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08-15-2013, 02:38 AM   #1
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Macro & Crop Factors

Having purchased a reversing ring for my kit 18-55mm lens. I have now thought about purchasing a 50mm or 28mm. I understand that a 50mm reversed gives approx. 1:1 (might be wrong?) and that a reversed 28mm will get me closer; shorter focal lengths giving greater magnification when reverse mounted.

What puzzles me is the crop factor aspect when using older 35mm lenses in reversed mode. For example a 50mm (Non-Di lens) on my K10D not reversed equals 75mm with the 1.5x crop factor, a 28mm would be more 'standard' (closer to 50mm) at 42mm with the 1.5x crop factor.

However, when I reverse these on the camera how does the crop factor work then? Does the focal length remain the same? Or is the reversed 50mm now reduced by the 1.5x crop factor to effectively become a 25mm 'back to front'? Someone may know all this and I'm a bit confused!

Also, given the above crop factor, what will the magnification be on a reversed 50mm (non-Di lens) be; 1:1 would only be in the 35mm 'full frame' format surely? I'd like to be able to work out what magnification I'm working at.

Thanks to anyone who can throw some light on this...and help my floundering!

08-15-2013, 02:43 AM   #2
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To keep things simple: the reproduction ratio and focal length stay the same, your DSLR just sees less of the frame than a 35mm camera would.

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08-15-2013, 03:45 AM   #3
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Thank you for the prompt response Adam, appreciated. So the focal length on a reversed 50mm (non-Di lens inclusive of the 1.5x crop factor) will be still 75mm even when reversed and the magnification remains 1:1 albeit a smaller image area? Thanks for the advice.
08-15-2013, 04:35 AM   #4
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The lens doesn't get any longer/shorter by changing formats. 50mm is 50mm.

The cropping factor of 1.5 is relative to FoV which does change. On the smaller sensor surface you are only measuring a portion of the image projected by the lens.

A 1:1 capable optical set up will still be 1:1.

08-15-2013, 11:59 AM   #5
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Yep. crop factor means just that - NOTHING changes optically. The Pentax APS-C sensor is 75% of the size of the 24mmX36mm image on the film ..... it is just like taking a pair of scissors and trimming the edges of the film - - - or if you have ever done any enlarging work in a dark room it is the same as projecting an image larger than the photo paper in order to <drum roll please> crop the original image size.

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