What is this focal length thing?

[Mark T]

Why is my Tamron 70-300, when zoomed out, only about 240 mm long?

Where's my missing 60mm?



Edit: added image as requested (good thing that delayed timer shutter release thingy)

[christiand]

Good question, I don't know.
How if you post a picture ?

Cheers
CD

[Photodude]

I have the Tamron as well
Zooms all the way to 300
Guess thats not much help in your case though

[fozzie]

Mark T,

Take off another 70mm, because it is the length of the lens from start to finish and not including the camera body .


Cheerio,

[Matt. K]

Your lens has internal focussing elements and does not need to be 300mm. It is a telephoto lens. Lenses that are the same physical length as their focal length are correctly called long focus lenses...but this is a fairly old and not often used term. A 500mm long focus lens is an awkward beast to hold.

[mudder]

G'day,
Hope you're worried about a small size thing

Cheers,
Mudder

[gstark]

G'Day.

What Matt K said.

Remember that it's the nature of optical glass to introduce disortions into the path. In this case the distortion takes the form of a magnification effect, which can be seen as a telephoto effect as described by Matt.

Fozzie, the focal length as measured does include most of the camera. If you look at top of the camera, from the back, and just to the right of the LCD, you'll see a small Plimsol sort of mark. It's clearly visible in the photo that Marfk posted.

That represents the fcal plane, and on film cameras, it's the location of the film plane. On the D70, it'll be the light sensing surface of the CCD.

This is the plane upon which the lens must focus, and it's the distance to this point that the focal length of any lens refers.

[birddog114]

Sorry to hijack this thread!

Gary,
Where's your first pics from HNL?

[fozzie]

Mark T,

Please have a look at the following for further explanation:

http://www.dpreview.com/learn/?/Glossar ... gth_01.htm


Cheers,

[Matt. K]

To simplify
The focal length refers to the magnifying power of the lens. If you take a magnifying glass and hold it up from your hand and get the smallest sun spot you can...where it starts to burn like hell...then the measurement from the spot to the lens is the focal length.

[gstark]

Sorry to hijack this thread!

Gary,
Where's your first pics from HNL?

In the camera.

[Mark T]

Thank you Matt. K (and others) who helped me on the way to understanding what focal length "really" is.

In my search for enlightenment I have found that the focal length (as Matt. K described) is not necessarily related to the physical length of the lens.

Furthermore, I found out how to measure the focal length of a lens. My Tamron 70-300 is only 293mm max.  PS: for those interested: take a photo of some stars for which you can determine their angular distance apart as viewed from earth, find the length in mm of the distance between these stars on your CCD, divide one into the other, presto - focal length.

for more detailed description see here.

Mark T

[Mark T]

By the way, if anyone would like me to calculate the focal length of their lens, post a high-res image of the stars in the belt of the Orion constellation (include all EXIF info).

Regards

Mark T