Newbie Q About Lenses

Darren, the real focal length of the Panasonic 20mm lenses for example is 20mm, that is a physical unalterable characteristic of the lens. All lenses produce a circular image which covers the sensor or film negative of the camera. If the Panasonic 20mm lens could produce an image that covered a 35 mm negative, it would still be a 20mm focal length lens.
When people say it's "effective" focal length is 40mm all they are saying is that it produces an image that is equivalent to the image that would be produced by a 40mm lens on 35mm format.

I have long argued that this practice has it's drawbacks and is confusing, I am in a minority, perhaps a minority of one. Your question shows just why I have my feelings about this practice.

Your 20mm lens would have a focal length of 20mm no matter what format it was used on, although of course it is designed to be used on :43: and no other, nor is it capable of being used on any other.

Barrie

 

Because the number given is the actual focal length. Take your 20mm lens outside and use it to project an image of the sun on a piece of paper-- you will get an image the same size as what you would get with a single glass lens element of 20mm focal length. The latter would need to be centered exactly 20mm away from the paper to produce a sharp image of a distant object like the sun.

Compared to a 35mm film frame, an m4/3 sensor is about 1/2 the height and width. This means that an object that is half the height and width of the frame in a 35mm photograph will completely fill the frame with m4/3 if you have the same lens attached. It's as if you are taking the 35 mm photo and cropping it--- or as if you are zooming in to twice the effective magnification. To get the same object to fill the full 35mm frame, you would have to either move closer or use a longer lens with about twice the focal length.

So, your 20mm lens "really" is a 20mm lens. With a m4/3 camera it gives the same field of view as a 35mm film camera would if it had a 40mm lens on it. Realistically it would make sense to specify the field of view directly in degrees of angle, but people got so used to thinking in 35mm terms that it has stuck.

 

Darren, the comparison is made so that those who are steeped in the 35mm format can make a comparison so that they can envisage what an image made on :43: would look like if made on 35mm. I used to shoot a lot of 6 x 9 so I could say that the 20mm lens on :43: was equivalent to a 90mm lens on 6 x 9, but it wouldn't make a lot of sense to most people, and I believe that once you've got some experience under your belt you should refer to a lens by it's actual focal length. There will be people taking up photography who do not have experience of other formats and will become confused by this practice.

Barrie

 

Exactly!

The field of view varies with the sensor size divided by the focal length. So if you want the same field of view and you halve the sensor size, you should halve the focal length.

(the relationship is non-linear. FOV = 2 arctan [1/2 sensor size / focal length] ... if this means nothing to you, don't worry, you can safely ignore it!)

 

Quick followup - so if a native m43 lens is not doubled up but an adapted lens is then what do you set the focal length to for the purposes of Image Stabilisation on an Oly ? For a native lens the electronics pass this info through to the camera for a non-native you manually set it.

For example if I put a Zuiko Pen 40mm lens on - do I set the Image Stabilisation focal length to 40mm or 80mm ?

Thanks in advance,
Raj.

 

Raj, with respect I think you've rather missed the point, an adapted lens of focal length say 40mm on :43: is still a 40mm focal length lens. It produces an image which is equivalent to a lens of 80mm on a 35mm film format camera. That is just a way that some people use to envisage the results that will be produced by that lens on a :43: camera. It is still however a 40mm focal length lens for all time and on all formats, that cannot alter.

So whilst I can claim no experience of the Olympus system I would deduce from that that you would set the image stabalisation for a 40mm focal length lens.

Barrie

 

correct

 

I Guess the term "Crop Sensor" is also a misplaced phrase...
The word "Crop" refers to the Factor associated with it to "Cropping" a 35mm frame to get a mind image of what lens would be used on a 35mm to achieve the angle of view..

Really, the "Crop Factor" has to with how much you have multiply your m4/3 lens to be equal in angle of view on a 35mm camera... which has been already explained above.

Man, aren't you glad 6x6 camera weren't used.....

Then a 4/3 sensor would be a 3.5x crop factor, not a 2x crop factor, since it compared to a larger format.

That's all "The Crop Factor" is, a comparison between 2 formats... a well known one to a new one, so we can figure out what lens will produce similar angle of coverage....

m4/3.. 25mm
APS-C 35mm (Canon)
35mm 50mm
6x4.5 70mm
6x6 80mm
6x9 105mm

All these focal lengths give the about the same angle of view with their format size, and the lenses are made to have a circle of coverage large enough to have no corners cut off.

 

It has just occurred to me that the "original" Pen cameras were ½-frame ... did all the same confusion occur then?

 

I don't know, I wish i did. I have voiced my misgivings about this constant reference back to 35mm format and why we should drop it.
I have used 35mm, 6 x 6 and my favourite 6 x 9 and never felt the need to cross reference one format to another in order to visualize what the results would be using any particular focal length lens.
The sooner you get used to the results obtained by any particular focal length lens on the format you are using the better, all this cross referencing is probably becoming more confusing to some than it is of value, particularly new comers to the hobby who have no concept of the results of any particular focal length lens on any format.
From previous discussions I doubt that many agree with me, but I'm going to keep plugging away , unless that is I do my usual trick in this life and just go off on my own and do my own thing.

Barrie

 

Barrie, you have your view and should continue to express it as you wish ... some people find the comparison confusing, and it never seems to be people who "grew up" with 35mm cameras (like me) ... so perhaps you're right, and this is a practice that many newcomers find confusing ... it's taken me 9 or 10 months, but I have to say I no longer internally "translate" FL from :43: to 35mm ... my thought process goes "Shall I take the 17, the 20 or the 28?" Instead of "Shall I take the 35, 40 or 56?" ... but to begin with, it did help me visualise what I would be in the frame with the different lenses ...

The trouble is, having got used to this m4/3 angle of view malarkey, when I sell my car, a kidney and and the rest of my furniture to buy an M9, I'll be all confused again ...

 

A convert, not overnight apparently, but every one counts, yippee

Barrie

 

I don't think so, the amount of 1/2 frame camera's were limited to Olympus (Mostly... the great Pen series that had like 10 models in it), and Canon had a cool Canon Demi with a spring wind) a few more, but not too popular. Well, Popular, but not a huge public campaign.

I think people just remembered that a 38mm was a normal 1/2 frame lens, 50mm for 35mm, 80mm for 6x6 and so on. No "Crop factor" comparisons that I can remember, and I am 57