[nickthetasmaniac]

So, on another forum on the interwebs an interesting comment was made, paraphrased below:

'Pentax fudged the max aperture numbers for the Takumar Bayonet 135/f2.5 and SMC 200/f2.5 because the filter size (and therefore front element) is too small for the stated aperture.'

For reference:
- Tak Bayonet 135/f2.5 - filter size 52mm, max aperture directly calculated 54mm
- SMC 200/f2.5 - filter size 77mm, max aperture directly calculated 80mm

Now my understanding is that the aperture number is a theoretical thing, rather than an actual physical measurement, but I admit the physics is beyond me.

Can someone clarify this (either way) in laypersons terms?

Tah!

[Sandy Hancock]

The way light travels through a lens is complicated. I'm not an optical engineer, but might it be possible that the maximal virtual diameter of a telephoto lens can be greater than the filter diameter?

[nickthetasmaniac]

The way light travels through a lens is complicated. I'm not an optical engineer, but might it be possible that the maximal virtual diameter of a telephoto lens can be greater than the filter diameter?

Yep this is what I’m trying to get someone to explain. It’s making my brain go mushy...

[biz-engineer]

- Tak Bayonet 135/f2.5 - filter size 52mm, max aperture directly calculated 54mm

If the lens is focused at infinity, with 52mm front diameter it's a 135mm f2.6. But if the lens has focus breath and is actually a 100mm lens at portrait distance, it can well be a 100mm f2 equivalent lens?

[MarkJerling]

So, on another forum on the interwebs an interesting comment was made, paraphrased below:

'Pentax fudged the max aperture numbers for the Takumar Bayonet 135/f2.5 and SMC 200/f2.5 because the filter size (and therefore front element) is too small for the stated aperture.'

For reference:
- Tak Bayonet 135/f2.5 - filter size 52mm, max aperture directly calculated 54mm
- SMC 200/f2.5 - filter size 77mm, max aperture directly calculated 80mm

Now my understanding is that the aperture number is a theoretical thing, rather than an actual physical measurement, but I admit the physics is beyond me.

Can someone clarify this (either way) in laypersons terms?

Tah!

No idea. But, both the 50mm F1:1.4 and the 50mm F1:1.7 have the same filter size of 49mm. So I'm not sure the "direct" calculation holds true.

[Fogel70]

F-number of a lens is the ratio between focal length and max entrance pupil.
Entrance pupil is the apparent aperture size when looking through the front of the lens, and can be magnified by the optics.

Zoom lenses may have variable f-number even though the physical aperture stay the same.

[nickthetasmaniac]

No idea. But, both the 50mm F1:1.4 and the 50mm F1:1.7 have the same filter size of 49mm. So I'm not sure the "direct" calculation holds true.

I probably didn’t explain clearly. The premise of the comment in the OP was simply that a 135/f2.5 lens with filter thread of 52mm couldn’t physically fit it’s theoretical maximum aperture (54mm). The filter thread bit is slightly misleading, what they’re really getting at is the front element diameter (which is always smaller than the filter thread).

By this math, a 50/f1.4 has a max aperture of 35.7mm and a 50/f1.7 29.4mm, so a 49mm filter would be fine in both cases.

[StiffLegged]

Yep this is what I’m trying to get someone to explain. It’s making my brain go mushy...

That’s the primary purpose of other interweb forums.

[biz-engineer]

'Fudging' aperture numbers

not fudging aperture numbers, but fidging focal length. f2.5 at 52mm gives a focal length of 130mm. We've had models reject the photos because the lens was not a true 135mm lens

[photoptimist]

The way light travels through a lens is complicated. I'm not an optical engineer, but might it be possible that the maximal virtual diameter of a telephoto lens can be greater than the filter diameter?

The optical aperture can be larger than the physical front opening but only for subjects closer than infinity. For subjects at infinity, the filter ring, front element, etc. must be at least as big as the focal length divided by the aperture number. In fact, if the lens is to deliver the stated aperture across the entire image circle, then the filter ring must be larger than that ratio to accept a full aperture's worth of light across the full cone of the image circle.

[Lowell Goudge]

i think the real answer is very simple. within all lens specifications there are tolerances on both focal length and aperture. in the good old days when photo magazines were somewhat independent of manufacturers. you would see lens test reports showing the focal length and aperture of the lenses as received. what we are discussing here is either a 1/6 of a stop error in aperture, or perhaps the lens only has 192mm focal length, both probably within the limits of standard lens nomenclature.

as an example, my vivitar 70-210/3.5 version 1 which is a well regarded lens, has a front element diameter, inside the beauty ring of 55 mm which would make it F3.8 even my nikon 300/4.5 which inside the lens retention ring measures 62mm in diameter works out to f4.8 clearly at the time this seems to be a common practice

note, i found some old Popular photography reports on line, and what i noticed is normal to wide angle lenses seem to be Longer than stated but by the time you get to medium tele to tele, they are shorter than stated. just a quick observation not sure if it is true for all lenses.

[zapp]

That’s why you call the lens a 200 and not a 200mm. Today the tolerances are tighter. 1% Offset I think. Aperture has different offset. Filter diameter is always wider than lens diameter, so differences are even worse.

[stevebrot]

'Pentax fudged the max aperture numbers for the Takumar Bayonet 135/f2.5 and SMC 200/f2.5 because the filter size (and therefore front element) is too small for the stated aperture.'

The F-number is a sure thing and a calculated value (focal length / entry pupil diameter*). Whether maximum aperture is correctly reported by manufacturers is another thing entirely. A good example might be my Tamron 70-150/3.5 (20A) where the maximum aperture was measured for test reviews at 4.0. The suggested rational at the time was to make the lens more competitive against a similar f/3.8 product from Tokina.


Steve

* Strangely, entry pupil diameter is independent of the front element diameter.

[MarkJerling]

I probably didn’t explain clearly. The premise of the comment in the OP was simply that a 135/f2.5 lens with filter thread of 52mm couldn’t physically fit it’s theoretical maximum aperture (54mm). The filter thread bit is slightly misleading, what they’re really getting at is the front element diameter (which is always smaller than the filter thread).

By this math, a 50/f1.4 has a max aperture of 35.7mm and a 50/f1.7 29.4mm, so a 49mm filter would be fine in both cases.

Thanks, that explains it better!

[MarkJerling]

That’s why you call the lens a 200 and not a 200mm.

What? Since when?