Actual light??

bobandcar

No longer a newbie, moving up!
Joined
Dec 18, 2011
Messages
451
Reaction score
52
Location
Newark, Delaware, USA
Website
www.flickr.com
Can others edit my Photos
Photos OK to edit
I currently own a canon 70-300 IS lens that is f5.6(f8is better) when I use it at 300mm.
The question is, the canon 300 mm f4 has better light abilities but by how much?

70-300 at 300 f5.6
300 at f4

Now direct stops is easy to calculate but how much actual light am I gaining?
I've heard that length of barrel and number of elements play a factor here also
 

tirediron

Watch the Birdy!
Staff member
Supporting Member
Joined
Jul 8, 2005
Messages
45,747
Reaction score
14,804
Location
Victoria, BC
Website
www.johnsphotography.ca
Can others edit my Photos
Photos OK to edit
The difference between f4 and f5.6 is one full stop, so therefore the difference in real-world terms is twice as much, since each stop, whether of shutter-speed, ISO, or aperture represents a doubling or halving of the amount of light striking the sensor (or in the case of ISO, a change in the sensor's sensitivity to light).
 

ronlane

What's next?
Joined
Aug 3, 2012
Messages
10,207
Reaction score
4,934
Location
Mustang Oklahoma
Website
www.lane-images.com
Can others edit my Photos
Photos OK to edit
You are comparing a zoom to a prime. The prime in most cases will produce a better image to begin with.

That being said, if I understand what you are saying the f/8 is the sweet spot of the 70-300. If this is the case, I would say that yes, there would be a big difference in light in favor of the 300mm because you can go to f/4.
 
OP
bobandcar

bobandcar

No longer a newbie, moving up!
Joined
Dec 18, 2011
Messages
451
Reaction score
52
Location
Newark, Delaware, USA
Website
www.flickr.com
Can others edit my Photos
Photos OK to edit
I understand stops of light.

Here's a comparison for ya.
My 24-105 at 50mm f4 ISO 100, scene calls for shutter of 1/250
My nifty 50 at 50 f4 ISO 100, same scene calls for shutter 1/350

So f4 is not always f4, it's a relationship of optic sizes I beleive. The difference has to be in the elements(I think)
 

tirediron

Watch the Birdy!
Staff member
Supporting Member
Joined
Jul 8, 2005
Messages
45,747
Reaction score
14,804
Location
Victoria, BC
Website
www.johnsphotography.ca
Can others edit my Photos
Photos OK to edit
I understand stops of light.

Here's a comparison for ya.
My 24-105 at 50mm f4 ISO 100, scene calls for shutter of 1/250
My nifty 50 at 50 f4 ISO 100, same scene calls for shutter 1/350

So f4 is not always f4, it's a relationship of optic sizes I beleive. The difference has to be in the elements(I think)
In fact, f4 is always f4; I am not going to pretend to understand the science of optics well enough to explain it (Helen B can do that), but in simplistic terms, the aperture is a ratio value with respect to pupil diameter. If you got f4 on one reading and f5.6 on another, then I'm going to guess that there was the slightest change in your metering position which caused that. Think back to the days of the hand-held light meter. There's NO provision there for focal length, you just have aperture, shutter-speed and ISO/ASA.
 

ronlane

What's next?
Joined
Aug 3, 2012
Messages
10,207
Reaction score
4,934
Location
Mustang Oklahoma
Website
www.lane-images.com
Can others edit my Photos
Photos OK to edit
Typed at same time. Lol

Is the 300mm fine at f4 with little improvement at f5.6 or f8?

From everything that I have learned in a year and a half, is that each lens has a different sweet spot for sharpness, which I think is where you are heading with this.

No two pieces of glass are exactly the same, just because of the nature of glass itself.
 

Gavjenks

TPF Noob!
Joined
May 9, 2013
Messages
2,976
Reaction score
588
Location
Iowa City, IA
Can others edit my Photos
Photos OK to edit
The OP is correct that f/4 does not always mean the same amount of light. (It is, however, always f/4, lol)

Tirediron, as you say, "the aperture is a ratio value with respect to pupil diameter" But f/stop is not the only thing that effects light. Have you ever put a lens on some extension tubes before? Neither the focal length nor the pupil diameter change when you do that, and therefore the f/stop is the same. But you get a hell of a lot less light. Because magnification, or how far the lens is (or its optical centers/nodes/whatever) from the sensor matters too.

It's easy to calculate exactly how much of an effect this has, in fact. The formula is:
(mathematical f/stop) * (1 + magnification) = effective f/stop in terms of actual amount of light.

This is NOT easy to apply to the 24-105 vs. 50mm, because they don't publish magnification numbers for the 24-105 at 50mm. I have one, and could measure it, but that is a PITA and the OP isn't even asking about it.

However, for the original question, those numbers are published!

70-300 max magnification at 300mm = 0.26x
300 prime max magnification at 300mm = 0.24x

(5.6) * (1 + 0.26) = 7.06
(4) * (1 + 0.24) = 4.96
7.06^2 / 4.96^2 = 2.026
log2(2.026) = 1.02 stops' difference

This is the LARGEST difference possible with those lenses at 300mm, mind you, and the 2% more light in the prime lens also comes at a cost of 8% or whatever less magnification. There's no free lunch. If you're focusing further away than the minimum focusing distances, then the difference is smaller, and there may not even be any difference (when you go away from the published numbers, then it depends ambiguously where the principle optical planes of the lens are, which we don't know since we aren't Canon. Unless you want to hook your lenses up to some high precision rigs and test it yourself)

Obviously, it's not going to actually matter at a 2% max deviation from the mathematical apertures, and this shouldn't influence buying decisions. This sort of thing matters mainly just for macro lenses and view cameras.
 
Last edited:

tirediron

Watch the Birdy!
Staff member
Supporting Member
Joined
Jul 8, 2005
Messages
45,747
Reaction score
14,804
Location
Victoria, BC
Website
www.johnsphotography.ca
Can others edit my Photos
Photos OK to edit
The OP is correct that f/4 does not always mean the same amount of light. (It is, however, always f/4, lol)

Tirediron, as you say, "the aperture is a ratio value with respect to pupil diameter" But f/stop is not the only thing that effects light. Have you ever put a lens on some extension tubes before? Neither the focal length nor the pupil diameter change when you do that, and therefore the f/stop is the same. But you get a hell of a lot less light. Because magnification, or how far the lens is (or its optical centers/nodes/whatever) from the sensor matters too.

It's easy to calculate exactly how much of an effect this has, in fact. The formula is:
(mathematical f/stop) * (1 + magnification) = effective f/stop in terms of actual amount of light.

This is NOT easy to apply to the 24-105 vs. 50mm, because they don't publish magnification numbers for the 24-105 at 50mm. I have one, and could measure it, but that is a PITA and the OP isn't even asking about it.

However, for the original question, those numbers are published!

70-300 max magnification at 300mm = 0.26x
300 prime max magnification at 300mm = 0.24x

(5.6) * (1 + 0.26) = 7.06
(4) * (1 + 0.24) = 4.96
7.06^2 / 4.96^2 = 2.026
log2(2.026) = 1.02 stops' difference

Obviously not worth worrying about. This is more of an issue for view cameras or macro lenses, etc.
Fair enough; there is a difference, however the real-world application to "normal" lenses ('normal' as in, 'everyday use', not 50mm FL) is such that this minor variation is utterly inconsequential, and since the OP was discussing two different lenses and didn't mention the use of extension tubes.... I felt that for the question raised, indicating that there was no difference was reasonable. I stand corrected.
 

Gavjenks

TPF Noob!
Joined
May 9, 2013
Messages
2,976
Reaction score
588
Location
Iowa City, IA
Can others edit my Photos
Photos OK to edit
I apologize if that seemed nitpicky or rude, Tirediron. I figured that since the OP already realized the differences were tiny and insignificant (e.g. 1/250 vs. 1/350), that maybe he was just curious about the science. Normally I would answer the same way you did =)
 
OP
bobandcar

bobandcar

No longer a newbie, moving up!
Joined
Dec 18, 2011
Messages
451
Reaction score
52
Location
Newark, Delaware, USA
Website
www.flickr.com
Can others edit my Photos
Photos OK to edit
I like the maybe end of it. Something can be proven.

I just didn't know If the difference got larger as the focal length got larger.
 

Most reactions

ClickASnap
Top