Real simple newbie lens question!

Hi all :hiya:

Had a question for a while - and its a simple one for you chaps I'm sure.

I'm confused about lenses - in so much as why would you have more than a couple?

I've got the stock 18-55 and a sigma 70-300 - but what do the lengths actually relate too? I've seen people with a 70-300 and like a 40-200 etc. Whats the point if you already have lenses that cover that range already? What does a 200mm give you that a 300mm doesnt?! :rotfl:


Well-known Member
Probably all down to speed.
I have a DA Pentax 16-45mm F4 lens which is an excellent performer but have recently snapped up a cheap Sigma 24-60mm F2.8 purely for using indoors.

I also have a Sigma 135-400mm which is fairly slow so am looking at a Tamron or Sigma 70-200mm f2.8 for better speed at shorter distances.


Active Member
It can be a combination of things, obviously a 40-200 has a wider angle than the 70-300, this can be substantial even if the difference is small....

Also a 40-200 is likely to have a better Fstop range, less glass means it can let in more light and work better in low light conditions...

one lense may have anti-shake the other may not

its all personal preference, if you are happy that the lenses you have get you the results you want then :smashin:

*please anyone feel free to correct me if I'm wrong, I work with video lenses and only have a little knowledge of photographic lenses*
I find this website pretty nice to compare focal lengths.

Tamron - Focal Length Comparison

The overlap in my lenses are generally because on lens is faster than the other, or the other is more convenient to have because of the range of zoom.

Thats really cool thanks for explaining and that link is ace.

So what does the "70mm" relate too? Is that the closest you can be to a subject?

God I sound so dumb! :rolleyes:


So what does the "70mm" relate too? Is that the closest you can be to a subject?

God I sound so dumb! :rolleyes:

If you don't ask you never know, so it's not a dumb question.

Basically the 70mm relates to the distance from the front of the lens (the front element) to the camera's sensor (in 35mm terms). Full frame cameras such as the Canon 5d or Nikon D700 means that a 70mm lens will be 70mm lens as the sensor does not crop the image. Cropped sensors extend the 70mm to what ever the crop factor is on that camera, say for example the Nikon D300 which has 1.5 crop you would multiply the 70mm by 1.5 (70 X 1.5 = 105mm). So a 70mm lens on a 1.5 crop would in reality a 105mm lens, hope this makes sense.

Please correct me if I'm wrong. :)
Last edited by a moderator:


Well-known Member
Please correct me if I'm wrong. :)

Close enough :smashin:

When talking about a simple lens, focal length of a lens is distance from the centre of the lens to the point at which parallel light (ie from infinity) is focussed. As imcarn says, this is where you put the sensor/film.

This illustrates it pretty well.

Understanding Camera Lenses

This focal length is a physical characteristic of the optical system, set in tablets of stone. The diameter of the lenses determines how big the imaging circle is. A 35mm film pretty much fits exactly in that circle. An APS-C sensor is smaller. You only take the centre portion of the image, so you effectively magnify it.

Light entering a long focal length lens is converged at a smaller angle than longer lenses, resulting in a smaller angle of view, and therefore more magification. Simple as that. Magnification (2x, x3 etc) is all relative, whereas focal lengths are absolute. A 100-200mm zoom is x2, as is a 10-20mm, ever though they have ridiculously difference angles of view. Once you realise this, it becomes obvious why focal lengths are a more useful term to photographers than magnification.

Correct me if I'm wrong folks, but the human eye is roughly equivalent to 50mm on full frame (ie or 35mm x1.5 for APS-C), so measuring magnification compared to that, an 70mm lens would be x2 magnification.


Distinguished Member
I'm confused about lenses - in so much as why would you have more than a couple?
There are several reasons for having different lenses:

1) It's useful to own different focal lengths - eg, you may use a wide angle lens (say 24mm) to fit a group of friends into a picture, or a telephoto lens (say 300mm) to fill your frame with a single animal. You may want even wider, even longer, or anywhere in between.
2) Quality - some lenses have better quality glass, which can result in sharper images. Prime lenses (they have just one focal length) are generally better quality than similarly priced zoom lenses.
3) Speed (= large aperture = small 'f' number). A lens with a large aperture will allow more light in, which allows you to use a faster shutter speed, or allows you to use the lens in lower light. Lenses like a 50mm f1.4 are useful for portraits indoors without using flash, and lenses like the 300mm f2.8 allow for fast shutter speeds and shallow depth of field (amount of the photo in focus) which is useful for wildlife and sports photography. Zoom lenses are not available with apertures as wide as f1.4, and zooms that come close are heavy and expensive (eg, a Nikkor 50mm f1.8 is under £76, and the Nikkor 28-70mm f2.8 is not as fast, weighs more, and costs over £1000.
4) Weight - the professionaly quality zoom (or prime telephoto) lenses are heavy. You may want a light lens for travelling/wandering

You may also have a dedicated macro lens for taking pictures of small objects, like flowers or insects. So many of us need a few lenses to take the sort of photos we're interested in.

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