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Old Feb 22, 2006, 3:26 AM   #1
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Looking at a forum post on another site I was surprised to see a message suggesting that.. F4 on a 5D was the same as F2.8 on a 20/350D because of the 1.6x effect. My understanding was that, whilst what consituted "normal" focal length (and a result DoF and the likes) was determined by the diameter of the image-area (ie approx 50mm for a 35mm film, 85mm for medium format and c35mm for DSLR/1.6) the F-stops (being a factor of EV) remained the same across all film formats. So, as rough rule of thumb and using a 100ASA setting, all image-size/lens combinations would respond to EVn in the same way; ie for a bright outdoor scene we might see either 125/1 @ F11 or 250/1 @ F8 or 60/1 @ F16, etc. So when I see an EF-S lens and in order to understand the lens in terms of 35mm photography, I need to convert both focal-length ("that's equivalent to a 35mm-85mm lens on a film camera") and I also need to convert the F-values as well ("that's equivalent to F5.6 to F8 in 35mm film terms")? Firstly this makes EF-S look unattractive - no wonder we see lots of cheap IS rather then expensive glass being added to these lenses - it also makes standard EF lenses quite attractive; if an EF/F4 is actually F2.8 when added to a 20/350d it's surely better to buy EF as opposed to EF-S lenses? Like I said, I'm confused. Can anyone have a go at trying to sort me out? Thanks/Jules.
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Old Feb 22, 2006, 6:14 AM   #2
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f/2.8 is always f/2.8: http://www.uscoles.com/fstop.htm
Quote:
... the f/stop is a ratio. The ratio is between the diameter of the aperture in the lens and the focal length of the lens.

Your understanding is correct, the EV value remains the same across all platforms - If one takes a reading of any lightmeter the same EV settings apply to all cameras.

-> Think about it this way: The projected image and intensity from the lens hasn't changed on the film plane - you're just capturing a different size image (the apparent DOF may be different though).
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Old Feb 22, 2006, 12:14 PM   #3
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Okay, so do you think the original article was comparing actual (say 18-55mm) with¬*comparative (@ 1.6x = 35-80) focal length? Creating a "comparative" F-value by determining the ratio between the actual diameter and the comparative focal length appears fallacious?¬*On that basis I can understand what they're saying, but an 18-55mm lens is always an 18-55mm lens so F4 is always F4 irrespective of which camera it is placed on?
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Old Feb 22, 2006, 1:49 PM   #4
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julianps wrote:
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... so F4 is always F4 irrespective of which camera it is placed on?
Yes - Otherwise a lot of folks here including myself would love to have the 70-200 f/4 to behave like an f/2.8 instead on a 1.6x camera... What a tremendous saving for everyone! :-) :lol:
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Old Feb 23, 2006, 2:01 AM   #5
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You will get a shallower depth of field with the 5D with the same viewing angle and aperture.
For example a picture framed at 80mm on the 5D would have the same perspective as one framed at 50mm on a crop camera taken from the same spot. If both were at f4 however the 5D would have a shallower depth of field due to the longer focal length. The 50mm would have to open up to f3-and-a-bit to get the same shallow depth of field.
Maybe this is what he was getting at.
This principle is why point and shoot cameras with tiny sensors struggle to take good background-out-of-focus portraits even though they have nominally very fast lenses. The tiny focal lengths of their little lenses mean deep depth of field even wide open.
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