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Question about light gathering potential in larger diameter lenses
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Feb 13, 2019 17:50:47   #
Blurryeyed Loc: NC Mountains.
 
f8lee wrote:
Then you are thinking wrong. Yes, the font element is much larger but that is in order to bring in the light (naturally!). But the calculation of f-stop has zero/nada/nothing to do with that front element size.

Otherwise, feel free to explain the Nikkor 6MM f2.8 fisheye lens from the early 1980s that actualy had an angle of view of 220 degrees (yees, it sees behind itself).

Wikipedia that, bosco!


LOL... that massive curved element is to take in light from all angles and bend it, if you look at the lenses directly behind it they are tiny.... I completely understand the calculation as explained, that does not mean that you don't have to engineer a lens that will make that math work, a large front element does not necessarily mean a fast f/stop, but with a long lens with a small front element will result in a small aperture. How well do you think a 500mm lens with a 58mm element in the front of the lens will work, and what to you think that the widest aperture will be?

Bosco! Really, are you that immature?

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Feb 13, 2019 18:53:16   #
reverand
 
The maximum aperture of a lens is the focal length of the lens divided by the effective maximum diameter of the lens. This means that it doesn't matter what the focal length of your lens is, whether its 50mm, 105mm, or 300mm. In each case, f/4 on each lens will let in the same amount of light. A lens with a maximum aperture of f/2.8 will let in twice as much light as a lens with a maximum aperture of f/4. Again, it doesn't matter what the focal length of the lenses are.

If you do the math, you'll quickly see why there's no such thing as an f/1.4 maximum aperture 500mm lens: the diameter of the lens would have to be enormous (357mm). By contrast, a 50mm lens only needs a diameter of 35.7mm to provide an f/1.4 maximum aperture.

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Feb 13, 2019 18:53:58   #
LWW Loc: Banana Republic of America
 
ek2lckd wrote:
yes,but ypu are confising the issue//include focal length and it comes out the same..that is why f stop was DEFINED!! the same f stop puts the same light n the film per unit area without regard to the focal lenght...


Let’s make this easy again.

A focal length of 300 mm and a maximum aperture of f4 would have a 75mm aperture. The front element would be 75mm plus a tad for filter thread clearance, wall of the lens etcetera.

Meanwhile a 135/4 would have an aperture of 37.5mm.

(300/75)=4

(135/37.5)=4

I hope this helps.

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Feb 13, 2019 20:25:02   #
ecurb1105
 
ek2lckd wrote:
yes,but ypu are confising the issue//include focal length and it comes out the same..that is why f stop was DEFINED!! the same f stop puts the same light n the film per unit area without regard to the focal lenght...


Correct. And the size of the aperture depends on the focal length of the lens. F2 on a 300 mm lens is 150mm while f2 on a 50mm is 25mm.

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Feb 13, 2019 21:02:35   #
LWW Loc: Banana Republic of America
 
Remove the front element and throw in a lit match, you’ll have all the light you want.*

* Actually, don’t do that.

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Feb 13, 2019 21:11:31   #
tomcat
 
GrahamO wrote:
Tomcat you are over complicating things. I and others have already explained that an f1.8 lens is better for your purposes than an f4 lens. A 300mm f4 lens due to its long focal length also has such a shallow depth of field that you’d find it impractical indoors for action including the difficulty of finding your target. Brush up on your focusing techniques which is a different question. An f1.8 lens can of course also be set to f2.8 or f4 if you need more depth of field but an f4 lens can’t be set to f1.8. The faster f1.8 lens should also auto focus better / faster even if it’s set to f4 than the 300mm f4 lens
Tomcat you are over complicating things. I and oth... (show quote)


I understand and agree. I'm trying to clean up some of the other's comments and make certain everyone is on the same page. It was an interesting thought earlier on about the 300's light gathering ability. I had not factored in the law of inverse squares and did not realize it would make that much difference at a short 12" distance. I believe that the 300 mm f/4 lens is more suited for outdoors in sunshine photographing a moose, etc. or sports players coming at you rather than lateral shots.

Thanks for the comments and support.

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Feb 13, 2019 21:13:19   #
tomcat
 
LWW wrote:
Let’s make this easy again.

A focal length of 300 mm and a maximum aperture of f4 would have a 75mm aperture. The front element would be 75mm plus a tad for filter thread clearance, wall of the lens etcetera.

Meanwhile a 135/4 would have an aperture of 37.5mm.

(300/75)=4

(135/37.5)=4

I hope this helps.



It's not a 135 f/4. It's a 135 mm f/1.8. So would you please re-do your calculations? thanks

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Feb 13, 2019 21:21:18   #
LWW Loc: Banana Republic of America
 
tomcat wrote:
It's not a 135 f/4. It's a 135 mm f/1.8. So would you please re-do your calculations? thanks


I was using that as an apples:apples comparison example.

A 135/1.8 would have a 75mm aperture.

A 300/1.8 ... if such a beast existed ... would be a 166.67mm aperture and way approximately as much as a MAZDA MIATA.

The math is the same in both examples.

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Feb 13, 2019 21:23:22   #
tomcat
 
tomcat wrote:
It's not a 135 f/4. It's a 135 mm f/1.8. So would you please re-do your calculations? thanks


Actually, I'll do it. The 135 mm at f/1.8 has lens opening of 75 mm, the same as the 300 mm at f/4. So this re-raises my point. Both lenses have the same opening of 75 mm and gather the same amount of light. However, due to the law of inverse squares, by the time that the light travels the further distance in the 300 mm lens it is loses some intensity by the time it gets to the sensor. The barrel length of the 300mm is about 6" versus 4.5" for the 135 mm lens. So there's not a lot of distance difference that the light travels from one lens to the other.

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Feb 13, 2019 21:26:27   #
LWW Loc: Banana Republic of America
 
tomcat wrote:
Actually, I'll do it. The 135 mm at f/1.8 has lens opening of 75 mm, the same as the 300 mm at f/4. So this re-raises my point. Both lenses have the same opening of 75 mm and gather the same amount of light. However, due to the law of inverse squares, by the time that the light travels the further distance in the 300 mm lens it is loses some intensity by the time it gets to the sensor. The barrel length of the 300mm is about 6" versus 4.5" for the 135 mm lens. So there's not a lot of distance difference that the light travels from one lens to the other.
Actually, I'll do it. The 135 mm at f/1.8 has l... (show quote)


How much light enters has no relevance to what reaches the sensor.

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Feb 13, 2019 21:26:34   #
tomcat
 
LWW wrote:
I was using that as an apples:apples comparison example.

A 135/1.8 would have a 75mm aperture.

A 300/1.8 ... if such a beast existed ... would be a 166.67mm aperture and way approximately as much as a MAZDA MIATA.

The math is the same in both examples.


I understand the math. But don't do an apples to apples comparison. Keep in mind I am comparing a 300 mm lens at f/4 to a 135 mm lens at f/1.8. These are the maximum apertures that I would always be shooting indoor gymnastics at. So my original question was to determine which of the two lenses would work better in such low light at ISO 18,000.

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Feb 13, 2019 21:27:19   #
tomcat
 
LWW wrote:
How much light enters has no relevance to what reaches the sensor.


I know, because the law of inverse squares will reduce what gets to the sensor.

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Feb 13, 2019 21:32:15   #
LWW Loc: Banana Republic of America
 
tomcat wrote:
I understand the math. But don't do an apples to apples comparison. Keep in mind I am comparing a 300 mm lens at f/4 to a 135 mm lens at f/1.8. These are the maximum apertures that I would always be shooting indoor gymnastics at. So my original question was to determine which of the two lenses would work better in such low light at ISO 18,000.

Obviously a 1.8 will be brighter than a 4.0.

Which would work better depends on your distance from the action.

If you can get with 20 ft I’d use the 135 and turn the ISO down.

I you are in the upper stands I’d use the 300.

The best choice, IMHO, is still neither.

A 80-200/2.8 splits the difference on speed and length, gives you a wider short end and the ability to zoom.

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Feb 13, 2019 21:51:05   #
Blurryeyed Loc: NC Mountains.
 
tomcat wrote:
It's not a 135 f/4. It's a 135 mm f/1.8. So would you please re-do your calculations? thanks


You must be shooting the Sigma Art, if it is not getting enough light you are going to have a hard time.

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Feb 13, 2019 21:56:56   #
DavidPine Loc: Fredericksburg, TX
 
The lens glass that you keep clean is only a passageway. The Aperture setting f/stop has everything to do with making the quantity of light available to be absorbed by the camera's sensor and the shutter speed determines how the quantity of light gets to the aperture opening to get passed onto the sensor. The Aperture and Sutter Speed work in conjunction with the sensor's measured ISO to produce the correct quantity and quality of light to help reach a proper exposure

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