Actual aperture diameter - DPChallenge Forums

05/04/2006 10:09:38 AM · #1

It's feeding time again! On information that is...

I was considering whether I should spend money on the 50mm f/1.4 or an 85mm f/1.8 that my friend is selling. I realize that the focal length is quite different, but there is another thing I am considering, as I like to do macro shots with reversed lens techniques. This means learning more about the actual dimensions of the apertures of different lenses.

I thought that the number was a ratio like this: focal length (ie, 50mm)/f-stop = actual aperture diameter.

Seems that my brain must be full of poo poo because this is NOT what I got when I measured my 50mm f/1.8.

50/1.8=27.77777e
50/1.4=35.714
85/1.8=47.222
85/2.8=30.357

I have a high precision measuring tool that I don't know how to use very well (does this make me a measurebator? yikes! - I think it's good to 0.05mm) and I noticed that my 50 f/1.8 has a diameter of around 23mm. That's 4mm short!!!

What the Chicken?

The 85 f/1.8 measured at 35.7mm give or take a very tiny bit. OY!

So it's pretty obvious to me that I'm an idiot who sucks at math and hasn't a clue about how this works.

I invite anyone who likes to play "whack a mole" to get out their nearest handy 2x4 and see if you can club some info into my sludgy grey matter on this subject.

THANKS!

PS. Don't spend consecutive nights with 2 hours sleep, then post technical threads ;)

Oh yeah, my purpose in this thread is that when I stick my camera on a reversed lens, I notice that with the 85mm f/1.8, the image circle is HUGE and a half! I only need to zoom in a little bit to get rid of vignetting. Using the 50mm f/1.8 reversed means that I need to zoom in almost to the limit.

I am guessing that if I can alleviate the burden on my glass with extreme magnifications, I can achieve better results.

Mr_Pants

SiPix Blink

05/04/2006 10:13:59 AM · #2

I seem to remember Kirbic posting recently that the aperture dimension that you can calculate in this way is the effective aperture at the front element of the lens.

Canon RF 24-70mm f/2.8 L IS

05/04/2006 10:23:31 AM · #3

Thanks man. I knew I was missing something really obvious like that.

>loser<

Sorry for the time wasting... I need more SLEEP!

kirbic

Canon EOS R5

05/04/2006 10:23:51 AM · #4

Ah, the ol' physical aperture question... yep, that one can be confusing. The confusion comes from the fact that people, even well-informed people, talk about the aperture as if it were the physical hole in the iris diaphragm. Though controlling the size of this hole is what changes the f-ratio, the size of the hole is not the size of the physical aperture. The physical aperture is determined by the size of the light cone entering the front element. For longer focal lengths, the front element will be slightly larger than the required physical aperture.
Example: the 70-200/2.8 needs a 71.4mm physical aperture (200mm/2.8 = 71.4mm). The filter thread is 77mm, and there's not much space inside it, so the front element is just over the required aperture. You'll find this to be true for any lens over about 50mm.
Once the light cone enters the front of the lens, it is refracted and condensed (converging rays). Because of this, the iris opening can be smaller than it would otherwise need to be.

Nikon AF Nikkor 50mm f/1.8D

05/04/2006 10:42:49 AM · #5

Yeah, that sounds a bit more like it... :)

I did indeed get the math right then...

Do you have any ideas then regarding the issue that brought me to this question in the first place?

Which lens will provide the largest image circle when used reversed?

I am going to venture a guess that the focal length has a direct relationship with the size of the rear element circle, so shorter lenses (like the 50mm) will allow the camera to get larger circles.

However, what I have discovered so far is that the rear aperture is so small on the 50mm f/1.8, it still causes significant vignetting and distortion.

I get an extra 200mm equivalent of room to play with the reversed 85mm f/1.8. Of course, this provides less actual magnification though....

I might be able to live with this though..

I was also considering (in another somewhat dead thread) the possibility that using a Medium format 50mm lens might help. I MIGHT be able to find a 50mm MF lens for a reasonable price right now as things are looking ROUGH for that market... Most MF gear has pretty fine optics.

Sadly, I noticed that many 50mm MF lenses have smaller natural apertures on the front end. Without handling the lenses (which might be somewhat rude when I don't intend to spend two grand on the lens which I would be manhandling), it is difficult for me to determine how their rear elements would look...

That 85mm is pretty funkarific for this purpose though...

As always Kirbic, your descriptions are clear and immediately comprehensible.

Just to clarify that I got the right terms from your description...

Iris = the aperture at the rear element
Physical Aperture = the aperture at the front element

Sound right?

Message edited by author 2006-05-04 10:46:34.

_eug

Nikon D50

05/04/2006 11:20:14 AM · #6

I think the answer to your question is that you want the one with the largest Angle of View.

Edit to add: 50mm = 46 Deg, 85mm = 26 Deg.

Message edited by author 2006-05-04 11:24:11.

kirbic

Canon EOS R5

Canon RF 24-70mm f/2.8 L IS

05/04/2006 11:39:58 AM · #7

Originally posted by eschelar:

...Just to clarify that I got the right terms from your description...

Iris = the aperture at the rear element
Physical Aperture = the aperture at the front element

Sound right?

The iris is the physical diaphragm that opens and closes to provide aperture control. It's composed of several blades, and is visible when the lens is stopped down.

When reversing a lens on another, you need to consider two things. For the "normally-oriented" lens (the one mounted to the camera), the shorter the focal length, the harder it will be to find a reverse-mount lens that won't vignette. That's because the shorter focal length lens has a wider angle of view, and sees the barrel of the reversed lens. In my experience, using a 100mm lens mounted normally with a 50mm f/1.4 lens reversed results in little no vignetting, at least with a 1.6-crop cam. As the clear aperture of the reversed lens gets smaller (smaller focal length or slower lens) vignetting may appear. As the physical aperture of the normal lens gets larger, vignetting also may appear. So there is a "sweet spot" where the angle of view and front element size of the mormal lens matches up with the clear aperture of the reversed lens.
Hope that helps!

Sony 24-70mm F4 Vario-Tessar T* FE OSS

05/04/2006 11:34:21 PM · #8

Ok, I went ahead and got myself some sleep. Hopefully I am reading a bit more clearly now...

So now I get Iris (same thing in your eye I'm guessing).

So what do we call the rear aperture? Rear Aperture?

So I guess what it boils down to is that as the angle of view increases, the rear aperture decreases.

Hence, a 50mm lens with a 46 degree angle of view, the rear element is quite small. With an 85mm lens, with a 26 degree angle of view, the rear element is quite a bit larger. 46/26=1.77, so it is probably 1.77 times larger.

So, 23mm rear aperture on the 50mm times 1.77=40.71.
The rear aperture on the 85mm is 35.7mm...

I'm still doing something wrong.

I tried instead with the focal length... 85mm/50mm x 23mm = 39.1.
Nope. Maybe there isn't such a strict relationship
However, the principle seems to be the same.

Checking BH, the specs on the 85mm list the angle of view as 28 degrees.
This is probably larger than the theoretical 26 degree angle of view due to the oversized physical aperture? If this is the case, then it's possible that the 50mm f/1.8 also has a 'fudged' listing of 46 degrees due to a larger front element. Of course, it's also possible that because the lens is cheaper, they include less glass and that extra bit of breathing room afforded for filters, focusing shifts and such (ie, why the 85mm f/1.8 has a 58mm thread instead of a 48mm...) is reduced, so the 46 degree angle might work out a bit smaller in practice.

46/28 x 23mm = 37.78. This is quite a bit closer to the actual diameter of the rear aperture on the 85mm lens.

Barring some minor errors in my own measuring, it seems that this could be a pretty close measure of rear element size.

So thank you _eug, as that cleared up my understanding a great deal on the matter.

That brings me back to the original problem of reverse lens technique.

The 85mm provides less magnification than a 50mm. That is plain obvious.

With a 200mm lens, magnification through a reversed 50mm is 4x. (200/50=4)
With a 200mm lens, magnification through a reversed 85mm is only 2.35x

A pretty significant difference indeed.

However, because of the tradeoff in angle of view, it is possible to get a MUCH wider image circle, which will make it a LOT easier to deal with lower magnifications. If you have a zoom lens that uses a range from 120mm+, you are likely to be able to shoot directly through the 85mm with magnifications from approximately 1.5x right on through to whatever else is provided by your lens zoom.

Not all shots require ultra-high magnifications of 4x+, so this can really help to fill in the middle ranges, when your subject is a little larger.

Now I went ahead and did some real world testing of this though and didn't find as much of a difference as I had imagined. This is likely due to the relationship of focal lengths.

The biggest difference can be seen in the vignetting.

I'm putting together some examples. Give me a minute.

Message edited by author 2006-05-04 23:46:10.

Bear_Music

Samsung Galaxy S24 Ultra

05/05/2006 12:13:44 AM · #9

Eschelar:

The published angle of view is the actual angle of view covered by the 35mm frame of film / FF sensor. In fact, the lens will normally have a somewhat larger field of view just to keep the film/sensor out of the edge effects at close to the circumference of the circle. Taken to extremes, you get lenses like view camera lenses, where the FOV of a 210mm lens is so large that that you can shift an 8x10 film 6 or 7 inches vertically or horizontally and have to be within the eimage circle. Mount the same lens on the 4x5 view camera and the amount of movement that is possible is extraordinary.

The EF-S lenses throw considerably smaller image circles than the EF lenses, and there is a corresponding decrease in the size of the elements.

R.

Nikon AF Nikkor 50mm f/1.8D

05/05/2006 12:47:30 AM · #10

Thanks for the heads up on that. I hadn't even considered this aspect of lenses.

None of the lenses I have tested so far are EF-S lenses (although I am borrowing an 18-55 kit lens, I've only used it once, and you are right, it has an itsy bitsy rear aperture). Incidentally, outside of the kit lens, I don't have any EF-S (or equivalent) lenses on my list to buy currently.

Wouldn't the Angle of View also relate to the relationship between Aperture and front element? Or is this just a coincidence?

I would guess that this is why certain lenses have such HUGE front elements. I wonder what the back end of the Peleng 8mm looks like...

As I am dealing with two different fields of view (1.6x and 6x on my S2), so this makes things too complicated. Other people reading might also have different FOV formats, so I thought it would be better sticking to just the characteristics of the lens.

Now having said that, I've just done some more in-depth testing and found that in fact, the image circle of the 85mm is roughly the same as the 50mm. I will post some more details in a minute when I'm through uploading my pics.

It seems that I'm not finished being corrected on this one :).

_eug

Nikon D50

05/05/2006 12:55:53 AM · #11

Originally posted by eschelar:

So thank you _eug, as that cleared up my understanding a great deal on the matter.

Woohoo! I actually made sense!

BTW I got my numbers from B+H. ;)

Canon EF 16-35mm f/2.8L USM

05/05/2006 01:16:05 AM · #12

B+H lists angle of view on the 85mm f/1.8 as 28 degrees. An easy mixup if you just read 46 degrees on the 50mm. Have a look. :)

Here are some results of my tests... I'm sorry if this is a bit disorganized, I'm about to be late for work!

I ran three sets of lens profiles.

1 - 85mm with a filter on it... this lens is not mine and is much more expensive than the 50mm. I don't want to scratch it.

2 - 50mm with a filter on it... this is useful for those wanting a rough idea that have a lens that is larger

3 - 50mm with no filter... my S2 is my preferred weapon of choice here currently. That will be dealt with in another thread when I have more time. The lens fits quite a bit closer because it can get inside the barrel of the 50mm f/1.8. Irrelevant to anyone not using an S2.

I tried to do most of the tests with focus at infinity. I did not find that changing the focus of the off-camera lens had any affect on vignetting.

The most significant difference is actually the vignetting, which in the 50mm reduces the usable portion of the image by quite a lot.

Wide open:
1

As you can see, the angle of view is pretty much identical. This makes sense because of the purpose of the optics to render a scene entering the front element to a specific size at the rear element where it will enter the camera. Wide open on this camera is a 6x crop of 6mm, so an equivalent of 36mm.

Then I tested a bit zoomed in so you could see the quality of the vignetting. This was taken at a 6x crop of 20.2mm or 121.2mm equivalent. This is important because at this length, I will be able to do a pretty decent square crop of the center.

20.2mm:
1

Then I zoomed in until I got a 100% clean frame from the 85mm. Note the top left corner has a bit in the frame because I wasn't perfectly centered. This is handheld stuff. Focal length was 34.3mm or an equivalent of 205.8mm

1st clean frame:
1

This is important because this is only half way through the zoom range of my camera. Further, when using the 80-200 on my 1.6x crop camera, this is only 1.6 of the way through my equivalent zoom range, meaning I have 200-320mm equivalent to play with magnification.

Note that I did not post those images for the 50mm. This is because I find that there is HEAVY vignetting with that lens, requiring severe cropping if I want a clean image.

The next test shows maximum telephoto on my camera which is an actual 72mm, but equates to 432mm 35mm equivalent.

Full TeleP:
1

So the conclusion from this is that angle of view doesn't really help as I originally thought it did because of course the lens is always shooting for the same relative view because the sensor is assumed to be a certain size.

The only real difference in looking through a lens will come, as Bear_music pointed out, when reversing an EF-S lens, which is only trying to make a relatively small image circle. This will affect what you see as you look through the reversed lens.

If you want less vignetting though, a lens like the 85mm might work which features drastically less vignetting. This is likely due to less 'compression' inside the lens due to a narrower angle of view.

I hope this is accurate and I hope someone else is benefiting from my learning journey!

I believe that a significant portion of my troubles MAY have to do with using the smaller lensed camera. I don't seem to get as much vignetting when I use the 80-200 looking through the 50mm, but I get other issues instead.

This also brings me right back to the MF lens too. If the Field of view allows less vignetting, a cheap 2nd hand MF lens might give the ability to use 50mm with very little vignetting, even at far more moderate magnifications.

Message edited by author 2006-05-05 01:41:31.

sir_bazz

Pentax K-5

Pentax FA 50mm f/1.4 AF

05/05/2006 01:46:22 AM · #13

I have no ideaa what your on about but thanks for sharing.

bazz.

oOWonderBreadOo

Canon EOS-5D

05/05/2006 02:15:32 AM · #14

I thought this was a superb explanation and I understood all of it! ;0P

crayon

Samsung Galaxy S24 Ultra

05/05/2006 02:30:09 AM · #15

I was wondering why kirbic seems to know so much,
so I went and checked his profile and found out the truth...
He is...