I just made the leap into the digital SLR arena with my Nikon D70 and am totally confused with the zoom factor.
With my Canon G5, G2 before that and my first digital camera, the Kodak DC280, they all spoke of nX zoom rating. Either it was 2x, 3x or 4x. I've seen Sony's as high as 10x. This I understand but when I get into the SLR lenses, they don't use this rating.
They use metric measurements. Which is fine, but I'm confused because when I went to Nikon's web site, it stated the ED lens I just bought, 70-300mm is 4.3x. Well, found an article on DPReview.com that has a review on the new forthcoming 18-70mm DX lens and it states it is 4.3x.
So how can the telephoto be the same as a mid-range? I'm totally confused. Is there a "chart" some where to explain this to those of us that have grown up in the digital focal range world?
Full Version: What Is Conversion Of Slr Zoom To Point-and-shoot?
First of all, congratulations for buying what should be the SLR of the year.
The zoom factor or magnitude is calculated the same way on all zoom: you take the highest focal lenght and you divide it by the lowest.
So, in your examples, a 70-300 is a 300/70 = 4.29 Zoom and the 18-70 is a 3.89 Zoom.
This number tells you that, when you start zooming from the wide-angle position, you can go nearer of you subject with that factor.
If you are at 3 meter of your subject at wide-angle, using a 3x zoom will get you at one meter of it, at full telephoto.
But there is one thing that vary greatly from a "normal" digital camera to a SLR camera, it is the field of view crop.
Please read http://www.dpreview.com/reviews/nikond70/page18.asp for more info. This has to do with the size of the CCD compared to the size of the normal 24x36 film size. It let you compare focal lenght of digital cameras as they were 24x36 film one.
The FOVC for your Nikon is 1.5, this mean that the normal lens is 50mm/1.5=33mm
For your G5 (not a bad one), the FOVC is 35/7.2 = 4.86 and the normal focal length is 10mm. When you read that the focal length of your G5 was 10 mm in the EXIF data of a photo, it was in fact a FOV equivalent to 48.6mm on a 35 mm film camera.
Hope it helps
P.S. I have a friend with a Digital rebel. It takes time before he admitted that his 18-55mm zoom provides a field of view equivalent to a 28.8-88 mm lens on a 35 mm film camera (FOV of 1.6)
The zoom factor or magnitude is calculated the same way on all zoom: you take the highest focal lenght and you divide it by the lowest.
So, in your examples, a 70-300 is a 300/70 = 4.29 Zoom and the 18-70 is a 3.89 Zoom.
This number tells you that, when you start zooming from the wide-angle position, you can go nearer of you subject with that factor.
If you are at 3 meter of your subject at wide-angle, using a 3x zoom will get you at one meter of it, at full telephoto.
But there is one thing that vary greatly from a "normal" digital camera to a SLR camera, it is the field of view crop.
Please read http://www.dpreview.com/reviews/nikond70/page18.asp for more info. This has to do with the size of the CCD compared to the size of the normal 24x36 film size. It let you compare focal lenght of digital cameras as they were 24x36 film one.The FOVC for your Nikon is 1.5, this mean that the normal lens is 50mm/1.5=33mm
For your G5 (not a bad one), the FOVC is 35/7.2 = 4.86 and the normal focal length is 10mm. When you read that the focal length of your G5 was 10 mm in the EXIF data of a photo, it was in fact a FOV equivalent to 48.6mm on a 35 mm film camera.
Hope it helps
P.S. I have a friend with a Digital rebel. It takes time before he admitted that his 18-55mm zoom provides a field of view equivalent to a 28.8-88 mm lens on a 35 mm film camera (FOV of 1.6)
Great reply Herve!
This is one of the annoying things about digital SLR cameras in general - wide-angle photography is difficult to achieve because of the field of view crop factor. My 17-40mm wide-angle lens actually gives a field of view equal to 28-62mm on a 35mm film SLR, which isn't very wide at all.
This is one of the annoying things about digital SLR cameras in general - wide-angle photography is difficult to achieve because of the field of view crop factor. My 17-40mm wide-angle lens actually gives a field of view equal to 28-62mm on a 35mm film SLR, which isn't very wide at all.
Herve,
Yes indeed, great reply. Thank you.
I did notice the FOV problem when comparing to my dad's Nikon film SLR over the Easter weekend. It wasn't too dramatic to me, but I was a little bummed. So, I understand that part.
Thank you for explaining the zoom factor. That makes total sense on what I read on Nikon's web site, now.
To clarify, my G5's standard focal point was 10mm, and thus when it states it has 4x zoom that would make it 40mm at full zoom? Thus my standard lens on my Nikon is nearly twice as powerful at 70mm?
Are all digital point-and-shoots the same focal point (10mm)? Or are they doing the same thing as Nikon does on their lenses and the zoom factor is based on the low to high focal point?
I totally appreciate your help. You totally provide excellent information that helps me better understand this new world of SLR.
Yes indeed, great reply. Thank you.
I did notice the FOV problem when comparing to my dad's Nikon film SLR over the Easter weekend. It wasn't too dramatic to me, but I was a little bummed. So, I understand that part.
Thank you for explaining the zoom factor. That makes total sense on what I read on Nikon's web site, now.
To clarify, my G5's standard focal point was 10mm, and thus when it states it has 4x zoom that would make it 40mm at full zoom? Thus my standard lens on my Nikon is nearly twice as powerful at 70mm?
Are all digital point-and-shoots the same focal point (10mm)? Or are they doing the same thing as Nikon does on their lenses and the zoom factor is based on the low to high focal point?
I totally appreciate your help. You totally provide excellent information that helps me better understand this new world of SLR.
I've been a bit quick about what is a normal lens 
A normal lens is the one which gives you the same field of view as your eyes. (I know, how a single lens reflex, even a digital one, can give you the same field of view than a guy with two eyes
. Thats why you have to look through the viewfinder ! ). Please read:
http://www.nikon-image.com/eng/LensGuide/t...l#Picture_Angle
Now what about your question:
Q1) To clarify, my G5's standard focal point was 10mm, and thus when it states it has 4x zoom that would make it 40mm at full zoom?
NO, the 4x zoom they are talking about is the total zoom factor taking the wide-angle position as the start position before zooming. The wide-angle position is 7.2 mm equivalent to a 35mm lens, NOT a 50mm one
If you want to know the telephoto factor of this camera you have to divide 28.8 by 10 equal 2.88. This number shows the telephoto power of the lens compared to your eyes and the wide-angle power of you lens is 7.2/ 10 = 0.72 (in this case you turn away from your subject).
Q2) Thus my standard lens on my Nikon is nearly twice as powerful at 70mm?
NO because the field of view crop factor is different (5 now becomes 1.5)
Same calculation for the 18-70 Nikon lens (27-105mm equiv.):
telephoto factor is 105/50 = 2.1
wide-angle factor is 27/50 = 0.54
so the lens of the D70 is evenly wide or telephoto and the G5 lens is more a telephoto lens.
Q3) Are all digital point-and-shoots the same focal point (10mm)?
NO because the field of view crop factor is different. High end SLR have CCD nearly the size of a negative film. The CCD of a G5 is 1/1.8" with a surface of 4.8*6.4 = 30.7 mm˛ and a normal negative is 24*36 = 864 mm˛
It's a big difference in size and you have to make a five megapixel picture of it So, in fact SLR are much easier to build
Q4) Or are they doing the same thing as Nikon does on their lenses and the zoom factor is based on the low to high focal point?
YES but I think you now understand how to compare lenses: take the equivalent focal length then you know that 50mm is the normal FOV, 100mm is a small telephoto, 200mm is 4 times nearer, 400mm is like a binocular (8x) but use a tripod. For the wide-angle, 28mm is good value.
I think the next step is to speak about the angle of view of all these lenses If you find a formula, post-it...
Regards,
A normal lens is the one which gives you the same field of view as your eyes. (I know, how a single lens reflex, even a digital one, can give you the same field of view than a guy with two eyes
. Thats why you have to look through the viewfinder ! ). Please read:http://www.nikon-image.com/eng/LensGuide/t...l#Picture_Angle
Now what about your question:
Q1) To clarify, my G5's standard focal point was 10mm, and thus when it states it has 4x zoom that would make it 40mm at full zoom?
NO, the 4x zoom they are talking about is the total zoom factor taking the wide-angle position as the start position before zooming. The wide-angle position is 7.2 mm equivalent to a 35mm lens, NOT a 50mm one
If you want to know the telephoto factor of this camera you have to divide 28.8 by 10 equal 2.88. This number shows the telephoto power of the lens compared to your eyes
and the wide-angle power of you lens is 7.2/ 10 = 0.72 (in this case you turn away from your subject).Q2) Thus my standard lens on my Nikon is nearly twice as powerful at 70mm?
NO because the field of view crop factor is different (5 now becomes 1.5)
Same calculation for the 18-70 Nikon lens (27-105mm equiv.):
telephoto factor is 105/50 = 2.1
wide-angle factor is 27/50 = 0.54
so the lens of the D70 is evenly wide or telephoto and the G5 lens is more a telephoto lens.
Q3) Are all digital point-and-shoots the same focal point (10mm)?
NO because the field of view crop factor is different. High end SLR have CCD nearly the size of a negative film. The CCD of a G5 is 1/1.8" with a surface of 4.8*6.4 = 30.7 mm˛ and a normal negative is 24*36 = 864 mm˛
It's a big difference in size and you have to make a five megapixel picture of it
So, in fact SLR are much easier to build Q4) Or are they doing the same thing as Nikon does on their lenses and the zoom factor is based on the low to high focal point?
YES but I think you now understand how to compare lenses: take the equivalent focal length then you know that 50mm is the normal FOV, 100mm is a small telephoto, 200mm is 4 times nearer, 400mm is like a binocular (8x) but use a tripod. For the wide-angle, 28mm is good value.
I think the next step is to speak about the angle of view of all these lenses
If you find a formula, post-it...Regards,
OK,
I've got the Canon 300D, bought in kit form with the EFS 18-55mm lens. Now I thought that the EFS lens was different, and designed specifically for the 300D, with the 'S' of EFS denoting a 'S'horter distance between the rear lens element and the sensor. This can be seen if I compare my EFS 18-55 with my EF50 or EF70-300 lenses, the rear part of the EFS lens extends into the body of the camera further.
It is safe to conclude therefore that the EFS lens is a "digital" lens, i.e. developed for the smaller, digital sensor. Does this not therefore mean that the 1.6x cropping effect does not apply with the EFS lens? And therefore that it is a true 18-55mm for the 300D? I thought that the cropping factor only came into the equation if you used a standard EF (i.e. 35mm based format) lens, as these are designed with a 35mm film frame in mind.
A confused Jools.
I've got the Canon 300D, bought in kit form with the EFS 18-55mm lens. Now I thought that the EFS lens was different, and designed specifically for the 300D, with the 'S' of EFS denoting a 'S'horter distance between the rear lens element and the sensor. This can be seen if I compare my EFS 18-55 with my EF50 or EF70-300 lenses, the rear part of the EFS lens extends into the body of the camera further.
It is safe to conclude therefore that the EFS lens is a "digital" lens, i.e. developed for the smaller, digital sensor. Does this not therefore mean that the 1.6x cropping effect does not apply with the EFS lens? And therefore that it is a true 18-55mm for the 300D? I thought that the cropping factor only came into the equation if you used a standard EF (i.e. 35mm based format) lens, as these are designed with a 35mm film frame in mind.
A confused Jools.
As far as I know that lens is also subject to the 1.6 cropping effect, turning it into a more conventional 28-90mm lens.
So any ideas what the advantages of the EFS lens are? 
The advantage of the EFS lens is quite simple: they are designed to put light only on the CCD and not on the negative film.
Because of the smaller size of the CCD, the lens can also be smaller so It cost less (and, for most of us it's a GREAT advantage...).
Regards,
Because of the smaller size of the CCD, the lens can also be smaller so It cost less (and, for most of us it's a GREAT advantage...).
Regards,
Very well answered Herve - that seems to make sense. Cheers. 
However, as it is a digital lens, specifically for the 300D and it's small sensor size, I wonder why Canon have quoted it's focal lengths in 35mm format, and not in native 300D format (i.e. with the cropping factor included), so that it would be classed as a digital ~ 28-90mm lens, and you'd know that that is exactly what you are getting. It's not as if you can also use the lens on a 35mm camera is it?
Does anyone know whether the smaller sensor size effectively makes it either impossible or too expensive to make a proper wide angle lens specifically for the 300D?
However, as it is a digital lens, specifically for the 300D and it's small sensor size, I wonder why Canon have quoted it's focal lengths in 35mm format, and not in native 300D format (i.e. with the cropping factor included), so that it would be classed as a digital ~ 28-90mm lens, and you'd know that that is exactly what you are getting. It's not as if you can also use the lens on a 35mm camera is it?
Does anyone know whether the smaller sensor size effectively makes it either impossible or too expensive to make a proper wide angle lens specifically for the 300D?
Oh, BTW i've found the formula to find the angle of view (AOV) if you know the focal and the diagonal of the film or CCD.
if
D is the diagonal (remember the diagonal of a rectangle is the square root of the sum of the squared side)
F is the focal
then
the AOV is arctang (D / 2F ) * 2
in excel you write this as =DEGREES(ATAN(SQRT(1872)/2/A2))*2
and you get:
F (mm) AOV Zoom Wide Factor
10 130,4 0,20 5,0
21 91,7 0,42 2,4
28 75,4 0,56 1,8
32 68,1 0,64 1,6
35 63,4 0,70 1,4
45 51,4 0,90 1,1
50 46,8 1,00 1,0
85 28,6 1,70
96 25,4 1,92
100 24,4 2,00
135 18,2 2,70
200 12,3 4,00
300 8,2 6,00
400 6,2 8,00
500 5,0 10,00
800 3,1 16,00
1000 2,5 20,00
if
D is the diagonal (remember the diagonal of a rectangle is the square root of the sum of the squared side)
F is the focal
then
the AOV is arctang (D / 2F ) * 2
in excel you write this as =DEGREES(ATAN(SQRT(1872)/2/A2))*2
and you get:
F (mm) AOV Zoom Wide Factor
10 130,4 0,20 5,0
21 91,7 0,42 2,4
28 75,4 0,56 1,8
32 68,1 0,64 1,6
35 63,4 0,70 1,4
45 51,4 0,90 1,1
50 46,8 1,00 1,0
85 28,6 1,70
96 25,4 1,92
100 24,4 2,00
135 18,2 2,70
200 12,3 4,00
300 8,2 6,00
400 6,2 8,00
500 5,0 10,00
800 3,1 16,00
1000 2,5 20,00
It is safe to conclude therefore that the EFS lens is a "digital" lens, i.e. developed for the smaller, digital sensor. Does this not therefore mean that the 1.6x cropping effect does not apply with the EFS lens? And therefore that it is a true 18-55mm for the 300D? I thought that the cropping factor only came into the equation if you used a standard EF (i.e. 35mm based format) lens, as these are designed with a 35mm film frame in mind.
thanks,
Queenie
Simulation pret immobilier
thanks,
Queenie
Simulation pret immobilier
Have to love marketers. Most people wouldn't understand 50mm or 200mm, but when you say 5x zoom they think wow, it must be able to zoom far. What everyone has said is correct about the multiplier and the ef-s lenses being optimized for the cropped sensors.
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