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07-04-2010, 07:27 AM   #1
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Field of view with small sensors

I know that focal length changes by a factor of 1.5 when using, say, an M lens on a digital camera. Does the field of view also change? I ask because a Cokin spot filter needs a fairly wide angle lens before it begins to 'work'.
Many thanks!
Jim

07-04-2010, 07:35 AM   #2
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The focal length never changes because the lens isn't changing. The 1.5x refers to the crop factor in that the sensor is 1.5x smaller than a full 35mm frame. So the image circle the lens projects remains the same between bodies but the digital sensor is smaller and thus crops the image. This results in a field of view equivalent to 1.5 x focal length but doesn't actually change the focal length of lens characteristics.
07-04-2010, 08:18 AM   #3
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QuoteOriginally posted by enoeske Quote
The focal length never changes because the lens isn't changing. The 1.5x refers to the crop factor in that the sensor is 1.5x smaller than a full 35mm frame. So the image circle the lens projects remains the same between bodies but the digital sensor is smaller and thus crops the image. This results in a field of view equivalent to 1.5 x focal length but doesn't actually change the focal length of lens characteristics.
Well said. Put more simply, a lens with a 50mm focal length will have its full 50mm field of view on a film camera. Put on an APS-C digital, it'll have a field of view equivalent to a 75mm lens (1.5x) on a film camera, but its focal length will still be 50mm.

This is why the 18-55 is the standard zoom on a digital...because it's field of view is equivalent to 1.5x(18) to 1.5x(55), or approximately 28-80mm.

The crop factor affects field of view directly and is why the focal lengths are cited to be different. In terms of Depth of Field, however, focal length is focal length, no matter what camera it's on
07-04-2010, 08:49 AM   #4
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Both enoeske and unixrevolution had wonderful explanations, however I ran across a set of diagrams that I though would help....

Crop Sensor (APS-C) Cameras and Lens Confusion



07-04-2010, 10:23 AM   #5
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There was one such comparison in the forums some time ago:
https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/99585-plots-fo...ield-view.html
07-04-2010, 11:58 AM - 1 Like   #6
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I dont want to be rude, but i think none of previous posters address the OP's question.

I believie with the spot filter, the lens has to be wide because of two reasons.
One is DOF - if the lens is too long, it's shallow dof will make the filter so much out of focus it wont be visible.
Other one is visiblity. The lens has to have wide enough field for the smeared edges of filter to be visible.

This makes me think, that just because of 2.) you still need to have a lens with wide view. So crop factor must be taken in account with any lens M, DA or m42.
Otherwise you'd simply crop out the sharp part/spot in center without showing the smear effect.
07-04-2010, 12:55 PM   #7
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QuoteOriginally posted by ytterbium Quote
I dont want to be rude, but i think none of previous posters address the OP's question.

I believie with the spot filter, the lens has to be wide because of two reasons.
One is DOF - if the lens is too long, it's shallow dof will make the filter so much out of focus it wont be visible.
Other one is visiblity. The lens has to have wide enough field for the smeared edges of filter to be visible.

This makes me think, that just because of 2.) you still need to have a lens with wide view. So crop factor must be taken in account with any lens M, DA or m42.
Otherwise you'd simply crop out the sharp part/spot in center without showing the smear effect.
I reread the question and you have a point. Although I really don't know how wide is fairly wide, so I thought that knowing the FoV of the lens might help. I don't know anything about spot filters, so I can't help any further.
07-24-2010, 06:48 AM   #8
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Thanks...

for taking the time and effort to reply and my apology for my slow response, (I had a serious family problem to deal with). All your replies were helpful and the last person gave me the information I needed. How wide is wide enough to include the smear is what I will experiment with.
Best wishes,
Jim

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