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Question about light gathering potential in larger diameter lenses
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Feb 12, 2019 12:32:52   #
tomcat (a regular here)
 
I have searched the internet for an answer to this question, but all I get is conflicting answers so I thought I would go to the experts that have the experience. Will a larger diameter lens like a 300mm or a 500mm fixed lens let in more light at their widest opening (f/4) than a smaller diameter lens like a 50mm or a 105mm lens at f/2.8? It would seem to me that the larger diameter lens with its larger blade opening and larger barrel diameter would let more light pass than a lens with a smaller opening and smaller diameter barrel.

The reason I ask this question is to determine if it is worth the investment to get an f/4 300mm lens for indoor basketball or to stay with my 135mm f/1.8? As an experiment one day, I shot basketball with my 50mm Sigma Art lens at f/1.4 and the images were a lot brighter than they were with my 70-200, which is f/2.8. The lens diameters are almost the same, so this was not a good test at diameter vs light transmission. I expected the larger aperture would be brighter, as it was.

So I need answers from you guys that shoot a 300mm lens. Can you really get a lot more light onto the sensor with this larger lens, compared to when you shoot your smaller diameter lens?

Thanks

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Feb 12, 2019 12:41:11   #
LWW (a regular here)
 
tomcat wrote:
I have searched the internet for an answer to this question, but all I get is conflicting answers so I thought I would go to the experts that have the experience. Will a larger diameter lens like a 300mm or a 500mm fixed lens let in more light at their widest opening (f/4) than a smaller diameter lens like a 50mm or a 105mm lens at f/2.8? It would seem to me that the larger diameter lens with its larger blade opening and larger barrel diameter would let more light pass than a lens with a smaller opening and smaller diameter barrel.

The reason I ask this question is to determine if it is worth the investment to get an f/4 300mm lens for indoor basketball or to stay with my 135mm f/1.8? As an experiment one day, I shot basketball with my 50mm Sigma Art lens at f/1.4 and the images were a lot brighter than they were with my 70-200, which is f/2.8. The lens diameters are almost the same, so this was not a good test at diameter vs light transmission. I expected the larger aperture would be brighter, as it was.

So I need answers from you guys that shoot a 300mm lens. Can you really get a lot more light onto the sensor with this larger lens, compared to when you shoot your smaller diameter lens?

Thanks
I have searched the internet for an answer to this... (show quote)


Depends on how close you can get, I use a 80-200/2.8 for indoor sports.

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Feb 12, 2019 12:43:15   #
Longshadow (a regular here)
 
If you're trying to compare the physical diameter of a lens with the amount of light let in, you can't.
An f/4 on a four inch lens lets in the same amount of light as an f/4 on a two inch lens.
Seems like it would be logical, but the amount of light transmitted depends on the design of the lens, not the physical diameter of the lens. Telephotos have to be a larger diameter by design.

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Feb 12, 2019 12:49:00   #
PHRubin (a regular here)
 
A 500mm lens is not necessarily a larger diameter lens, the f stop is its aperture, not diameter. There is a major difference. Although not usually mentioned in the name, (i.e. 300mm f/4L IS USM) the front element size (usually the filter size) is analogous to its diameter.

For brightness, you need to compare apertures only.

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Feb 12, 2019 12:49:34   #
rook2c4 (a regular here)
 
At same aperture and shutter speed setting, then same amount of light. Doesn't matter if lens is big or small, wide or narrow.

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Feb 12, 2019 12:55:55   #
f8lee
 
F stop is a simple mathematical approximation of how much light passes through the lens- it is simply the ratio of focal length to aperture. So a 400MM lens with an aperture that is 100MM in diameter is f4, as is a 100MM lens with an aperture that has a 25MM diameter. The measure known as T-stop is based on actual measurements of light transmission through the lens- useful for complex lenses like zooms that have many elements and groups, each of which detracts a little from light transmission.

Thus, longer focal lengths necessitate larger diameter diaphragms as, generally speaking, larger diameter glass in front.

As for why your Sigma is making for “brighter photos” (whatever that means), perhaps the gym lighting isn’t sufficient to use smaller aperture without slowing shutter speed or bumping up ISO. But certainly you can take shots with the Sigma set to f1.8 and then f4 (without altering the shutter or sensitivity) and see how that works.

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Feb 12, 2019 13:13:16   #
Strodav
 
fstop = focal length / diameter or diameter = focal length / fstop

So, focal length is an important part of understanding what is happening. The longer the focal length, the larger diameter front lens you need to get the same fstop as a shorter focal length lens.

Let's do an example. Let's say we have a 50mm f1.4 lens so the diameter needs to be 50 mm / f1.4 = 35.7mm. What aperture would we need to get f1.4 at a 500mm focal length? 500mm / f1.4 = 357mm. BTW, 357mm = 14 inches. One hell of a big lens.

One more example. Let's look at a 500mm f4 lens. The diameter would need to be 500mm / f4 = 125mm or 4.92". The spec for the 500mm f4 lens says 5.5" diameter. The difference between 4.92" and 5.5" is the barrel that holds the front lens in place.

Hope I got this right.

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Feb 12, 2019 13:15:33   #
tomcat (a regular here)
 
f8lee wrote:
F stop is a simple mathematical approximation of how much light passes through the lens- it is simply the ratio of focal length to aperture. So a 400MM lens with an aperture that is 100MM in diameter is f4, as is a 100MM lens with an aperture that has a 25MM diameter. The measure known as T-stop is based on actual measurements of light transmission through the lens- useful for complex lenses like zooms that have many elements and groups, each of which detracts a little from light transmission.

Thus, longer focal lengths necessitate larger diameter diaphragms as, generally speaking, larger diameter glass in front.

As for why your Sigma is making for “brighter photos” (whatever that means), perhaps the gym lighting isn’t sufficient to use smaller aperture without slowing shutter speed or bumping up ISO. But certainly you can take shots with the Sigma set to f1.8 and then f4 (without altering the shutter or sensitivity) and see how that works.
F stop is a simple mathematical approximation of h... (show quote)


See, this is the part that is confusing to me. The lens with a 100mm opening at f/4 is 4 times wider opening than the other lens that has a 25mm opening at f/4. So why would the larger lens not be transmitting 4x as much light?

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Feb 12, 2019 13:17:26   #
tomcat (a regular here)
 
Strodav wrote:
fstop = focal length / diameter or diameter = focal length / fstop

So, focal length is an important part of understanding what is happening. The longer the focal length, the larger diameter front lens you need to get the same fstop as a shorter focal length lens.

Let's do an example. Let's say we have a 50mm f1.4 lens so the diameter needs to be 50 mm / f1.4 = 35.7mm. What aperture would we need to get f1.4 at a 500mm focal length? 500mm / f1.4 = 357mm. BTW, 357mm = 14 inches. One hell of a big lens.

One more example. Let's look at a 500mm f4 lens. The diameter would need to be 500mm / f4 = 125mm or 4.92". The spec for the 500mm f4 lens says 5.5" diameter. The difference between 4.92" and 5.5" is the barrel that holds the front lens in place.

Hope I got this right.
fstop = focal length / diameter or diameter = fo... (show quote)


You did, but you didn't answer my question about the light transmission through the smaller vs larger lens openings at f/4 in both lenses.

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Feb 12, 2019 13:20:03   #
f8lee
 
Strodav wrote:
fstop = focal length / diameter or diameter = focal length / fstop

So, focal length is an important part of understanding what is happening. The longer the focal length, the larger diameter front lens you need to get the same fstop as a shorter focal length lens.

Let's do an example. Let's say we have a 50mm f1.4 lens so the diameter needs to be 50 mm / f1.4 = 35.7mm. What aperture would we need to get f1.4 at a 500mm focal length? 500mm / f1.4 = 357mm. BTW, 357mm = 14 inches. One hell of a big lens.

One more example. Let's look at a 500mm f4 lens. The diameter would need to be 500mm / f4 = 125mm or 4.92". The spec for the 500mm f4 lens says 5.5" diameter. The difference between 4.92" and 5.5" is the barrel that holds the front lens in place.

Hope I got this right.
fstop = focal length / diameter or diameter = fo... (show quote)


Not quite. The diameter of the front lens element has nothing to do with f stop calculus- it’s the focal length divided by aperture diameter - that’s it.

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Feb 12, 2019 13:20:41   #
BebuLamar (a regular here)
 
tomcat wrote:
You did, but you didn't answer my question about the light transmission through the smaller vs larger lens openings at f/4 in both lenses.


More light goes thru larger opening but because the focal length is longer and so the nodal plane is further away from the sensor thus enlarges the image more. The light intensity is less when the image is enlarged.

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Feb 12, 2019 13:21:53   #
lamiaceae (a regular here)
 
tomcat wrote:
I have searched the internet for an answer to this question, but all I get is conflicting answers so I thought I would go to the experts that have the experience. Will a larger diameter lens like a 300mm or a 500mm fixed lens let in more light at their widest opening (f/4) than a smaller diameter lens like a 50mm or a 105mm lens at f/2.8? It would seem to me that the larger diameter lens with its larger blade opening and larger barrel diameter would let more light pass than a lens with a smaller opening and smaller diameter barrel.

The reason I ask this question is to determine if it is worth the investment to get an f/4 300mm lens for indoor basketball or to stay with my 135mm f/1.8? As an experiment one day, I shot basketball with my 50mm Sigma Art lens at f/1.4 and the images were a lot brighter than they were with my 70-200, which is f/2.8. The lens diameters are almost the same, so this was not a good test at diameter vs light transmission. I expected the larger aperture would be brighter, as it was.

So I need answers from you guys that shoot a 300mm lens. Can you really get a lot more light onto the sensor with this larger lens, compared to when you shoot your smaller diameter lens?

Thanks
I have searched the internet for an answer to this... (show quote)


You need to start with the definition of F-Stop and Aperture. A f/ stop is a ratio. As usual Google is your friend (if you know how to use it). Also older photo books tended to give all the appropriate optical equations.

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Feb 12, 2019 13:22:39   #
f8lee
 
tomcat wrote:
You did, but you didn't answer my question about the light transmission through the smaller vs larger lens openings at f/4 in both lenses.


Remember the inverse square law- the 400MM lens’ focal point is 4 times as far away from the image plane as the 100MM- so only 1/16th of the light makes it to the plane.

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Feb 12, 2019 13:23:50   #
nadelewitz (a regular here)
 
The f/ number is the FOCAL LENGTH divided by something, not the lens diameter divided by something. Lens diameter is not relevant to your question.

F/8 on a long lens and f/8 on a short lens provides the same amount of light to the sensor or film plane. Even if you want to talk about how much light gets into the front of the lens and that f/8 on a long lens is a bigger diameter hole in the diaphragm, the distance to sensor/film plane is longer on a longer focal length lens. Light intensity drops off with longer distance.

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Feb 12, 2019 13:25:37   #
LWW (a regular here)
 
rook2c4 wrote:
At same aperture and shutter speed setting, then same amount of light. Doesn't matter if lens is big or small, wide or narrow.


The same light reaching the sensor plane is correct.

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