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Forum: Pentax Lens Articles
05-25-2013, 06:26 PM
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Thanks to Adam and rparmar. I understand my error now.
I'm going to use the word 'resolution' and when I do, consider the light that the lens is refracting - do not think about sensor resolution!
The simplest way I can think of to summarize (dangerous, I know):
When designed for different sensor sizes, the same focal length will have a different field of view because
a) the light arriving at the sensor plane is covering a different sized area (full frame cm^2 > APS-C cm^2) and
b) the resolution of the image arriving at the sensor plane is the same for each lens.
ergo, the field of view for an APS-C designed lens is smaller than the field of view for a full frame designed lens at the same focal length.
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Forum: Pentax Lens Articles
05-25-2013, 10:28 AM
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Thanks, Adam. I see what you mean about focal length saying nothing about the angle of view unless we know the sensor size. Knowing that some lenses are designed for full frame sensors and some are designed for APS-C sensors means that we can have a focal length of 35mm, for example, for each type of sensor, each giving the SAME angle of view...
I don't shoot Nikon, but I found this interesting: Camera Lens Explained | DX & FX Format Lenses from Nikon from Nikon
It shows how some lenses have the same focal length but are designed for different sensor sizes. Unfortunately, the article is not technical enough to explain how they design DX (APS-C) lenses with the proper angle of view.
So I still wonder what Sigma is doing, marketing a lens for APS-C and stating an angle of view based on a full frame sensor with a different focal length. It seems like you can put this lens on a full frame sensor and get the 30mm angle of view.
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Forum: Pentax Lens Articles
05-25-2013, 09:33 AM
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Sigma makes a "30mm f/1.4 DC HSM" where DC denotes a lens designed for APS-C sized sensor, HSM is the focus drive 'Hyper-Sonic Motor'.
From what I understand about lens focal length and sensor sizes, this statement, by Sigma, is very confusing:
"...this large-aperture standard lens with an angle of view equivalent to 45mm on a 35mm camera..." (emphasis is mine)
30mm F1.4 DC HSM | A | Sigma
If the lens is designed for an APS-C sensor and they say it is 30mm then should the angle of view not be the same as a 30mm full frame lens on a full frame sensor? Or are they marketing a full frame lens as an APS-C lens... If I'm not missing something obvious then I'll pose the question to Sigma.
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