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  #16  
Old 10th October 2009
EH1
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Smile Re: Trying to explain Focal Length, Field of View and Crop x2

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Originally Posted by NSS View Post
Seems my not silly question has stired up some debate.

I must however add that the explenation has cleared up my original question.

Thanks

Kev
I wish more people asked `silly questions` that way we would all learn so much more! ..............................the only silly question is the one that you don`t ask!
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  #17  
Old 10th October 2009
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Re: Trying to explain Focal Length, Field of View and Crop x2

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As for Canikonites using the term 'crop factor' in a derogatory manner is a little short sighted as most of the Canon and Nikon range are either of a 1.4 or 1.6 crop factor. Only the top of their range are FF cameras.

Ask them about their in-camera IS...
And be sure to point out the dust bunnies on their images An innocent question about sensor cleaning should be good entertainment.
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  #18  
Old 10th October 2009
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Re: Trying to explain Focal Length, Field of View and Crop x2

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Originally Posted by jdal View Post
Nicely put together article John, but I'm still not sure about this . Is the image circle of a 4/3 100mm lens, as measured at the sensor plane, the same in the 4/3 system as it is for a FF 100mm lens in an FF system? That's what your article implies. I've had a scour of the net for the relationship between focal length, distance to focal plane and the size of the image circle but without success.
The 'image' circle is quite arbitrary. The projected image from a camera lens is limited by the size of the rear lens element and the baffles. So it will only usually project a circle (sometimes a circle with the corners chopped) shape that is slightly larger than the sensor or film frame. Designers deliberately do this to minimise light projected that isn't part of light that forms the image. Too much excess light could reflect and cause ghosting.

So a 100mm lens designed for a full frame 35mm camera will cover a circular area of twice the radius of a 100mm lens designed for a Four Thirds camera. This is why you can use 35mm legacy lenses on Four Thirds cameras, but you will get vignetting (dark corners) if you tried to use a Four Thirds lens of the same focal length on a 35mm camera.

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  #19  
Old 10th October 2009
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Re: Trying to explain Focal Length, Field of View and Crop x2

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From four-thirds.org: "The Four Thirds Specification defines the standard diagonal length of the 4/3-type image sensor, suitable image circle of lens and an interface between lens and body. This will make it possible to secure compatibility among camera bodies and lenses regardless of manufacturer or model"

i.e. the size of the image circle of a 4/3 lens is defined by the 4/3 standard. It isn't a function of the focal length of the lens, although for any particular lens it's constant and in theory it should be the same for all 4/3 lenses. So 4/3 images are no more cropped than FF images.

I think the point is that the images are cropped compared to FF dslrs (or the 1.3/1.6 crop bodies) but only because of the design of the 4/3 system, there is nothing which actually crops things.

Of course FF is 'cropped' compared to Large Frame or medium frame cameras. I saw a 150mm lens for a bronica described as negative crop the other day 'like an 85mm on ff'

The only real issue over quality is down to the sensor density, but most peole arguing about it don't have a clue (hmm, could be setting myself up here) and are regurgitating something they dont understand from a theoretical review on a website somewhere.

Different lenses of the same focal length can be designed to have differnt image circles at a given sensor/mount distance. I assume that the difference between the full frame and crop lenses on the Canon/nikon ranges is down to this - using the crop lenses on a ff body would I think result in terrible vignetting.

I dont know, but I assume that larger frame glass is more expensive (and heavier). Olympus have always been a lightweight option.

I'm a firm believer that it is the end result which matters.

edit: it took me a long time to write that (including coffee) just to see that Ian has delivered a far more succinct reply in the meantime.
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  #20  
Old 10th October 2009
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Re: Trying to explain Focal Length, Field of View and Crop x2

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Originally Posted by mas View Post
I think the point is that the images are cropped compared to FF dslrs (or the 1.3/1.6 crop bodies) but only because of the design of the 4/3 system, there is nothing which actually crops things.

Of course FF is 'cropped' compared to Large Frame or medium frame cameras. I saw a 150mm lens for a bronica described as negative crop the other day 'like an 85mm on ff'

The only real issue over quality is down to the sensor density, but most peole arguing about it don't have a clue (hmm, could be setting myself up here) and are regurgitating something they dont understand from a theoretical review on a website somewhere.

Different lenses of the same focal length can be designed to have differnt image circles at a given sensor/mount distance. I assume that the difference between the full frame and crop lenses on the Canon/nikon ranges is down to this - using the crop lenses on a ff body would I think result in terrible vignetting.

I dont know, but I assume that larger frame glass is more expensive (and heavier). Olympus have always been a lightweight option.

I'm a firm believer that it is the end result which matters.

edit: it took me a long time to write that (including coffee) just to see that Ian has delivered a far more succinct reply in the meantime.
I liked your description too

It's also worth noting that the brightness for a given area will be the same for two lenses even when one is designed for a different sized frame.

Ian
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  #21  
Old 10th October 2009
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Re: Trying to explain Focal Length, Field of View and Crop x2

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Originally Posted by Ian View Post
The 'image' circle is quite arbitrary. The projected image from a camera lens is limited by the size of the rear lens element and the baffles. So it will only usually project a circle (sometimes a circle with the corners chopped) shape that is slightly larger than the sensor or film frame. Designers deliberately do this to minimise light projected that isn't part of light that forms the image. Too much excess light could reflect and cause ghosting.

So a 100mm lens designed for a full frame 35mm camera will cover a circular area of twice the radius of a 100mm lens designed for a Four Thirds camera. This is why you can use 35mm legacy lenses on Four Thirds cameras, but you will get vignetting (dark corners) if you tried to use a Four Thirds lens of the same focal length on a 35mm camera.

Ian
Absolutely, Ian, and this is not a recent phenomenon brought about by different sensor sizes in digital imaging. It's always been a part of photography and a characteristic of the differences between various film format sizes. The relationship between Four Thirds and "Full Frame" digital is almost exactly the same as the relationship between 35mm and 6x7 film formats.

Four Thirds is approximately one quarter the surface are of a full frame sensor and 35mm film negatives are approximately one quarter the size of 6x7 negatives.

Thus, a 50mm lens on full frame digital is the same as a 50mm lens on 35mm film. However, on Four Thirds it is the equivalent of 100mm and on 6x7 the equivalent of 25mm. Thus the same focal length is a standard lens on full frame/35mm, a telephoto on Four Thirds and a wide-angle on 6x7. To take things a stage further, on 5x4 inch Large Format sheet film a 50mm lens would be practically a fish-eye lens!

Because of the difference in diameter of image circles in lenses designed for different film formats you could, if an adapter was available, fit a Medium Format lens to a 35mm camera but not vice versa, due to the severe vignetting that would result. In fact, Pentax produced an adapter which allowed lenses for their 645 format camera to be used on their 35mm camera bodies.

It's perhaps an easier concept for me to grasp because for many years before the advent of digital I was making mental conversions of focal lengths between 35mm film and Medium Format.

It's ironic that many of the people who now bang on about "cropped sensors" in derogatory terms previously used 35mm film quite happily and pooh-poohed the advantage of increased image quality that Medium Format offered.

It's also somewhat ironic that I "moved up" to Medium Format many years ago as I was unhappy with 35mm quality (and aspect ratio) but I now find Four Thirds digital quality so good that I feel no need to go "Full Frame!"
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Last edited by Zuiko; 10th October 2009 at 08:50 PM. Reason: text added
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  #22  
Old 10th October 2009
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Re: Trying to explain Focal Length, Field of View and Crop x2

Following on from my previous post, using 6x7 film literally gave twice the resolution of 35mm film, because the film emulsion was a constant factor shared by both formats.

That isn't quite the case with digital comparisons. In terms of pure resolution it would be, if Four Thirds sensors were populated with one quarter the number of pixels of full frame sensors; i.e. if a FF sensor has 24MP then the 4/3 sensor would have 6MP. But hang on, they don't, they currently have 12MP! Of course, as we all know, the trade-off is a small deterioration in high ISO noise performance but the advantage is that 4/3 punches well above its weight in terms of resolution!

At the beginning of this post I did say, "because the film emulsion was a constant factor shared by both formats." Curiously, that was not always the case. Before the advent of Velvia, Kodachrome 25 was the undisputed leading slide film, finer grained and much sharper than any other film. It was available in 35mm but not 120 roll film, used by Medium Format cameras. Thus, if you were a colour worker there was a strong arguement that 35mm Kodachrome 25 slides were possibly superior to Medium Format (at least 6x6 or smaller) and certainly negated much of the advantage of Medium Format. Of course that all changed with Velvia, which was (is!) available in all formats, but it does make an interesting parallel with digital where certain factors of Four Thirds technology (not least lens quality) negate much of the theoretical advantage of Full Frame in much the same way as Kodachrome did with the film formats!

Do you think I should start a thread with this line of reasoning on dpreview or talk photography?
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  #23  
Old 11th October 2009
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Re: Trying to explain Focal Length, Field of View and Crop x2

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Do you think I should start a thread with this line of reasoning on dpreview or talk photography?
Don asbestos/nomex underpants first

Rod
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  #24  
Old 11th October 2009
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Re: Trying to explain Focal Length, Field of View and Crop x2

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I wish more people asked `silly questions` that way we would all learn so much more! ..............................the only silly question is the one that you don`t ask!
Are we nearly there yet?

Heard many a time from my children while still on the M25 and with 450+ miles yet to go to reach the Scottish grandparents

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  #25  
Old 4th April 2012
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Re: Trying to explain Focal Length, Field of View and Crop x2

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Originally Posted by NSS View Post
Seems my not silly question has stired up some debate.

I must however add that the explenation has cleared up my original question.

Thanks

Kev

John, this is a TRULY EXCELLENT EXPLANATION ! 10/11 and ******
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  #26  
Old 4th April 2012
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Re: Trying to explain Focal Length, Field of View and Crop x2

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John, this is a TRULY EXCELLENT EXPLANATION ! 10/11 and ******
You're very welcome Chevvyf1.

The responses are also very informative and detailed. Ian's post about the light being scattered inside the camera housing is of particular interest.
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Old 4th April 2012
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Re: Trying to explain Focal Length, Field of View and Crop x2

Yes, I re-read this thread and it is all good stuff.

You don't half see some rubbish on this subject. I once read that the 4/3 system has a 2 stop advantage over full frame 35mm because the lenses have to concentrate the light onto a smaller sensor, so they are 2 stops brighter that they should be... This was stated as a fact.

Erm, in which universe would that be?

BTW - Regarding image circles: I see we have some 8mm f1.4 CS CCTV lenses at work. Ooh - I want to try one of those on my EPL3! Why pay a fortune for some giant Zuiko glass when you can pick up a neat little eight mil for fifty quid.

Guess what: It doesn't turn out to be a cheap fish eye. You only get a small circular image in the middle of the frame. All else is darkness.

You get what you pay for..

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  #28  
Old 4th April 2012
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Re: Trying to explain Focal Length, Field of View and Crop x2

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Originally Posted by theMusicMan View Post
You're very welcome Chevvyf1.

The responses are also very informative and detailed. Ian's post about the light being scattered inside the camera housing is of particular interest.
John, I have to STOP myself from becoming too interested in the "nitty gritty" because I did that with Golf and Horses ... and got bogged down in it ... IF I get too bogged down with photography I may lose my creativity (as many friends who have become quite obsessed with kit) !

To me, my Olympus kit is "MARVELOUS" and I love it, it facilitates ... and sometimes I "push it" a little too much ...

... but I really did "wonder" about lens and capture and in comparison to "others" ... so very grateful !
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Old 4th April 2012
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Re: Trying to explain Focal Length, Field of View and Crop x2

Although I had a conceptual understanding of crop factor as it relates to sensor size, this conversation pulls it all together. Thanks to all, especially Chevvy for introducing it, and to jdal and the Music Man for elaborating. If only I had had this lucid explanation when I fell out with a mate who fiecely asserts the superiority of the FF sensor, in spite of himself never having owned a camera - his conclusions are drawn from a fixation on those obscene white lenses he sees in news footages ! I have concluded that arguing the merits of one make of camera over another is like trying to convert agnostics into true believers ... or vice versa !
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  #30  
Old 7th April 2012
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Re: Trying to explain Focal Length, Field of View and Crop x2

Hi. One question that never seems to get asked on this subject and one which kind of grates on me is this.

Why do fourthirds users even bother to try and make a comparison to 35mm cameras and even more bizarrely, those crop 35mm cameras that some manufacturers make? It never ceases to confuzzle me. I have friends with medium format and large format cameras and they don't sit working out their own respective, comparative focal length multipliers and "crop" factors. In fact they couldn't care less.

Fourthirds isn't a "crop" camera. The lenses are designed for the sensor so there is no crop. If you want to compare the angle of view to a 35mm camera then fair enough, apply a focal length multiplication.

I've never really heard of an intention to challenge 35mm cameras with fourthirds. The reason I like fourthirds is I like the format. I like the squarer frame and I dislike the elongated 35mm frame. I don't understand the obsession with comparing the formats ... i never understood the term "full frame" either, full what frame? 35mm, medium format? Does someone with a leaf back on their camera refer to theirs as a full frame and a 35mm Canon as a half frame. It's bizarre.

The term "crop" is based on the fact that 35mm photographers were using 35mm lenses on a sensor that was too small for the resulting image circle and therefore the image was literally "cropped" (or maybe i recieved incorrect information - please correct me if i did).

Fourthirds has none of those characteristics. It is a unique format with lenses designed natively for that sensor. It's not a crop camera any more than a Canon 5d with a Mamiya lens fitted is.

Why in the case of fourthirds is it so important to constantly ensure everyone understands this comparison? If someone is so interested in 35mm photography its easy and cheap to buy a 35mm camera ... even cheaper if you buy film. If it is so important, why do we not also make sure everyone understands Mamiya, Hasselblad, Pentax 645 sensor crops and FLMs compared to fourthirds?

Maybe we should just focus (excuse the pun) on our chosen format and where we are going with it and let users of 35mm cameras get swept up by the marketing bumpf of other manufacturers.

I shoot in fourthirds. I shoot in 35mm film. I shoot in 120mm film. I don't care what my 11-22mm lens is in 35mm terms or 120mm terms.
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