Debunking the Myth of the Loupe

[ckiang]

Hi,

Well, Jed and I just hit upon something. The truth to all the loupes (ok, let's just say "most" to be safe ;p )

We probably hear/read that a 4x loupe on a 35mm slide gives magnification equivalent to a 4R print. Likewise, a 8x loupe gives the equivalent of an 8RC.

The reason behind that is simple. A slide is roughly 1 x 1.5", so 4x on both side gives 4x6", and 8x on both sides gives 8x12". Logical right?

No! Loupe magnification is usually based on magnification of area, not a single dimension (i.e. not linear magnification). A 4x loupe on a 35mm (36x24mm) slide therefore gives an image size of only 72mm x 48mm, which is 4 times the area of a 35mm slide. Similarly, a 8x loupe will give 144 x 96mm.

Do this experiment. Put 4 slides in 2 by 2 layout. Does it equal the size of a 4R print? No right?

If you looked through a 4x or 8x loupe, you would have found that it isn't quite the same size as a 4R or 8R print. For the same reason, the image through an 8x loupe does not look twice the size as a 4x. It's only 1.4x more.

So here it is, the myth debunked. Feel free to post if you don't agree. :rbounce:

Regards
CK

 

[Jed]

I think the real problem comes from the fact that some labs call 8R prints 8x magnification, which is right only as linear magnification (since a 35mm neg is about 1" x 1 1/2"). And similarly 4R prints as 4x magnification.

 

[Tweek]

That is a helpful piece of info, magnification is in terms of area, and not length. I think KC (mpenza) clarified that sometime ago, just that he used zoom (3X, 10X etc) as the context instead of loupe.

 

[ckiang]

Originally posted by Tweek
That is a helpful piece of info, magnification is in terms of area, and not length. I think KC (mpenza) clarified that sometime ago, just that he used zoom (3X, 10X etc) as the context instead of loupe.

Zoom is different - the how-many-times zoom is with respect to the minimum focal length. Eg. 35-70mm is 2x zoom. 35-105mm is 3x, 50-500mm is 10 times, etc.

Regards
CK

 

[Tweek]

Originally posted by ckiang


Zoom is different - the how-many-times zoom is with respect to the minimum focal length. Eg. 35-70mm is 2x zoom. 35-105mm is 3x, 50-500mm is 10 times, etc.

Regards
CK

As in, an object in a 2X zoom will be twice as large in area as an object with 1X zoom, that kinda thing. Yeah I know zoom is wrt focal length as a convention, but it can be looked at from the image area point of view as well, right?

 

[Tweek]

Originally posted by Tweek


As in, an object in a 2X zoom will be twice as large in area as an object with 1X zoom, that kinda thing. Yeah I know zoom is wrt focal length as a convention, but it can be looked at from the image area point of view as well, right?

oh wait, i might have stepped on my own toe. Is an object at 70mm (focal length) twice in area as the same object at 35mm, or twice in length?

:dunno: :dunno:

 

[mpenza]

Originally posted by Tweek


As in, an object in a 2X zoom will be twice as large in area as an object with 1X zoom, that kinda thing. Yeah I know zoom is wrt focal length as a convention, but it can be looked at from the image area point of view as well, right?

no no. 2x zoom for lens, means 4x in area. both length and breadth are increased by 2x each.

 

[mpenza]

Originally posted by Tweek


oh wait, i might have stepped on my own toe. Is an object at 70mm (focal length) twice in area as the same object at 35mm, or twice in length?

:dunno: :dunno:

twice in length.

 

[Jed]

Oooh Tweek.... me confused now.

Zoom factor refers to the number of times a lens is able to "zoom", but this is completely dependent on the base focal length of a zoom. Hence, a 50-500mm lens is a 10x zoom lens. A 28-300mm lens is a 10.7x zoom lens, even though it only has 272mm of "zoom" compared to the 450mm for the former lens.
Focal lengths:

Every doubling of focal length results in a halving of the angle of view. So a 20mm lens has double the angle of view of a 40mmm lens, which has double that of an 80mm lens and so on. This is measured as a diagonal.

If you still got questions ask... I dunno whether the above has answered anything!

 

[Tweek]

Originally posted by mpenza


no no. 2x zoom for lens, means 4x in area. both length and breadth are increased by 2x each.

yep yep...silly me...just did a check, yep a 2X zoom means 2X magnification in LENGTH not area. Sorry for the misinfo......

/me nurse stubbed toe.... :embrass: :embrass:

 

[Tweek]

Originally posted by Jed
Oooh Tweek.... me confused now.

Zoom factor refers to the number of times a lens is able to "zoom", but this is completely dependent on the base focal length of a zoom. Hence, a 50-500mm lens is a 10x zoom lens. A 28-300mm lens is a 10.7x zoom lens, even though it only has 272mm of "zoom" compared to the 450mm for the former lens.


Yup in my argument I was referring to the same base focal length, in the case of 1X zoom vs 2X zoom. Sorry for not making that clear.


Focal lengths:

Every doubling of focal length results in a halving of the angle of view. So a 20mm lens has double the angle of view of a 40mmm lens, which has double that of an 80mm lens and so on. This is measured as a diagonal.

Yup that makes sense, the angle of view is prob linearly and inversely related to the object length, so doubling of focal length results in halving of angle of view, and doubling of object length.

Understood liao!