Have I got this right?

[ericmark]

http://www.photography.ca/Forums/members/ericmark/albums/odd-pictures/326-lens-length-diagram.jpgThe focal length of the lens is in theory measured from where the light waves cross with pin hole camera to where the film is placed. Because that will depend on film size the amount of seen captured with a 35mm film with 400mm lens will be same as 23.5mm film with 270mm lens as the lens can be closer to the film plane as shown. But with the D-SLR the lens is not closer but the light is projected to non receptive area. This allows the same lens to be used with both film and CCD cameras. In some cases the diameter of the lens will only allow it to be used with smaller film plain so it is marked with effective length. Therefore a lens marked 18mm – 55mm on a 23.5mm CCD can give exactly the same results as a lens marked 27mm – 82mm on a 35mm film (Compare black and green lines). Conversely a 400mm lens from a 35mm film camera becomes a 600mm lens on a 23.5mm CCD camera (Red lines).
Just checking I have got this right before handing in my work?

[Mad Aussie]

No ... you spelt 'length' wrong! :)

As far as the rest is concerned ... dunno ... I've never bothered to look into it this closely.

[mbrager]

I assume the question is what is the difference between full frame and crop frame cameras. Full frame cameras have a sensor the same size as the 35mm film cameras. Crop frame cameras have smaller sensors and there is a wide variety of these. Your diagram is attempting to explain the differences when using the same lens on different size "film" or sensors.
Try this link for another explanation of the same thing: DSLR Magnification (http://luminous-landscape.com/tutorials/understanding-series/dslr-mag.shtml)
You an see for yourself where your work and the article's information match up. Hope this helps and isn't more confusing.
Mike

[casil403]

No ... you spelt 'length' wrong! :)

As far as the rest is concerned ... dunno ... I've never bothered to look into it this closely.

MA...you spelled "Spelled" wrong! Lol...spelt is a type of flour/grain! :laughing:

[ericmark]

I assume the question is what is the difference between full frame and crop frame cameras. Full frame cameras have a sensor the same size as the 35mm film cameras. Crop frame cameras have smaller sensors and there is a wide variety of these. Your diagram is attempting to explain the differences when using the same lens on different size "film" or sensors.
Try this link for another explanation of the same thing: DSLR Magnification (http://luminous-landscape.com/tutorials/understanding-series/dslr-mag.shtml)
You an see for yourself where your work and the article's information match up. Hope this helps and isn't more confusing.
Mike
Seems to match up OK. Since it will be part of my exam though it would be good to check first so seems I just need to correct spelling.

[Mad Aussie]

MA...you spelled "Spelled" wrong! Lol...spelt is a type of flour/grain! :laughing:
That's only 1 meaning. Spelt is also the past tense of Spell
Spelt | Define Spelt at Dictionary.com (http://dictionary.reference.com/browse/spelt)

;)

[casil403]

That's only 1 meaning. Spelt is also the past tense of Spell
Spelt | Define Spelt at Dictionary.com (http://dictionary.reference.com/browse/spelt)

;)

Lol ya got me...but I'm still taller than you...lol! :evil2:

[JAS_Photo]

Poor Eric. At least someone answered his question. Very good link btw mbrager. So what we are saying here, in very basic terms is that a lens that gives a normal view on a 35mm or full frame camera over extends its range on a crop frame camera and gives the appearance of a longer lens. So tecnically, if I have a very good crop frame camera and take an awesome photo of an eagle, and say Micaelaw has a very good full frame camera and takes an awesome photo of an eagle with the same lens from the same distance, he should be able to crop his photo down to the have the subject the same size as mine without losing resolution because technically mine is already a crop of sorts. Does that make sense?
http://luminous-landscape.com/tutorials/understanding-series/dslr-mag.shtml

[ericmark]

That is my understanding. The second point is if you used a cropped cameras lens on a full screen camera likely the corners of the picture will have no image. With mine at wide angle this is the case.
So a 35mm lens will fit a 23.5mm CCD camera but a 23.5 CCD lens will not fit a 35mm camera without some cropping.

[mbrager]

JAS: It makes sense like butter on bread. Unless it makes you want to run out and buy a full frame camera. Except for the part about resolution and cropping which is also related to the number of megapixels and the density of pixels in each camera.
Ericmark:
I use Canon equipment and they manufacture lenses for both full frame and crop frame cameras. They only manufacture one or two full frame cameras anyway. But the full frame lenses will also fit their crop frame DSLR cameras. But the crop frame lenses, called EF-S lenses, are only suitable for the crop frame cameras and won't fit the full frame cameras. So what you are saying in your post is accurate, but the camera manufacturers have already prevented this from being able to happen. Good luck with your exam. When is it?
Mike

[ericmark]

Thanks for confirming Exam Monday and Tuesday 22 and 23 of March. Out of interest I tried fitting my D-SLR lens on old 35mm camera. And as I zoom out you can see the round edge of lens. Fitted OK but no film so did not try taking a photo. What I do wonder is with third party lenses years ago they came with 42mm screw thread onto which one fitted the bayonet mount for your camera.
However the size of the CCD seems to vary make to make so since my Pentax is one of the larger CCD's at 23.5mm I wonder if any of the third party lens would crop some picture? Also width to hight old was 36mm x 24mm and new is 23.5mm x 15.7 which is nearly the same relationship but not sure if all are like this? Of course when working out lenses one has to go diagonal like with TV so film was 43.26mm and new CCD is 28.26mm with film the 24mm was limit as width of film but with D-SLR there is no such constraint.
Best for lens is 1:1 but surb is 1.618 (Golden ratio) so 24mm x 14.8mm would have same diagonal but follow golden ratio. Seems the ancient Greeks knew all about surb but monitor and paper manufactures haven't caught up!
Oh yes other subject I study is maths. Bet you never guessed?

[edbayani11]

the focal length remains the same. if it is 300mm it is 300mm when used in any camera. it is the camera format that changes.
like the 90mm is considered as wide angle in 4x5 view cameras, it remains a 90mm lens. what changes is the covering power of the lens like the 90mm for a 4x5 can cover more than the diagonal of the film giving room for tilts and swings
while the 90 of a 35mm camera will cover only the size of the film or ccd. that is why view camera lenses are more expensive.
focal length produces the image size.

[ericmark]

You are right and once you pointed it out it clicked. Thanks for all your help.