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2x zoom lens with same absolute aperture instead of extender ?

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mermaidkiller

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Using an extender (tele converter) has the advantage that the absolute aperture (in mm, not as f/ number) remains the same with the same lens which allows a relative bright f/nubmer without T/C and yet acceptable with T/C. E.g the bright 200mm f/2.8L with T/C results in a yet acceptable 400 f/5.6.

A zoom alternative (100-400 f/4.5-5.6L) results in a dim f/5 at 200mm.

Why not make a 200-400 f/2.8-f/5.6 which replaces a T/C and a prime lens ?

There is a 200-400 lens f/4L, but that is not a real alternative in both price and weight / portability.

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Ricoh KR-5 ... Pentax ME Super ... Canon T90 ... ... ... 40d ... 7d ... 6d
 
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Solution
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MitchAlsup said:
But you are not measuring the size of the entrance pupil! You are measuring how much of the lens can be seen through the viewfinder--which needs to be the whole lens (or the manufacture would save money by making the lens smaller.

You can observe the size of the entrance pupil by opening up the aperture all the way and see how much of the view finder you can see from the front of the lens. Hint: its a lot smaller than the size of the objective. The objective is large because it has to fit multiple entrance pupils across its face in order to fully illuminate the off axis portions on the sensor.
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Not for long lenses. The 300/4L IS for example has a front element which measures 75 mm. The various 200/2.8s (71.4 mm entrance pupil...
mermaidkiller said:
Using an extender (tele converter) has the advantage that the absolute aperture (in mm, not as f/ number) remains the same with the same lens which allows a relative bright f/nubmer without T/C and yet acceptable with T/C. E.g the bright 200mm f/2.8L with T/C results in a yet acceptable 400 f/5.6.

A zoom alternative (100-400 f/4.5-5.6L) results in a dim f/5 at 200mm.

Why not make a 200-400 f/2.8-f/5.6 which replaces a T/C and a prime lens ?

There is a 200-400 lens f/4L, but that is not a real alternative in both price and weight / portability.
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You raise a good point. There are any number of excellent 2x zooms that could be made in this fashion, and I'm not sure why it isn't done.
 
mermaidkiller said:
Why not make a 200-400 f/2.8-f/5.6 which replaces a T/C and a prime lens ?
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Because the entrance pupil of the zoom lens is inside the optical train, Zoom lenses have to have a larger front lens than the linear aperture, typically by 20%-25% so you would be holding 5" of glass at the front of the 200-400. Thus, the cost will not be what you are anticipating.
 
MitchAlsup said:
mermaidkiller said:
Why not make a 200-400 f/2.8-f/5.6 which replaces a T/C and a prime lens ?
Click to expand...
Because the entrance pupil of the zoom lens is inside the optical train, Zoom lenses have to have a larger front lens than the linear aperture, typically by 20%-25% so you would be holding 5" of glass at the front of the 200-400. Thus, the cost will not be what you are anticipating.

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Mitch
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I measured the 70-300L aperture (entry pupil, the *actual* value) by lighting a flashlight through the eyepiece and then measuring the diameter of the projection circle. That appears to be 56mm, exact the same size as I measure the front lens.

The same is with the 200mm f/2.8L. But the 24-105L is indeed much smaller than the front lens diameter.



80dadf437c764f3b9444abf63d29153d.jpg



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Ricoh KR-5 ... Pentax ME Super ... Canon T90 ... ... ... 40d ... 7d ... 6d
 
But you are not measuring the size of the entrance pupil! You are measuring how much of the lens can be seen through the viewfinder--which needs to be the whole lens (or the manufacture would save money by making the lens smaller.

You can observe the size of the entrance pupil by opening up the aperture all the way and see how much of the view finder you can see from the front of the lens. Hint: its a lot smaller than the size of the objective. The objective is large because it has to fit multiple entrance pupils across its face in order to fully illuminate the off axis portions on the sensor.
 
Unless I misunderstand , my 17-55 2.8 is over 2 X and my 70-200 mk II is also . If I put the 2X II on the 70-200 , it puts it at about 5.6
 
and focuses about as slow as a wet week, where as the all in one has a much faster focus eg 200 2.8 + tc2 against a 400 f5.6 and the 400 f5.6 wins every time.
 
MitchAlsup said:
But you are not measuring the size of the entrance pupil! You are measuring how much of the lens can be seen through the viewfinder--which needs to be the whole lens (or the manufacture would save money by making the lens smaller.

You can observe the size of the entrance pupil by opening up the aperture all the way and see how much of the view finder you can see from the front of the lens. Hint: its a lot smaller than the size of the objective. The objective is large because it has to fit multiple entrance pupils across its face in order to fully illuminate the off axis portions on the sensor.
Click to expand...
Not for long lenses. The 300/4L IS for example has a front element which measures 75 mm. The various 200/2.8s (71.4 mm entrance pupil size) are similar. The effect you describe is increasingly true as the lens covers a wider field of view - the EF-S 10-22mm f/3.5-4.5 for example has an entrance pupil of approximately 3 mm (at 10 mm zoom) but it has a 50 mm front element.
 
Solution
mermaidkiller said:
Using an extender (tele converter) has the advantage that the absolute aperture (in mm, not as f/ number) remains the same with the same lens which allows a relative bright f/nubmer without T/C and yet acceptable with T/C. E.g the bright 200mm f/2.8L with T/C results in a yet acceptable 400 f/5.6.

A zoom alternative (100-400 f/4.5-5.6L) results in a dim f/5 at 200mm.

Why not make a 200-400 f/2.8-f/5.6 which replaces a T/C and a prime lens ?
Click to expand...
I'm not sure this will be much different in price & weight compared to the 200-400 f/4.
 
mermaidkiller said:
Using an extender (tele converter) has the advantage that the absolute aperture (in mm, not as f/ number) remains the same with the same lens which allows a relative bright f/nubmer without T/C and yet acceptable with T/C. E.g the bright 200mm f/2.8L with T/C results in a yet acceptable 400 f/5.6.

A zoom alternative (100-400 f/4.5-5.6L) results in a dim f/5 at 200mm.

Why not make a 200-400 f/2.8-f/5.6 which replaces a T/C and a prime lens ?
Click to expand...
I would take it even further: Why not replace such 2x zoom lenses with primes and high pixel density sensors that are cropped accordingly? For example your 200/2.8 on a 5Ds would be equivalent to a 200-400/2.8-5.6 if 12.6 MP resolution are enough. Not long ago many photographers (especially with Nikon DSLRs) thought that 12MP is all they will ever need and I'm sure 100MP sensors will arrive in the not too distant future.
 
MitchAlsup said:
But you are not measuring the size of the entrance pupil! You are measuring how much of the lens can be seen through the viewfinder--which needs to be the whole lens (or the manufacture would save money by making the lens smaller.

You can observe the size of the entrance pupil by opening up the aperture all the way and see how much of the view finder you can see from the front of the lens. Hint: its a lot smaller than the size of the objective. The objective is large because it has to fit multiple entrance pupils across its face in order to fully illuminate the off axis portions on the sensor.
Click to expand...
 
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mermaidkiller said:
MitchAlsup said:
But you are not measuring the size of the entrance pupil! You are measuring how much of the lens can be seen through the viewfinder--which needs to be the whole lens (or the manufacture would save money by making the lens smaller.

You can observe the size of the entrance pupil by opening up the aperture all the way and see how much of the view finder you can see from the front of the lens. Hint: its a lot smaller than the size of the objective. The objective is large because it has to fit multiple entrance pupils across its face in order to fully illuminate the off axis portions on the sensor.

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Mitch
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That is exactly the objective / entrance pupil diameter. This IS the test to measure the objective diameter. This website tells how to measure the diameter of objective of binoculars. This is valid for telescopes as well and ... cameras. A DSLR woth OVF is just a.... telescope with objective and eyepiece, regardless of how many mirrors, lens elements or baffles are in the light path. Any obstruction will be shown in the projection image on the objective site.

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I believe you are measuring the "effective" aperture. In other words what diameter aperture you would need if the lens was composed of a single element, and the aperture was inside that element.

Most photographic lens are composed of multiple elements, the physical aperture is not co-located with the front element. A 100mm lens at f/2 has an effective aperture diameter of 50mm, but the physical opening may be a different size.

This is one of the factors that make constant aperture zoom lenses practical. The physical aperture is located such that the effective aperture does not vary wildly with focal length.
 
This is why I like my 70-200 w / 2 X II better than the 100-400 mk 1. I have a 135-400 and my 70-200 w / 2 X II is far better .

The 100-400 II is not 2000 + better . I am looking to compare the 2 [ 3 ] 150-600 but so far the store has not had both to demo at same time .
 
Michael Fryd said:
mermaidkiller said:
MitchAlsup said:
But you are not measuring the size of the entrance pupil! You are measuring how much of the lens can be seen through the viewfinder--which needs to be the whole lens (or the manufacture would save money by making the lens smaller.

You can observe the size of the entrance pupil by opening up the aperture all the way and see how much of the view finder you can see from the front of the lens. Hint: its a lot smaller than the size of the objective. The objective is large because it has to fit multiple entrance pupils across its face in order to fully illuminate the off axis portions on the sensor.
Click to expand...
Click to expand...
Click to expand...
 
Steve Balcombe said:
Steve Balcombe said:
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Yes, but as per my earlier post the front element of a long lens is the size of the entrance pupil. The shorter the lens, the less true this is.

...
Click to expand...
The aperture is the weakest link in the chain. Typically, that will be the size of the front element, but that's an engineering decision, not a requirement of physics.

Thus with a zoom lens, there are times when the effective maximum aperture is smaller than the front element.
 
Michael Fryd said:
Steve Balcombe said:
Michael Fryd said:
...
Click to expand...
Yes, but as per my earlier post the front element of a long lens is the size of the entrance pupil. The shorter the lens, the less true this is.

...
Click to expand...
The aperture is the weakest link in the chain. Typically, that will be the size of the front element, but that's an engineering decision, not a requirement of physics.
Click to expand...
I'll say one more time - *not* with a long lens.

When the most oblique rays are arriving from angles of less than two or three degrees from parallel with the lens axis, making the front element significantly bigger would just result in collecting light which could never reach the sensor. It *is* physics which determines this.
 
Michael Fryd said:
Steve Balcombe said:
...

Yes, but as per my earlier post the front element of a long lens is the size of the entrance pupil. The shorter the lens, the less true this is.

...
Click to expand...
The aperture is the weakest link in the chain. Typically, that will be the size of the front element, but that's an engineering decision, not a requirement of physics.
Click to expand...
The diameter of the aperture (entrance pupil) is typically significantly less than the diameter of the front element, but the diameter of the front element will tend to approach the aperture diameter as the lens gets longer.
Michael Fryd said:
Thus with a zoom lens, there are times when the effective maximum aperture is smaller than the front element.
Click to expand...
Not "there are times", but pretty much always. The only exception *might* occur when a long zoom is wide open at the longest focal length when the aperture diameter and front element diameters are equal.
 
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