Bill_de Loc: US

Maybe because there is more light lost as it travels through and gets bent more going through the longer lens?

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Strodav Loc: Houston, Tx

I believe (notice I am not saying I know) that an f stop is an f stop. So, a 50mm lens at f4 gets the same amount of light to the sensor as a 500mm lens at f4. I can't see how the system would work otherwise. I believe this has to do with field of view (FOV). A smaller FOV gathers light from a smaller area. Now the perspective and DOF will be significantly different between the two.

PHRubin Loc: Nashville TN USA

Because it uses 1/4 as much of the light as the 25mm lens because it makes an image that covers 1/4 the area of the scene.

Blurryeyed Loc: NC Mountains.

f/4 is f/4 no matter if it is on a 50mm lens with a 49mm diameter front element or on a 500mm f/4 with a 180mm front element.

tomcat

Aaahhh. Now the light bulb comes on. I am familiar with the inverse square law from moving my studio lights closer or further back from the subject's face. When I move my studio lights, it's usually a distance of several feet forward or backward and never 12". So 12" just doesn't seem like much of a distance for a lot of light to be lost. But then again, I suppose we aren't talking about a lot of light being transmitted either.

So having an f/4 300mm lens is not going to get me any more light to the sensor than my 135mm f/1.8 can send to it. And perhaps even less light then. Just seems strange

Thanks for the primer

tomcat

True, but the physical diameter is larger on the bigger lens

lamiaceae Loc: San Luis Obispo County, CA

Problem is only us people with science or engineering backgrounds seem to understand that type of stuff today. I learned first about inverse square relationships in science and nature back in HS in 1972.

CHG_CANON Loc: the Windy City

Others will point you to the arithmetic that is the dimensionless ratio of the focal length over the lens diameter known as the f-number, wiki has several basic entries on this and related subjects.

But to your question about lens options, consider the issue of the maximum aperture of the lens. Here is where there are definite performance improvement opportunities. When the camera focuses, the aperture of the lens is opened to the maximum. The wider the aperture, the more light that passed onto the AF sensors. In more light, the AF is faster and more accurate, all other things being equal. Of course, the quality of the lens design, their AF motors, their IS / VR capabilities and the physical materials make lenses quite unequal beyond how much light they can gather.

Also impacting results is the marriage of the lens to the camera, where a manufacturer's lenses should always exhibit better overall performance where the design team of the lens has access to the secret sauce of the camera system in a way a third-party never will.

Regarding a 300 f/4 vs a 200 f/2.8, the longer lens passes 1-stop less light to the sensor at the maximum aperture. Upping the ISO by 1-stop should yield an image at the same "brightness" as an image from the f/2.8 lens. Depending on the ISO / light / camera body involved, as well as the distance to subject, the 300mm lens may yield a much better finished image if the longer lens helps to fill the frame and minimize the cropping to the finished image.

Also impacting the decision is the wide-open performance of the lenses involved. If the light does not support stepping down for more depth of field and / or improved sharpness, which lens design and focal length creates the better end-results for the camera and shooting situations involved?

You might rent a candidate lens to test the performance for a weekend or week making a more informed decision.

Rich1939 Loc: Pike County Penna.

Blurryeyed Loc: NC Mountains.

Yes I know, I shoot with both. The mathematical explanation above is the best I have seen.

User ID

OK. Here's the catch, the thing that seems "wrong"
to you. A 100/4.0 really DOES gather much more
light than a 25/4.0 ... not 4X as much but 16X ! So
why is the image NOT 16X brighter ?

Note that verb: "gathers" [same as "captures"]. Not
all light GATHERED will get DELIVERED to the sensor.

If you've ever set up a projector, you know that if
the image is too big for the screen you move it closer.
If you've ever moved it very much closer I'm sure
you noticed that the smaller the image, the brighter
it is. But the light source has not dimmed ! If you
cram the same bunch of photons into a smaller
area, you do NOT have MORE photons, that means
you do not have MORE light. But within the confines
where the photons are sent, there is MORE ENERGY
PER SQUARE INCH [or sq mm, sq yard, whatever].
OK, more energy per square unit means a brighter
image ! Got that ?

So the 100/4.0 gathers 16X as much energy as the
25/4.0 ... but then that energy is spread out over
16X as much area. Thaz why the 100 is a tele and
the 25 is a normal or a WA. The 100 takes the side
of a barn and projects it over more distance ... here
you hafta recall setting up a projector ... and so the
image has less energy per square unit. With 100/4,
only 1/4 of the side of the barn fits on the sensor.
With the 25/4.0 the whole side of the barn fits on
the sensor. Thaz why the 100 is a telephoto.

So I hope you can see where a lens that GATHERS
more light does not FIT more light energy into the
confines of the sensor. As to where the rest of it
went, it hit the inside of the lens barrel, the rear
baffle of the lens, or whatever ... but the point is
that it is energy that never landed on the sensor.

This post is too long, cuz a diagram is worth a
thousand words and I don't have a diagram to
offer. If you got the idea of what I'm explaining
but some of it remains unclear, you now know
a bit more than before and that should help you
google up a diagram :-)

Longshadow Loc: Audubon, PA, United States

Are you talking about a 25mm lens and a 100mm lens? If so, that is the focal length of the lens, not the diameter of the lens.

speters Loc: Grangeville/Idaho

The diameter of the lens itself, has nothing to do with it, it is the aperture and size of the element in the lens mount and flange distance that matters. For example, I do have a 55mm/3.5 lens (that I like a whole lot), it is quite substantial, because it has a very large diameter, the front element takes 100mm size filters, so it's a beast! Then I have this little pancake 40mm, it is an f/2.8, so it lets in way more light, than the 50/3.5. The pancake is a tiny little thing with a tiny diameter (the front element is about 5/8 to 3/4" in size. How is that for a comparison?

f8lee Loc: New Mexico

This is incorrect. F stop is the ratio between aperture diameter and the lens’ focal length. Size of the front element is irrelevant

tomcat

Setting up the projector was the perfect illustration. Also the inverse square calculations for the increased distance that the light has to travel from the business end of the lens to the sensor helped me understand. So the f/4 300mm lens would not be ideal for trying to reduce the ISO in my dim gym shots by getting more light to the sensor. So I should stay with my 135mm f/1.8 lens. Images taken with the 135mm for basketball don't need to be cropped too much as I stay on the floor under the basket or near the corners of the floor. So I'll stick with the 135mm for basketball. Regarding the gymnastics sessions, often I am 50' or more from the action, so I'll just have to deal with the increased ISO from my 200mm and just not crop as tightly. I was hoping that the 300mm would have helped get me more light to the sensor. Next time the opportunity arises, I will shoot with the 135mm at f/2.8 and compare it to the 200mm at the same f/2.8. I'll have to be very selective with the background though to make certain that the 200mm won't zero in on a darker part of the background, since I shoot with spot-metering.

Thanks for all your comments and help guys, Tom