Understanding Magnification factor, zoom range and focal length range of a lens

[rollercoaster]

Hi all,

I am new to photography and have just bought a canon 500D which I gather has a 1.6x Magnification factor. That makes the standard kit lens of 18-55mm more like a 28.8-88mm. I am planning to get a 70-300mm Tamron DI lens, which will become a 112-480mm on my camera.

According to what I have read on the internet the 18-55mm lense gives me 3x zoom (55/18) and similarly 70-300mm gives me 4.3x zoom. That doesnt look much to me.

I guess what I am trying to say is, is buying the 70-300mm lens really getting me much closer to distant objects? Is it worth buying just for 1.3x more magnification? (it now makes sense why the 18-200mm is so much more costly). Unless It does get me closer and I have incorrectly understood the equations above. What more does the 70-300mm gets me against the 18-55mm?

The second thing I want to understand is, consider two hypothetical lenses 10-500mm (=5x), 100-5000mm (=5x) . What would be the benifit of the second lens over the first? If I had to guess I would say the second one lets me focus sharply in its range i.e 100mm to 500mm ?? or something like that. but whoulding the 10-500mm also focus at anything further when set at infinity?


Sorry if my noobness boggles your mind. Thanks for your time.

[Digidave]

Whoa Nelly!! Take a step back here.

1) Forget about the sensor cropping factor. I mean it! Forget it, don't even think about it. Unless you have used a film camera for many years & want to know what the difference will be when migrating to digital, this crop factor thing is ridiculous in my opinion.

According to what I have read on the internet the 18-55mm lense gives me 3x zoom (55/18) and similarly 70-300mm gives me 4.3x zoom. That doesnt look much to me.

2) Your looking at that all wrong. Yes, 18-55 is a 3x zoom & yes, 70-300 is 4.3x. When you want an idea of how different these 2 lenses will be you have to compare the short end to short end(18mm compared to 70mm) & long end to long end(55mm compared to 300mm).

Use your kit lens for a while.

3) If you are constantly saying to yourself, "Boy, I wish I didn't have to constantly walk forward 100yds in order to fill the frame." Then you need a longer lens. 300mm will get you 5.4x closer without having to walk forward.

4) If you are constantly saying to yourself, "Boy, I wish this wall wasn't here because I need to back-up more to get all 3 of my friends in the shot." Then you need a wider lens. I don't know exactly what it would look like but just for the sake of argument, let's say that you are 3 feet away from your buddy & you want to take a picture of them. You can't back-up anymore because you are crowded in. You have your 70-300mm lens on set at 70mm. You aim & shoot. Well, you now have a picture of your buddies nose & upper lip. As nice of a nose as it may be, it's not exactly what you were looking for. You change lenses by throwing on your 18-55mm. Set it to 18mm & snap away. Now you have a very nice shot of your buddies face, shoulders, & upper chest. All is good!

5) Click Me for a focal length comparison.

[sybren]

Originally Posted by Digidave

Forget about the sensor cropping factor.

I feel the same way. I'm using a camera with a 1.6x crop factor, and I won't switch to a different crop factor any time soon. This means that I know what a 300mm lens looks like on my camera. I know that 400mm will be more zoomed in, 200mm will be more zoomed out, and 10mm is the widest I can go with my wide-angle lens. That's enough info for me, without having to calculate back and forward between different crop sizes.

[sybren]

Originally Posted by rollercoaster

The second thing I want to understand is, consider two hypothetical lenses 10-500mm (=5x), 100-5000mm (=5x) . What would be the benifit of the second lens over the first?

It zooms in 10x as much. The compact camera salesmanisms of "how much zoom" a camera has is just for people who don't like numbers, and is quite unusable when comparing lenses.

My "camera" has "30x zoom", because I can put on a wide angle lens and zoom it out to 10mm, and I can put a tele lens on it and zoom in to 300mm. 300/10 = 30x. However, this 30x still does _not_ tell me how I can bring far objects closer. If I had a theoretical 1mm lens, my "camera" would have a 300/1=300x zoom. Still, the most zoomed in setting would be 300mm, which hasn't changed despite the fact I have "10x more zoom".

10mm is zoomed 10x further out than a 100mm lens. This is regardless of the rest of the lens. It really doesn't matter if those focal lengths are in a 10-100mm lens or in a 5-100000mm lens - 10mm is zoomed out 10x further than 100mm.

[milosh]

Originally Posted by rollercoaster

According to what I have read on the internet the 18-55mm lense gives me 3x zoom (55/18) and similarly 70-300mm gives me 4.3x zoom. That doesnt look much to me.

I guess what I am trying to say is, is buying the 70-300mm lens really getting me much closer to distant objects? Is it worth buying just for 1.3x more magnification? (it now makes sense why the 18-200mm is so much more costly). Unless It does get me closer and I have incorrectly understood the equations above. What more does the 70-300mm gets me against the 18-55mm?

As far as I understand this matter, I think you are missing a point here. Your 18-55 lens does have a 3x zoom. And the 70-300 lens does have a 4.3x zoom, but that is from 70 mm, and not from 18 mm. With both of these lenses you would have the range from 18-300 mm covered, and that is a zoom of 300/18 = 16.6x.
Imagine this: you put your 18-55 lens on and you look through the viewfinder at 18 mm. You see a very wide angle and a tree that is far away. If you zoom to 55 mm, that tree will look 3x closer to you. Then you take your 70-300 mm lens and look through the viewfinder at 70 mm. The tree now looks even closer to you than it did at 55 mm. If you zoom all the way to 300 mm, the tree looks like it's very close to you. Now you are looking at that tree with a zoom ratio of 16.6x (300/18=16.6). I hope I got this right and I hope I helped you a bit.

[Elmo]

Crop factors are for old farts like me. It relates to old 35mm film users. I still see a scene and think “I need a 28mm lens for that”. If I shoot a crop factor camera I need to divide the 28 by 1.6 = 18mm.

The zoom factor is only a ratio between the longest focal length divided by the shortest focal length. This ration has nothing to do with what we often think of a magnification power.

Magnification power is determined by dividing the focal length being used by the “normal” focal length for the camera. The normal focal length for a camera closely approximates normal vision. This normal focal length has been determined to me the distance diagonally across the capture area. 35mm and FF cameras use an image size of 24mm x 36mm giving a diagonal length of 50mm (actually 48mm). Then the normal lens for a 1.6 crop sensor should be 30mm. Then for magnification power we divide the focal length used (300mm) divided by the normal (30mm) and we get a 10x magnification power.

[dcclark]

Originally Posted by rollercoaster

The second thing I want to understand is, consider two hypothetical lenses 10-500mm (=5x), 100-5000mm (=5x) . What would be the benifit of the second lens over the first? If I had to guess I would say the second one lets me focus sharply in its range i.e 100mm to 500mm ?? or something like that. but whoulding the 10-500mm also focus at anything further when set at infinity?

Let me take another stab at this.

The important numbers are the focal length measurements (100mm, 500mm, etc.), not the "zoom ratio". The focal length measurements are an absolute measurement of how "close" a lens will make an object appear.

For example, 10mm is very wide angle -- things will appear distant. 50mm is much closer, a bit closer than normal human vision. 100mm is very close. 300mm is telephoto -- very very close indeed.

These numbers are the same, whether you look at a 10-20mm lens, or at a 50-100 lens. A 10-20mm lens will "magnify" things from very far (10mm) to moderately far (20mm). Meanwhile a 50-100 lens will "magnify" things from moderately close (50mm) to very close (100mm) -- even though they have the same "zoom ratio" of 2x!

The upshot is: zoom ratio alone means nothing. You have to know what level of zoom you're starting at, which is why we use millimeter measurements. Try to just use focal lengths, and you'll start to get used to ideas like "10 - 20 is wide angle" and "70-300 is all very telephoto".

[Trader]

For many years I shot with a full frame film camera starting out back in the mid '60s and became very familiar with composing a shot. I had no trouble switching over to a digital camera. I actually prefer a cropped frame now and even see added benefits to shooting with one. I guess someday I will have to switch back to full frame again but I am not looking forward to that time.

[rollercoaster]

Originally Posted by milosh

As far as I understand this matter, I think you are missing a point here. Your 18-55 lens does have a 3x zoom. And the 70-300 lens does have a 4.3x zoom, but that is from 70 mm, and not from 18 mm. With both of these lenses you would have the range from 18-300 mm covered, and that is a zoom of 300/18 = 16.6x.
Imagine this: you put your 18-55 lens on and you look through the viewfinder at 18 mm. You see a very wide angle and a tree that is far away. If you zoom to 55 mm, that tree will look 3x closer to you. Then you take your 70-300 mm lens and look through the viewfinder at 70 mm. The tree now looks even closer to you than it did at 55 mm. If you zoom all the way to 300 mm, the tree looks like it's very close to you. Now you are looking at that tree with a zoom ratio of 16.6x (300/18=16.6). I hope I got this right and I hope I helped you a bit.

Wow that answers my question very clearly. 70mm is already offset so much further that 18mm. the 4.3x zoom will be from there on.

I am also going to take the advice that you good folks have given to me and wait a bit before buying the zoom lens. I dont even have a tripod yet.

Thanks a lot.

[rollercoaster]

@everyone Thanks for all the info. I understand things much better now. My main question was how much closer the 70-300mm would get me in comparison to the 18-55mm kit lens. milosh really covered it very well without the additional PhD level science

On a different topic, I made a different post(my first here) earlier and didnt get any answers for quite some time. I was worried there for a bit that noobs like me might be left out to dry!

But all of a sudden I check in today and its like you guys come on in herds

I am happy to have join this forum.