Help me understand this aspect of aperture, please

I have a Canon 85mm 1.8 and a canon 40mm 2.8.

Because they are different focal lengths, is the actual hole (iris) a different size when both are set to the exact same aperture?

For clarification: Is the opening the exact same diameter when both an 85mm lens and 40mm lens are set to F4?

Thanks

The expression "f/4" gives the effective physical diameter of the aperture of a lens as a fraction where "f" is the focal length of the lens. For a 50mm lens, this would be f/4 = 50/4 = 12.5mm diameter physical aperture. For a 200 mm lens, f/4 = 200/4 = 50mm diameter physical opening.

An elementary study of optics would show that a 12.5mm aperture on a 50mm lens passes the same amount of light per mm² as a 50mm aperture does for a 200 mm lens. Because of this, you need not concern yourself at all with the actual diameter of the aperture, just the relative light-passing property of the aperture as measured in f/stops.

In simple terms, f/4 with one lens is the same as f/4 with another lens.

Unless you're really keen on physics/optics/mechani​cal design, then you're really best served with going by 20droger's last line: An f-ratio on one lens is effectively the identical to the same ratio on a lens of a different length.

The actual physical openings involved may differ from what the math states due to issues in lens design. (I've not torn one apart or gotten around to looking at details on the lens designs, but I'm told that most of Canon's constant aperture internal focusing telephoto lenses are doing some really neat things.) The rabbit hole goes a little deeper when you start wandering into the issue of effective light transmission values, as most lenses will not actually preform exactly the way focal ratio itself suggests it should due to losses and inefficiencies in how light is transmitted through the glass elements. So transmission stop values are more precisely measured, but this is rarely actually an issue, and it is unlikely that you're ever going to notice.

It becomes an issue when you're dealing with high precision things, and it can make some tasks easier, but I've not come across it outside of cinema or lab grade equipment. (For movie production it can be important when you're shooting the same scene with multiple cameras, or doing zoom shots. With still photography you're unlikely to notice the small difference in exposure, but the dimming effect may become apparent in a movie, and such balance issues become more of a headache to adjust.)

20droger wrote in post #17279165
The expression "f/4" gives the effective physical diameter of the aperture of a lens as a fraction where "f" is the focal length of the lens. For a 50mm lens, this would be f/4 = 50/4 = 12.5mm diameter physical aperture. For a 200 mm lens, f/4 = 200/4 = 50mm diameter physical opening.

An elementary study of optics would show that a 12.5mm aperture on a 50mm lens passes the same amount of light per mm² as a 50mm aperture does for a 200 mm lens. Because of this, you need not concern yourself at all with the actual diameter of the aperture, just the relative light-passing property of the aperture as measured in f/stops.

In simple terms, f/4 with one lens is the same as f/4 with another lens.

The was an outstanding explanation. Thank you. Now I understand.

EOS-Mike wrote in post #17279287
The was an outstanding explanation. Thank you. Now I understand.

You be most welcome.

Questions - and answers - such as these serve to educate many of us. I also appreciate the information and the education.

Hillbille

+1 well appreciated

Concept well explained by the first two OPs.

In short and theoretically all same number F numbers on various focal length lenses permit same amount of light to pass through to get over sensor/film. But this is not the reality, nevertheless we wont noticed it on still photography.
Most importantly, never have concern about the diameter, but pay attention to amount of light that passes through.

Actually, f/stops would reflect reality if optics were perfect. Alas we live in an imperfect world, and those imperfections can affect performance to a noticeable degree, particularly when comparative images are used. That's why cine lenses have T/stops, which are basically f/stops with the imperfections accounted for.

If you want an example of the losses incurred with some lenses, consider this: if a lens were perfect, it would have no light losses through either absorption or reflection, which would make it invisible. Canon makes some good glass, but not quite that good.

One thing to consider with the constant aperture zoom lenses is that the physical aperture diameter doesn't vary as you zoom. In these designs the aperture is near the front of the lens, so that as the lens is zoomed, the magnification of the diaphragm opening changes along with that of the image. So the apparent physical size of the opening seems to change as you zoom in. when working with complex lens designs it is the apparent size of the opening when looking out from the image plane in the camera that actually matters when calculating the aperture.

Someone mentioned that aperture values are stepped and zoom is infinitely variable. which causes complications. This isn't really true. F Stops are stepped, so that they always double/halve the amount of light reaching the sensor/film. The actual f ratio is also infinitely variable, and the diaphragm in a lens can vary in just the same way that the lens can zoom. In a modern electronically controlled (as in the EOS system) variable aperture zoom the maximum aperture would vary in relation to the focal length as its effective diameter is constant. The display readout for maximum aperture value will change as you zoom probably at 1/3 stop intervals. As the electronic controls are capable of moving the aperture to any desired diameter, all of the aperture stop values will be correct at any of the lenses possible focal length, well within the margin of error of the control systems.

This was not the case with old mechanically operated diaphragm. In those old lenses the control ring would only be marked with the f ratios of the lens at the widest zoom setting. With those lenses the diaphragm would be stopped down to the same physical diameter at all focal lengths, so the change in f ratio would not be indicated as you zoomed, and the error, which would be constant across the range of stops, would apply to all of the possible f ratio stop settings. I guess that this would be one of the factors in the fact that variable aperture zoom lenses really only became popular after TTL metering became common. As the TTL metering would take care of the error for you by increasing the shutter speed a bit. The wide latitude of colour negative film would have also helped.

Alan

20droger wrote in post #17279165
In simple terms, f/4 with one lens is the same as f/4 with another lens.

simple enough explanation

I've been accused of being simple more than once.

20droger wrote in post #17280132
Actually, f/stops would reflect reality if optics were perfect. Alas we live in an imperfect world, and those imperfections can affect performance to a noticeable degree, particularly when comparative images are used. That's why cine lenses have T/stops, which are basically f/stops with the imperfections accounted for.

If you want an example of the losses incurred with some lenses, consider this: if a lens were perfect, it would have no light losses through either absorption or reflection, which would make it invisible. Canon makes some good glass, but not quite that good.

This is true enough, but any discussion of T-stops with modern lenses should stress that with today's coatings the actual losses are quite tiny, generally in the range of 1/6th stop or so. So the phenomenon is real but not of a huge importance to folks.

Think of it this way, the modern 24/1.4L II might in reality pass as much light as a f/1.5 in the center of the lens. In the corners, it passes about as much light as f/5.6.

In other words, the lens loses 1/6th of a stop to transmission and a whopping four stops to vignette.

EOS-Mike wrote in post #17279287
The was an outstanding explanation. Thank you. Now I understand.

This is what makes this forum so good. People willing to share their knowledge and experiences.

Next question - how does the sensor size affect this. I'm assuming a crop sensor will have less light hit it than a full frame sensor at f4 with the same lens. True?