Okay, Now I am REALLY confused about DX
Barry Nichols
Registered Users Posts: 55 Big grins
I don't know if anybody gets "PC Photo", but in the MAR/April 2008 copy or online at http://www.pcphotomag.com/gear/photography-gear/lenses-designed-for-digital.html, there is an artilcle "Lenses: Designed for Digital." In that article the writer says about small sensors and "digital" lenses:
HuH! In the third paragraph he comes up with the same number I would using a 1.5x magnification factor, assumingly talking about FX lens's.
But his last statement has be baffled. What happened to my 12-24 DX being more like a 21-36? Is it worse? Does this mean it is still better to use a 14-24 FX lens on a DX Body? Help me understand the math :scratch
And since the original design standard for many SLR-turned-D-SLR lenses is 35mm film, the image circle they create is intended to cover the area of a 35mm frame: 24x36mm. With the Nikon D3 or the Canon EOS-1Ds camera line, there’s no need to apply a magnification factor to focal length because their sensors are the same size as 35mm film.
However, if the sensor is smaller than a full-frame, and a majority of them are, a portion of the frame becomes cropped because the image circle is much larger than the sensor actually needs. If the sensor is two-thirds the size of a full-frame, a 28mm focal length will look like a 42mm focal length, as if you had zoomed into the scene you’re photographing.
To maintain the same angle of view, which is especially important with wide-angles, manufacturers have to shorten the focal length. That’s why you often see a 35mm-equivalent focal range listed with interchangeable lenses. Because so many of us still think of focal range in terms of the 35mm standard, manufacturers include that as a reference.
That way we know a 12-24mm DX Nikkor, designed for Nikon’s DX sensors (15.8x23.6mm) will have a 27-52.5mm perspective, or a Canon 17-55mm EF-S, designed for its smaller APS-C-sized sensors (15x22.5mm), will perform like a 27-88mm EF lens , and so on.
HuH! In the third paragraph he comes up with the same number I would using a 1.5x magnification factor, assumingly talking about FX lens's.
But his last statement has be baffled. What happened to my 12-24 DX being more like a 21-36? Is it worse? Does this mean it is still better to use a 14-24 FX lens on a DX Body? Help me understand the math :scratch
Barry Nichols
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Nikon DX cameras have a "crop factor" of 1.5x, which means that "either" an FX or DX lens will have a focal length equivalence equal to the actual focal length times the crop factor in term of field-of-view.
DX lenses will only work properly on DX cameras, but the actual focal length is the same as the same focal length FX lens. Only the "image circle" is affected, the DX image circle being smaller since it only has to cover a smaller sensor.
A 12-24 DX lens will have the equivalent field-of-view on a DX camera as an 18-36mm lens on an FX camera.
An FX lens may experience less vignetting on a DX body, but there is otherwise little advantage to an FX lens on a DX body versus a DX lens of the same actual focal length.
If you were to upgrade your body to an FX body then you could continue to use any FX lenses already in your kit, but DX lenses would not work properly. Some folks use this as a reason to only buy FX lenses, but I buy what I need and sell what I don't need.
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Okay, I am with you, but where does this writers calculation come from for a 12-24 DX Nikkor?
When you say are you and he talking about something different or the same thing? This is where my confusion lies.
What the previous writer is saying does not make sense to me either.
In order to calculate the equivalent field-of-view you multiply the focal length by the crop factor so for the 12-24mm zoom you multiply the 12mm by 1.5 and get 18mm equivalent for the wide end, and multiply the 24mm by 1.5 to get 36mm at the longer end.
Since an FX sensor has no crop factor, any lens mounted on it has the measured focal length with no crop/conversion needed.
The DX imager is very much like using a teleconverter of 1.5x magnification, except that a DX imager has already been compensated for exposure (meaning there is no exposure penalty like an optical teleconverter would incur.)
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