Zoom and telephoto lens

[boris_activia]

<p >I still use my kit lens 18-55mm and looking at purchasing more lenses, however I'm confused what’s the difference between a zoom and telephoto lens?</p>

<p > </p>

<p >I'm getting the grasp of Focal lengths am I right in saying my 18-55mm lens has a focal length of 88mm? After reading in my manual the crop factor has something to do with focal length and as my camera is canon 450 my crop factor is 1.6.<br>

 

 

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[john_a5]

<p>Crop factors are actually just a way for us all to talk about how a lens will work on a given 35mm format. Your 18-55mm lens has a focal length of 18-55mm! It is just that it would be EQUIVALENT to say an 85mm or so lens, set at 55mm, on a full frame 35mm. Equivalent as to the field of view or what the sensor will capture standing at the same spot.</p>

<p>Telephoto lenses are simply lenses designed to allow you to use a longer focal length on a fixed camera, but a longer focal length, say a 300mm is not necessarily a telephoto lens--it is how the optics are designed. For instance, a 300mm lens on an 8x10 camera is neither a telephoto lens nor is it considered a long lens, it is in fact a normal lens. In order for a 35mm camera, which has a fixed distance between the lens mount and the film plane to use long lenses and not end up with extremely (physically) long lenses, they use an optical design that is telephoto. A 300mm lens constructed as a normal lens requires 12 inches of space between the camera focal plane and the center of the lens optics for infinity and even more for closer focus--24 inches for a 1:1 magnification!</p>

<p>Zoom lenses are just lenses that generally cover a wider range of focal length equivalents and may use wide angle or telephoto design. But a "Telephoto" lens is generally termed such if it has one focal length or you would refer to the zoom as a telephoto zoom--like a 100-300mm on a 35mm camera.</p>

[James G. Dainis]

A zoom has variable focal lengths, 18-55mm, 70-200mm, etc. A telephoto has only one focal length, 200mm, 300mm etc. Telephoto also designates a longer than normal (50mm) lens.<P>

 

The 18-55 lens on the Canon 450 has a 35mm camera equivalent of 29 - 88mm due to the 1.6 cropping factor. The entire lens image circle doesn't fall on the camera sensor as it would with a 35mm film frame..<P>

<center><img src="http://jdainis.com/tree3.jpg"></center>

[boris_activia]

<p>So a telephoto lens can belong to the prime lens family and the the zoom lens family?</p>

[tommyinca]

<p>c. all of above.</p>

<p>ie. Telephone prime lens like a 300mm f4. or Telephoto zoom lens like a 70-300mm f4.5-5.6.</p>

 

[phule]

<p>A "prime lens" (generally speaking) is simply a lens with a single focal length. A better term would be fixed-focal-length. So a telephoto is just a sub-set of prime (or fixed-focal-length) lenses.</p>

<p>There are telephoto zoom lenses, of course. A telephoto zoom being a zoom lens that has focal lengths in the telephoto range.</p>

[geoffs1]

<p>Apologies in advance for the pedantry. Just a quick correction. <em>Telephoto </em> is commonly used to mean a lens with a field-of-view narrower normal human vision (i.e. a focal length longer than the diagonal of the sensor/film). </p>

<p>More correctly, a telephoto lens is one with a focal-length longer than the physical length of the lens. Wikipedia has a pretty good discussion: http://en.wikipedia.org/wiki/Telephoto_lens</p>

<p>For example, the length of a simple (or ideal) lens is exactly the focal-length (at least at infinity focus). A telephoto lens would be shorter than that. </p>

<p>FWIW, there is also an <em>inverted telephoto</em> lens design (aka <em>retrofocus</em> ) that is used in many SLR wide-angle lenses to move the optical-elements forward (i.e. longer than the focal-length) to clear the swinging mirror: http://en.wikipedia.org/wiki/Ang%C3%A9nieux_retrofocus</p>

[andrewg_ny]

<p>To put another way--for common usage of the terms:<br>

Angle of view: Wide angle > normal/standard > telephoto<br>

Prime = fixed/single focal length<br>

Zoom = adjustable/variable focal length</p>

<p>The confusion comes because sometimes people say "zoom into" to refer to using a longer focal length telephoto lens to use a narrower, more selective angle of view. </p>

<p>The meaning of 'magnification' can also be a source of confusion. In common usage, some might say that a telephoto will 'magnify' distant objects. But in other photographic usage magnification refers to the relative size of the subject to the recording medium (film or digital sensor) - in this respect, a macro lens that allows very close focus produces much greater magnification than a telephoto lens that may not allow focusing on subjects closer than 1.5 meters.</p>

<p>As for what sort of telephoto zoom you might want for your Canon 450D (without knowing more specifically what you're hoping to photograph), the first one I would consider is the Canon EF-S 55-250 f/4-5.6 IS. A good, reasonably-priced, general-purpose telephoto zoom that will partner nicely with your 18-55 kit lens. Significantly longer-focal-length or faster (greater maximum aperture) lenses will be considerably more costly and bulky.</p>

<p> </p>

[boris_activia]

<p>Thanks, so my understanding would be a F/2.8 lens whould have an F/2.8 through all the focal lengths!</p>

[marsha_milne]

<p>I think James had the best way of saying this for a newbie !!!!</p>

[boris_activia]

<p>thanks, I remember watching a programe on Alberto Korda and his famous quote stuck with me “<strong >Forget the camera, forget the lens, forget all of that. </strong><strong >With any four-dollar camera, you can capture the best picture.</strong>” He made Che Guevara a star</p>

[James G. Dainis]

Roland,

 

No. That is why the Canon EF-S 18-55mm is marked f/3.5-5.6. It is f/3.5 at 18mm and f/5.6 at 55mm. More expensive zoom lenses can maintain a constant aperture at all focal lengths but you pay for that.

 

The aperture f/stop is determined by the focal length divided by the aperture opening.

 

100mm focal length/ 25mm aperture opening = f/4

 

Zoom to 200mm

 

200mm focal length/ 25mm aperture opening = f/8

 

To avoid that change in f/stop, more expensive lenses are built so the 25mm aperture opening would open to 50mm as you zoomed.

 

200mm focal length/ 50mm aperture opening = f/4

[john_a5]

<p>"To avoid that change in f/stop, more expensive lenses are built so the 25mm aperture opening would open to 50mm as you zoomed."</p>

<p>Actually, that is not the case at all. It is more that the actual "optical" focal length of the lens design does not change while the elements change position to achieve the effective telephoto focal length we refer to in MM's. Maybe there are some that change, but I haven't seen any myself. The lens design to achieve this consistent aperture is more difficult than a variable aperture lens, thus the extra $$ for them.</p>

[James G. Dainis]

John,

 

You are quite correct. I should have thrown the word "effective" in there. I try to keep things simple and easier to understand when explaining things to new comers.

[john_a5]

<p>James, I understand and I know my first post here was probably very confusing, but hopefully will stimulate some research if the OP is really interested in learning about optics. The problem with simplistic answers that are misleading at best, is that they start many of the misconceptions that people have about photography these days. Soon, these simplistic metaphors are being spouted around as facts and gospel. I think it better to present what is fact and allow the person to be confused and seek clarification if it is important to them. I find that there are so many "old wives tales" spouted about on these sites that have no basis in fact. We should look to curtail these things not promulgate them!</p>

[boris_activia]

<p>Thanks to one and all, I think another way to learn is get out there and experiment and borrow lens to understand how they work.</p>

[mike_hitchen]

<p>Ignore crop factor. For you, it is totally irrelevant. It only comes into play when you want to move between APS-C format (the so-called 'crop sensor') and the 35mm sensor (same size as 35mm film).</p>

<p>Buy a lens, get out there and shoot a lot to learn how its properties affect the image in different circumstances. If you decide to go to 5D or 1D series then the different size sensor will affect the width of picture you can capture with the same lens.</p>