some sort of measurement?

Focal length: This one I'm totally confused on.
80mm, 120mm. I think it's the distance from the lenses to the sensor (in a digital camera). So a larger number gives you a higher zoom rate?
How do the numbers equate to a zoom number, eg, 120mm equals 12x (or what ever)

Eventually I'll be purchasing two lenses one for close ups (for flowers and such) and one for telephoto (I want good wide angle zoom lens if there is one). The close up lens is call a macro lens (not sure why) and I want to get a feel of what all of the numbers mean so I can an informed consumer. These lenses, especially the Nikon branded lens for the D70 are quite expensive.

If you can provide some detail or provide some links to help me learn, that would be excellent.
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Old Oct 19, 2004, 8:47 AM   #2
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hm...i can answer some(if not all) of these

ok, yes, the arperture is the iris, that you can make bigger and smaller (like you said)
the smaller the number the larger the hole...

the fstop numbers are measurements, i'm not sure if it's inches/millimeters though...

so far you are right about the focal length, the larger the number the more "zoom" you have...

ok, to get how many "times" it is, you take the focal length, multiply it by two, then take off the zeros...so, a 300mm lens would be (x2=600-00=6x) 6x

i would suggest checking out the site


it's got a bunch of tutorials and articles that will help you out...

hope this helps

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Old Oct 19, 2004, 10:22 AM   #3
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I would suggest you read the definitions of terms here:

Many of them say it better than I can (including the one on aperture, which answers you question.)

The concept of f-stop is the same with the old Sony you had as with the Nikon D70. So don't worry about your knowledge not being transferable (you might not be, but I thought I'd point this out.) The apertures are physical larger with the D70's lenses than with the lens on the Sony, but the use of them are the same.

The one about focal length is also nice (and uses pictures, which I can't.) So read that too. It will answer parts of your question.

The way I think about focal length is this. Ignoring that the sensor is a difference size on the D70 for a second, the human eye sees the same as a 50mm lens. This means that if you put a 50mm lens on a 35mm film camera and take a picture the objects in the picture will appear the same size as you saw them. If you use a 100mm lens, things will appear twice as big. If you use a 25mm lens, things are twice as small.

Lenses with a focal length smaller than 50mm are usually called "wide angle lenses" and lenses with a focal length larger than 50mm are usually called "telephoto lenses". (I say usually because when you are close to 50mm the difference can be so little that people don't consider the difference significant. Most wouldn't call a 80mm lens a "telephoto" although technically it is. But a 400mm lens is clearly a telephoto.

When you say "zoom number" what are you referring to? The number that a binocular uses? (i.e. how much magnification?) or are you referring to the zoom range (the DSC-P72 was a 3x zoom)?

For the first, just divide the focal length by 50. So a 400mm lens is an 8x lens (so you would see the same thing through 8x binoculars as through a 400mm lens.)

For the second thing, divide the longer focal length by the closer focal length. Your old Sony has a 39-117mm lens. Doing the division that comes to 3x. It should be said that this way to describe a lens in the SLR/DSLR realm is basically useless. A 200-400mm lens is 2x. A 20-40mm lens is 2x. But they are completely different lenses.

Now, do you know about the effect that the size of the sensor has on focal length? It's something you didn't have to deal with when using your old Sony. And you'll need to know it.

A macro lens will let you take very close pictures of flowers and such. Nikon makes some very good Macro lenses, but Sigma makes some that are good that also work with that camera. Macro lenses can be expensive, but their quality is very high. The longer the focal length of the macro lens, the further you can be away from your subject. Doesn't matter as much with flowers, but with bugs you might want to be futher so you don't disturb them. www.dpreview's glossary also has some info on macro.

I hope that helps, feel free to ask more questions if this didn't give you want you needed (or just raised more question.)

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Old Oct 19, 2004, 10:23 AM   #4
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Cool thanks.

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Old Oct 19, 2004, 12:59 PM   #5
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eric s:

Even though someone else asked the question(s), I wanted to say your explanation was very good. I'm a definite newbie, and I read as many of these posts as possible so I can learn. Not to presume to speak for all newbies here, but this newbie appreciates the efforts you "experts" put forth to educate the rest of us. Thanks.
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Old Oct 20, 2004, 4:30 AM   #6
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frontier02 wrote:
eric s:

Even though someone else asked the question(s), I wanted to say your explanation was very good. I'm a definite newbie, and I read as many of these posts as possible so I can learn. Not to presume to speak for all newbies here, but this newbie appreciates the efforts you "experts" put forth to educate the rest of us. Thanks.
I have to agree. Eric (as well as everyone else) is pretty awesome.
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Old Oct 20, 2004, 5:35 AM   #7
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I just want to add a few comments to the aperture definition which I believe is very critical, but often ignore by the novice lens buyers (may be for cost reason). The aperture is a ratio of the lens opening diameter to its focal lenght, which means on most inexpensive zoom as one zooms this ratio changes because the focal lenght is varied while the barrel of the lens stays constant -> this change can be up to several stop sometime! :?

Why is it important? The lens can slow down considerably when you try to shoot under the shade of trees or the sky becoming overcast. This could result in the difference of between being able to take a picture or not. Also if you're on manual in a studio environment changing the zoom setting can now affect your exposure as well...

If you can afford it go for the constant aperture zooms, theses lenses usually cost more because they are designed to better control the aperture, but are well worth it! Also the "bokeh" is different, ie the amount of defocus when a lens is open up (something you can't do with a slow lens): Fashion photographers do this all the time to isolate their subject from the background
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Old Oct 20, 2004, 6:49 AM   #8
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And a further clarification about why the aperture number is higher for a smaller opening:

The aperture number is thediameter of the hole expressed as a fraction of the focal length of the lens, as NHL pointed out. If the lens' focal length is 50mm and the aperture diameter is 25mm, the f-stop would be half the focal length "f," or f/2. If the aperture is 10mm, the f-stop is one-fifth of "f," or f/5. Notice the "f-number" is the denominator of the fraction-- which means that a higher "f-number" means a smaller aperture diameter.

I'm often guilty of expressing apertures as "f8" or "f5.6" when I really should use "f/8" and "f/5.6."

Why the weird numbers like 2.8, 5.6, etc.? Aperture stops are approximate multiples of the square root of two-- because the amount of light that can get through a lens depends on the AREA of the aperture, and multiplying the diameter by the square root of two will double the area. The usual sequence of apertures (from biggest to smallest on a common SLR lens) is (f/)1.4, 2, 2.8, 4, 5.6, 8, 11, 16, and 22; each step (or stop) lets in half the light of the one before it.

Since the amount of light reaching the film or sensor is a DIRECT function of shutter speed, shutter speeds are approximate multiples of two (1/30, 1/60, 1/125, 1/250, 1/500, etc.). So, having apertures set to double or halve the amount of light getting through the lens makes it very easy to trade off between aperture and shutter speed. For a given amount of light, you can reduce (or increase) the shutter speed and increase (or decrease) the aperture size by the same number of clicks and the exposure will stay the same.
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Old Nov 2, 2004, 11:45 AM   #9
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Big hole= lotta light = fast shot

Small hole = little light = slow shot

This is a technical distillation of the previous replies (HeHe).

Go shoot a bunch and it will become second nature to you !
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