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Old Oct 20, 2009, 4:53 PM   #1
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Default Focal length question

I'm such a novice I'm not even sure I'm asking the right question or using the correct terminology. I currently have a Panasonic Lumix with a 12X optical zoom. On the side of the lens it says it goes up to 420mm "35mm equivalent."

Recently I saw a Panasonic camera with a 12X optical zoom and it said it was 300mm. I find this zoom stuff confusing. How can you tell when looking at digital camera specs if they are talking about 35mm equivalent or are using digital measurements?

The reason for the question is I often take pictures at the maximum zoom on my camera and now I'm thinking about upgrading to a SLR. Specifically a Canon T1i and would like to get a lens that has the ability to zoom out to at least 400mm "35mm equivalent." So when I see a lens for a Canon digital camera that says 75-300mm are they talking about "35mm equivalent" or the digital equivalent and how can you tell? And if they are using digital measurements is there a conversion equation so I can help my old 35mm film brain figure out how close I'll be able to zoom in?
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Old Oct 20, 2009, 5:26 PM   #2
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First, the 35mm equivalent focal length is the specification that's used to provide a consistent reference to the angle of view. Since different cameras, especially P&S camera have different size image sensors, a lens with the same actual focal length would have a different angle of view on each of them, By stating a 35mm equivalent focal length, you can compare apples to apples, so to speak. So a lens with 35mm equivalent focal length of, say, 100mm would have an angle of view of 24, regardless of what it's actual focal length is..

Second, the 'X' has nothing to do with the actual focal length or the 35mm equivalent focal length. It simply identifies the lens as a zoom lens (which allows you to vary the angle of view) and is the ratio of the shortest focal length (widest angle of view) to the longest. So a 10-100mm lens is a 10X lens, but so is a 5-50.

So, both values are useful. For instance, if you're looking for the greatest flexibility, you may want to get the largest 'X', if you want to shoot landscapes, you may want to get the shortest 35mm equivalent focal length, and if you want to shot wildlife, you may want to get the longest 35m equivalent focal length.

In the case of SLRs and dSLRs, the lenses are always identified with the actual focal length, not the 35mm equivalent focal length. That's because the same lens could be mounted to different cameras and would provide different angles of view, depending on the camera, not the lens.

The Canon APS-C dSLRs have image sensors that are 22.3mm by 14.9mm, while 35mm film exposures are 36mm by 24mm. So a 35mm film exposure is about 1.6 times larger than a Canon APS-C image sensor, to get the 35mm equivalent focal length of a lens on a Canon APS-C dSLR, multiply the actual focal length by 1.6. So a 75-300 lens (which would be a 4X zoom, btw) would have a 35mm equivalent focal length of 120-480 on a Canon T1i.
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Last edited by TCav; Oct 20, 2009 at 5:37 PM.
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Old Oct 21, 2009, 6:48 PM   #3
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Thanks for the info. So if I understand you correctly I can calculate the 35mm equivalent (roughly) by dividing the length of one side of the sensor into 36 and then multiplying that by the mm specs for the lens. Sounds simple enough.
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Old Oct 21, 2009, 6:54 PM   #4
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I forgot to add this thought. Using your example the 18 to 55mm lens that typically comes with the Canon T1i is the equivalent of roughly a 29 to 88mm lens on a 35mm film camera. Also I meant dividing the length of the longer of the two sides of the sensor into 36 in the above post.
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Old Oct 21, 2009, 7:20 PM   #5
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Quote:
Originally Posted by Gary S View Post
Thanks for the info. So if I understand you correctly I can calculate the 35mm equivalent (roughly) by dividing the length of one side of the sensor into 36 and then multiplying that by the mm specs for the lens. Sounds simple enough.
Sort of. For P&S digicams, it's more complicated than that, and is affected by the aspect ratio of the image sensor and how the manufacturer has determined the value. And most manufacturers don't actually report the size of the image sensors in their P&S digicams.

See http://en.wikipedia.org/wiki/35_mm_e...t_focal_length and http://en.wikipedia.org/wiki/Image_sensor_format for more info.
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Old Oct 21, 2009, 8:20 PM   #6
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Using the sensor height instead of width is a more correct way of doing the conversion, as it accounts for the difference in aspect ratio.

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Old Oct 22, 2009, 5:11 AM   #7
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Using the sensor height instead of width is a more correct way of doing the conversion, as it accounts for the difference in aspect ratio.
But different manufacturers use the diagonal measurement to calculate the result.

And, once again, P&S manufacturers usually don't report the actual dimensions of the image sensor, so we've got nothing to calculate with.
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