18-70 or 27-105 or 28-105 AHHHH

[dan park]

Alright I'm just plain confused now. I know that there is a 1.5(4)

multiplier for all existing 35mm based nikon F mount lenses on a Nikon

DSLR. OK I get that. So here I am looking to purchase the D70 Kit

lens for my D50. The 18-70 is made specifically for the smaller aps-c

sensor size not the full frame 35mm size. So if it is made for the

aps-c sensor why does the multiplier apply? Strait off the nikon site

I get the following qoute.

 

"A Nikon DX format exclusive lens covering the most frequently used

focal range of 18-70mm (35mm equivalent: 27-105mm/ digital 28-105mm)"

 

I see three focal ranges here

1. 18-70

2. 27-105

3. 28-105

 

Based on this I would think that no matter what F-mount camera you

stick this lens on you would never get 18-70 out of it. Is this true?

 

Another question is if this lens is made specifically for the APS-c

sensor and would give you a focal length of 28-105 on a DSLR then why

not just call it a 28-105? Or is the 18-70 number still based on the

rear elements min/max distance from the film plane?

 

Please someone help me clear this up.

 

Thanks

[uli_theune]

Optically/physically the lens is constructed such that the focal length is 18 to 70 mm - independent of the film/sensor format. Since digital cameras have a smaller sensor the apparent focal length is longer. You see only a small portion of the entire frame, similar to what you would see when using a 27/28-105 lens on a 24x35 mm frame. The difference between 27 and 28 is rather artificial: 18x1.5 = 27, and the closest standard focal length is 28 mm. Anyway, it is rather "psychologial" than a physical difference between 18-70 mm and 28-105 mm ...

[dan park]

Being that the rear element of this lens is just slightly wider than the aps sensor wouldn't that somehow correct for the 1.5 multiplier?

 

Please excuse that I'm just a layman when it comes to optics.

[larrybc]

Focal length isn't related to the film or sensor size. It is only related to the lens parameters. The image circle it produces also isn't part of the focal length. Consider that on a given film format, the image circle of a telephoto is much wider than a wide angle lens.

 

Large format camera users have been used to dealing with this for a long time.

 

 

larsbc

[jbq]

The lens is physically an 18mm lens at the wide end, i.e. (when focused at infinity) for an object that's a small angle alpha off the optical axis (measured in radians), the image is off the center by alpha*18mm. That's pretty much a definition of the focal length.

 

The focal length isn't directly related to the distance from the rear element to the image plane. The size of the image circle isn't related to the diameter of the rear element.

 

The issue of the multiplier comes from the fact that most people think of image angles in terms of focal lengths, not in terms of angles:

 

An 18mm (rectilinear) lens on a 23.7mm sensor covers a horizontal angle of 66.7 degrees - that's a medium-wide angle (horizontal angles between 50 and 80 degrees are considered "wide"). Wider than 80 degrees is "ultra-wide". 30 to 50 degrees is a "normal" lens. 15 to 30 degrees is a "moderately long" lens, 7 to 15 degrees is a "long" lens, narrower than 7 degrees is a "super-long".

 

If people were used to thinking like that, there'd be no discussion about "35mm equivalents", which are actually never entirely equivalent anyway.

 

An 18-70mm on a 23.7mm sensor (assuming that the image circle is large enough) covers horizontal angles between 66.7 and 19.2 degrees, i.e. from the middle of the "wide" to the middle of the "moderately long" angles. Those angles are approximately the same as you would get with a 28-105mm lens on 36mm film (once again assuming that that the image circle is large enough).

[dan park]

So Larry you are saying that the image circle created by the 18-70 is the same whether it is mounted on a 35mm film camera or a DSLR. But since the aps sensor is smaller it pics up less of the circle than the full frame. I've got that.

 

I guess what my question is is why the special digital lenses if they do nothing to correct for the multiplier? Are they cheaper to make because they require less glass (don't have to create a circle that covers the full frame??)? What is the advantage?

 

Thanks

[dan park]

Jean-Baptiste I wish I would have paid more attention in my college level math courses. Wow do you have a diagram cause the words aren't drawing any pictures in my head. :)

 

Thanks

[Ed_Ingold]

Lenses for 35mm and DSLR cameras are customarily specified in the actual focal length, probably because they tend to use the same lenses. The focal length is the distance from the rear node (not element) and the focal plane at infinity. Do some homework - there are some excellent essays on basic photographic optics and nomenclature on Photo.Net.

 

Experienced 35mm shooters tend to visualize their shooting with respect to focal length. It is easy to mentally apply the "cropping factor" of about 1.5 to visuallize the equivalent focal length for this purpose. Point and shoot cameras digital cameras have a much smaller sensor, with many variations. Consequently it is not easy to visualize the effect of a 9-35mm lens (for example) in 35mm terms, so the "equivalent" values are usually given.

 

Nikon DX lenses have a smaller image circle, which will just cover the APS-sized sensor. These lenses are smaller, easier to design and potentially perform better than standard 35mm lenses in a DSLR. The image circle is, roughly, the diameter of the spot of light cast on the focal plane by a lens focused at infinity. It varies with f/stop and for zoom lenses, the focal length.

 

This phenomena is well known to large format photographers. A 150mm lens is available, for example, in two or three different versions with different image circles - normal, wide and super-wide. A 150mm lens with a 170mm image circle is a "normal" lens for a 4x5 camera. A 150mm lens for an 8x10 camera will be a "wide-angle" lens and have an image circle of 250mm to 400mm. The 8x10 version will also be twice as big and 3x as heavy (and 4x as costly).

[dan park]

Ahh I got it. Thanks Edward. The lenses are smaller(always good) and cheaper and easier to manufacture(good as well at least for my wallet). Those are the answers I needed.

 

Thanks all.

[yeux tortu]

Just remember when you are buying your Nikkors that the DX format will not work well with film cameras or full frame DSLRS of the (not too distant) future.

[greglyon]

Just to clarify (hopefully not confuse) a point. The term 'multiplier' is part of what muddies the waters here. Really something like 'crop factor' would be more useful. The Nikon DSLRs could be considered to be cropping out 1/3 of the picture area as compared to 'full frame' (35mm film).

<p>

So, as the other posters have already stated, a smaller than Full frame sensor results in a narrower angle of view for a given focal length, which then gets represented in 35mm 'equivalents' because we photographers don't think in terms of angle of view.

<p>

If we did, that new 28-105 lens we bought might be called a <em>74-23 degree/35mm, 53-15degree/Dx f/3.5-4.5D AF</em> instead of a <em>28-105 f/3.5-4.5D AF</em>.