SumanV

EOS M6 Mark II
Sep 25, 2016
53
18
Not really. In the f-number fraction, the f stands for the focal length, so yes, the larger f is, the bigger the opening is physically. But the denominator is the real measure (more or less) of the exposure. So an f/4 lens will give you more or less the same exposure as another f/4 lens, regardless of focal lengths. (And, yes, I realize there are a lot of yes-buts and more technical ways of saying this. But I'm just addressing main practical purposes.)

In general, you can get faster lenses in the 35mm to 50mm range than you can with telephotos. Fast telephotos are large, heavy and expensive for the most part, so somewhat less practical. And even with zoom lenses, many of them will have variable f-numbers, say f/3.5 at the wide end and f/5.6 on the telephoto side, so that difference can be built-in. Wider lenses are also more practical in low light because of greater depth of field at a given f-stop.

Also, most low-light shooting situations, other than concerts, will call for wider lenses, for example, kids playing in the family room, or a lot of night street photography.
Thanks, @stevelee. I would like to clarify that I meant faster lenses. For the longer focal length lenses I was thinking of 85 f1.2/1.4, 135 f2 or 200 f2.8. Wouldn't these lenses be better than the 35/50 mm ones (faster lenses).
I agree with you that composition would also play a role in the decision making process. But if IQ is the only consideration then I feel that longer focal length, faster lenses would be better due to their larger apertures.

Regards
Suman
 

stevelee

FT-QL
CR Pro
Jul 6, 2017
2,159
860
Davidson, NC
Thanks, @stevelee. I would like to clarify that I meant faster lenses. For the longer focal length lenses I was thinking of 85 f1.2/1.4, 135 f2 or 200 f2.8. Wouldn't these lenses be better than the 35/50 mm ones (faster lenses).
I agree with you that composition would also play a role in the decision making process. But if IQ is the only consideration then I feel that longer focal length, faster lenses would be better due to their larger apertures.

Regards
Suman
No, I still don't follow your logic.

The only way that the physically larger size of the aperture would give you better quality, that I can think of, is in terms of diffraction. You can get away with stopping down a longer lens more before diffraction softens the image too much, I think. And of course, in low light, you're probably not that interested in shooting at f/32 whatever the lens.
 

SumanV

EOS M6 Mark II
Sep 25, 2016
53
18
No, I still don't follow your logic.

The only way that the physically larger size of the aperture would give you better quality, that I can think of, is in terms of diffraction. You can get away with stopping down a longer lens more before diffraction softens the image too much, I think. And of course, in low light, you're probably not that interested in shooting at f/32 whatever the lens.
The logic I used is that the light collected by the sensor is related proportional to the square of aperture. So, I thought the longer f-length would perform better than a smaller f-length lens (200mm f2.8 vs 50mm f1.4). Please note that I do not have any evidence/images to confirm my guess.

Regards
Suman
 

stevelee

FT-QL
CR Pro
Jul 6, 2017
2,159
860
Davidson, NC
There is a reason that for exposure purposes the ratio with the focal length is used rather than the physical size of the diaphragm opening. That doesn't mean that I can explain it, however. Maybe one way to think about it is that the longer lens is already letting in less of the light in the scene than is the shorter lens. So you need a bigger opening to let in the same amount of light with a longer lens. At least in real life, it works out that way. Perhaps someone who knows what they are talking about can straighten us out here.

Maybe a way to think about it is to look at it from the other direction. At f/4, the size of the lens opening is equal to the focal length divided by 4. It follows then that the 4 is equal to the focal length divided by the physical size of the opening, when expressed in the same units (normally millimeters in our range of thinking).
 

SteveC

R5
CR Pro
Sep 3, 2019
2,436
2,305
There is a reason that for exposure purposes the ratio with the focal length is used rather than the physical size of the diaphragm opening. That doesn't mean that I can explain it, however.

I'll give it a shot as well.

A 100mm versus a 50mm gives you an image half the width--not on the sensor but out there. If your 50mm lens just fits an object a meter wide, the 100mm will show you only half a meter of that object. (given same sensor size, same distance, of course). That is 1/4 the area. That pretty much means the 100mm lens is only collecting 1/4th the light as well, though, since it's throwing away 3/4 of what the other lens does. The shutter speed has to be four times slower.

If you make the aperture of the 100 mm lens four times the <i>area</i>, you can compensate for this. But that's just increasing the aperture <i>diameter</i> by 2 times.

Since you increased the focal length by doubling it, and doubled the width of the aperture as well, to keep the same shutter speed, it stands to reason that what matters is the ratio between the two, if you double one and then double the other, the ratio is still the same--you've multiplied numerator and denominator by the same thing. (Of course this assumes that all else is the same--you have to make sure the glass absorbs exactly the same fraction of the light, and so on.)

They could have made it the aperture diameter over the focal length, or the focal length over the aperture. Since it's usually a challenge to make the aperture big, and people prefer to think in numbers over zero, they went with focal length over aperture, so its: F/A. This does mean that "bigger" numbers are actually "smaller", though. f/4.0 represents a smaller opening than f/2.0 on the same lens. (I try to always write the / to make it clear the number is a denominator, many just write F4.0.)

Because going from f/2.0 to f/4.0 is reducing the diameter of the aperture by half, it reduces the area by 4 times, and that is not one, but two stops. So to halve your aperture you really need to go from f/2.0 to f/2.8; then you have to double your ISO or halve your shutter speed (double its duration) to compensate.
 
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SumanV

EOS M6 Mark II
Sep 25, 2016
53
18
There is a reason that for exposure purposes the ratio with the focal length is used rather than the physical size of the diaphragm opening. That doesn't mean that I can explain it, however. Maybe one way to think about it is that the longer lens is already letting in less of the light in the scene than is the shorter lens. So you need a bigger opening to let in the same amount of light with a longer lens. At least in real life, it works out that way. Perhaps someone who knows what they are talking about can straighten us out here.

Maybe a way to think about it is to look at it from the other direction. At f/4, the size of the lens opening is equal to the focal length divided by 4. It follows then that the 4 is equal to the focal length divided by the physical size of the opening, when expressed in the same units (normally millimeters in our range of thinking).
Thanks @stevelee. Actually, this logic is used by astrophotographers all the time and they swear by it. According to them, for the same subject, a larger aperture lets in more light. However, if, how and when it applies to portraits is something I do not know yet.

Regards
Suman
 

SumanV

EOS M6 Mark II
Sep 25, 2016
53
18
I'll give it a shot as well.

A 100mm versus a 50mm gives you an image half the width--not on the sensor but out there. If your 50mm lens just fits an object a meter wide, the 100mm will show you only half a meter of that object. (given same sensor size, same distance, of course). That is 1/4 the area. That pretty much means the 100mm lens is only collecting 1/4th the light as well, though, since it's throwing away 3/4 of what the other lens does. The shutter speed has to be four times slower.

If you make the aperture of the 100 mm lens four times the <i>area</i>, you can compensate for this. But that's just increasing the aperture <i>diameter</i> by 2 times.

Since you increased the focal length by doubling it, and doubled the width of the aperture as well, to keep the same shutter speed, it stands to reason that what matters is the ratio between the two, if you double one and then double the other, the ratio is still the same--you've multiplied numerator and denominator by the same thing. (Of course this assumes that all else is the same--you have to make sure the glass absorbs exactly the same fraction of the light, and so on.)

They could have made it the aperture diameter over the focal length, or the focal length over the aperture. Since it's usually a challenge to make the aperture big, and people prefer to think in numbers over zero, they went with focal length over aperture, so its: F/A. This does mean that "bigger" numbers are actually "smaller", though. f/4.0 represents a smaller opening than f/2.0 on the same lens. (I try to always write the / to make it clear the number is a denominator, many just write F4.0.)

Because going from f/2.0 to f/4.0 is reducing the diameter of the aperture by half, it reduces the area by 4 times, and that is not one, but two stops. So to halve your aperture you really need to go from f/2.0 to f/2.8; then you have to double your ISO or halve your shutter speed (double its duration) to compensate.
Thanks @SteveC. I understand what you are saying but in "astrophotography, for the same subject, the larger aperture will always collect more light". I think I will experiment with my camera and see if that's the case. I actually compared two images shot at different focal lengths and same apertures (?) and the longer focal length shot looked cleaner to me. Please note that that test is completely unscientific and has to be done in a correct manner. That prompted me to investigate further.

Regards
Suman
 

SteveC

R5
CR Pro
Sep 3, 2019
2,436
2,305
Thanks @SteveC. I understand what you are saying but in "astrophotography, for the same subject, the larger aperture will always collect more light".

I believe that whoever wrote this was assuming you'd be using the same lens at two different apertures, not swapping lenses while changing apertures.
 

SumanV

EOS M6 Mark II
Sep 25, 2016
53
18
I believe that whoever wrote this was assuming you'd be using the same lens at two different apertures, not swapping lenses while changing apertures.
:). I think I may be wrong and you are right. Keeping the composition the same, one has to move farther from the subject for the longer focal length lens. This could decrease the light gathering power. Apologies if I created a confusion.

Thanks and regards
Suman
 

CanonFanBoy

Purple
CR Pro
Jan 28, 2015
5,658
4,086
Irving, Texas
It sounds, or reads, as though you believe the aperture is the size of the front lens element?

I have a related question. Are longer focal length primes better for low light photography than smaller focal length ones?
Shouldn't 85/135/200 mm lenses do better than 35/50 mm? I think that these would do better as the former's aperture is larger than the latter thus collecting more light(?)

Regards
Suman
 

CanonFanBoy

Purple
CR Pro
Jan 28, 2015
5,658
4,086
Irving, Texas
Isn't aperture defined as the lens focal length divided by the diameter of the exit lens opening? When the f-stop increases, the diameter of the rear lens element reduces and vice versa.

Regards
Suman
Aperture is variable and is controlled by how open or closed the aperture blades are. You can't vary the size of the rear glass element.
 
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privatebydesign

I post too Much on Here!!
CR Pro
Jan 29, 2011
10,502
5,734
Isn't aperture defined as the lens focal length divided by the diameter of the exit lens opening? When the f-stop increases, the diameter of the rear lens element reduces and vice versa.

Regards
Suman
No.

Aperture is, in the real world, front element focal length divided by apparent diameter of the entrance pupil this is a simplification because camera lenses are not simple lenses but it basically holds close to true.

The entrance pupil size is the size of the aperture hole when measured from the front of the lens, this is because the objective lens will enlarge or reduce the actual size of the opening.
 
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