90, 100, 105 macros not "constant f/2.8"?

*WMBP* · 09-27-2009, 08:33 PM

This is embarrassing - my memory is so bad! I was reading about one of the three macro lenses in the 90/100/105 range (Tamron, Pentax, Sigma respectively). I was reading customer reviews over at Amazon. I don't remember which lens I was reading about - I'm pretty sure it would have been the Sigma or the Tamron. Somebody had an interesting complaint about the lens and (this is what's especially embarrassing) I can't precisely remember the complaint. I am pretty sure that the complaint was that the lens wasn't a constant f/2.8. The complainer later made a change to his post saying that he had learned that this problem was a normal feature of a macro lens like this.

Again, I'm not sure, but I think the poster was saying that it was f/2.8 when you shoot in macro mode, but if you switch to non-macro mode, it's NOT a max f/2.8 - it's f/4 max or something like that.

I've spent half an hour searching for this review on Amazon.com again and I can't find it. Hence this post.

Can anybody help me with this? Any of this make sense? I don't know a lot about macro lenses myself.

Here's why I care. I'm thinking of buying one of these lenses. I'm not very interested in the macro capabilities. What I'm looking for is a telephoto lens with a focal length around 100mm for normal shooting - say, portraits, or other normal shooting where this focal length would be useful. And I want the lens to be f/2.8. Would these three lenses meet my needs?

Thanks in advance.

Will

Lowell Goudge · 09-27-2009, 09:31 PM

Will

I believe the answer is as follows.

Aside from infinity focus none of the lens proprties really hold any more. But over the normal focusing range you don't really notice this because the change is not great.

For example, from infinity to minimum focus distance, my K50F1.4 only moves the entire lens grouping 7mm.

Now think about the use of an extension tube, the lens is now extended 50mm, at 1:1, and the light from that image is spread out over a very large area, yet the lens opening has not changed, and in the limiting case, the diameter of the front element has not changed.

At 1:1 we are doubling the distance from the lens to the film plane, therefore you should consider the lens focal length is doubled, and with the aperture not changing, you have now lost 2 complete stops

*WMBP* · 09-27-2009, 10:02 PM

Thanks, Lowell. But forgive me: I'm not sure I followed your reply.

If I'm shooting a portrait (say) and I'm (say) 15 ft away from the subject, can I still get f/2.8?

Will

Motorman · 09-27-2009, 10:38 PM

The answer to your question Will is yes,it will stay at 2.8 till you change it.Your focal length is a constant with the lenses you listed and f stop set will not change once set,2.8 will stay 2.8 no matter your subject distance but your DOF will change with distance but not F stop.
Hope this helps,Lowell may have misunderstood your question......and I hope I did

netuser · 09-28-2009, 04:09 AM

Some sigma lens have this problem, they are marked as constant f2.8 but they don't stay there.
I have a friend shooting nikon that had to send back 2 copies with this "problem".

My DFA 100mm works perfectly

Class A · 09-28-2009, 05:10 AM

Originally Posted by Motorman

The answer to your question Will is yes,it will stay at 2.8 till you change it.

The f-ratio will stay at f/2.8 and in non-macro shooting conditions you'll get f/2.8 as the effective f-ratio.

Originally Posted by Motorman

Lowell may have misunderstood your question...

No, he didn't misunderstand the original question.
He correctly commented that in close focusing situations, the effective aperture (f-ratio) will be different. This holds true for all macro lenses, not just Sigmas. It is not a problem though, as most macro shots at f/2.8 would be pretty useless. Approaching 1:1 you'd want to be at f/8 the least to get some usable DOF.

axl · 09-28-2009, 05:32 AM

I have come across this in Nikon land. My wife has Micro Nikkor 60 at her work. And if I remember correctly, that one changes f stop to something like 4.5 when close focusing.

Lowell Goudge · 09-28-2009, 05:37 AM

Will

I may also have misunderstood your question.

I was assuming you were talking about the change in aperture at close focus, i.e. when the subject is typically less than ten times the focal length away.

If you are talking about changing aperture as you focus in on something that is still distant, and for a 100mm lens 15 feet is distant (45 x focal lenght) then the only thing I could guess at would be the actual lens design.

Specifically if these lenses are achieving close focus with internal focusing, not moving groups, and the placement of the aperture blades.
'
Then maybe, the change in focus does cause change in aperture (physical diameter), or more likely the focal length of the lens, but again I would be surprised if it is significant until you really get close. You could test this point by taking photos of a known height subject, at different distances from the front element and then looking at the image height on the sensor. Check and see if the following formula is met (again for image distance >> focal length)

Image size = Subject size x focal length / subject distance

Lowell Goudge · 09-28-2009, 05:51 AM

Originally Posted by Motorman

The answer to your question Will is yes,it will stay at 2.8 till you change it.Your focal length is a constant with the lenses you listed and f stop set will not change once set,2.8 will stay 2.8 no matter your subject distance but your DOF will change with distance but not F stop.
Hope this helps,Lowell may have misunderstood your question......and I hope I did

Just to be clear, and I might have mis-understood wills question as I had assumed all along that he was talking about macro work not normal photography, all lenses have slight change in exposure as you focus closer simply because you are changing the magnification ratio (increasing it) and as a result the light passing through the front element is spread out more. (but you have no additional light entering the lens. This is just optics.

The reason you don't notice it until yoou get really close is that exposure is a square function, i.e. doubling or halving the light is one F stop. As a result to get a 1 stop change (from infinity) you need to have a 1:2 magnification ratio. This requires about 50mm of lens extension with a 100mm macro and focused in to about 1 foot. at 15 feet in focus, which will has commented about, the change in aperture is almost negligable, unless there is something very funny in lens design internally

Motorman · 09-28-2009, 07:21 AM

I am no "expert" on lenses but I have 2 -1:1 macro prime lenses,a 2.5 and a 2.8,and have shot these both at their largest apertures at 1:1 and at infinity,2.5 and 2.8 respectively, and have not noticed a f stop change from what what was set.I have experienced exposure differences but nothing I could not compensate for while shooting.As for some kind of magic I am unaware of 2.8 going to 4 while still at 2.8 well I do understand the effective loss of light with extension but 2.8 is 2.8 is it not?Am I missing something here?
Kinda reminds me of the argument that a 100mm lens is a 100mm on FF and 150mm on a cropped sensor,the lens is still a 100mm focal length.What I have noticed is that the DOF changes with distance and if there is a light change my cameras seem to handle it well shooting in A mode with minor EC adjustments.
Please note I was not trying to be argumentative here and was just trying to give an answer to the OP and my experience.So I would like to apologize if I clouded the water with my reply it was not intended to do so........

*WMBP* · 09-28-2009, 07:33 AM

Originally Posted by netuser

Some sigma lens have this problem, they are marked as constant f2.8 but they don't stay there.
I have a friend shooting nikon that had to send back 2 copies with this "problem".

Well, one thing I remember clearly about the user comment at Amazon.com that I mentioned when I started the thread, is that the user who complained about the non-constant f/2.8 came back later and amended his comment by saying that he had learned that this is normal for this type of lens. In other words, it's not a problem you could send the lens back for.

Will

Lowell Goudge · 09-28-2009, 07:33 AM

Originally Posted by Motorman

I am no "expert" on lenses but I have 2 -1:1 macro prime lenses,a 2.5 and a 2.8,and have shot these both at their largest apertures at 1:1 and at infinity,2.5 and 2.8 respectively, and have not noticed a f stop change from what what was set.

no you won't.

the change in F stop results in a change in exposure not a change in any setting on the lens.

I have experienced exposure differences but nothing I could not compensate for while shooting.

that is usually managed by the TTL metering, but gets difficult if you are doing manual or "auto" flash, because the true aperture is not what is set on the lens

As for some kind of magic I am unaware of 2.8 going to 4 while still at 2.8 well I do understand the effective loss of light with extension but 2.8 is 2.8 is it not?Am I missing something here?

yes, you are missing something. F2.8 on any prime lens sets a specific diameter of the aperture, which relates to the mathematical calculation of F Stop, specifically

F-Stop = focal length / diameter

But note, focal length is only the length at infinity focus.

In reality, you need to consider not the focal length in an F-Stop calculation, but the equivelent lens to focusing plane distance.

Kinda reminds me of the argument that a 100mm lens is a 100mm on FF and 150mm on a cropped sensor,the lens is still a 100mm focal length.

no not the same thing cropping the middle out of an image (FF vs ASP-C) does not change the exposure of the cropped section or any other characteristic, but close focusing does cause a change in magnification ratio of the lens, making the image bigger on your sensor, making an image bigger means projecting the light entering the front of the lens over a bigger surface, and since the light entering the front is constant, spreading it out over a bigger area means that the light intensity hitting any one area is getting less and less as the image gets bigger and bigger

What I have noticed is that the DOF changes with distance and if there is a light change my cameras seem to handle it well shooting in A mode with minor EC adjustments.

yes the camera metering should sort this out

Please note I was not trying to be argumentative here and was just trying to give an answer to the OP and my experience.So I would like to apologize if I clouded the water with my reply it was not intended to do so........

no problem, and no offence taken.

Discussions like this are healthy

*WMBP* · 09-28-2009, 07:40 AM

Originally Posted by axl

I have come across this in Nikon land. My wife has Micro Nikkor 60 at her work. And if I remember correctly, that one changes f stop to something like 4.5 when close focusing.

Ah, this sounds like what the commenter over at Amazon might have been talking about.

Originally Posted by Class A

The f-ratio will stay at f/2.8 and in non-macro shooting conditions you'll get f/2.8 as the effective f-ratio.

Great, thanks. So any one of these three lenses (Tamron 90, Pentax 100 or Sigma 105) could be used simply as a fast telephoto lens, right?

No, he didn't. [i.e. Lowell G. didn't understand my question]
He correctly commented that in close focusing situations, the effective aperture (f-ratio) will be different. This holds true for all macro lenses, not just Sigmas. It is not a problem though, as most macro shots at f/2.8 would be pretty useless. Approaching 1:1 you'd want to be at f/8 the least to get some usable DOF.

Class A, so let me make sure I understand what you're saying. I gather that there's a switch of some sort on the lens that causes it to switch from normal telephoto mode to macro mode. In normal telephoto mode, I can get a "normal" f/2.8. Or to put it differently, I could use the Pentax 100 (say) to take a picture of somebody 25 ft away at f/2.8 and get a result very similar to the result I'd get if I took the same picture with my Pentax 50-135 f/2.8. Right?

And when the lens is switched into macro mode, then it does NOT have an aperture as wide as f/2.8 any more, right?

I had never thought about it before I read that comment on Amazon, but with these 1:1 macros, I can easily understand why this would be the case: you have to get so close to the subject to take the photo that depth of field would be literally paper thin.

Will

*WMBP* · 09-28-2009, 07:47 AM

Originally Posted by Lowell Goudge

the change in F stop results in a change in exposure not a change in any setting on the lens. ...

But note, focal length is only the length at infinity focus.

In reality, you need to consider not the focal length in an F-Stop calculation, but the equivelent lens to focusing plane distance.

Thanks for adding that, Lowell. Interesting. I think I understand what you're saying - and also your understanding.

I started a thread recently here about shooting with threads and in one post I described a fantasy of mine: a lens that isn't a zoom, but has two or three distinct focal lengths and you can move from one to another. In other words, it's like two or three prime lenses in one housing so you would not have to change lenses so often. It sounds like these macro lenses are similar to my fantasy. IN macro mode the incoming light is handled one way and in normal tele use the light is handled differently, with the effect being similar to what you might get if you were using 2 distinct lenses. Is that about right?

Will

Lowell Goudge · 09-28-2009, 08:04 AM

Originally Posted by WMBP

Thanks for adding that, Lowell. Interesting. I think I understand what you're saying - and also your understanding.

I started a thread recently here about shooting with threads and in one post I described a fantasy of mine: a lens that isn't a zoom, but has two or three distinct focal lengths and you can move from one to another. In other words, it's like two or three prime lenses in one housing so you would not have to change lenses so often. It sounds like these macro lenses are similar to my fantasy. IN macro mode the incoming light is handled one way and in normal tele use the light is handled differently, with the effect being similar to what you might get if you were using 2 distinct lenses. Is that about right?

Will

Will,

I remember that post, as I responded there that I think Leica made a lens or a P&S like that once. The idea is that it is easier to optimize 2-3 specific focal lengths, lets imagine 24-50-75 then to make every focal length over the entire range acceptably sharp. Perhaps easier is not the right wording here, maybe it would be possible to make a lens exceptional at 3 specific points than meerly acceptable over the range.

ALso, vivitar did this with the Series 1 70-210 F3.5 Version 1. On that lens, you pulled back on the zoom collar, and then pushed a button and rotated a collar on the lens to put it in macro mode. the focusing changed in that mode such that focus was achieved by moving the zoom collar in and out, as opposed to turning it as you would in normal operation.

One lens, 2 specific functions. They recognized that by changing the way the internal elements moved, they could achieve very close focus, and still have good optical quality.

I have one of these and still use it from time to time as it is one of the best for image quality, it just weighs a ton. (Kilo actually)