Understanding DSLR zoom lens spec from a compact camera users perspective

[nagais]

I am very new to the DSLR world - recently migrating from the compact camera world!
One of the biggest confusions I had is looking for the X factor in the specification of optical zoom capability of a DSLR (lens) - which of course I didn't find. I read through various forums and posts - here is the compilation of my understanding and a (novice's) method for understanding DSLR lens specs from a compact camera user's perspective :

http://www.geekytalk.snaga.net/2011/...-sense-of.html

Please let me know what you think of this.

[Geoff F]

Well I suppose it all makes sense; but lens sizes and the variation between different camera sensors is difficult to explain in simple terms.

For most people, it is more a case of I already have . . . but would like something bigger/wider etc.

If you are thinking about getting any lenses just ask, and give a few details of your camera and what you wish to photograph. We are all experts at spending other people's money!

[Sonic4Spuds]

This looks like good information. I'm just getting a feel for the practical application of lens measurements.

-Sonic

[Sonic4Spuds]

Geoff I think he was writing it for others who maby haven't found the forums.

-Sonic

[AlanP]

Point and shoot cameras have large X factors because it is convenient. Image quality is traded off for convenience. If you never print a picture because you use your point and shoot as a convenient way to show pictures you will never notice the loss in picture quality. If you want to make an 16 X 20 inch enlargement you don't want the loss therefore DSLR owners buy zoom lenses with an X factor around 3 to get better picture quality.

Two point and shoot cameras both having an X factor of 10 will not necesarily have the same field of view at the extremes. One may provide a little more telephoto and the other a little more wide angle. Camera manufacturers will generally convert the field of view of a point and shoot to the equivalent field of view of a full frame (or film) 35 mm camera. This allows a comparison of field of view for cameras with very different sized sensors. For example brand A point and shoot may have a 35mm equivalent zoom of 35mm to 350mm giving it an X factor of 10. A second brand may have a 35mm equivalent zoom range of 28 mm to 280 mm. Also a X factor of 10 but a little wider angle at the lower number and less telephoto at the higher number. There is a good deal of overlap between the point and shoots given the same x factor. The difference between brand A and brand B is noticibly but not too large.

In order to get roughly the same field of view range in a DSLR you will need 2 zoom lenses. DSLR cameras have a "crop factor" of 1.5 to 2 for non full frame cameras. Multiplying this number by the lens focal length gives the 35mm full frame equivalent. A four thirds DSLR camera will a zoom range of 14mm to 45mm will have a 35mm equivalent of 28 to 90 mm (wide andgle to slightly telephoto). This would be X factor of three, but it is actually referred to a "zoom range" of 3. In order to get the equivalent of an X factor of 10 the four thirds DSLR owner will buy a second zoom. In this case the zoom may be 45mm to 200mm (35mm equivalent of 90mm to 400mm). This is a telephoto zoom with a zoom range (X factor) of appromimately three.

The two point and shoot cameras which had a X factor of 10 will have similay but not identical field of views with a lot of overlap. The two zoom lenses used with the DSLR will have zero overlap. Saying the two DSLR zoom lenses both have a zoom factor of 3 says almost nothing about them. That is why X factor is not used in the DSLR world.

[rpcrowe]

I rephrased my comment because I didn't realize that the OP was also the author of the article. My new comment is that the article appears to make a fairly simple subject rather complicated and is sometimes hard to follow.

[nagais]

Thank you all for your comments. My intention in this post is to come up with magnification factor that will explain the magnification in the "same way as a binocular spec does" - in terms of magnification compared to human eye.

I could have gone a bit overboard with math rambling - sorry for that.

[Dave Humphries]

Hi Nagarajan,

First off, let me admit I haven't had time to read the linked article from the original post

My intention in this post is to come up with magnification factor that will explain the magnification in the "same way as a binocular spec does" - in terms of magnification compared to human eye.

If you talking of magnification factor as in say; "10 x 50 binoculars", where the first figure is the magnification factor and the second the diameter of the lens, then the effect on camera images is related to focal length and crop factor of the camera.

It is generally accepted that for 35mm film (or FF digital) that a lens of 50mm focal length represents a 'life size' capture, so it would take a 300mm telephoto lens to produce a similar angle of view/magnification that you would get with a modest set of "6 x" binoculars.

However, if you put that same lens on a 1.5 crop factor camera, it equates to 450mm in terms of angle of view, or about "9 x" in binocular terms.

However, the effect when viewed through the viewfinder may be somewhat different due to the fact that a viewfinder rarely shows a 100% image and its eyepiece magnification factor may also not be 1.

Cheers,

[nagais]

Exactly !! Thats excatly what I am driving at in the post.

I have assumed that the reader is not gong to have the idea that 50mm (for 355mm camera) is considered normal for photgraphy and he may not be aware of the sensor sizes and crop factor as well. Explaining them made the article lengthy. Not sure, If I have done a good job of explaining them in simple manner, though