# Understanding Lens specs for a dummy

Discussion in 'Digital Photography' started by Dave, Nov 11, 2007.

1. ### DaveGuest

I find it very difficult to understand lens specs.
Such as: 500mm lens.
In down to earth plain language, how does this translate into everyday
useful information?
Is this a zoom lens?
How much zoom?

For example:
http://www.usa.canon.com/consumer/c...categoryid=154&modelid=7318#ModelTechSpecsAct
EF 500mm f/4L IS USM
Super Telephoto Lens

Item Code: 2532A002

Specifications

Lens
Focal Length & Maximum Aperture
500mm 1:4.0

Lens Construction
17 elements in 13 groups

Diagonal Angle of View

Inner focusing system with USM

Closest Focusing Distance 4.5m / 14.8 ft.

--------------------------------------------------------------------------------------------------------------------------------------------------------
Anyway, does anyone know of a website that explains lens specs in simple
down to earth, non-esoteric language
that is useful and understandable to a non-professional such as myself? Or
does anyone have simple down to earth
notes to that effect?

---------------------------------------------------------------------------------------------------------------------------------------------------------
I did a test of sorts with my lousy DMC-FZ50 P&S camera. With my camera on
a tripod at a distance of 18.3 ft.
(measured from the front of the lens) to the subject (a foot long ruler
horizontally positioned), I took 3 photos:
1) first with the lens set at 35mm equiv. 2) second with the lens set at
210mm equiv. (6X). 3) Third with the lens set at 420mm equiv..
I measured the width of the ruler in photoshop using pixels as the unit of
measurement
1) first with the lens set at 35mm equiv. measured 206 pixels 2) second with
the lens set at 210mm equiv. (6X). measured 1195 pixels 3) Third with the
lens set at 420mm equiv. measured 2157 pixels.

I don't get how the X value equates to the image size differences, since
1195/206 = 5.8 and 2157/206 = 10.47
Wouldn't the actual X values be 5.8X and 10.47X?

Dave, Nov 11, 2007

2. ### Paul FurmanGuest

500mm is a very long telephoto lens.

f/4 is the max aperture, pretty fast for such a long lens.

L is 'Luxury' built for abuse with the best specs.

IS = Image Stabilized gyro built into the lens for hand held without a
tripod. In-lens stabilizing, makes the viewfinder image stable versus
sensor stabilized.

USM means UltraSonic Motor in the lens (instead of a crank shaft from
the motor in the body) for fast quiet autofocus.
On the lens they write 1:4 or on paper, they write f/4
Hard to get much meaning out of this without a detailed discussion
Inherent in a 500mm lens
This is useful info for doing closeups. Perhaps more useful is the
magnification where 1:1 is a macro lens.
The x value in a zoom lens refers simply to the relative range 35-420 is
420/35=12x this is not like the terminology for binoculars or
telescopes. Magnification in photo gear is usually defined with the
focal length, 500mm in 35mm equivalent field of view (fill frame/film)
or simply the angle of view: 5 degrees diagonal.

Most DSLRs have about a 1.5x crop factor so that 500mm lens looks like a
750mm lens. The angle of view is stated for a full frame camera.

P&S compact cameras have much more depth of field than DSLRs so that's
something which is going to look different at the same 35mm equivalent
focal length.

Paul Furman, Nov 11, 2007

3. ### mianilengGuest

The mm part of a lens spec is the focal length. It's the distance
between the film or sensor and the lens when it is focused on a
very distant object - technically at an infinite distance.

The higher the mm number (long focal length), the bigger the
subject appears in the photo. IOW, a long focal length can pull
in distant objects so that they appear closer.

500mm falls well into the telephoto category - those that make
objects appear substantially closer than when viewed with the
human eye.

However, focal lengths may be given in actual or equivalent
values. Equivalent focal lengths are given in terms of 35mm film
cameras. Most digital cameras have sensors that are physically
smaller than 35mm film, so that they need a shorter focal length
for the same apparent image size. For example, for a particular
camera, a lens with a focal length of 10mm may produce the same
apparent image size as a 35mm film camera with a 60mm lens. This
camera may be cited as having a 60mm (35mm equivalent) lens.

Zooms are a different matter. A zoom lens is one whose focal
length can be varied so that objects can be made to appear closer
or further away.

A typical compact camera may have a zoom range from 35-105mm
equivalent (the actual focal length range may be 6-18mm). The
longest focal length is three times the shortest, and is specced
as having a 3x zoom range. A 3x zoom lens may also be 28-84mm or
40-120mm or any other value with a 3:1 ratio.

mianileng, Nov 11, 2007
4. ### Jürgen ExnerGuest

The focal length is 500mm. The 'normal' lens is generally considered around
50mm(*), so that 500mm lens has a magnification or tele-factor of about 10.
In short: a very powerful tele-lens which typically you won't be able to
hand-hold.

*: important note: the 50mm are for a full-frame SLR. For a DX sensor it
would be smaller by a factor of 1.5 or 1.6, i.e. around 32 to 33mm and this
tele becomes a 15x or 16x tele.
No it isn't. If it were a zoom lens, then both, the minimal and maximal
focal lengths, would be listed, e.g. 200-500mm would be a zoom lens with a
zomm factor of 2.5x where even the shortest lenght would still give you a
magnification of 4x.
The f/4 means that the maximum apperture of this lens is 500/4mm, i.e.
125mm.
In laymen terms: the maximum apperture tells you how much light the lens
will capture and thus indirectly how fast the shutter speed can be.
All lenses with the same apperture can use the same shutter speed given the
same light conditions regardless of their different focal lengths. This is
what makes the apperture information so important.
IS is Canon's designation for lenses with active vibration reduction.
Got that part covered already
The lens has a total number of 17 glass elements, some of them glued/merged
together, such that only 13 individual pieces are actually mounted inside of
the barrel.
The diagonal on the picture will correspond to a 5 degree angle of view in
nature. Again this number is probably for a full-frame and has to be reduced
by 1.5 or 1.6 for a DX sensor.
'Inner focus' means the lens does not extend (i.e. does not change length)
when focussing.
No idea what the USM designation is, I don't use Canon.
You cannot but an object into sharp focus unless it is at least 4.5m away.

For Nikkor lenses there is a good overview of the cryptic designation on
http://bythom.com/lensacronyms.htm. I suppose something similiar may exist
for Canon, too.

jue

Jürgen Exner, Nov 11, 2007
5. ### Jürgen ExnerGuest

Well, sorry, no. The number on the lens is _always_ the actual focal lenght.
There is nothing like an equivalent focal lenght for a lens. Putting it the
way you did it just confusing.
Well, poor terminology. Not the lens is equivalent (much less the 35mm) but
it's observable behaviour. Better to say
On this camera this lens has an angle of view/magnification factor/behaviour
that is equivalent to a 60mm lens on a full-frame camera.

jue

Jürgen Exner, Nov 11, 2007
6. ### Don Stauffer in MinnesotaGuest

The 500 mm figure is the focal length. The image size of a distant
object is directly proportional to the focal length. On a 35mm camera
or most digicams 500mm would be thought of as a telephoto, these
days. The name used to mean something else, but now means a long lens
meant for "blowing up" size on a distant object. Note that on a large
camera, such as an 8 x 10 film or view camera that would only be a
mild telephoto.

f/# is the maximum aperture diameter divided into the focal length.
So the actual diameter of a 500 mm f/4 would be 125mm.

17 elements means there are 17 seperate pieces of glass, most cemented
together but with air gaps between two of threee sections (each
section is several elements glued together.

Don Stauffer in Minnesota, Nov 11, 2007
7. ### mianilengGuest

I don't know what point you're trying to make here. I tried to
explain the difference between actual and "35mm equivalent"
focal lengths as the latter is often - perhaps *more* often -
cited in camera specs, especially with P&S cameras. The
terminology is not mine. It is well established in the trade a
well as in the field.

mianileng, Nov 11, 2007
8. ### mianilengGuest

I somehow missed seeing the rest of the OP's post after the
part I cited. Otherwise I would have concentrated more on
explaining the figures and terms in the rest of the message,
as has been done by others.

In any case, the zoom ring on my FZ30 is graduated in 35mm
equivalent values, and I believe it's the same on the OP's FZ50.
"35mm equiv. 35-420" is also printed on the side. The actual
focal length is printed on the front of the lens where it is
much less conspicuous, and much less relevant for most users.

There are so many different sensor sizes, with different
results obtained with the same focal lengths, or, conversely,
different focal lengths are required to get the same apparent
image size. Therefore, it is often more convenient, and less
confusing, to cite "35mm equivalent" focal lengths.

mianileng, Nov 11, 2007
9. ### RonGuest

We need to get away from referring Lens length to film cameras. You re
referencing something they have never used. Be it a 35mm, 6x7cm or 4x5"
camera its irrelevant most new photographer have never used any of these
camera so they don't have a clue what you are talking about.

Explain it in terms of Magnification like 15x magnification of a distance
object when used with a Cannon camera.

Ron, Nov 11, 2007
10. ### Jürgen ExnerGuest

I do not know if this terminology is established or not. My point is, that
it is missleading at best.

A single lens can have many different "35mm equivalent" focal lenghts,
depending upon which camera it is used with. For arguments sake let's take
e.g. a Sigma 200mm.
On a full-frame camera it behaves like a 200mm lens, on a Nikon DX like a
300mm lens, and on a Cannon DX like a 320mm lens. Therefore it is just
meaningless to assign _a_single_ "35mm equivalent" number to this lens.

Same with the 500mm UMS IS the OP cited except that mounting it on anything
but Canon is a bit more involved.

jue

Jürgen Exner, Nov 11, 2007
11. ### mianilengGuest

How big an object appears to be in the image depends on the
ratio between lens-to-subject distance and sensor-to-lens
distance.

The focal lengths given are the distances between lens and
sensor for subjects at an infinite distance. When the subject
is closer than infinity, the lens has to move away from the
sensor to maintain proper focus. The movement will be more for
longer focal lengths. This changes the magnification factor.
Your 18.3 ft distance is much less than even an approximation
of infinity.

Again, even when the "actual" focal length is cited (7.4-88.8mm
in your FZ50), this is still an equivalent number. A practical
camera lens is made up of many separate lenses of different
focal lengths, and their combination gives a single equivalent
focal length, with an equivalent single, virtual lens
positioned somewhere on the axis.

A lens with multiple elements can be more compact than a single
lens of the same focal length, though this is not the only
reason for using multiple elements.

The Canon lens you're interested in is 15.2" long. If this
were a single lens, it would have to be almost 20" in front of
the sensor when it's focused at infinity. To focus at 18.3 ft,
the lens would have to move outwards by about 2", making it
necessary for the whole structure to be nearly 2 ft long.

If such a single lens were actually used, then the focal length
and the sensor-to-lens distance will be the actual physical
values (but the optical quality will be poor).

mianileng, Nov 11, 2007
12. ### Jürgen ExnerGuest

[I missed the second part of your post below those awfully long horizontal
lines at first]
That would be the front focal lenght. Distance for photographic lenses is
measured at the rear principal plane which is typically somewhere near the
end of the lens. See http://en.wikipedia.org/wiki/Focal_length for details.
Forget about "real" focal lenght or "equiv" focal lenght, it is not relevant
in this context.
I would say those values are amazingly close to what you would expect.
35mm being your base unit, then 210mm is a magnification of 6 times (as you
noticed already) and 420 a magnification of 12 times.
Well, yes, but:
- there is always some inaccuracy in us more mortals doing simple
experiments like that
- you were using the front focal point
Given that you were measuring from the front of the lens instead of from the
pincipal plane your numbers match up _very_ nicely.

jue

Jürgen Exner, Nov 11, 2007
13. ### Jürgen ExnerGuest

I totally agree. The notion of "35mm equivalent" is antiquated and
Well, I'm afraid unfortunately it's not that easy because magnification is
not an inherent property of the lens alone. So saying 15x on Canon is wrong
because it can be correct only for either the full-frame or the DX, but not
for both.

Listing all the combinations of a particular lens with all possible bodies
is just not feasible, either. Who would update the specs and information for
a lens every single time a new body is released?

Actually the focal length of a lens is the only inherent property of a lens
you can go by. And if people would just learn the "normal" focal distance of
their camera (50mm for full-frame, 33mm for Nikon DX, 32mm for Canon DX,
....) then all the rest like magnifaction factor etc. of lens-body
combinations becomes trivial to calculate.

jue

Jürgen Exner, Nov 11, 2007
14. ### Marty FremenGuest

Magnification WRT to what though? You have to define what 1x magnification
is. With things like binoculars it's easy because you're talking about the
view through the eyepiece, which you compare to the naked eye view, but
this is irrelevant with a camera (and indeed meaningless if you don't have
an optical viewfinder).

The only objective and sensor-independent way of describing the field of
view you get with a lens is the actual angle of view. But whilst this is
fine for medium to wideangle optics, people have difficulty visualising
things like a 1° or 5° angle of view. Nevertheless if manufacturers would
switch to it people would start to get a feel for the figures I think.

The magnifying ability of a lens is not quite the same thing as field of
view (though in practice the two tend to be closely related). Ultimately it
comes down to its angular resolving power which determines how big you can
make the image when viewing it. For instance I was just comaring lens tests
for a camera with a 35-350mm lens and one with a 35-390mm lens. Now you
would think the latter would give you 10% more magnification, but its lens
was slightly softer with 10% less resolution, so in the end they came out
the same in terms of the degree of magnification that was achievable in the
final image. Indeed the 350mm could end up the better bet, since the
shorter focal length would give a slightly wider field of view at maximum
magnification.

Marty Fremen, Nov 11, 2007
15. ### mianilengGuest

I agree that the present situation can be confusing to someone
who's not familiar with the pre-digital era.

But "15x magnification" relative to what ? The term "15x
magnification" by itself, even when applied to a particular
camera, is meaningless.

Even the actual focal length or magnification ratio has to be
related to *something* in order to have an idea of how it will
perform. If you tell a complete novice that model X comes with
an 18-54mm kit lens, he will still have no idea what to expect
in terms of image size even if he understands that the zoom
range is 3x. After using that lens for some time, he will know
what to expect from a 70-135mm lens on the same camera.

The point is that everything is relative. But there's no
universal standard to relate to, except the 35mm film camera,
however unsatisfactory that might be.

mianileng, Nov 11, 2007
16. ### DaveGuest

Thanks Paul.

Dave, Nov 13, 2007
17. ### DaveGuest

Thanks.

Dave, Nov 13, 2007
18. ### DaveGuest

I somehow missed seeing the rest of the OP's post after the
part I cited. Otherwise I would have concentrated more on
explaining the figures and terms in the rest of the message,
as has been done by others.

In any case, the zoom ring on my FZ30 is graduated in 35mm
equivalent values, and I believe it's the same on the OP's FZ50.
"35mm equiv. 35-420" is also printed on the side. The actual
focal length is printed on the front of the lens where it is
much less conspicuous, and much less relevant for most users.
Yes, what you are saying about the numbers on the camera itself is exactly
right:
35mm equiv. 35-420" is also printed on the side (rotating adjustment ring)
and
7.4-88.8 ASPH on the front of the lens.
Thanks for taking the time and trouble!

There are so many different sensor sizes, with different
results obtained with the same focal lengths, or, conversely,
different focal lengths are required to get the same apparent
image size. Therefore, it is often more convenient, and less
confusing, to cite "35mm equivalent" focal lengths.

Dave, Nov 13, 2007
19. ### DaveGuest

So, in actuality would the 200-500mm lens would really be a 4x-10x zoom
range lens?

Dave, Nov 13, 2007
20. ### Jürgen ExnerGuest

Hmmmm, well, I guess. That 200-500mm lens has a zoom factor of 2.5x
(500/200) and on a _on_a_full_frame_(!!!) camera a magnification factor of
4x to 10x. If that's what you mean with zoom range, then yes.
Normally people would call the 200-500mm the zoom range of this lens,
because the zoom range is independant of the camera while the magnification
factor is not.

jue

Jürgen Exner, Nov 13, 2007