Lens question

[Starboard Tack]

A lens that is referred to by two numbers like that is a zoom lens (lens that can change it's focal length between a range of focal lengths. In this case, it is from 12mm to 24mm. The smaller the focal length, the wider the angle of the view at that focal length. Focal length is the distance from the optical centre of the lens to its focal point (where an object at infinity is in focus - usually the focal plane where the film is or sensor in a dSLR).

Why is this? Think of this analogy as it relates to angle of view. When you are looking out a window, the closer you are to the window, the greater the angle of view. If you (the focal plane) moves closer to the window (the optical centre of the lens), you are able to see more. As you adjust your distance from the window (focal length), you will be able to see more or less depending on whether you move closer to the window (shorten the focal length) or move further from the window (longer focal length).

That's a great analogy Warren.

Mike

 

[victor]

Good Thread.
The following might help others.
The standard lens on a 35mm SLR film camera was the 50mm, less than 50 was a wide angle, greater than 50 was a telephoto, 135mm was a good portrait lens, 200+ was good for animals and birds, 35 or 28 for landscapes etc.

Now with digital SLR's I need to adjust for the sensor size.
Olympus multiply by 2, e.g. 25mm behaves like a 50mm on a film camera
Nikon multiply by 1.6 so a 35mm behaves like a 50mm
Cannon multiply by 1.5 or 1, depending on the model.

Well I think I understand it now. I still need to think before I can imagine what a lens means.

 

[Warren_L]

Good Thread.
The following might help others.
The standard lens on a 35mm SLR film camera was the 50mm, less than 50 was a wide angle, greater than 50 was a telephoto, 135mm was a good portrait lens, 200+ was good for animals and birds, 35 or 28 for landscapes etc.

Now with digital SLR's I need to adjust for the sensor size.
Olympus multiply by 2, e.g. 25mm behaves like a 50mm on a film camera
Nikon multiply by 1.6 so a 35mm behaves like a 50mm
Cannon multiply by 1.5 or 1, depending on the model.

Well I think I understand it now. I still need to think before I can imagine what a lens means.

The crop factor is actually 1.5 for Nikon dSLRs and 1.6 for Canon dSLRs (with the exception of full frame). Also remember it is the angle of view that changes. A 50mm lens on an SLR, Canon dSLR or Nikon dSLR is still a 50mm lens, though the viewing angles will be different.

 

[Charlie99]

Charlie, your last statement doesn't make much sense:



The notion of a "3x zoom" for a "17mm" lens doesn't work. The x3 (or x5, or whatever) zoom terminology is something as was introduced by marketting departments of point and shoot cams for average joe to measure genital extenions.

In fastmarc's original question, his "12-24mm" lens is a x2 zoom. Ie, the long end (24mm) is two times the focal length of the wide end (12mm).

A fairly standard 3x zoom on compact camera usually refers to a 35mm equavalent focal range of a 35-105 zoom. (35 x 3 = 105).

My point is that the magnification level and field of view depend upon both the focal length of the lens and the size of the sensor/film.

For my point and shoot camera, a 17mm focal length gives the same field of view as would a 105mm lens with a 35mm camera. Sorry about the confusion created by using the term "3x zoom".

 

[alcina]

Warren_L's great explanation is now in the Pink Link in post #4 -> http://www.scubaboard.com/showpost.php?p=2187232&postcount=4

Fabulous!

 

[Warren_L]

Wow, thanks. I am honoured!

 

[victor]

The crop factor is actually 1.5 for Nikon dSLRs and 1.6 for Canon dSLRs (with the exception of full frame). Also remember it is the angle of view that changes. A 50mm lens on an SLR, Canon dSLR or Nikon dSLR is still a 50mm lens, though the viewing angles will be different.

Drats I new I was going to get 1.5 and 1.6 the wrong way around.

Though these replies helped, I still a little confused by it.
I assume that the experienced can look at the 17-55 for e.g. and know what it will be suitable for?

Yes the 50mm lens is still a 50mm lens but it behaves, on the olympus, as a 100mm lens would on a 35mm film camera, e.g a telephoto suitable for portraits.

The 17-55 on the oly would be simillar to a 35-110 on a film camera so slightly wide angle to slight telephoto.
On the Nikon it would be simillar to a 25-80 on a film SLR.
On the full frame Cannon it would be the same on both the film and DSLR so 17-55.

I definitly have to think twice when I think of lens.

 

[victor]

Wow, thanks. I am honoured!

Warren
Can you please give as clear a description of diopters, I still cannot get my head around them.

 

[bladephotog]

The crop factor is actually 1.5 for Nikon dSLRs and 1.6 for Canon dSLRs (with the exception of full frame). Also remember it is the angle of view that changes. A 50mm lens on an SLR, Canon dSLR or Nikon dSLR is still a 50mm lens, though the viewing angles will be different.

Actually Canon has three crop factors - Full Frame - 5D and 1Ds MK II
1.3x - 1D MK IIn and ID MK II
1.6 - All others

And don't forget that Nikon's D2x has two crop factors - 1.5 and 2x in the high speed cropped image mode.

It sure was simpler when a 20mm was a 20mm in conversation. Lord, I sound old.

 

[Warren_L]

Warren
Can you please give as clear a description of diopters, I still cannot get my head around them.

A diopter is really just a magnifying glass that you can use to adjust the refractive power of a lens. Practically, what this means for photography is that it allows your lens to focus closer (if you use a + diopter), which is useful in particular in macro photography as it will allow you achieve a higher magnification of your subject than if you were to use your macro lens alone. I guess technically you can also have negative diopters which would make your lens focus further away, but there's no real practical purpose for using one as far as I know.

For example, for macro, I have two lenses I use. One a 60mm and one a 105mm. Both can reproduce a subject at 1:1 magnification (i.e. at closest focus for both lenses, shooting the same subject will be the same level of magnification). The difference really is how far I am from my subject. The 60mm does so while being closer to the subject than the 105mm, which is further away.

The implication of being further away varies depending on conditions. Further away might mean more water to contend with (and if water quality if bad, this might affect your lighting as well as an increased chance of things like particles in the water getting in the way). On the flip side, though, if you're further away, you may have better success getting certain shots (particularly of skittish animals) since you do not have to be as close to get the same level of magnification.

So why add a diopter? Simply because you want to achieve a greater magnification than your native macro lens will allow. Which lens would you add it to? Well, technically you can add it to either lens, but it would likely be better to add it to a longer focal length macro since you have greater working distance to begin with. The diopter will shorten your working distance on both lenses, but in the case of the 60mm, it will be much closer than with the 105mm with a diopter on in either case. The problems associated with being too close are simply you might scare away whatever it is you're shooting or even accidentally bump into it with your port. As well, when you're that close, lighting might get a bit tricky. With the 105mm, you're going to have more working distance and these problems, while they may still exist to some degree, will be easier to work with.

You can add either a wet diopter or dry diopter. You may have heard about people having "Woodys" - they are hopefully referring to an add-on diopter that you attach to the front of your port. It is a wet lens that allows you to focus closer. A dry diopter is a lens (looks like a regular filter) you screw onto the front of your lens, so it is inside the housing. You have to make sure that this diopter does not interfere with the port as it extends the length of the lens, so when focusing, you have to check to see that the port is long enough to accomodate it's addition.

As well, you can get varying degrees of power on diopters. I don't know if the wet diopters are available in varying degrees of power, but I have a set of 3 dry diopters that I use with my 105mm (+1, +2. and +4). The greater the power, the closer the focus is.

Does this help?