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Old Mar 7, 2006, 1:30 PM   #1
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Does it bother anybody else that camera manufacturers have started printing "equivalent" focal lengths on the lenses of point-and-shoots and EVFs? I hate to hear people claiming that the 200mm lense they used to use on their film rig is now a 300mm supertelephoto lense on their digital rig. The scaling factorfor DSLRs is all about angle of view, not magnification!A 200mm lense is going to give you the same amount of magnification on a digital body as it will on a film body, howevever, your angle of view will be more narrow on the digital body due to the lack of real estate where the image is collected (i.e. 1.5 crop factor for Nikon CCD). Is anybody in this boat with me?

-Blake
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Old Mar 7, 2006, 1:47 PM   #2
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Not really with you on this. Yes it is a crop not a strictly a magnification. However the result is much the same. If I put a 300mm lens on a Film SLR and a 200mm lens on a DSLR I will get the same angle of view in both shots so what I see on the digital image will be the same as on the film image. Also since a 50mm lens is considered to give an angle of view roughly equivalent to the human eye and on a DSLR a 35mm lens will give about the same then a 100mm Lens and a 70mm lens will each give half the angle of view so how else do we define magnification in a photographic image.

When it comes to digital compacts the only thing that makes any sense to me is the 35mm equivalent focal length. 10x zoom is meaningless if you don't know where it starts and ends. Quoting the actual focal length say 7mm - 70mm means nothing to meas sensor sizes are different and I can't really relate to them anyway. Telling me the 35mm equilvalent focal lengths gives me something I can relate to 20mm is wide angle and 200mm is a telephoto.


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Old Mar 7, 2006, 1:49 PM   #3
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"The imaging area is physically smaller. Less of the image circle projected by the lens is used, therefore it is a crop. The image remains the same size at the film plane for a given lens and subject distance – it is in no way magnified. It does, however, take up a larger proportion of the (smaller) frame and so it is easy to see why some people call it a magnifying effect." - http://www.luminous-landscape.com/tu...dslr-mag.shtml

It is true that it isn't really magnifying the image but the result is very similar. Since the "cropped" image takes up a larger proportion of the frame and the sensor records only that proportion at it's rated amount lets say 8 MP then that cropped portion comes out to have a resolution of 8MP.

If the same shot with the same distance and lens was used but on a FF camera that also was 8MP the same "cropped" portion of the image would have less Megapixels.

To sum up it isn't true magnification in the purest sense of the word but it is effective magnification as it yeilds more detail in a similar fashion to true magnificaiton. That said I have no problem with people saying 300mm instead of 200.

After all my Fuji S5100's actual focla length is 6.1-61mm and effective equals 37-370. I can garuntee you that a 370mm zoom 4MP pic of a subject has more detail (and looks larger at a given ppi) than a 61mm actual shot at 4MP and then cropped to the same angle of view as the 370mm shot.
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Old Mar 7, 2006, 3:18 PM   #4
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I agree with your arguments that a cropped image, enlarged to the same proportions as a FF print, is an effective magnification, but there is nothing physically equivalent about a 200mm lense and a 300mm lense. The focal length is a measure of the extreme lense element separation. This figure truly determines the magnification of the image, cropping and filling space is just enlarging the optical image...so in a sense, the crop factor is more like a built-in digital zoom. I know this is complicated, especially when you start considering sensor resolution, but based purely on the physics of the problem, I still disagree with "equivalent" focal length.

-Blake
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Old Mar 7, 2006, 3:57 PM   #5
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What do you propose we use to describe the crop factor?? Since there is no standard (canon has 1.6x and 1.3x, nikon 1.5x, olympus 2x) how should we refer to lenses on digital bodies???

Really, the term is used to give a point of reference so folks understand what they're getting. There are many marketing gimmicks ("x" zoom comes to mind) but this is not one of them. I cannot think of a better term to describe the crop factor.
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Old Mar 7, 2006, 5:47 PM   #6
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Advertising a crop factor is fine. I just don't think it's entirely clear (maybe I don't have it clear in my head) that this crop factor, and it's relationship to focal length, is in regards to angle of view. If I am looking at a lense and I know that the wide end of the zoom is 25mm, and that my crop factor is 1.5, then I know that this lense will produce an angle of view equivalent to the angle of view of a 37mm lense on a FF camera. The crop factor is very useful in this sense (as a reference to 35mm film as common ground),but I think the average consumer gets lulled into a false pretense as to the lense'soptical performance.

-Blake
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Old Mar 7, 2006, 6:24 PM   #7
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I think most people generally think about the field of view associated with the focal length more than the focal length itself. When a particular focal length is mentioned, people think about what "they see" and the crop factor or 35mm equivalent does a better job of conveying this than field of view. Field of view has never been a reference point in photography (at least for my generation)...lenses have always been described by focal length....that's why continuing the terminology and using that as a point of reference makes sense to me.
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Old Mar 8, 2006, 3:33 AM   #8
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I appreciate what you say about the optical magnification of a 200mm versus a 300mm lens.

However in photography when we talk about magnification or wide angle what are we talking about if not the field of view as compared to the human eye.

The sensor in the human eye is not 35mm so a 200mm telescope would give more magnification than 4x.

You can't really compare a the digital crop factor to digital zoom as digital zoom is lowering the resolution whereas with the digital crop the sensor is working at it's full resolution. Whether or not that resolution is equivalent to 35mm film is another issue and not really relevant to this discussion.


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Old Mar 8, 2006, 12:27 PM   #9
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No, the perspective game is played based on where you are standing, and how much the lens takes in (angle of view). Stand one foot from a person and fill the frame with thier face using a 28mm lens and your likely to be looking up thier nose. Stand back sixty feet and fill the frame using a 400mm lens and your perspective is different, because of where you are standing. Take the same picture from sixty feet using a 28mm lens, then crop just the persons face, and it will look the same as the 400mm shot (resolution differences aside).

Now to compare in less practical (but more common) terms, say a 100mm lens on a 35mm film slr lets you fill the frame with a persons face at 20 feet. Put that lens on a typical DSLR and you have to back up to get the whole face in the frame, changing your perspective. Same lens, different cameras, different location, different perspective.

Stay in this same place and use a 150mm lens on the film camera, and all is equal. 100mm digital lens, 150mm film lens, same location same composition same perspective. Alas, the 100mm lens does equal an equivalent 150mm in film terms.

Don't worry about magnification, that's an internal mechanical technicality that has no meaning in real-world use. If you crop a 35mm negative and print an 8x10, or print a full-frame 8x10 from a digital file, they are the same (assuming same camera location and setup). If your concerned about sensor pixels per inch, or theoretical film resolutions, and resolving power of the lens, then the 100mm and 150mm lenses may have some slight physical differences, but the argument is pedantic. I don't work for nasa and I don't have a 82mp sensor, so the horribly minute optical differences between using 1 square inch of glass per megapixel or 2 square inches of glass per megapixel are unimportant. Here in the real-world, they do the same thing. You can use any amount of magnification, and any size image capture media, but when you print to final media the magnification is UNDONE. The only differences are resolution and field of view.


If you wanted to have clear marketing, the real focal length should be displayed along with effective field of view angles. Then sensor size differences and crop factors would be irrelevant, and we'd all use a 28 degree normal lens (whether that's 50mm focal length on a film slr, 35mm focal length on a DSLR, or 11.4mm focal length on a prosumer digicam). BUT since nobody talks in degrees of view, and a few short years ago everybody used 35mm film, we now talk in 35mm equivalent terms. By saying "35mm equiv" we are saying, "in practical terms this is what your going to get."

Its like measurements in feet....
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Old Mar 8, 2006, 6:33 PM   #10
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Very good point. I can buy that explanation...also, tomorrow is national Get-Over-It day (in the U.S., seriously) and this will be my pet peeve to get over. Thanksto everyone for participating in this discussion.

-Blake
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