Aperture vs entrance pupil

[Abitconfused]

What is the difference between an aperture and an "entrance pupil?" In a 600mm f/4 lens, is the aperture diameter truly 150mm, as given by the f/D=N formula?

[pschlute]

What is the difference between an aperture and an "entrance pupil?" In a 600mm f/4 lens, is the aperture diameter truly 150mm, as given by the f/D=N formula?

In a 600mm f4 lens the "entrance pupil" will indeed be 150mm, but that is not the same thing as saying the physical aperture is 150mm.

My understanding is that the magnifying effect of the elements in front of the physical aperture will change the apparent size of the latter, and it is that size, as observed from the front of the lens, that gives the entrance pupil, and by calculation, the f-stop.

This is not my field of expertise so welcome any corrections.

[xpatUSA]

In a 600mm f4 lens the "entrance pupil" will indeed be 150mm, but that is not the same thing as saying the physical aperture is 150mm.

My understanding is that the magnifying effect of the elements in front of the physical aperture will change the apparent size of the latter, and it is that size, as observed from the front of the lens, that gives the entrance pupil, and by calculation, the f-stop.

This is not my field of expertise so welcome any corrections.

Yes, agreed and the entrance pupil is also known as the effective aperture ...

https://photo.stackexchange.com/ques...ctive-aperture

and the f-number is indeed the focal length divided by either one ...

https://en.wikipedia.org/wiki/F-number

HTH.

[Manfred M]

Agreed Peter, but there are a couple of other "gotchas".

The focal length of a lens is determined when focused at "infinity". The focal length can (and does) change when focusing close up (say the minimum focusing distance). This can make a difference in the actual f-stop. In real world applications, for most fixed focal length lenses this difference is quite minor. The only exception here are macro lenses where the shift in focal length can be fairly extreme; a 100mm macro could be at a focal length approaching 70mm at the minimum focusing distance. The same thing is true for a zoom lens. My 70-200mm zoom has a focal length of 135mm, rather than the stated 200mm, at the minimum focusing distance.

The other issue is that lens manufacturers round the actual focal length and maximum apertures to whole numbers in 5 or 10 mm increments. As measured, a lens can have a 48.5mm focal length, but the lens will still be marketed as a 50mm lens.

The f/D=N is what all this is based on, but there is another "gotcha" here. The formula does not compensate for inefficiencies in the lens design that reduce light transmission, so two different lenses marketed as f/2.8 lenses will not necessarily transmit the same amount of light. Cine lenses are marked in T-stops, when the loss of light is taken into account. An f/2.8 cine lens could be marked as a T-3.5.

[DanK]

a 100mm macro could be at a focal length approaching 70mm at the minimum focusing distance.

Most sources say the reverse, e.g., the tutorial on this site, which says that focal length roughly doubles at minimum focusing distance with a macro lens:

, the lens becomes so far from the sensor that it actually behaves as if it had a longer focal length. At 1:1 magnification, the lens moves all the way out to twice the focal length from the camera's sensor:

I don't have a Nikon, but I believe this shows up in larger effective f-stops at minimum distance on Nikon cameras.

I haven't tested this myself.

[revi]

Would that depend on whether the lens has 'internal focusing' or just moves the whole glass block back and forth?

If the former, the apparent focal lenght would indeed change with object distance, appearing as a shorter lens when focusing close up.
If the whole glass moves, it'l act more like a single lens, and then indeed a 100mm lens would be at 100 mm from the sensor when focused at infinity, and
at 200 mm at 1:1 magnification (using the thin lens aproximation).

Also, I think two different definitions of focal length are used above:
- for the 100 -> 200 mm macro exemple from Dan, they treat the objective as a single thin lens and take the distance between the focal plane and that thin lens as the focal length of the lens, (and this would be consistent with a light loss of 2 F-stops at 1:1 magnification, i.e. half the effective diameter of the diaphragm).
- in Manfred's exemple, they compare the behaviour of the lenses: where at infinity it has the field of view of ("behaves as") a 200 mm lens, at closest focusing distance, the FoV is that of a 135 mm focused at that distance. My guess is that those lenses hardly change lenght when focusing...

[DanK]

yes, I think this refers to internally focusing macro lenses.

[pschlute]

The focal length of a lens is determined when focused at "infinity". The focal length can (and does) change when focusing close up (say the minimum focusing distance). This can make a difference in the actual f-stop.

Yes I used to own a 60-250 zoom, constant f4. At closest focus distance it lost a large amount of FL and was approx 180mm

[DanK]

I don't want to hijack this thread to make it about macro, but since this has already come up:

I've been searching online for sources about changes to focal length when approaching minimum focusing distance with macro lenses, and what I have found is inconsistent. Some sources clearly state what Manfred did, which is that focal length decreases. Others (like the tutorial here) say the reverse, that focal length increases.

I haven't found a credible technical explanation yet. However, I think the problem may lie in part on the vague terms "effective f-stop" and "effective focal length." Here's my hypothesis about what is going on:

1. Actual focal length decreases, as Manfred (and also Brian Valentine, one of the best macro photographers I have found) says.
2. The amount of light transmitted also decreases, enough to more than compensate for #1 and to increase "effective" f-stop in terms of light transmission.

I don't know if this hypothesis is correct, but it fits with two things I have observed. First, the effect of adding a given length of extension tube to my 100mm macro at minimum working distance is much greater than it ought to be if the actual FL were 100mm. It's more consistent with 70mm or so. I actually took measurements for this but no longer have them. Second, if the tutorial here is right, one should see a big increase in diffraction:

This has all the usual characteristics, including an increase in the depth of field, a longer exposure time and a greater susceptibility to diffraction.

I haven't attempted to measure diffraction, but I haven't noticed that effect. I routinely shoot bugs at f/13 nominal, which should be f/26 actual or greater if I use an extension tube, without evident diffraction, and I have shot some macros with narrower apertures yet.

One way to test this partially is to test the angle of view at different a few distances. If #1 is correct, then the AOV should increase as you approach minimum working distance.

If anyone knows the answers to this and has some technical explanation, I'd love to see it.

[William W]

What is the difference between an aperture and an "entrance pupil?" In a 600mm f/4 lens, is the aperture diameter truly 150mm, as given by the f/D=N formula?

I believe the answer to this second question is: possibly but very rarely when considering modern lenses.

It is worth noting that, lenses, which we now term as "Telephoto" were originally classed as a subset of "Long Lenses".

The factor determining a Telephoto Lens as a special Long Lens, is indeed the element grouping, known as the telephoto group, which accommodates, (amongst other things as mentioned by Peter), the physical length of the lens to be shorter than the focal length.

As an example: the Canon 600mm f/4 lenses are about 470mm long, and not 600mm long.

Similarly, Telephoto Lenses have a special subset, "Zoom Telephoto Lenses".

It appears few of these traditional classifications remain in either Camera Manufacturers' Literature and most other sources, except Photography Text Books, mainly those with an strong optics focus (pun intended).

***

This type of classification system is common in all Mathematically inclined studies - a simple example of a Classification and the subsets containing special features is:

Quadrilateral - four sides two dimensional
Kite - (is a special quadrilateral) and has adjacent sides equal
Trapezium - (is a special quadrilateral) and has one pair of opposite sides parallel
Parallelogram - (is a special quadrilateral and Trapezium) - has opposite sides parallel and equal.
Rhombus - (is a special quadrilateral, special Trapezium and Parallelogram) - all sides equal and opposite sides parallel
Rectangle - (is a special quadrilateral, special Trapezium and Parallelogram) - and all angles are 90 degrees
Square - (is a special quadrilateral, special Trapezium, Parallelogram, Rectangle and Rhombus) - all sides equal and all angles are 90 degrees.

WW

(gee - I hope I got those correct, else Ted will never let go of the error!)

[xpatUSA]

Re macro lenses, I do recall the max/min f-numbers varying on a Micro-Nikkor 60mm f/2.8 (G model).

There's a table here:

https://www.imaging-resource.com/len...nikkor/review/

All normal when focused at infinity but at the macro end:

min is f/4.8 max is f/57 (sic). If I recall correctly, my D50 did report those numbers on the LCD -puzzled me at first until I read the manual ...

[Manfred M]

Bill - If I recall correctly, a telephoto is in fact a very specific design that does exactly what you mention, it reduces the overall length of the lens, which ends up making it shorter (in some cases, much shorter) than the focal length. I suspect that the catdioptric (mirror) lenses were the most extreme in reducing length as the design inherently reduced the physical length by a factor of 3 when compared to the focal length. A 500mm "Cat" would be around 167mm long.

My first "long lens" was a Leitz f/6.8 400mm lens and it really is 400mm long and is optically very simple; two optical elements in a single group.

[xpatUSA]

My first "long lens" was a Leitz f/6.8 400mm lens and it really is 400mm long and is optically very simple; two optical elements in a single group.

By coincidence, here's my first "long lens" an M42 Takumar 400mm f/5.6 purchased in November this year.

Seen here on the Panasonic µ4/3 Lumix DC-G9, hence 800mm equivalent:

[William W]

...it appears as if it is having a little nap, on a comfy bed.

[William W]

Dan's post #9

If anyone knows the answers to this and has some technical explanation, I'd love to see it.

-

Me too. This has got me thinking.

WW

[pschlute]

By coincidence, here's my first "long lens" an M42 Takumar 400mm f/5.6 purchased in November this year.

Seen here on the Panasonic µ4/3 Lumix DC-G9, hence 800mm equivalent:

Nice lens Ted. I have the later M version of that lens. "K" bayonet mount. It doesn't get used very much.

[xpatUSA]

Nice lens Ted. I have the later M version of that lens. "K" bayonet mount. It doesn't get used very much.

Thanks, Peter. My first-ever collar-mount lens. I was surprised by the quality at 400mm. I found my Giottos 'Silk Road' carbon-fiber tripod to be less that satisfactory on sandy soil with it's rubber feet and no sand-bag. So, if I get serious, e.g. moon-shots, I'll at least put the spiked metal feet on and place it on concrete ...

[pschlute]

Interesting point Ted about stability, probably deserving of it's own thread.

My experience when using long "footed" lenses is that a tripod is not the rock-solid platform many expect it to be in all circumstances.

The use of a camera which suffers from "shutter shock" (all SLRs do to some degree, especially a FF camera), together with a long footed lens creates a fulcrum effect where the combination vibrates on shutter trip. Usually only noticeable in the 1/30-1/200 sec range.

[William W]

I think the relative lighter weight has an impact too, combined with the relative longer length of a 'long lens', has an impact on the "fulcrum effect" you mention.

WW

[xpatUSA]

Yes, Peter and Bill, the "fulcrum effect" is a variation of the good old spring-mass-damper system that we learned in Mechanics class at School. You may recall that the system has a natural frequency of vibration hence the significance of shutter-time.