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Old Aug 30, 2004, 5:57 PM   #1
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Hi all,

A theoritical question that has been bugging me for a while. I had a similar question a while back but never quite got the clarification I was looking for me.

1) Does the DLSR multiplier add magnification to a lens?

From what I have read I believe that if a 35mm lens is used on a DSLR which has a multiplier the only effect is a field of view of crop. If the lens was used on a 35mmcamera effectively the same picture could be achieved by cropping the picture.

For example, if a DSLR has a 1.6 multplier, and a 100mm lens for a 35mm cam is used, the field of view would appear to be that of a 160mm lens. However the apperent magnification is only a result of the field of view crop. Optically there is no magnification.

Is this understanding correrct? Why then do people talk up the multiplier. Even for a telephoto lens it is still bad, no? All it means is less field of view, and a crop/framing that could have been achieved with some software editing?

Thanks for any clarification. Hope I am on the right track with this one.

Cheers, Tim
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Old Aug 30, 2004, 6:31 PM   #2
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You are correct that it's only cropping the image. That's why you see the term "crop factor" used more and more often now (versus "focal length multiplier").

But, you still have the same amount of data in the image, based on the resolution of the CCD, even though it's not using the entire image circle.

In other words, you could have a full frame (35mm film size) sensor with 6 Megapixels and it would contain the same number of pixels as an APS-C size sensor that only used part of the frame.

There is also a crop factor for non-DSLR models (which is even higher, because the sensors are much smaller). For example, a camera like the Canon Powershot A80 has a 35mm Equivalent Focal Length of approximately 38-114mm. Yet, the lens on this model has an actual focal length of 7.8-23.4mm. Note the photo of the lens on this page:

http://www.steves-digicams.com/2003_reviews/a80.html

So, it's crop factor is approximately 4.9 (38/7.8 ).

This is also the reason that you have dramatically more depth of field on a non-DSLR Camera model, for any given focal length, aperture and focus distance; compared to a 35mm SLR (or even compared to a DSLR with a 1.5 or 1.6 crop factor). This is because depth of field is based on the actual focal length of the lens (versus the 35mm equivalent focal length).


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Old Aug 31, 2004, 12:16 AM   #3
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Hi Jim,

Thanks for your answer. Helped to clarify.

However I didn't quite catch your point about the sensor:

"In other words, you could have a full frame (35mm film size) sensor with 6 Megapixels and it would contain the same number of pixels as an APS-C size sensor that only used part of the frame."

Thanks for your help. Tim


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Old Aug 31, 2004, 7:43 AM   #4
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Tim:

I threw the sensor part in because of your intial post referencing the cropping of 35mm film.

I wanted to make sure that nobody thought that a crop factor means less detail captured when you're talking about a Digital Camera Sensor. Just because you're not using the entire image circle, you still get just as many pixels with a small sensor, compared to a larger sensor, if thepixel count of the sensor is the same.

Actually, you do have full frame models without a crop factor, and other models with a smaller crop factor.

For example, the Canon EOS-1D and Canon EOS-1D Mark II models have a little larger sensor compared to the entry level DSLR models, with a crop factor of 1.3x.

There are also some "full frame" DSLR models with 35mm size sensors: Canon EOS-1Ds, Kodak DSC-14n, Kodak DSC-SLR/c, Kodak DSC-SLR/n. These "full frame" models do not have any kind of crop factor.

Now, they do have more megapixels compare to the entry level models, but you could build a larger CCD that's only 6 Megapixels, too. In this were case, you'd be capturing just as much resolution ina smaller 6MP Sensor (using a crop factor), as you would with a larger 6MP Sensor that did not use a crop factor.

In other words, you're not cropping part of the sensor with entry level DSLR models -- the sensor is only using less of the image circle being projected by the lens.


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Old Sep 1, 2004, 12:12 AM   #5
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got it. Thanks for your help
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