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10-13-2010, 11:54 AM   #1
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Newbie lense question

Can someone please explain the zoom capabilities of dslr lenses. Coming from point and shoot world where zoom is explained as 5x, 10x etc.. how far would a 200mm and 300mm lense zoom (5x, 10x ?) Is there a formula to determine?

Thanks in advance...

10-13-2010, 12:30 PM   #2
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QuoteOriginally posted by xr4kw Quote
Can someone please explain the zoom capabilities of dslr lenses. Coming from point and shoot world where zoom is explained as 5x, 10x etc.. how far would a 200mm and 300mm lense zoom (5x, 10x ?) Is there a formula to determine?

Thanks in advance...
I'll give a quick try, and others can do better later. :-)

Zoom refers to the range between the shortest ("widest") focal length and the longest. So a 5x zoom is five times as long at the long end than at the wide end.

The Pentax kit lens is an 18-55, which is just over 3x zoom. The new Pentax 18-135 is just over a 7x zoom. The Pentax 55-300 is over a 5x zoom, but if you have the 18-55 and the 55-300, you have the equivalent to over a 15x zoom, but you have to change lenses in the near-telephoto range.
10-13-2010, 12:34 PM   #3
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Also note that a 300mm or a 15mm lens is not a zoom lens; they each have a fixed field of view. So if you had just those two lenses, it would be as if you had a 20x zoom but had to be either zoomed out all the way or zoomed in all the way, nothing in-between. Which is one reason some of us prime lens users collect lenses . . .
10-13-2010, 12:35 PM   #4
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First, Welcome Aboard.

Second, forget about 5x 10x etc. That would be best. But if you insist, here is a simple way of thinking about it.

Normal, on a 35mm camera is around 50mm. 200mm would be 4x, 300mm>6x etc.

Normal on a Pentax dSLR is about 35mm so go from there. I don't really know what the P&S cameras use in their multiplication factors.



10-13-2010, 12:40 PM   #5
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your whole definition of zoom using the 'x' factor is wrong. as stated above, Zoom refers to the range between the shortest focal length and the longest, same way it is with P&S's. for example, I own a P&S camera that it's field of view range is equivalent to 25-250mm zoom on an APS-C DSLR (as all Pentax DSLRs are) and my friend has one that it's field of view range is equivalent to 18-216mm zoom on an APS-C DSLR. mine is labeled 10X and his is 12X.
as you can see, the X doesn't tell us nearly as much info as one would need before using a camera.
10-13-2010, 02:21 PM   #6
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In the P&S world, zoom X-factor is important, because sensor sizes and actual lens focal lengths vary in confusing ways. So saying that a lens is 10X is less confusing than giving all the actual numbers. Many P&S's zooms also have numbers giving their 'equivalence' to 35mm lenses, but those are very misleading. The Angle-of-View or Field-of-View (AOV or FOV) may be similar, but the Depth-of-Field (DOF) is very very different.

In the SLR world, sensor sizes and focal lengths are much more standardized. Anyone familiar with a 35mm or APS-C or m4/3 camera knows how different focal lengths look in that camera's format. You'll see references to "crop factor" -- IGNORE THOSE! Crop factor is only significant to someone familiar with one format, who wants to mentally translate a lens's AOV between formats.

Anyway, with SLRs we don't worry much about an X-factor, except that small-X zooms are usually thought to have better image quality (IQ) than large-X zooms. Zooms like 10-20mm, 35-70mm, 75-150mm, are all 2X, but that 2X is pretty meaningless. We talk about the focal lengths, not the X-factor. We don't care that an 18-55mm is 3.06X, that 18-200 is 11.11X and 18-250 is 13.89X. We care that a lens might or might not show softness or distortion at some extreme of its range, that's all.
10-13-2010, 02:51 PM   #7
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Thank you all for the answers, they make sence...

I'm use to my 3.7x Canon point and shoot which my wife always complains about as not having enough "zoom" to pull an image close. I want to be sure I get the appropriate "zoom" capability for my kids sporting events.

This transition over to dslr is still confusing and intimidating but I'm extremely determined to learn. Thanks for the info, great site!
10-13-2010, 08:22 PM   #8
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I'd say think about a 55-300 lens. 300 at the long end should get you pretty close to the action.

This is presuming you'll be at an outdoor event, during the daytime. If you're inside or a night game outside, you'll have a hard time getting good results since the lens is pretty slow.

"slow" versus "fast" refers to the Aperture of the lens, and that's another story.

Perhaps we should have an "Intro to Photography" section or thread or front-page article. A video podcast i look at, the Art of Photography, recently aired an episode that talks about focal length; maybe it will make sense to you: The Art of Photography Podcast (it's episode 43, Choosing the Right Lens)

10-13-2010, 11:03 PM   #9
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Read the manual of your current camera. It will state something about the focal length like 10 - 37mm (37/10 = 3.7x zoom) or you can find it on the camera. It will probably also state something about the 35mm equivalent focal length; if not, you might be able to find it on the net (possibly on Digital Camera Product Reviews: Digital Photography Review). The 35mm equivalent focal length is what is important.

Let's assume that your camera has an equivalent focal length of 30 - 111mm.

Pentax DSLRs use an APSc sensor with a crop factor of 1.5 . So to get the same field-of-view, you need a lens with a focal range of 20 - 74mm.

Because your wife wants to 'pull things closer', you need a so-called 'longer' lens; this translates to a change of the '74mm' number to something higher (e.g. 100mm). It's difficult to say how much higher but if you make it double (150mm) a subject will be twice as big in the picture.

Post the model of your current Canon and the calculations that you've made and we can verify if you're on the right track.
10-13-2010, 11:05 PM   #10
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QuoteOriginally posted by RioRico Quote
In the SLR world, sensor sizes and focal lengths are much more standardized. Anyone familiar with a 35mm or APS-C or m4/3 camera knows how different focal lengths look in that camera's format. You'll see references to "crop factor" -- IGNORE THOSE! Crop factor is only significant to someone familiar with one format, who wants to mentally translate a lens's AOV between formats.
In this case, OP comes from another system (Canon P&S) and therefore can't ignore the crop factor in my opinion.
10-14-2010, 04:05 AM   #11
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QuoteOriginally posted by sterretje Quote
In this case, OP comes from another system (Canon P&S) and therefore can't ignore the crop factor in my opinion.
You're joking, right? Translating the (obscured) focal length of a zoom on a 1/1.7" P&S sensor into APS-C terms is about meaningless. I'll argue that formatfaktor misunderstandings lead many who are new to dSLRs to think that a 100mm lens mounted onto their Kx magically transforms into a 150mm lens. It sure fooled me the first time I put a 400mm lens on a half-frame SLR. Oh wow, a 600mm lens! I quickly learned my error; it's still a 400mm lens, but the image ends are sliced off.

I'll repeat: Crop factor is only significant to someone familiar with one format, who wants to mentally translate a lens's AOV between formats. And maybe not even then. In my film days I worked simultaneously with 135 half- and full-frame, 6x6cm medium format, and 9x12cm large (ha!) format. Not to mention 16mm spy cams. My peers and I didn't apply a formatfaktor calculation to lenses; we just learned what each focal length would do with each format.
10-14-2010, 04:56 AM   #12
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No, I'm not joking but I might be missing your point.

For me to understand your view or to make my point (I'm not sure which one it is):

Let's say that I currently have an Olympus SP500UZ with a 6.3-63mm (35mm equivalent 38-380mm) that allows me to get an elephant occupying the full frame at distance X and 63mm setting.
I want to buy a dSLR and a lens that allows me to do the same (fill the full frame with the same elephant at the same distance).
Which lens would you advise me to buy in combination with, let's say, a Pentax DSLR, to achieve the above?
10-14-2010, 05:07 AM   #13
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QuoteOriginally posted by sterretje Quote
No, I'm not joking but I might be missing your point.

For me to understand your view or to make my point (I'm not sure which one it is):

Let's say that I currently have an Olympus SP500UZ with a 6.3-63mm (35mm equivalent 38-380mm) that allows me to get an elephant occupying the full frame at distance X and 63mm setting.
I want to buy a dSLR and a lens that allows me to do the same (fill the full frame with the same elephant at the same distance).
Which lens would you advise me to buy in combination with, let's say, a Pentax DSLR, to achieve the above?
Obviously, that would be an 18-250 lens (a 25-250 would be more nearly identical, if it existed). But it is equally obvious that the DoF characteristics of the Pentax would differ markedly from the Olympus . . .
10-14-2010, 06:13 AM   #14
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Thanks Impartial. I know that DOF characteristics will change although I've never cared to study the theory behind it.
I'm glad that you come to the same conclusion as me with regards to the 250mm (380/cropfactor).

I however like to see RioRico's thoughts on this because there seems to be a misunderstanding between RioRico and me.
10-14-2010, 07:36 AM   #15
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To the OP: remember, that with the very long focal length, it becomes hard to get clear handheld shots. Any camera movement is magnified, along with the image.

The rule of thumb for an APS-C camera is that the shutter speed should be at least 1.5 times the focal length. That is, a 200mm lens should not be handheld at any shutter speed slower than 1/300 second. SR will give you a couple of stops, to maybe 1/100 second or so, but don't think you can hold it at 1/30 second, even with SR. At that speed, a tripod or some other camera support is needed.

With the shutter speed limitation in mind, and the relatively small aperture size of most affordable long zooms (f/5.6 is common at the long end), you may have to bump the ISO way up to be able to keep the shutter speed up. This can risk introducing electronic noise into the image, although the new cameras, such as the K-r are reportedly much better than cameras of only a few years ago.

One trick I've used successfully is to shoot in RAW, rather than jpeg. RAW images preserve much more data than do jpegs. With this in mind, I sometimes deliberatly underexpose by as much as two stops, to keep the shutter speed up. Then, in post-processing, the image can be lightened without sacrificing image quality. You can't do that with jpeg, or at least not as well. This doesn't work in all cases. A scene with high dynamic range may result in blown highlights with this technique, but if the scene has that much DR, you probably have a problem anyway.
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