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DSLR Lens "Physical Aperture"?

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#1 Agreegator

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Posted 13 April 2021 - 03:19 AM

How can I measure the actual Aperture of a DSLR lens ? I know I can divide FL by F # and get the theoretical aperture. But having dealt with telescopes all my life, it just really bugs me that I do not know the physical aperture of primary lens.

 

I know the physical aperture will be pointless because Iris changes accessible aperture - but I still wanna know anyway. For a start, I do not even know if the glass in the front is the actual lens  ? Like a telescope, can I just put a tape measure against the front element of DSLR lens and take that as the actual aperture? If not, then how ? 

 

 

I'll really appreciate some input. Thanks.


Edited by Agreegator, 13 April 2021 - 03:19 AM.


#2 Tapio

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Posted 13 April 2021 - 03:26 AM

Aperture is lens/mirror diameter.

(Don't know why Iris would need lens diameter/aperture - fl is more important).



#3 sg6

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Posted 13 April 2021 - 03:53 AM

I do not think there is a direct measurement, well not a fairly simple one. Camera lens are way more complex, for a start they have a lot more lens and so a lot more light loss along the path from reflections. And something tells me that the loss at all the faces is taken into account.

 

The other "oddity" is camera lens are defined by focal length and focal ratio, people buy a 200mm f/4 lens. How it gets to be an f/4 lens is not part of the description. All it is saying is that the light level received at the sensor is equivalent to f/4. So my odd memory that light loss is accounted for would mean that the physical measurements could mean it begins as say f/3.5 and the light loss at all faces reduces the final result to f/4.

 

What would happen at Canon, Sony, Nikon, Sigma, Tamron etc is they input in a calibrated source and measure the final image brightness, then calibrate the diaphram to correspond to the "normal" focal ratio values. Or a lot simpler use more or less of the front objective area with a simple aperture ring in there somewhere.

 

On a camera lens of say 200mm f/4 is an effective aperture of 50mm, but at a guess a real aperture of 54mm.


Edited by sg6, 13 April 2021 - 03:55 AM.

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#4 Agreegator

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Posted 13 April 2021 - 04:10 AM

I do not think there is a direct measurement, well not a fairly simple one. Camera lens are way more complex, for a start they have a lot more lens and so a lot more light loss along the path from reflections. And something tells me that the loss at all the faces is taken into account.

 

The other "oddity" is camera lens are defined by focal length and focal ratio, people buy a 200mm f/4 lens. How it gets to be an f/4 lens is not part of the description. All it is saying is that the light level received at the sensor is equivalent to f/4. So my odd memory that light loss is accounted for would mean that the physical measurements could mean it begins as say f/3.5 and the light loss at all faces reduces the final result to f/4.

 

What would happen at Canon, Sony, Nikon, Sigma, Tamron etc is they input in a calibrated source and measure the final image brightness, then calibrate the diaphram to correspond to the "normal" focal ratio values. Or a lot simpler use more or less of the front objective area with a simple aperture ring in there somewhere.

 

On a camera lens of say 200mm f/4 is an effective aperture of 50mm, but at a guess a real aperture of 54mm.

This makes a lot of sense .. I took apart an old lens, a month ago to see what's inside and the largest lens at the very beginning of the train did seem to be 15% - 20% larger than what numbers should make it out to be. That's what really confused me ...  I guess your idea about light loss is quite correct. Thank you.



#5 sharkmelley

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Posted 13 April 2021 - 04:48 AM

For a start, I do not even know if the glass in the front is the actual lens  ? Like a telescope, can I just put a tape measure against the front element of DSLR lens and take that as the actual aperture?

No you can't do that.  For example take a look at this ray diagram of a Canon 28mm f/2.8 lens:

https://www.photonst...5,AxisO,OffAxis

 

Even an on-axis star (i.e. the blue lines) is using a small fraction of the front element glass.  So the diameter of the front element would be totally misleading for this lens.

 

Mark


Edited by sharkmelley, 13 April 2021 - 04:48 AM.

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#6 Sandy Swede

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Posted 13 April 2021 - 05:21 AM

An excellent question, Agreegator.  Wondered that myself a few times.  IIRC, Astronomy Tools did not have camera lenses in the database, but hope to verify that later.



#7 SteveInNZ

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Posted 13 April 2021 - 05:25 AM

The focal ratio isn't the ratio of focal length to the objective diameter. It's the ratio of focal length to the "entrance pupil" which is the diameter of the aperture as seen from the front of the lens.

In a simple lens or mirror (like a telescope), the entrance pupil is the diameter of the lens or the mirror. With a more complex lens, the entrance pupil is seen through one or more lens elements.

Have a look through the front of a constant aperture zoom lens (eg. 70-200mm f/2.8) while you zoom the lens. You'll see that (obviously) the front diameter stays the same but the aperture blades appear to change size as you change the focal length, keeping the focal ratio the same.

 

So for the purpose of comparing a lens to a telescope, you can divide the focal length by the focal ratio to arrive at an effective aperture. If you wanted to actually measure something, take the lens off the camera and take a photo from the front with a white surface behind it. Use something like photoshop to measure the diameter of the apparent aperture.

Reflection losses are ignored as they are of no consequence in photography with TTL metering. You'll see cinematography lenses marked in T stops which takes lens efficiency/reflections into account.

 

 

Steve.


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#8 kevinbreen

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Posted 13 April 2021 - 06:27 AM

Stopping down the lens changes the aperture. This is a good question and one I've often wondered about myself. From what I understand though, each increase in f-stop decreases the aperture by a factor of 1/1.414 or 1/sqrt(2). Please correct me if I'm wrong.



#9 DAG792

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Posted 13 April 2021 - 06:29 AM

The method I'm describing may not be perfect, as I only have experience with using it to find the effective aperture of binoculars. However, I think that it should give you a useful ballpark figure.

What you need to do is use the camera lens as an 'objective lens' and make a telescope. Use a medium focal length eyepiece(10-15 mm). Next, adjust this 'telescope' so that it focuses at infinity.

Now, what you need to do is shine a collimated beam of light through the eyepiece, such that it makes an evenly illuminated circle of light upon exiting the objective A.K.A the camera lens. Place a screen to capture the illuminated circle, and measure its diameter, this is the effective aperture of your telescope, or the effective aperture of your camera lens.

 

P.S. A useful way to make a collimated beam of light (if you don't have a flashlight), is to use another long FL Plossl eyepiece, an put a light source(i.e. a cellphone flashlight) at its focal point.

 

Try this and let me know if it works. 



#10 ccs_hello

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Posted 13 April 2021 - 06:56 AM

The way I handle the aperture number, (yes, it's a derived number from F.L. and F-number), is to verify if there is anything that does not make sense.

Turns out for DSLR lenses, no one is lying.

Say, for a zoom lens wih a 55-300mm F 4-5.6 spec, we know it's going to be 300mm fl at F 5.6.  In such case, the front lens (the largest and widest lens) will be no smaller than 300/5.6 in diameter.

 

BTW, usually a higher grade lens will bump everything larger such that only the sweetspot of the optics got utilized. 



#11 TOMDEY

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Posted 13 April 2021 - 07:14 AM

How can I measure the actual Aperture of a DSLR lens ? I know I can divide FL by F # and get the theoretical aperture. But having dealt with telescopes all my life, it just really bugs me that I do not know the physical aperture of primary lens.

 

I know the physical aperture will be pointless because Iris changes accessible aperture - but I still wanna know anyway. For a start, I do not even know if the glass in the front is the actual lens  ? Like a telescope, can I just put a tape measure against the front element of DSLR lens and take that as the actual aperture? If not, then how ? 

 

I'll really appreciate some input. Thanks.

Hi, Agreegator!

 

If the lens is properly characterized in its tech-specs, then the diameter of the Entrance Pupil is accurately and identically F/F#. If the lens is further designed and built to be free of vignetting over all used fields, then the ~used aperture~ varies with where you measure it everywhere from the front of the complete lens on back to the image. Those diameters are overtly manifest in the traditional broadside ray-trace renditions that comprise the trajectories of rays from central and limiting field edge. This is actually what the designer uses to size the diameters of all the lens elements to be free of vignetting (or to intentionally-compromise that ideal in order to block offending rays that would otherwise diminish resolution).

 

That's really all that can be said on the topic. Beyond that, terms like "Actual Aperture" and "Physical Aperture" lack meaning or specificity. The Lagrange Invariant (intimately bonded to étendue and Emmy Noether's Theorem) defines how these apertures and associated solid angles invariantly combine as light cascades from object-space --- through the lens --- to the image.    Tom

 

Etendue
Etendue or étendue is a property of light in an optical system, which characterizes how "spread out" the light is in area and angle. It corresponds to the beam parameter product in Gaussian beam optics.



#12 whwang

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Posted 13 April 2021 - 09:00 AM

Many people gave quite useful and correct information already. What I can add here is that for camera lenses, especially wide-angle ones, “aperture” is an illy defined term. How large is the aperture actually depends on what you want.

Like many had said, if what you want is a number that can tell you the image brightness (light collecting power), then it should be the size of the entrance pupil. Same for diffraction limit (although most lenses are not diffraction limited unless stopped down to F8 or even F11).

On the other hand, if what “aperture” means to you is how large the first glass is, then it can be very different from the entrance pupil. In practice, not many people care about such a number. The only exception is probably ultra-wide lenses that don’t have front filter threads. For such lenses, it’s sometimes useful to know the diameter of the first glass element so one can purchase a suitable square filter. Otherwise one just looks at the size of the filter thread in the lens spec without thinking about the size of the “aperture.”

#13 bclaff

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Posted 13 April 2021 - 09:41 AM

No you can't do that.  For example take a look at this ray diagram of a Canon 28mm f/2.8 lens:

https://www.photonst...5,AxisO,OffAxis

 

Even an on-axis star (i.e. the blue lines) is using a small fraction of the front element glass.  So the diameter of the front element would be totally misleading for this lens.

 

Mark

FWIW, the size of the diaphragm/iris is unknowable (without disassembly).
In the example you gave it's known because the full optical prescription is taken from a patent and then ray traced.

At f/2.86 the diaphragm is 13.110mm (labeled AD). while the entrance pupil is 10.00mm (PD) at the focal length of 28.6mm


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