Focal Length Help!!

[pgosling]

Hi All.

I currently have a Fuji s8100fd bridge camera with a fixed lens.

It has an 18x optical zoom on the exif data shows the focal length at maximum zoom as 84.2mm.

I am looking at getting a Sony Alpha A33 with a Tamron 70-300mm DI Lens as I want to upgrade to a "real" DSLR.

How would the zoom of this lens compare to my current Fuji camera??

Many thanks in advance!!

[RichardTaylor]

70-300 is a little over 4x

The largest range of a DSLR lens that I am aware of is the Tamron 18-270mm or 15x.

[pgosling]

Ok

As the focal length is 300mm it will still have a better overall magnification than my Fuji camera (84.2mm) as it is a 4.6mm-84.2mm lens?

Or am I completely misunderstanding things?

[RichardTaylor]

The Fuj1 equivalent is 486mm

On the Sony 300mm is equivalent to 450mm

The image quality of the Sony combination will be lots better especially in low light where you may be using be using higher ISO.

[pgosling]

Thanks Richard

Thats a little confusing!!

Think I'm clearer now though!

Ta!

[RichardTaylor]

The zoom range is from wide angle to telephoto.

The maximum focal length is basically how close the subject will appear in the viewfinder.

The old 35mm film cameras (like colour slide) are used as a common reference for lenses on different types of cameras (that's the equivalent).

[pgosling]

I guess another difference will be the speed at which the camera focuses.

I tried some airshow photography (on my Flickr) which I found really difficult as the zoom is controlled by a switch around the shutter button and focus was too slow to be able to get decent close up shots.

[OsmosisStudios]

Stop looking at it as a "#x zoom": that doesnt work with an SLR.

You have to look at "35mm equivalency", which is what Richard was alluding to. Because of the small size of the sensor in your fuji, the lens is a 4.2-84.2, but the 35mm equivalency, which is the de facto standard, means you end up with a 24-486 zoom. So, to cover the same range, you'd need something like a 16-320 (on most digital SLRs). This does not exist, so you'll likely end up with two or more lenses.

The greater the range of a single lens, the more compromises that are made in its design.

[pgosling]

Thanks for that.

I'm looking at getting the Sony A33 or A390 with an 18-55mm lens included and then a Tamron 70-300mm additional lens.

Why can't it be straightforward!!!

[inkista]

Marketing. They know everybody only wants one number to compare. The zoom factor is a relative measurement of the zoom range. It's just the longest focal length divided by the shortest focal length. Both a 10-100mm lens and a 1-10mm lens are 10x zooms. But they'd give you very different effects. But it was a way that marketing could make a camera look better, even if the extension of the zoom range was really on the wide end, not the telephoto. Zoom doesn't mean more reach. Zoom means that the focal length can change. And a zoom factor is a measurement of how much it can change, not how far it can reach.

Focal length, however, is more of an absolute measurement of the magnification a lens gives you. But sensor size also plays into things.

You'll generally hear the sensor-size-factor called a "crop factor", because it's rather like zooming by cropping in. The smaller the sensor, the more crop, and the more magnification you get from shorter focal lengths.

Your Fuji's "35mm equivalent" focal length is 27-486mm, but the actual focal lengths of the lens, given the 1/2.3" sensor size, is 4.6-84.2mm. Your crop factor is 5.77x. That means a frame of 35mm film is 5.77x larger than the sensor, so the lens you'd use to get the same field of view on 35mm film would be 5.77x longer.

You'll notice that Richard also said that the 70-300 lens would give you a 450mm equivalent. That's because the Alpha A33's sensor is also smaller than a frame of 35mm film, with a crop factor of 1.5x. (300x1.5=>450).