aperature question re: Tamron 90mm macro...

rob1116 · Feb 5, 2005

Hello hello,

Just for the record:

<------ "New" Thanks

Anywho, I have a question regarding max. aperature labelling on some lenses. Example: The "Tamron 90mm f/2.8 DI macro" . I understand the basic concept of f/numbers, (ie. aperature diameter is the focal length over the f/number... ie. 100mm f/4 lens should have a max. aperature diamter of about 25mm, right?)

So I'm in the local camera store the other day trying this lens out on my D70. The D70 powers up and I smile as I see the aperature listed at 2.8 . (what can I say, I have only ever used my 18-70mm f3.5-4.5 nikon, and my Tamron 70-300mm 3.5-5.6, so 2.8 seems so fast to me!)

Then, to my horror, after focusing on something moderately close the aperature reading starts to drop as if I'm zooming in and losing stops!?!? I'm guessing this is because of the large focus barrel required for proper 1:1 macro, but WTF? Are all macro lens like this? What about other lenses that are labelled with constant max. ap?

Is this normal? If so, then this lens can't really shoot 1:1 from 90mm at 2.8 can it?

Okay, this is the part when half a dozen of you gurus chime in and enlighten me to the one piece of the puzzle I'm obviously missing... please!? :hail:

Rob · Feb 5, 2005

Was it in Auto, Program or Shutter Priority mode(s) and the camera was adjusting the d-o-f to compensate for less light on a closer object? Stick it in aperture priority mode at f2.8 to check this.

Rob

rob1116 · Feb 5, 2005

sorry, I should have said that... I tried it in Ap Priority and Manual, and it would only allow me to use f/2.8 when I wasn't focusing on something relatively close, like maybe 1:2 mag max, and then it start stopping down, or at least the camera read that it was.

So this isn't normal?

Bob_McBob · Feb 6, 2005

1:1 macro lenses close focus by extension, which is essentially the same as using extension tubes. You lose some light because the lens barrel is extended. Be happy your camera auto-adjusts for the aperture change, it's a real pain in the ass on manual cameras!

rob1116 · Feb 6, 2005

bob : So you're saying this is normal with macro lenses... hmm.

I kinda wondered how the aperature could remain constant when the lens extends so far out.

So, my next question: What about other lenses listed as having a constant max. aperature? Ex. AF-S VR Zoom-Nikkor 70-200mm f/2.8G IF-ED . If it really is constant, then the aperature would have to open up as you zoomed in right? So if that's what it's doing, and the aperature can already open that far, why not leave that larger aperature setting available at shorter focal lengths... turning it into a 70-200 f/1.4 - 2.8 or something... ???

I'm definitely missing something here. I'm gonna have a look for some detailed lens construction diagrams. If anyone knows of any decent sites explaining/showing these things I'd really appreciate it!

deepakram · Feb 6, 2005

maybe i'm going to sound stupid saying this, but i'll say it anyway!
the max-aperture listed on a lens pertains to the min-zoom of that lens, ie., the min. focal length. so, if i have a tamron 28-200 f3.8-5.6, then it means that the max aperture at 28 mm is f/3.5. right? now, if you keep your lens at its min focal length and focus on anything, no matter how far or near, the aperture should remain the same! while typing this reply out, i tested my statements on my camera, and it works as expected, atleast at my end!!

rob1116 · Feb 6, 2005

yes, I understand and agree with that deepakram... but how, when the length of this macro lens doubles when focusing on something very close (because the focus barrel [correct terminology?] protudes waaaay further than a regular lens to allow focusing from 1:1 to ∞ ) can it physically be the same f/number?

Digital Matt · Feb 6, 2005

I don't own a true prime macro lens. I use extension tubes. There is a light loss, but I'm still able to open up to my max aperture.

You won't be able to use f/2.8 for anything when you are focusing in the macro range anyway. Your DOF will be way too shallow. Most macro shots are done at f/11 and below, to have anything in focus. The 2.8 comes in handy if you are using that as a portrait lens, for which it will work very well also. You will not be focusing in the macro range for portraiture, so the 2.8 will be available.

rob1116 · Feb 7, 2005

Excellent point Matt; the DOF is extremely shallow shooting macro at max. aperature... but I'm more interested in getting an idea of the mechanical workings of the lens.

Bob_McBob · Feb 9, 2005

Digital Matt said:
I don't own a true prime macro lens. I use extension tubes. There is a light loss, but I'm still able to open up to my max aperture.

Yes, you can open the lens to its maximum marked f/stop, but the reason you lose light with extension tubes is because the effective aperture size decreases as the length of the barrel increases. You get the same effect with bellows extension in large format cameras. It's called the bellows effect.

rob1116 · Feb 15, 2005

hmm... tried the Sigma 105mm macro on my D70 today. It allows me to choose f/2.8 regardless of focus distance and subsequent barrel extension. Still not getting it... how is the aperature the lens length/2.8 when the length of the lens is obviously changing? This must have something to do with the focusing elements moving without effecting the focal length, but I still can't picture it, and I definitely DON'T get how you can have a constant aperature on a zoom lens, like a 70-200mm f/2.8.

Anyone care to continue this discussion for me? The girl at Henry's looked at me funny for a second after I asked her about it, then said she was sorry but she didn't have the answer

Kodan_Txips · Feb 16, 2005

The iris is a small round source of light, as far as the film is concerned. The further away it is, the less light gets to the film. (A bit like the Sun and the stars. Good, or the sky would be too bright!)

To make the calculations easy, let's pretend that it is 100 mil from the film plane, and you are able to use f2.8. By focussing on something that moves the aperture out to about 140 mmm, you have lost one stop, and your max F stop is F4. At 173 mm it is F5.6. If the focussing racks the iris mount out to double the length between the iris and the film, you will have lost a massive 3 stops, and the max aperture is F8. (Or is it 4 stops, and f11? That is even worse!)

Awful, isn't it?

Some zoom lenses have the same problem, as the iris is mounted within the front clump of elements. My 100-200 MD rokkor reduces the effect of this by keeping the iris in the same place, but as the iris is using the lenses at the front of a tube in front of itself, I still lose a stop or two as I lengthen the tube by zooming out and then focussing. The damned thing is so long and heavy, I intend to use it to beat up on muggers!

However, some zooms might be about a fifth as long, for a much wider zoom range, yet still keep the same aperture throughout the range. They do this by moving the iris backwards AND forwards as you focus and zoom. (It also helps that digicams usually use smaller sensors than the 35 mil neg size, which increases the apparent length and range of the zoom) Some zooms may even open the iris up as it moves away from the film plane, to make it appear the same size to the film.

Sorry if all this may seem like talking down to you, but it is a fault in many teachers like myself, and the only reason I am able to type such a lengthy title is that I am off work with bronchitis, which is making me even more ponderous. Also I was reasoning the whole thing out as I went along, as up till now I hadn't realised that the fixed iris position on my long zoom still lost at least one f stop at the 200 mm position.

Does anyone want to buy a telescopic drainpipe?

rob1116 · Feb 16, 2005

Thanks for the valiant attempt Kodan, but I think I'm gonna need a diagram or three

I understand everything you said, and I have a good grasp on the concepts of aperature and exposure 'stops' , but I still can't picture some of these things, and I would like too. I realize knowing exactly how a lens and it's elements work isn't that important, but I still have to know.

I guess the information that would help me the most is: How, physically, does a lens provide a specific focal length, and how does it focus. I have read more than my share of articles/books on what these things are, (ie. angle of view, what focusing IS, etc.) so I'm not looking for the 'circles of confusion' , or the 'angle of view', or the 'focal plane to nodal point' type tutorials.

Does that make any sense? Could someone recommend a book/site/etc. ? Thanks again everyone!

Rob · Feb 16, 2005

Rob,

Just to start out, the f number is the ratio of the length to the width of the lens - an f1.4 50mm lens is about 36mm wide. A 200mm f2.8 lens is therefore about 71mm wide.

A 50mm lens is approximately the same magnification as the human eye, whereas a 200mm lens is about a 4x magnification compared with what you see.

The FAQ sticky post at the top is worth reading and explains many of these things more eloquently and accurately.

Rob

Kodan_Txips · Feb 16, 2005

Well, I googled it, and spent ages being reminded of how at school I loved fiddling with the lenses, but hated having to draw all the damned diagrams, cos I was such a grubby kid.

The point about apparent focal length may be more understandable if you think of the path of the light from the circle that is the iris to the tiny dot that is the focussed "pixel" on the film as having the same shape as the beam of light from a torch or car headlight, but travelling in the opposite direction.

In the case of a single element convex lens, the focal length will be the distance from the centre of the lens to the film plane, when it is in perfect focus.

A long distance will be a sort of narrow beam effect, a short distance a sort of wide beam / wideangle effect, BUT ONLY IF THE LENS IS OF THE SAME DIAMETER. A bigger lens further away might have the same sort of beam angle.

So to give a camera lens a shorter focal length than its physical length, they might put a concave lens behind the first convex one, to spread the beam out. Then the light beam may even be spreading out so it doesn't focus, so they put another CONVEX lens behind that concave one, to bring the light back to a point again. 10 element lenses muck around with the light path a lot, and the nature of the glass can dim the light as well, so it is really problematic for manufacturers. Especially as red green and blue light is bent at different rates, so you have to sort it so they all get back together at the same angle before they hit the film...

I can't type much more atm, but instead of all those diagrams with formulae, I found this site that actually shows what the light may be doing on its way through the individual elements of a compound lens.

http://www.physics.umd.edu/lecdem/services/demos/demosl2/l2-00.htm