Help undstanding lenses

[theshortstack]

Afternoon all,

I've been reading a lot about lenses recently and I was wondering if someone could tell me what the mm figures relating to any lense refers too. I can understand that any lense with a large range of numbers (Eg 70-200) is going to be a fairly strong lens, but how can I tell whether I'm looking at a wide angle lens for example, or a macro lens?

Thanks very much,

[LaPistola]

The MM numbers you see are the lenes focal length or range of.

You will know a lens is a macro as it will be advertised as so and will have the macro icon on the lens body.

The smaller the number in mm the wider the angle of view or the further away the subject will seem from the camera. So a wide angle lens would be say 10-24mm. The larger the number the closer the subject will seem to the camera (like a zoom on a point and shoot camera).

There are two different types of lens. Lens that have only one mm number ie: 50mm is a prime lens and a lens that has a range number ie: 70-200mm is a zoom lens.

Now its the number or range of numbers that determine what a lens is typically used for.

10-27mm Wide angle
27-80mm Portrait's
70-*** Telefocal (zooming in)

[theshortstack]

That's awesome - thanks very much for the reply. Much appreciated!

One last question though, what do the mm measurements actually refer to though? So for example, what does the 50 in a 50mm lens mean? It obviously can't be how far ahead you're able to see but I'm not sure what else it could be.

Thanks

[LaPistola]

MM stands for millimeters. I think what your asking requires a far from basic answer and unless you own a full frame camera it gets more complicated.

How it helps you is it will allow you to work out the min distance between lens and distance in front of the lens required before light starts to focus. Or think of it as how far away you need to be before the lens can focus on a subject.

With a cropped sensor camera (which ill assume you have?) you'll also need to factor in the crop factor. For this example ill assume you have a canon camera and either have a 7D or under which means you will have a crop factor of 1.6. This means your focal lengths will need to be times by 1.6 so with a lens with a focal length of 50mm will have a true focal length of 80mm meaning the lens (which is an internal part of the whole lens) will need to be 80mm away from the subject before it can focus.

[greenwing]

Originally Posted by LaPistola

MM stands for millimeters. I think what your asking requires a far from basic answer and unless you own a full frame camera it gets more complicated.

How it helps you is it will allow you to work out the min distance between lens and distance in front of the lens required before light starts to focus. Or think of it as how far away you need to be before the lens can focus on a subject.

With a cropped sensor camera (which ill assume you have?) you'll also need to factor in the crop factor. For this example ill assume you have a canon camera and either have a 7D or under which means you will have a crop factor of 1.6. This means your focal lengths will need to be times by 1.6 so with a lens with a focal length of 50mm will have a true focal length of 80mm meaning the lens (which is an internal part of the whole lens) will need to be 80mm away from the subject before it can focus.

Please, LaPistola... the OP has no idea of focal lengths, so to confuse matters by bringing in crop factors and saying that a 50mm lens has a 'true' focal length of 80mm on a certain camera (which I hope you realise is complete rubbish) is not going to be helpful to him. He has no experience to base the 50mm or 80mm on. It only gets more complicated on a non-full-frame camera if someone decides you need to know something else that has no relevance to you.

I'm having trouble following your logic on the 'how far away before it starts to focus' thing, too. It is patently rubbish, anyway. That 50mm lens will 'start to focus' at the same distance regardless of what camera it's on. Different 50mm lenses will 'start to focus' at different distances.

theshortstack... The focal length of a lens, 15mm, 50mm or 500mm for example, show how far behind the lens rays of light from a distant point are brought to focus. This is a technical property of the lens, which determines how much of the scene can be recorded on the sensor. It's tricky to explain why a short focal length gives a wide-angle view, and the angle of view gets narrower as focal length increases. It's because, as focal length increases, the cone of light needed to cover a given sensor size becomes narrower, so it originates from a corresponingly narrower cone in the original scene.

Chris

[hssutton]

This will probably explain understanding lenses a little better as it's shown in pictures.

http://www.photoaxe.com/understandin...-and-aperture/

Harry

[LaPistola]

Originally Posted by greenwing

Please, LaPistola... the OP has no idea of focal lengths, so to confuse matters by bringing in crop factors and saying that a 50mm lens has a 'true' focal length of 80mm on a certain camera (which I hope you realise is complete rubbish) is not going to be helpful to him. He has no experience to base the 50mm or 80mm on. It only gets more complicated on a non-full-frame camera if someone decides you need to know something else that has no relevance to you.

I'm having trouble following your logic on the 'how far away before it starts to focus' thing, too. It is patently rubbish, anyway. That 50mm lens will 'start to focus' at the same distance regardless of what camera it's on. Different 50mm lenses will 'start to focus' at different distances.

theshortstack... The focal length of a lens, 15mm, 50mm or 500mm for example, show how far behind the lens rays of light from a distant point are brought to focus. This is a technical property of the lens, which determines how much of the scene can be recorded on the sensor. It's tricky to explain why a short focal length gives a wide-angle view, and the angle of view gets narrower as focal length increases. It's because, as focal length increases, the cone of light needed to cover a given sensor size becomes narrower, so it originates from a corresponingly narrower cone in the original scene.

Chris

I understand the OP is new but he seemed to show an interest and I have no idea on how to explain it in a basic way. Regarding your "rubbish" comments this is what I was tort and after reading your comments I did a google search and it seems what I said is right. If a 50mm lens isn't 80mm on a 1.6 cropped sensor camera then what is it??

I never said this is focal length but how it will help the OP which was one of his questions and if the OP wishes to know how it helps them they more than likely need to know about the crop factor.

http://www.great-landscape-photograp...al-length.html
http://www.digital-photo-secrets.com...ean-on-a-lens/

Now normally I would bow to others superior knowledge but in this case I don't think im to far off the mark at all and the two above links support what I said and explain the other.

Originally Posted by From a google search

In a 55mm lens, there are 55 millimeters between the lens and the camera’s image sensor. You can’t see this distance because the lens is housed in a protective shell, but if it weren’t, you would be able to measure it.

There are also 55 millimeters between the lens and front of area of the camera where light focuses.

Personally I think explaining what the focal length is instead of how it helps you is confusing. Focal length the measurement between a lens and the image sensor but this isn't always true on all lens there for confusing.

[hssutton]

"If a 50mm lens isn't 80mm on a 1.6 cropped sensor camera then what is it??"

Simple answer it's still a 50mm lens. The image as not been enlarged in any magical way, all that happens is the field of view reflects the smaller sensor. i.e cropped. That's why it's called a cropped sensor.

If you look on Wikipedia you will see a diagram showing the difference.
If you crop a full frame image to the the cropped sensor size you will see an identical image.

http://en.wikipedia.org/wiki/Crop_factor

[greenwing]

Originally Posted by LaPistola

If a 50mm lens isn't 80mm on a 1.6 cropped sensor camera then what is it??

I never said this is focal length but how it will help the OP which was one of his questions and if the OP wishes to know how it helps them they more than likely need to know about the crop factor.

A 50mm lens is a 50mm lens. End of subject. On crop-sensor cameras, we can use the crop factor to calculate a '35mm equivalent' focal length at which a full-frame camera would have the same field of view, but that's really not much use unless you're familiar with what focal lengths look like on 35mm formats, or want to compare different formats - compact, cropped, Nikon 1, medium format could all be compared this way if you want. The vast majority of digiatl SLR users have never used a 35mm camera and don't need to compare their camera to anything else, so the 35mm equivalent is not a lot of use to them. It's like telling someone who's never experienced gravity that they'll be able to jump 6 times as high on the moon than on the earth.

Note that it's a 35mm equivalent, not an 'effective' or 'true' focal length.

Originally Posted by LaPistola

http://www.great-landscape-photograp...al-length.html
http://www.digital-photo-secrets.com...ean-on-a-lens/

Now normally I would bow to others superior knowledge but in this case I don't think im to far off the mark at all and the two above links support what I said and explain the other.



Personally I think explaining what the focal length is instead of how it helps you is confusing. Focal length the measurement between a lens and the image sensor but this isn't always true on all lens there for confusing.

Digital photo secrets is confused. To try to explain focal length by using the close focus distance of a lens defies any form of logic. Very few lenses can focus at a point their own FL in front of them.

No, focal length isn't always the distance between the lens (when focused at infinity) and the sensor. It's always the distance between an imaginary plane and the sensor, I think the plane might be called the Nodal Point, but could be wrong and don't have time to check. In the case of extreme wideangles it's well behind any point of the lens, or it would interfere with the mirror. In the case of long telephotos it's well beyond the front of the lens, otherwise the lens would be unmanageably long. Don't ask me how lens designers acheive these tricks.

Chris

[theshortstack]

Thanks everyone for your thoughts - it makes interesting reading. I'll be sure to go through it all...