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Old Jul 11, 2004, 5:06 PM   #1
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Hi all,

Can someone please help me with the following questions (or point me to a link that may help)? I am looking at buying a DSLR and are wonderring about lenses. My introduction to photography has been through point and shoot cameras and I am a little confused on a few things.

First, I am trying to understand the relationship between focul length of a lense and zoom.
I have read that a 50mm focul lenth on a 35mm film is similar to what the eye sees. Does this also mean that this is 1X zoom?
If a buy a lense with a max focul length of 100mm can I calculate the zoom factor?
And say for example the zoom is 3X does this mean that the object I am taking a picture of appears three times as large?

Second, with the mulplier for a DSLR I read an article at athttp://www.luminous-landscape.com/tu...dslr-mag.shtml about the Focul Length Multiplier.

It comments that about a crop of the resulting image for a given lens when taken on a DSLR compared to 35mm: "...the imaging area is physically smaller. Less of the image circle projected by the lens is used, therefore it is a crop. The image remains the same size at the film plane for a given lens and subject distance – it is in no way magnified. It does, however, take up a larger proportion of the (smaller) frame and so it is easy to see why some people call it a magnifying effect. This is also why a tele lens appears so much more powerful – the field or angle of view has been reduced."

What does this mean for calulation zoom of a lense? Is it that the actual telephoto capability is not increased when using a lens on a 35mm camera vs a digital SLR? What is the effect on wideangel?

Third, does wide angle imply less than 1X zoom? Is a lens with a focul length of say 30mm wide angel (because it is less than the 50mm equivalent of the eye?

Thanks anyone for the help. I know I have probably messed up here with my questions and confused many things so if someone can set me straight that would be great.

cheers, Tim

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Old Jul 11, 2004, 5:57 PM   #2
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Ok, lets see if I can explain this without getting complicated.

In 35mm film, a 50mm lens captures enough scenery to mimic what we normally see with our own eyes. That is why 50mm lenses are called "normal lenses", because their field of view is normal to what we see.

So let us start with the humble 50mm and use that as our starting point. Lower focal ranges are considered wide angles. The lower the focal length, the more the lens will see, thus you get an increased field of view. The ultimate wide angle lens iscalled a circular fish-eye lens. Regardless, all wide angle lenses will change the aspect of a photograph. For instance, if you take wide angle in the 28mm range, and take a closeup of a friend, you will note some distortions. This is the fun of wide-angles lenses. The greater the field of view, the more distortion you will get.

Anything above a normal lens is considered a telephoto lens. Telephoto means magnification. You also get different ranges of telephoto lenses. A good rule of thumb is that the higher the focal length, the shallower your depth of field will be, and the greater your need for a heavy tripod support. For example, ages ago, the standard was a 70-210mm zoom range, and everyone owned one of these. It gave you a low to medium telephoto range. If your looking to take photos of birds and wildlife, then you will need to look at something that starts at 500mm and up. Lenses are also rated by how much light they allow in. Faster lenses with smaller "f" numbers can decrease your depth of field, and allow greater leeway in shutter speed in low light conditions. Take a look at the wildlife forums for some awesome photos that ultra telephoto lenses are capable of.

Now as you stated, there is a cropping factor with most DSLRs. This is good and bad. The good part is you get a nice increase in your focal length. Consider this as a bonus. The bad part is this; you will not be able to get into the ultra-wide angles with DSLRs because your working against the cropping factor.

For example, I will use a Canon Digital Rebel because I own one and am most familiar with it. the DRebel has a cropping factor of 1.6. This means any lens I put on it, I will have to multiply the lens's focal length by a factor of 1.6. A 100-300mm focal length lens (on a 35mm camera) now turns into a 160-480mm lens. You can see how ultra wide angle photography is difficult with DSLRS. For example, a 16mm ultra wide now turns into a 29mm wide angle lens (not so great as that expensive 16mm could get you with 35mm film. Likewise, lets say you already have a 500mm lens, and you want to use it with your DSLR, it now turns into a 800mm beast that is very touchy to vibration. This can be a good or bad thing depending upon how you look at it. The nice thing, is that your f stop or speed of the lens doesn't change. If it was f5 at 500mm, then it remains f5 at 800mm on a DSLR with a cropping factor of 1.6.

Now depending upon the DSLR, the cropping factor will differ. The most expensive DSLRs don't have a cropping factor as their sensors are full size.

Now zoom lenses allow you to change the focal length to frame your photo. Prime lenses (any lens with only 1 focal length) often tend to be faster and sharper because there is less of a compromise in their construction. Zooms however give you flexibility. Now, any lens that offers you the ability to change focal length is called a zoom. Note that if the focal length is greater then 50mm, it is considered a telephoto zoom. If the focal range is lower then 50mm, it is considered a wide angle zoom. Now if the focal range brackets 50mm, then it can be considered wide angle to telephoto zoom.

Examples of zoom lenses are: 17-40mm, 70-200mm. 75-300mm, etc.

Examples of prime lenses are: 16mm, 28mm, 50mm, 100mm, 600mm, etc

Note that zoom lens and prime lenses span a large range of focal lengths.

I can get into more detail..but I hope this answers some of your questions.

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Old Jul 11, 2004, 7:28 PM   #3
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With digital cameras 10X usually refers to the widest divided into the longest zoom. So a 38 to 380 zoom is considered 10X. This doesn't take "normal" into consideration.

An actual 1:1 lens is 57mm. Minolta 40 or so years ago used 57mm as their normal lens for that reason but it never caught on. 50mm is a better normal lens anyway IMO. If you want actual power that would relate to say binoculars or telescopes divide the 35mm equivalent focal length by 57. Unless binoculars cheat and use the inaccurate 50mm as normal.

covered the conversion multiplier well. The only thing I would add is that you are buying more glass than you need if there is a conversion factor. As you point out you are effectively cropping out the image by having a smaller sensor than the lens is designed for. You get a more compact lens if you get one designed for your sensor size.
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Old Jul 11, 2004, 8:43 PM   #4
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slipe wrote:

With digital cameras 10X usually refers to the widest divided into the longest zoom. So a 38 to 380 zoom is considered 10X. This doesn't take "normal" into consideration.
Yup, for instance, a 38-380mm, a 42-420mm, and a 28-280mm are ALL 10x zoom cameras.

The "nX zoom" factor is more of a video term as before digicams you'd see it only used on video cameras...and as the digicam is a combination of the two twchnologies (film and CCD) you see things from both types of cameras on the digicam.
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Old Jul 11, 2004, 9:57 PM   #5
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Ignore the zoom multiplier, it really has no information compared to refering to something like 42-127mm as a specification. That is a 3x zoom ratio. The same ratio as 450-1350 which is a very different camera.
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Old Jul 12, 2004, 10:27 AM   #6
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thanks everyone for the detailed answers. especially chako. Very much appreciated. It has cleared things considerably and I realise I have been thinking about optical zoom and focul length all wrong... which is kinda funny.

One extra little question related to the effect of a teleconvertor: If I have a 34 - 136mm lense and add a teleconvertor 1.7X does this turn the lense into a 57,8 - 231,2 lense? Are teleconvertors to be avoided?

Thanks again. Tim

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Old Jul 12, 2004, 11:36 AM   #7
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timmyB wrote:
One extra little question related to the effect of a teleconvertor: If I have a 34 - 136mm lense and add a teleconvertor 1.7X does this turn the lense into a 57,8 - 231,2 lense? Are teleconvertors to be avoided?

Yes, a 1.7x Teleconverter would make a 34-136mm lens, appear to be a 57.8 - 231.2mm lens from a focal length perspective. However keep Chako's post in mind, too (to make sure you don't already have a focal length multiplier from cropping).

For example: a DSLR like the Canon Digital Rebel (EOS-300D), or Nikon D70, have crop factors (i.e., focal length multipliers) of 1.6x and 1.5x respectfully.

Teleconverters do have their downsides. One is vignetting (darkening of corners), which tends to be much worse as greater multiplifaction factors are used. One other downside, is that teleconverters can impact the light gathering performance of your lens (sometimes reducing it by as much as 1 or 2 stops), requring you to use slower shutter speeds for the same lighting conditions, compared to the lens without the teleconverter installed.

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Old Jul 12, 2004, 11:43 AM   #8
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Tim -

Converters work exactly the way you describe, whether it be a tele- (as in your example) or wide-angle. They are, in my opinion, nice adds to your photo kit bag, especially if you have a "permanently attached" (non-removable) zoom lens. I don't know if there are any converters for DLSRs (which you mentioned in your first message.)

A couple of notes on converters - you usually don't get a "useable" full range of zoom when you use a converter lens. In most case, for example with a tele-converter, you are likely to encounter significant to mild vignetting (tunnel vision is what I call it) as you move from "wide angle" towards the telephoto end of your camera's zoom range. With my C-730 and a Oly B-300 tele-converter, I get vignetting out to about 60 % of the zoom range. After that, I get a fully usable frame.

Second, don't be lulled into some of the cheaper lens options available out on the 'net. Olympus, Raynox, and Sony are several quality brand names. Avoid anything that has "titanium" in its name. It's usually a case of you get what you pay for.

Hope this helps.

Paul in NoVA
C-730 B-300 WCON-07
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Old Jul 12, 2004, 2:30 PM   #9
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ok thanks again everyone for the help. very nice people in these forums :-)

cheers, Tim
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