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08-27-2012, 10:31 AM   #1
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F stops on different lenses

If this has been explained before sorry I have not seen it please give me a link Thank you

The question I have two lenses a kit lens with a 52mm diameter front lens and a FA100-300 with a 58 mm front lens.
They can both be set to f22 but would that aperture be the same diameter for both, or has it a relationship to the diameter of the front lens?
Thank you hope that makes sense Steve

08-27-2012, 10:41 AM   #2
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the filter diameter is not 100% connected to the F number of the lens.

Remember F number is focal length / diameter. so when you consider the filter there are a lot of other determining factors.

at best, the F number MUST BE smaller than maximum focal length / Filter diameter because at some limit, this is where the rubber meets the road, you simply can't have a better F number than this ratio. Usually between the filter / accessory threads and the front element however is some form of either beauty ring or screw in lens retainer. since these need a minimum amount of space the front element is typically smaller than the accessory thread.

also consider mechanical assembly. different makers implement the focusing etc differently and as these may need more diameter than the glass for the lens, they can dictate the filter diameter also Conisder any wide angle lens., a 28mm F2 lens by definition has a 14mm front element requirement, yet many have 49 or 52 or larger filters. some of this is based upon the placement (front to back ) of the filter ring relative to the front element, the edge of the filter has to be outside the field of view of the lens also

so you answer your question the only real relation is where I started, the F number is ultimately limited by front element diameter only at the long end. and for this, only the wide open aperture. the blades make a smaller circle when stopping down
08-27-2012, 10:43 AM   #3
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The diameter of the aperture is related to the geometry of the lens.

QuoteQuote:
Aperture area
The amount of light captured by a lens is proportional to the area of the aperture, equal to:

Where f is focal length and N is the f-number.



The focal length value is not required when comparing two lenses of the same focal length; a value of 1 can be used instead, and the other factors can be dropped as well, leaving area proportion to the reciprocal square of the f-number N.
If two cameras of different format sizes and focal lengths have the same angle of view, and the same aperture area, they gather the same amount of light from the scene. In that case, the relative focal-plane illuminance, however, would depend only on the f-number N, so it is less in the camera with the larger format, longer focal length, and higher f-number. This assumes both lenses have identical transmissivity.
08-27-2012, 11:23 AM   #4
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Leaving out the math - f number is related to diameter and focal length. So - 2 lenses of the same length with the same f number will have the same size/diameter opening. Two with different lengths and the same f number will have different size openings.

08-27-2012, 11:31 AM   #5
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The f-stop is not directly related to the front element size on lenses from the past 60 years or so. I have a Super Takumar 35mm f2.0 lens from 1967. It has a front element measuring about 2.1" diameter (w 67mm filter thread). I also have a Leica 35mm f2.0 Summicron from the early 1960s. It has a front element measuring about 0.85" diameter (w 39mm filter thread). Both give very good results, and offer the same brightness at f2.0, but the Leica is clearly superior.
An SLR needs a retrofocus design for a 35mm lens to move the lens out far enough to clear the mirror. The complex design behaves very differently than a non-retrofocus lens. The Leica was a rangefinder lens and could use a more symmetrical design.
Within a few years Asahi redesigned their 35mm f2 lens to a much smaller front element, allowing a 49mm filter thread. It also performs well.
Which lens lets in more light wide open? These f2.0 lenses are essentially the same, regardless of front element size.
08-27-2012, 11:32 AM   #6
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Simplified... Maximum Diameter or widest opening divided by the focal length is the given number for widest the lens can go.

Looking at a lens that is 200mm long and the widest element or opening is 50mm wide and that opening fits into the length 4 times and so the lens is called an f4.

So if you had a lens that is 50mm long and the maximum wide opening is about 42mm wide or about 1.2 times... 50mm f1.2.

f-number... focal length

f-stop... f= fenestra or window; value derived by opening relative to focal length.
08-27-2012, 11:42 AM   #7
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QuoteOriginally posted by TomB_tx Quote
The f-stop is not directly related to the front element size on lenses from the past 60 years or so. I have a Super Takumar 35mm f2.0 lens from 1967. It has a front element measuring about 2.1" diameter (w 67mm filter thread). I also have a Leica 35mm f2.0 Summicron from the early 1960s. It has a front element measuring about 0.85" diameter (w 39mm filter thread). Both give very good results, and offer the same brightness at f2.0, but the Leica is clearly superior.
An SLR needs a retrofocus design for a 35mm lens to move the lens out far enough to clear the mirror. The complex design behaves very differently than a non-retrofocus lens. The Leica was a rangefinder lens and could use a more symmetrical design.
Within a few years Asahi redesigned their 35mm f2 lens to a much smaller front element, allowing a 49mm filter thread. It also performs well.
Which lens lets in more light wide open? These f2.0 lenses are essentially the same, regardless of front element size.
no disagreement, which is why i said that the front element diameter or filter thread is a limiting case, not an absolute value, but when discussing telephoto lenses especially, where there is no need for a retrofocus design, the front element diameter DOES define the maximum aperture.
08-27-2012, 12:15 PM   #8
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Thank you to everyone who has answered, it may take me some time to digest .
Steve

08-27-2012, 03:16 PM   #9
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QuoteOriginally posted by TomB_tx Quote
I have a Super Takumar 35mm f2.0 lens from 1967. It has a front element measuring about 2.1" diameter (w 67mm filter thread). I also have a Leica 35mm f2.0 Summicron from the early 1960s. It has a front element measuring about 0.85" diameter (w 39mm filter thread).
The reason the Super-Takumar has the larger front element compared with the "same speed" Summicron is that the front group of the Super-Tak is afocal. It does not produce a usable image by itself but causes the rays to diverge. This divergence causes a light loss, as some of the light is lost into the baffling of the lens barrel.
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