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08-06-2012, 12:38 PM
manthing manthing is offline
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Say i had a FX lens and took that photo on a FX body, i would get the whole scene - ie everything inside the green square?


if i used the same FX lens, but on a DX body, i would only get what is inside the red square - correct?
that is the crop factor - for arguments sake say 1.5x crop factor

i'm understand that fully. where i'm unsure is the "reach".
eg a 400mm FX lens becomes a 600mm lens on a DX body.

now, a 400mm FX lens on a FX body has certain characteristics. like certain minimum focussing distance, a certain DoF etc. so when it becomes a 600mm lens, how do all those things change?

will i really get a further reach? ie a larger magnification of the bird/wildlife i'm trying to photo?

i'm googling around for this info but am getting nowhere. virtually all sites only show the crop factor change.
so, you have any ideas? could you point me to a website with the info?

ta for your help.


Edit / Update:

i think i've finally found the answer.

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that shows the image for a 50mm on a FX body and then on a DX body. so that shows the "reach" we get - ie the 50mm becomes a 75mm.
there will also be changes to the min/max focussing distances and DoF for which i think i can use http://www.dofmaster.com to work out what's what.

by all means please add whatever knowledge you have on this. i'm learning all the time.

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08-06-2012, 12:55 PM
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kpmedia kpmedia is offline
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The size of the sensor changes the angle of view, because it's still relative to the length of the 35mm body. Less image is recorded by the smaller sensor, because it's smaller. However, the size (depth) of the 35mm body is still the same. If you simply removed a FX sensor, and put a DX sensor in its place, you'd see light hitting outside the sensor. That light is not recorded.

The 35mm lenses are created at a size to match the 35mm film or sensor. So more light gets in. But it's not recorded by the smaller sensor, because it's APS-C sized (~25mm), not 35mm sized. The sensor can only "see" the inner 2/3rds (1.5x crop) of the light. The "DX" lenses only allow light from the inner 2/3rds, and you can see this when you mount a crop lens on a full-frame body. I have several images like this, where I had used a 10.5mm DX fisheye on a FX body. Only the inner 2/3rds of the photo has image data. The outer 2/3rds is a shadow.

Because it records a small portion of light, it's "effectively" the same as 1.5x distance. But it's really not much different from cropping in Photoshop, and then claiming you've zoomed in. The biggest issue with modern high-megapixel crop bodies is that lens flaws become more obvious, because you're now just using the inner portion of the lens. Older film lenses don't always do well magnified to 18MP (where only the inner 12MP is recorded by the 12MP crop body DSLR).

Lens quality is currently a primary limiter in digital photography -- not megapixels. We're now resolving beyond the detail capturing ability of once-popular zoom glass. Only primes (non-zoom lenses) have stood up well to this abusive optical magnification.

Because the angle of view is smaller, it does give different perception of depth compared against full-frame use of the entire lens. So you'll get higher depth of field on the crop body. The same distance would be shallower on the full-frame body. You can use this to your advantage, sometimes -- with macro and semi-macro photography, for example.

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