50mm Digital/Film Crop Factor example

[Corpsy]

ennacac wrote:

Focal length never changes no matter what camera the lens is on, it is a fixed physical characteristic of the lens.

" The focal length of a lens is defined as the distance in mm from the optical center of the lens to the focal point, which is located on the sensor or film if the subject (at infinity) is "in focus"."

http://www.dpreview.com/learn/?/Glos..._Length_01.htm

Only FOV changes!

Who was suggesting the focal length of the lens changes? Ken was pointing out that the 1.5x crop factor creates a longer equivalent focal length. This was pretty much the entire point of your original post. Nobody argues that the crop factor creates a new equivalent focal length, something you can read about in practically every DSLR review ever published.

The point argued was that the pictures you posted as examples of different equivalent focal lengths were not accurate representations of the effect, but rather illustrated the kind of differences you'd expect from vastly different perspectives. However, since the pictures appear to have been taken down, or the image links have since died, I suppose it is now a moot point.

people , you are not getting the point. here's how it is. an image taken with the digital and the 35mm will be exactly the same size on cameras. the only difference is that with the 35mm there will be more of an image above, below and to each side. the crop factor does not magnify. it just shows less of the scene.

lets look at a 1:1 macro. same size in life as it is on the image be it a digital or a 35mm. the only difference is that on the 35mm image it will show more background. the images are still the same size. 1:1 is 1:1. the crop factor does not multiply or magnify. on the digital there is just less area covered.

roy

[Corpsy]

This is still parsing language and depends on your definition of "magnify". The crop factor of a digital camera effectively captures a smaller area in higher detail, with the exact same results as placing a 1.5x teleconverter on the film camera. That might not meet your technical definition of magnification, but if the results are the same, I don't see the difference.

you're wrong..
there is no magnification. it's just that you are getting less of an image than the 35mm will get. a 300mm is still a 300mm. thats optics.

tom, i know what you mean.. it's the same image circle with just less of the area presented. a 300mm is a 300mm at no matter what the size that's captured by the film plane or the sensor plane.. it's the same size wheather digital or 35mm. it's the amount of the scene that's represented that is the crop factor.

a 50mm still gives the same image on a 35mm as a 1.5 crop factor digital image. the 35mm just shows a larger capture.

roy

[Corpsy]

robar wrote:

you're wrong..
there is no magnification. it's just that you are getting less of an image than the 35mm will get. a 300mm is still a 300mm. thats optics.

tom, i know what you mean.. it's the same image circle with just less of the area presented. a 300mm is a 300mm at no matter what the size that's captured by the film plane or the sensor plane.. it's the same size wheather digital or 35mm. it's the amount of the scene that's represented that is the crop factor.

a 50mm still gives the same image on a 35mm as a 1.5 crop factor digital image. the 35mm just shows a larger capture.

roy

I still maintain that you are parsing language. Nobody has claimed that the crop factor somehow magnifies the size of an image, only that the effective focal length is increased, and that a 200mm lens on a DSLR with a 1.5x crop factor would obtain the same image detail as a 300mm lens on a 35mm film camera. If you're doing telephoto photography as one would in birding, the effect could be considered magnification of the detail of the subject, but not of the entire image area.

Or are you claiming that that isn't the case because a film camera captures all the detail of the DSLR plus all the image area that was cropped off? Basically, if a 10mp photo from a DSLR makes a sharp 12" x 8", you could achieve an identical image with identical detail by putting that lens on a 35mm SLR, shooting the photo, printing it at 15" x 10.6" and then trimming off the edges.

Let's break it down into some numbers:

Let's say we're using the Canon XTI. The camera captures a 10mp image, 3888 pixels wide by 2592 pixels high. If you take that lens and put it on a 35mm film camera, you'll capture the exact same image, plus 1/3 more width and 1/3 more height, so 5184 x 4356 pixels, equaling 17.9mp.

Now of course I realize detail on film wouldn't technically be measured in pixels, but if you're saying that a film camera captures all the detail the DSLR does plus whatever was cropped off, then this argument is perfectly valid and a film camera should be capable of producing the equivalent of a 17.9mp image.

I don't know about you, but I have yet to see anyone argue that anything less than medium format film would capture that kind of detail. In fact, JimC recently posted some links to a couple older articles comparing 3mp and 6mp images to 35mm film. Here they are:

http://www.luminous-landscape.com/re..._vs_film.shtml

http://www.luminous-landscape.com/re.../d60/d60.shtml

Here are a couple links to some decent explanations and examples of how the crop factor works:

http://www.millhouse.nl/digitalcropfactorframe.html

http://www.luminous-landscape.com/tu...dslr-mag.shtml

[nadnerb]

Here's some shots I took recently just for comparison purposes.
Both the shots were taken from the same point with the same lense, a Pentax A 50mm 1.4 @ f8

First is the film shot:



And this is the digital shot:



The above shots are uncropped and when printed were the same physical size, 15cmx10cm

This shot is the film shot with the digital shot on top (reduced to 70%) to show the crop effect.



As you can see, they match up perfectly.
I'm not sure what this proves with regards to the arguments above, but I think it proves Tom's pictures were taken from a different spot.