Olympus UK E-System User Group
Olympus UK E-System User Group

Join our unique resource for Olympus Four Thirds E-System DSLR and Pen and OM-D Micro Four Thirds photographers. Show your images via our free e-group photo gallery. Please read the e-group.uk.net forum terms and conditions before posting for the first time. Above all, welcome!


Go Back   Olympus UK E-System User Group > Site news and information > Tutorials, Informative & Classic threads

Tutorials, Informative & Classic threads A new e-Group area for all the wonderful tutorials and helpful threads put on here by our members. Tutorials on using software, camera hardware - and feel free to request a tutorial if you need assistance!

Reply
 
Thread Tools Display Modes
  #1  
Old 10th October 2009
theMusicMan's Avatar
theMusicMan theMusicMan is offline
E-3 Enthusiast....:)
 
Join Date: Nov 2007
Location: South Wales
Posts: 5,878
Thanks: 143
Thanked 267 Times in 178 Posts
Likes: 8
Liked 30 Times in 19 Posts
Trying to explain Focal Length, Field of View and Crop x2

I posted this in response to NSS's not silly question in that thread too - but on reflection I feel it might help as a separate thread on its own... thus this one.

How the Oly 4/3 crop factor x2 works.

A lens's focal length is fixed and doesn't change, regardless of what camera body it is used on. If it's a 70mm lens, then it's a 70mm lens on a Canikon, Oly or Sony; or if it's a 35-100mm, then it's still a 35-100mm on whatever body it is used on.

All that changes on each body (in essence for purposes of this thread), is the amount in the center of the image that the sensor actually sees.

Let me see if I can explain this with the aid of a few diagrams, in terms of sensor size and crop factor.

Let's say this strange image of a piggy on a marble floor is what the lens actually sees. IMPORTANT: Remember, a lens projects a round image - not a square one!!


This round 'image' is then ultimately projected through the rear of the lens, and physically onto the cameras sensor. This next diagram illustrates two sensors; green being what the FF camera sensor might see, and blue being what the Oly sensor sees. It's simple physics here folks... the 4/3 sensor is smaller and thus less of the image is projected onto it by the lens as compared to the FF sensor.



And now, just to separate what each sensor 'sees'. Physically, the FF is larger and contains more of the overall picture i.e. a greater field of view whereas the Oly shows less of the overall picture i.e. less field of view.



However, if we now take a look at what we see through the viewfinders of each camera... we see a bigger pig in the Oly camera compared to the FF camera because of the reduced field of view as explained in the text above. Thus the apparent increase in focal length.



Which hopefully explains why there's the x2 crop factor in 4/3 equipment. The same lens but different sensor size has yielded less of the overall image being seen by the sensor and an effective increase in focal length. However, if it is 'zoom-in' you want (as opposed to 'zoom-out') - in the case of crop sensors, this can be interpreted as... less is more!!

Of course, there are many other factors that come into play here; photosite density, noise, capability of the lens to resolve more detail (which in ZD glass - is so) - but for a simple explanation - these diagrams should explain why we as Oly users get in closer to the subject as compared to FF cameras.

Please shout if there are questions...

Enjoy the piggie!
__________________
John

Reply With Quote
  #2  
Old 10th October 2009
snaarman's Avatar
snaarman snaarman is offline
Full member
 
Join Date: Feb 2008
Location: Baaarkshire UK
Posts: 6,838
Thanks: 494
Thanked 417 Times in 325 Posts
Likes: 492
Liked 1,291 Times in 498 Posts
Re: Trying to explain Focal Length, Field of View and Crop x2

Excellent explanation.

Good pig too.
__________________
Look, I'm an old man. I shouldn't be expected to put up with this.


Pete's photoblog Misleading the public since 2010.
Reply With Quote
  #3  
Old 10th October 2009
j.baker j.baker is offline
Banned
 
Join Date: Nov 2007
Location: North Wiltshire
Posts: 1,071
Thanks: 0
Thanked 20 Times in 11 Posts
Likes: 0
Liked 0 Times in 0 Posts
Re: Trying to explain Focal Length, Field of View and Crop x2

Nice article John.

Thanks for posting it.


I also think the pig is good
Reply With Quote
  #4  
Old 10th October 2009
jdal's Avatar
jdal jdal is offline
Full member
 
Join Date: Dec 2007
Location: Northumberland
Posts: 4,046
Thanks: 129
Thanked 503 Times in 439 Posts
Likes: 202
Liked 665 Times in 283 Posts
Re: Trying to explain Focal Length, Field of View and Crop x2

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.
__________________
John

m4/3: E-P2, EM-5, 100-300, 14-42mm 12-50mm, 45mm, panny 14mm. 4/3: 7-14 + Flashes & tripods & stuff

"Take nothing but pictures and leave nothing but footprints".

Flickr gallery
Reply With Quote
  #5  
Old 10th October 2009
theMusicMan's Avatar
theMusicMan theMusicMan is offline
E-3 Enthusiast....:)
 
Join Date: Nov 2007
Location: South Wales
Posts: 5,878
Thanks: 143
Thanked 267 Times in 178 Posts
Likes: 8
Liked 30 Times in 19 Posts
Re: Trying to explain Focal Length, Field of View and Crop x2

Quote:
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.
Hi John

Yes, it is. The image circle presented by any lens is fixed..
__________________
John

Reply With Quote
  #6  
Old 10th October 2009
jdal's Avatar
jdal jdal is offline
Full member
 
Join Date: Dec 2007
Location: Northumberland
Posts: 4,046
Thanks: 129
Thanked 503 Times in 439 Posts
Likes: 202
Liked 665 Times in 283 Posts
Re: Trying to explain Focal Length, Field of View and Crop x2

Quote:
Originally Posted by theMusicMan View Post
Hi John

Yes, it is. The image circle presented by any lens is fixed..
Cheers, so just for my thick self the only variable in evaluating the size of the image circle is the lens's focal length? I think that's what I was trying to establish.

I wish I'd stuck in at physics at Uni, but I don't think we'd have done anything useful like compound lenses.
__________________
John

m4/3: E-P2, EM-5, 100-300, 14-42mm 12-50mm, 45mm, panny 14mm. 4/3: 7-14 + Flashes & tripods & stuff

"Take nothing but pictures and leave nothing but footprints".

Flickr gallery
Reply With Quote
  #7  
Old 10th October 2009
EH1
Guest
 
Posts: n/a
Smile Re: Trying to explain Focal Length, Field of View and Crop x2

Here`s my stupid question as physics is`nt my strong point either! I understand (and already knew) everything you have said, BUT does it mean that say a 10 megapixel Olympus would have the same image quality as say a 20 megapixel FF Camera ? very
Reply With Quote
  #8  
Old 10th October 2009
theMusicMan's Avatar
theMusicMan theMusicMan is offline
E-3 Enthusiast....:)
 
Join Date: Nov 2007
Location: South Wales
Posts: 5,878
Thanks: 143
Thanked 267 Times in 178 Posts
Likes: 8
Liked 30 Times in 19 Posts
Re: Trying to explain Focal Length, Field of View and Crop x2

Hi Edward

Good question, to which the answer is... no.

Taking a FF and a 4/3 camera with say 10MPx sensor... there will be positives and negatives for both.

There will be less noise on the FF camera image - the individual photo-sites are physically further apart, and are prone to significantly less noise.
There will be more pig detail captured on the 4/3 camera image because there is more of the pig in the shot. On the FF shot, only roughly 40% of the 10M pixels are of the pig, whereas on the 4/3 shot its about roughly 80%.
To get the same frame fill on the FF camera as the shot taken with the 4/3 camera, one would have to significantly crop the FF shot - thus using significantly fewer pixels to make up the final image. Of course, this being a FF image, the quality wouldn't be proportionally less, thus if you cropped the image by 50%, the resulting image wouldn't be only half as good quality as the 4/3 image.

Not sure if that explains what you're after Ed...?
__________________
John

Reply With Quote
  #9  
Old 10th October 2009
jdal's Avatar
jdal jdal is offline
Full member
 
Join Date: Dec 2007
Location: Northumberland
Posts: 4,046
Thanks: 129
Thanked 503 Times in 439 Posts
Likes: 202
Liked 665 Times in 283 Posts
Re: Trying to explain Focal Length, Field of View and Crop x2

Quote:
Originally Posted by EH1 View Post
Here`s my stupid question as physics is`nt my strong point either! I understand (and already knew) everything you have said, BUT does it mean that say a 10 megapixel Olympus would have the same image quality as say a 20 megapixel FF Camera ? very
Half the length and half the height is 1/4 the area. so 10 megapixels would give you a higher pixel density, but poorer "quality" pixels (allegedly!). But I see what you mean, the Oly 10mpx image should be comparable with a 20 mpx FF crop of the same size. Other factors being the same, of course.
__________________
John

m4/3: E-P2, EM-5, 100-300, 14-42mm 12-50mm, 45mm, panny 14mm. 4/3: 7-14 + Flashes & tripods & stuff

"Take nothing but pictures and leave nothing but footprints".

Flickr gallery
Reply With Quote
  #10  
Old 10th October 2009
EH1
Guest
 
Posts: n/a
Wink Re: Trying to explain Focal Length, Field of View and Crop x2

Yes! I`ve totally got it now! Thanks John & jdal. BTW. It is an excellent article John, I hope this becomes a regular thing.
Reply With Quote
  #11  
Old 10th October 2009
theMusicMan's Avatar
theMusicMan theMusicMan is offline
E-3 Enthusiast....:)
 
Join Date: Nov 2007
Location: South Wales
Posts: 5,878
Thanks: 143
Thanked 267 Times in 178 Posts
Likes: 8
Liked 30 Times in 19 Posts
Re: Trying to explain Focal Length, Field of View and Crop x2

Quote:
Originally Posted by EH1 View Post
Yes! I`ve totally got it now! Thanks John & jdal. BTW. It is an excellent article John, I hope this becomes a regular thing.
Glad you found it helpful Edward.
__________________
John

Reply With Quote
  #12  
Old 10th October 2009
jdal's Avatar
jdal jdal is offline
Full member
 
Join Date: Dec 2007
Location: Northumberland
Posts: 4,046
Thanks: 129
Thanked 503 Times in 439 Posts
Likes: 202
Liked 665 Times in 283 Posts
Re: Trying to explain Focal Length, Field of View and Crop x2

I've just had another think about this and it's this word 'crop' I have trouble with

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'm, being picky on this because on a climbing forum I frequent, the Canikonites have been using "crop factor" as a derogatory term. I've managed to bore most of them into submission by now though...

cheers
__________________
John

m4/3: E-P2, EM-5, 100-300, 14-42mm 12-50mm, 45mm, panny 14mm. 4/3: 7-14 + Flashes & tripods & stuff

"Take nothing but pictures and leave nothing but footprints".

Flickr gallery
Reply With Quote
  #13  
Old 10th October 2009
theMusicMan's Avatar
theMusicMan theMusicMan is offline
E-3 Enthusiast....:)
 
Join Date: Nov 2007
Location: South Wales
Posts: 5,878
Thanks: 143
Thanked 267 Times in 178 Posts
Likes: 8
Liked 30 Times in 19 Posts
Re: Trying to explain Focal Length, Field of View and Crop x2

Hi John

I could be wrong of course, but one other thing that could be brought into the mix here is the distance between the rear of the lens and the sensor. The further away this is then less of the projected image is going to be seen by the sensor. I think the 4/3 standard defines what the effective distance should be for the 4/3 sensor.

The one thing that is for sure is that the focal length of a lens does not change regardless of what camera body or sensor it is used on. A 200mm lens is always a 200mm lens.

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...
__________________
John

Reply With Quote
  #14  
Old 10th October 2009
jdal's Avatar
jdal jdal is offline
Full member
 
Join Date: Dec 2007
Location: Northumberland
Posts: 4,046
Thanks: 129
Thanked 503 Times in 439 Posts
Likes: 202
Liked 665 Times in 283 Posts
Re: Trying to explain Focal Length, Field of View and Crop x2

Quote:
Originally Posted by theMusicMan View Post
Hi John

I could be wrong of course, but one other thing that could be brought into the mix here is the distance between the rear of the lens and the sensor. The further away this is then less of the projected image is going to be seen by the sensor. I think the 4/3 standard defines what the effective distance should be for the 4/3 sensor.
Yes, I'm sure the standard does that - the m4/3 standard has a shorter distance. But regardless of that, all of the 4/3 lenses are meant to have the same image circle and the standard defines the image circle at the plane of the sensor.

Quote:
Originally Posted by theMusicMan View Post
The one thing that is for sure is that the focal length of a lens does not change regardless of what camera body or sensor it is used on. A 200mm lens is always a 200mm lens.
Absolutely, and your article illustrates that well. It's a remarkably common misconception.

Quote:
Originally Posted by theMusicMan View Post
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.
...
Aye, they're keen on ignoring these little things when it suits!

cheers
__________________
John

m4/3: E-P2, EM-5, 100-300, 14-42mm 12-50mm, 45mm, panny 14mm. 4/3: 7-14 + Flashes & tripods & stuff

"Take nothing but pictures and leave nothing but footprints".

Flickr gallery
Reply With Quote
  #15  
Old 10th October 2009
NSS's Avatar
NSS NSS is offline
Full member
 
Join Date: Oct 2009
Location: Diss. Norfolk. UK
Posts: 131
Thanks: 2
Thanked 5 Times in 3 Posts
Likes: 0
Liked 0 Times in 0 Posts
Re: Trying to explain Focal Length, Field of View and Crop x2

Quote:
Originally Posted by theMusicMan View Post
I posted this in response to NSS's not silly question in that thread too - but on reflection I feel it might help as a separate thread on its own... thus this one.
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
__________________
Today I am ready for yesterday...

http://www.flickr.com/photos/kevinfairgrieve/
Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Stupid questen RE focal length. NSS Legacy Lenses 9 22nd November 2009 01:27 PM
Depth of Field Preview with Live View? Jim Ford Olympus E-3 4 23rd June 2009 09:27 PM
IS & "Legacy" Lens focal length setting Graham_of_Rainham Olympus E-3 14 24th January 2009 06:37 PM
Focal Length Field Of View Comparison Zuiko Lens focus 9 22nd November 2008 09:26 AM
Focal length incorrect in exif ScubaRoy Lens focus 2 26th June 2008 08:11 PM


All times are GMT. The time now is 06:13 AM.


The Write Technology Ltd, 2007-2019, All rights reservedAd Management plugin by RedTyger