[zeebanker]

Noob here, so please educate me. I have a new 50mm F1.8 SMC DA lens that I bought with my camera and an older 50mm F1.4 SMC M lens that I bought on Craigslist. Looking at the two lenses, the old F1.4, I can see the diameter of the objective (my term for the front element) is much larger than the new F1.8. The older lens is about 38mm vs. about 30mm for the new lens. So when fully open, the F1.4's aperture diameter is about 27mm and the F1.8's aperture diameter is 16.7mm. So I'm assuming the light gathering ability of the older lens is more than the newer lens (I also own a couple of telescopes, and in that world, objective diameter is king). Is this assumption of mine correct? When buying a lens, should we be looking at the physical diameter of the lenses as well as the focal length and minimum F-Stop? Why is the physical diameter not widely discussed/publicized?


I was looking to shoot some asteroids tonight, and would like as wide a physical aperture as possible. Naturally I'm leaning towards the older lens. I'll try shots with both lenses at the same F-stop and compare results.


Thanks in advance for your thoughts!

---

Last edited by zeebanker; 09-12-2020 at 08:03 PM.

[MetalUndivided]

should we be looking at the physical diameter of the lenses as well as the focal length and minimum F-Stop

Not really, the nomenclature for f stops means focal length divided by number, so in case of 50 mm f/1.4 you get 50/1.4=35.7mm. 50/1.8=27.8, so you don't need as big of a front element compared to 1.4, therefore it is smaller.

Why is the physical diameter not widely discussed/publicized?

Because it is redundant. The light gathering ability is fully reflected in the f number. You can't make a 50mm focal length lens with a 28 mm front element and call it a 50 mm f/1.4.

[tuco]

I think the max aperture says it all for a given focal length. Clearly a f1.2 50mm is a much faster lens than say a f2.8 lens without having to measure anything. But a variable you should also consider is not all lenses are equally sharp edge-to-edge wide open.

[swanlefitte]

50mm/1.4=35.7mm
50mm/1.8=27.8mm
If you measure the aperture opening, not the front objective, from the front wide open you should have close to the above.
If the aperture opened up to 30mm the max fstop possible is
50mm/30mm= 1.67 or f/1.67 so you couldn't get a f/1.4 lens with that objective.
Edit. If a front objective is a meter wide but the aperture only opens to 35.7mm the rest of that glass is not seen through the lens so it doesn't matter. It is still an f/1.4 lens.

---

Last edited by swanlefitte; 08-10-2020 at 09:05 PM.

[zeebanker]

Got it, thanks. I was actually doing the wrong math - dividing the objective diameter with the F-stop instead of the focal length.

Now I see that the faster lens' max aperture is 35.7mm and my rough measurement of the objective diameter was 38mm. Similarly the slower lens' max aperture is 27.8mm and my measurement was 30mm. Both are about 2mm more than the aperture. Would it be reasonable to think that the manufacturer would match the diameter of the objective to equal the max aperture + one or two mm, to minimize material consumption? Perhaps they actually make them equal (and my diameter measurements being rough, I could have easily eye-balled 2mm more than actual diameter).

So what I'm understanding is, faster => more light gathering capability => less exposure needed => wider aperture => wider front objective. Its beginning to make sense. So for low light photography, we need a smaller F-stop as the book says, which means the physical lens diameter will be more.

[stevebrot]

Why is the physical diameter not widely discussed/publicized?

The short answer is that it is not as important as one might think. The width of the entry pupil is the actual "choke point" and even at f/1.4, is sometimes much narrower than the front element size.* I have multiple 50mm lenses, f/2.0 and wider maximum aperture; all but one of which cover the 35mm image circle. For the f/1.4 models, physical front element diameter varies from 35mm to 42mm. For the f/2.0 models, the range is even wider with narrowest at 25mm and the widest at 35mm. Despite the differences, all the f/1.4 lenses have the same entry pupil diameter wide open with the same true for all the f/2.0 lenses at their maximum aperture.

One would think that the larger elements would squeeze more light through the opening, yet the TTL meter says otherwise. For a given subject distance, they sample the same object frame.

Moving to a shorter focal length, I have two 28mm f/2.5 lenses. One is a Vivitar with an impressive 45mm diameter front element. The second is a Tamron with a rather petite 22mm diameter front element. Both present similar viewfinder brightness and meter readings. Strangely enough and as noted above, both sample the same object frame at any given distance, even close up.


Steve

* For ease of discussion, the entry pupil is the lens opening as it is viewed (looking through) from the front of the lens. The f-number is the focal length divided by the entry pupil diameter, hence the f/ notation.

[pschlute]

So what I'm understanding is, faster => more light gathering capability => less exposure needed => wider aperture => wider front objective. Its beginning to make sense.

You are on the right track, although "exposure" itself consists of three factors: Shutter speed/aperture/Sensor sensitivity. Using a very fast lens means you can use a higher shutter speed or less sensitivity (lower ISO), but your "exposure" will be the same.

On a side note regarding focal lengths, this is a measurement from the point of light convergence within the lens, to the sensor. Note these two lenses, both 50mm 1.4 lenses, but very different physical sizes. Yet both will allow the same amount of light through to the sensor.

Attached Images

   

[swanlefitte]

. Would it be reasonable to think that the manufacturer would match the diameter of the objective to equal the max aperture + one or two mm, to minimize material consumption?

The lens also has to bend the light. Condider the 8mm lens with a 24mm wide sensor has an angle of view of 180 degrees or close to that. On a pentax camera the sensor is 45.6mm inside the camera. Somehow that light from the side has to curve around and in that hole. The lens does that and different designs do it differently. The 8mm fisheye front objective is very large and bulbous. So the minimal material is different for different designs.

[PJ1]

You have all the good info in the answers here. I use a quick and dirty measurement of focal length divided by objective diameter to give an approximate maximum aperture. For the 50mm f1.4 where you measured 38mm objective diameter, that gives a maximum aperture of 1.316. If you deducted a couple of mm to allow for possible internal baffles of some kind (even if they are not visible when looking through the lens), 50mm divided by 36mm would give a maximum aperture of 1.389 - call it 1.4.

You will find occasional complaints that this or that lens is not really as fast as the manufacturer claims - for example, that you don't get the correct f-number when you do this kind of calculation. The problem is often the focal length of the lens. For example, a 135mm lens may really only be 132mm. That makes a difference when you do the quick and dirty calculation. The longer the focal length, the more variation there seems to be. With lenses that have internal focusing or lots of fancy elements it is much more complicated and the simple calculation won't work.

I sometimes do those calculations myself just for interest. But I don't lose any sleep over it. Usually (but not necessarily always) manufacturers label a lens 85mm, 135mm, 200mm etc because the lens measurement is the nearest "accepted" length. Similarly, they use f1.4, f2, f5.6 etc because that is the nearest "accepted" aperture.

[D1N0]

If you really want to know the amount of light gathered you need the T-stop, not the F-stop. It is dependent on the amount of surfaces the light needs to pass through and the coatings. So not easily calculated by looking at the specs. It needs to be measured with a light source of known brightness.

[photoptimist]

1) Apertures: As other have said, the numerical aperture and physical aperture are related by the focal length. In fact, that's why many write the F-stop as "f/" some number and might refer to a 50mm f/1.4 lens as a 50/1.4 lens.


2) Light gathering power: When it comes to image brightness and setting exposures in photography of surfaces, numerical aperture is king. It's all you need to know for getting the right exposure. A 50mm lens set to f/5.6 and a 500mm lens set to f/5.6 will both render a surface (skin, paint, leaf, etc.) as the same brightness despite the huge difference physical aperture.

When it comes to object brightness and setting exposures in photography of point light sources (e.g., stars), physical aperture is king because the area of the light-gathering aperture determines the total star light collected. A 500mm f/5.6 lens (i.e., a small telescope with a 90mm objective and physical aperture) will gather over 6X more star light (over 2.66 stops more star light) than a 50mm set at f/1.4 (only a 36mm physical aperture). In fact, for a given exposure time, the 500/5.6 will render the stars as brighter and the sky blacker than will the 50/1.4. (However, the 50/1.4 will render diffuse objects such as nebulae and comets brighter than will the 500/5.6).

Note: the effect of numerical and physical aperture on the brightness of line-like sources of light (e.g., meteors) is also determined by the physical aperture more so than the numerical one but the strength of the effect if half way between that of area and point light sources.


3. Objective element size versus physical aperture: For longer focal length lenses, the physical size of the front element and the optical entry pupil are almost the same. For wider angle lenses, the wider angle of view means the fan of light rays from the scene are wider where they enter the first element than when they pass through the aperture deep inside the lens. Thus, the front element is wider than the aperture. This phenomenon can be quite extreme in ultra-wide angle lenses. For example, the physical aperture optical entry pupil of the Samyang 14 mm f/2.8 is a tiny 5 mm but the front objective element is 77mm across. That huge bulbous element is needed to bend a 114° wide field of view into a bundle of rays that are projected on to the sensor.

[photoptimist]

Ok the main pupil difference in newer lenses vs older is the newer lens is what is considered a crop sensor lens meaning due to crop sensors having a smaller imaging area( for Pentax roughly 23.5 x 15.6mm) lenses were developed to project the image precisely to that smaller area so using the same Fstop the opening will be smaller than a full size lens...


Where as the other lens is a full film size imaging lens meaning it is designed to project an image to a sensor or film roughly 35 x 24mm in size, so the aperture pupil opening is larger. Using this lens on a crop sensor camera the image will over shoot the sensor lessoning the capture field of view hence the term "crop factor". The upside to using these are many lenses are soft on the edges so using them on a smaller sensor camera one captures at the sweet spot center of the lens.

Both virtually provide the same Fstop light gathering and field of view as each other when used on a crop sensor. Using a crop sensor lens on a full frame camera the image in most cases will have vignetting in the captured image.

Sorry, this is false. The F-stop opening of a lens is only a function of focal length and has nothing to do with sensor size.

A 50 f/1.4 lens designed for a crop sensor (or even a micro4/3rds camera) will have the identical aperture pupil diameter as will a 50 f/1.4 lens designed for a full frame sensor.

What may be true is that the diameter of the front element may be larger on lenses for larger sensor cameras because they are wider-angle lenses.

[leekil]

When it comes to object brightness and setting exposures in photography of point light sources (e.g., stars), physical aperture is king because the area of the light-gathering aperture determines the total star light collected. A 500mm f/5.6 lens (i.e., a small telescope with a 90mm objective and physical aperture) will gather over 6X more star light (over 2.66 stops more star light) than a 50mm set at f/1.4 (only a 36mm physical aperture). In fact, for a given exposure time, the 500/5.6 will render the stars as brighter and the sky blacker than will the 50/1.4. (However, the 50/1.4 will render diffuse objects such as nebulae and comets brighter than will the 500/5.6).

Are there examples of the same focal length with the same aperture that have differences in light-gathering power (independent of T-stops)? This doesn't immediately make sense to me, unless the tele lens gets more light because of the larger image in the viewfinder. Would a wider-angle lens with a large front element also have brighter stars?

[zeebanker]

Wow, lots of interesting information and food for thought. Thank you all!

[Apet-Sure]

Congratulations on owning two fine lenses. The average user rating for both is in the 9s. In case you haven't run across it, there are reviews of both here:
SMC Pentax-M 50mm F1.4 Reviews - M Prime Lenses - Pentax Lens Reviews & Lens Database
SMC Pentax-DA 50mm F1.8 Reviews - DA Prime Lenses - Pentax Lens Reviews & Lens Database