18-70 AF-S DX

Why does this lens give 35mm angle of view for specs? The lens is

designed for aps-c sized sensor so it should be marked as 27 to 105 on

the barrel not 18-70

I have lenses for the Olympus Pen FT (half frame) which give their actual focal lengths just like the 18-70 lens.

 

Same applies to the lenses for the Pentax 110 SLR where the 50mm f/2.8 is the telephoto lens.

Peter, I don't think that's such a good idea. It would be even more confusing, cause 'regular' fullframe lenses can be used on digital as well. The "27-105" (now 18-70 DX) would then give a completely different field of view then the 28-105 mounted on a digital SLR.

Factors such as cropping due to the format used and the image circle of a lens don't determine the actual focal length. Nikon does include the effective focal range for most of its lenses in some product catalogs, but this is for convenience.

 

This convention has been standard for many years. When a rollfilm back is used on a 4x5 camera, the focal lengths aren't redefined. Ditto when Pentax 6x7 lenses are cobbled onto a Pentax 35mm SLR or dSLR. Or when the 35mm adapter is used in a Mamiya 7. The 80mm lens in my Yashica 635 TLR is the same whether I shoot 120 or 35mm film with the adapter.

Lenses for any format camera are given as their actual focal length. 100mm is 100mm, the field of view depends on the format, i.e. short telephoto for 35mm, slightly wide angle for a 4x5. Lenses have never been described as their "equivalent focals lengths" in other formats. You have to figure that out for yourself! ;)

but you wouldn't use the dx on a 35mm camera because it vignettes so whats the point of labeling using a 35mm angle of view. a standard 35mm lenses are designed for 35mm cameras it makes sense lable with a 35mm angle of view.

<i>

whats the point of labeling using a 35mm angle of view

</i>

 

<p>

None of my Nikkor lenses are labeled with angles of view. They're all labeled with focal lengths.

</p>

 

<p>

--Bill

</p>

It would make more sense to give a "Field of View" indication - The focal length is the focal length no matter what camera or medium is being used, while the effective field of view can be different depending on the size of the film/sensor it is projected on.

 

Check on http://www.photo.net/learn/fov/ for more info on this.

Peter, the lens isn't labeled using any field of view, just it's actual focal length. Focal length is independent of sensor/film size. It's only when you consider the image circle that the lens projects that sensor/film size becomes important.

 

The 18-70 really has a Focal length of 18mm to 70mm, no matter what camera it's mounted on. The point of including the 35mm equivalent in specs is that many people are used to Focal length in 35mm terms...more people 'know' that 50mm is 'normal' in 35mm than what is 'normal' for other sensor/film sizes.

 

The trick is that it takes an 18mm lens to give the same field of view on aps-c that 27mm gives on 35mm.

As it has been said above a number of times, field of view for a particular focal length depends on format. Don't confuse that with image circle. The same focal length lens can be designed to have a different size image circle to correspond to the format it is to be used with. You couldn't use a small format lens on a 4x5 camera (even if you could mount it) because the image circle would be too small. Same thing between DX lenses and 35mm film camera lenses, although their mounts are the same.

Peter - as others have noted, the focal length is a lens' PHYSICAL PROPERTY... just like your height might be 6 feet and 2 inches -- THAT DOES NOT CHANGE.

 

Due to the sensor size difference, that 18-70mm on a DSLR will now have a diagonal FOV of 77.4 deg to 23.3 deg, the same diagonal FOV of a 27-105mm mounted on a ful-frame 35mm body.

 

It is pointless to change the naming convention to FOV since it is a variable -- yes, Nikon currently only has full-frame (film) and DX (1.5 "crop"), but what if they add a 1.3x or 1.7x crop? What would you call the 18-70mm lens then??

 

The most technically correct and consistent is to call it using its PHYSICAL properties.

 

KL

Hmmm, me things a basic book on photography would be more helpful to Peter than

Nikon normalizing the focal length labels of their DX lenses to the (nearly defunct) 35mm

film format standard.

<p><i>35mm angle of view for specs?</i>

<p>As others said, it's the focal lenght, not the angle of view.

<p>This has been common practice for more than a century in photography. Digital changes

some things but not everything :-)

<br>When you buy a medium-format lens, the focal length is given... as the focal length not

as the angle of view in 35mm equivallent. And a 50mm on a medium format does not give

you the same angle of view as a 50mm on a 35mm lens.

<p>--ben

I can understand why it might seem logical to express focal lengths in terms of their relationship to the film/sensor format.

 

The problem is that this tends to be a 35mm-centric point of view. And some photographers still consider 35mm (let alone APS sized and smaller digital sensors) to be miniature or "puny" format.

 

Forcing relativism onto our expressions instead of using a standard set of conventions for everything just leads to even more disorder and misunderstandings.

 

Or something like that ... ask me again after I've had more coffee. I may be fulla beans.