what does the lens length tell you?

It's the focal lenght of the lens. The focal length an image size are directly proportional, so a 500mm lens will give you a 10x larger image than a 50mm lens.
 
vaizki said:
It's the focal lenght of the lens. The focal length an image size are directly proportional, so a 500mm lens will give you a 10x larger image than a 50mm lens.
Click to expand...

Not quite .... image size will stay the same :cool:
 
To put it another way... focal length is a measure of how 'zoomed-in' the lens is. If it's a 200mm lens, it'll get closeups of faraway things, and if it's a 20mm lens, it's a wide lens for fat people standing right in front of you.

A 'zoom lens' is one where you can change the focal length, i.e. zoom in and out, by turning or pulling a ring on the lens. Accordingly, a zoom lens will be called by its range of focal lengths it can offer: '30-300mm'.
 
Higher the number, the more zoomed in it will be. However there's way more difference between 10mm and 20mm compared to 400mm and 500mm for example. The bigger the number the less difference it makes. Try it with your zoom lens. start at the wide angle and go to the other end. You'll see the difference you're making to what you see through your viewfinder diminish as you go.
 
Macro lenses can be wide or long (short or long focal length) - the clever thing about macro lenses is that they can focus on things really close to them, where most lenses can only focus on anything between a few feet away and the far distance (infinity).

As it happens, most macro lenses are fairly long, because that way they allow you to do close-ups without being TOO close to the bee or the flower you're shooting.
 
Here's some nomenclature for you: a lens where the focal length is always one number (say 50mm) is called a prime lens. If you want to get less or more in your picture then you have to walk backwards or forwards. A zoom lens does the walking backwards and forwards for you, if you like!

(The effect of zooming in is actually subtly different from the effect of walking closer, because the viewpoint changes so the things behind the subject will look different, but that's a technicality for now.)
 
mm = millimetres.
 
The focal length in mm is just a number; there's nothing actually physically that length in the lens or anywhere else. It's just a way of describing the lens mathematically. Just learn roughly what different focal lengths mean in real life terms: e.g. 10mm is really really wide, 20 is wide, 30 is sort of normal, 50-70 is portrait, 70 up is telephoto, more than 300mm is for bird-spotting geeks.

(If anyone talks about '35mm equivalent' or 'full-frame equivalent', don't take any notice. All it means is that focal lengths have a slightly different effect on film cameras than they do on your D200 and our mainstream DSLRs, so people used to film cameras think of 30mm as being wide, not 20.)
 
Llama said:
:|


kk ty i was the answer i was looking for, seems odd to use mm if there is no distance being referd to though ?
Click to expand...

It is referring to a distance, but it's a theoretical distance that you'd need a physics lab and ray beams to measure. :)
 
focal length.

Basically, it's the distance in which the lens focusses the rays of light.

A longer focal length (but NOT always a longer lens - see 70-200 vs 70-300 for instance) means a higher magnification of faraway objects.

http://en.wikipedia.org/wiki/Focal_length
 
The focal length is the magnification of the lens - the size of the image it projects. It is measured from the nodal point (the 'optical center') to the sensor, when the lens is focused on infinity.

The nodal point of a simple one-element lens (eg magnifying glass) is also the physical centre, but modern camera lenses are far more complex and the nodal point of most wide angles for example is actually outside the physical lens. That is known as a retrofocus or inverted-telephoto design, which is necessary because the distance from the lens to the sensor has to be greater than the focal length in order to clear the mirror. Eg lens mount to sensor distance on a Canon DSLR is 44mm, so any lens with a shorter focal length than that has to be retrofocus design.

Here is a good focal length comparison tool from Tamron. Give it a moment to load and then input the sensor size, ie crop format or full frame. The format affects the field of view even though it looks like it affects the focal length. That's the crop vs full frame 'equivalent' focal length thing that confuses the heck out of some people, but that a different subject ;) http://www.tamron.com/lenses/learning_center/tools/focal-length-comparison.php
 
HoppyUK said:
That's the crop vs full frame 'equivalent' focal length thing that confuses the heck out of some people, but that a different subject
Click to expand...

I understand it perfectly and it still manages to confuse the hell out of me.
 
The "optical center" is a mathematically calculated equivalent to having a single simple optical element inside your lens. Lens designers use different shaped elements and exotic materials (high refraction etc) to achieve the same optical "formula" (and to control flare, CA, distortions etc).

For example my 70-200/2.8L IS II has 23 elements (pieces of optical glass) in 19 groups (meaning some of those pieces are "glued together").
 
then the magification of the image is directly proportional to the ratio of the focal length and the length of the diagonal of the sensitive material.

e.g. 35mm film has a diagonal of 43mm - so a standard lens is 50mm (nearest lens size to the diagonal). 100mm gives twice the maginfication in terms of what fist on the sensitive material.

A crop sensor has a diagonal of 27mm - so 28mm is often said to be standard as they give the same image in terms of perspective and what is visible on the sensor. This leads to the crop factor which is 1.6 for Canon cameras and the often misquoted idea of a 250mm lens being the equivalent of a 400mm lens of a full frame camera.

Cheers,

Graham
 
grahamsewelluk said:
then the magification of the image is directly proportional to the ratio of the focal length and the length of the diagonal of the sensitive material.

e.g. 35mm film has a diagonal of 43mm - so a standard lens is 50mm (nearest lens size to the diagonal). 100mm gives twice the maginfication in terms of what fist on the sensitive material.

A crop sensor has a diagonal of 27mm - so 28mm is often said to be standard as they give the same image in terms of perspective and what is visible on the sensor. This leads to the crop factor which is 1.6 for Canon cameras and the often misquoted idea of a 250mm lens being the equivalent of a 400mm lens of a full frame camera.

Cheers,

Graham
Click to expand...

I think I know what you're saying here, but focal length and image size is entirely independent of format. The length of the diagonal is just a shorthand way of establishing roughly what focal length will give a similar field of view on different format cameras. It's only rough because the shape of the format affects the distance, eg square vs rectangular.

Either way, the image projected by a 100mm lens has the same physical dimensions whether it's on crop format, or full frame or medium format or whatever.

The format just dictates how much of the image circle projected by the lens is recorded.
 
trencheel303 said:
How does that work then? :thinking:
Click to expand...

As Jukka said.

BTW, it is the retrofocus section of wide angles on DSLRs which adds considerably to their complexity, and in theory, this leads to a slight reduction in performance.

This is the reasoning behind claims that wide angles for Leica cameras are inherently superior, because they are not constrained by the mirror getting in the way and the retrofocus design that this dictates. In the same way, this also applies to the new mirrorless interchangeable lens cameras (MILC - like the Pano GF1).
 
HoppyUK said:
I think I know what you're saying here, but focal length and image size is entirely independent of format. The length of the diagonal is just a shorthand way of establishing roughly what focal length will give a similar field of view on different format cameras. It's only rough because the shape of the format affects the distance, eg square vs rectangular.

Either way, the image projected by a 100mm lens has the same physical dimensions whether it's on crop format, or full frame or medium format or whatever.

The format just dictates how much of the image circle projected by the lens is recorded.
Click to expand...

Hoppy,

What I was trying to say was that a "standard" lens to use an old fashioned name, projects an image not dissimilar to what is perceived by a persons normal vision it terms of what is senn and perspective. If the sensor is smaller, then the focal length decrease to fit the same image onto the smaller sensor.

This works for all but fisheye lenses (probably). However, I also believe that although the image may have the same perspective, the other properties of the lens such as depth of field and hyperfocal distance do not change as significantly - but he do change due to the smaller circle of confusion peresnt on the smaller sensor.
 
grahamsewelluk said:
Hoppy,

What I was trying to say was that a "standard" lens to use an old fashioned name, projects an image not dissimilar to what is perceived by a persons normal vision it terms of what is senn and perspective. If the sensor is smaller, then the focal length decrease to fit the same image onto the smaller sensor.

This works for all but fisheye lenses (probably). However, I also believe that although the image may have the same perspective, the other properties of the lens such as depth of field and hyperfocal distance do not change as significantly - but he do change due to the smaller circle of confusion peresnt on the smaller sensor.
Click to expand...

Maybe I'm not reading that properly but it doesn't seem quite right.

Perspective is a function of distance, and nothing else. The only similarity between our eyes and a 'standard' lens (with a field of view roughly around 45 degrees) is that with a lens like that the way we use them tends to result in an image that is framed from such a distance that often looks fairly normal. So if you just raise the camera to your eye, Cartier-Bresson style, the perspective looks natural.

This might actually be what you're saying, so please accept my apologies if so ;)

Trying to compare human vision to a camera is difficult, as while there are obviously similarities, trying to compare a moving 3D scanning device with 180 degrees field of view (which only ever looks directly at a two degree spot at any one time) doesn't bear a very close relationship to a still camera image.

Depth of field depends on f/number and magnification. Magnification is a combination of focal length, distance and sensor size. Is that what you're saying also?
 
hehe it's funny sometimes how what appears to be a simple and basic question can often unravel and command a long and complex answer!
 
gman said:
hehe it's funny sometimes how what appears to be a simple and basic question can often unravel and command a long and complex answer!
Click to expand...

Haha yes! The OP's question was actually correctly answered in post #2. Then it all went wrong... :nono:
 
Chaz Photos said:
i think the right answer is:-
the length of a lens affect the angle of view.
Shot length ie 10mm will give a very wide angle of view and a 400mm will give a very narrow filed of view
Click to expand...

A 10mm lens is quite 'long' on a compact. A 300mm lens is 'standard' on a 10x8in camera, approx 45 degrees angle of view.

Field of view is a combination of focal length and format size.
 
HoppyUK said:
A 10mm lens is quite 'long' on a compact. A 300mm lens is 'standard' on a 10x8in camera, approx 45 degrees angle of view.

Field of view is a combination of focal length and format size.
Click to expand...
it is all relative on the same cAMERA WHAT I PUT IS CORRECT
 
ShakyUK said:
Thanks for that link, I found that pretty informative, especially the difference in shot between the large sensor of the 1D and the smaller sensor of the 400D, at least I think that's what makes the difference in image size.
Click to expand...

You're welcome bud, yes it's a good comparitor.

And quite right, it is the format change that alters the field of view with the crop vs full frame camera, when used with the same focal length lens. Which was the point I was trying to make in post #31 above!
 
You must log in or register to reply here.
Back
Top