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Defining a fast lens - B

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Sergey_Green said:
mcabato said:
yeah, the gap is closing ... it seems you are getting towards the right path ...
it is all about being precise.

and equivalence is a misleading term, if you do not clearly state what are the side definitions you are implying.

so, better stop talking about equivalence if you stress only one property of the equivalence.

rgds gusti
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Gusti,
I believe the answer was in one of the links you once posted,
http://en.wikipedia.org/wiki/Lens_speed

A lens may be referred to as "fast" or "slow" depending on its maximum aperture compared to other lenses of similar focal length designed for a similar film format.

Similar format. In other words the f/2 is fast on FT, and it is faster than the f/2.8 on the same (similar) format. Is it faster than f/2.8 on APS-C ? - No, they are about the same, as f/5.6 on FF format. On large format, on the other hand, the f/5.6 is a VERY fast lens, and is it any wonder that 135mm f/5.6 (Nikkor W) is considered a fast portrait lens for the format. Would you ever call it a fast portrait lens for FT? I bet not ;). I do not think the likes of f/1.4-1.8 (that are so common for various dSLR formats) even exist for large format. They simply are not needed there.
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- sergey
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this is exactly what i am critizise. making more out of a definition that it is.
a 5.6/135mm lens is never a fast lens. a 5.6/800mm lens is one.
a lens being fast does not mean anything about DOF or iso speed.
a fast lens with 135mm is one of having f/2 or f/2.8
a lens with 4/135mm is for sure not fast.

a lens being fast comes historically from shutter speed with normal film iso speed in comparison to using standard lenses of the same angle of view.
a fast portrait lens for FT would be a 1.4/50mm or the zuiko 1.2/55mm
a sufficiently fast (apologize LOL) portrait lens for FT is the 2/50mm

my 2.8/90-250mm is a very fast protrait lens, whereby i would not generalize it to call it a portrait lens g

a fast lens is one you would use if your standard lens is not fast enough.

yes, many photographers beleive to be able to make their mediocre lenses faster by attaching them to a FF cam. but this is the wrong point of view.
rgds gusti
 
mcabato said:
..
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mcabato said:
a lens being fast comes historically from shutter speed with normal film iso speed in comparison to using standard lenses of the same angle of view.
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And on the same format.
mcabato said:
a fast portrait lens for FT would be a 1.4/50mm or the zuiko 1.2/55mm
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Ok, but the same can be achieved by smaller aperture lenses on larger formats. They are equally just as fast. Or not?
mcabato said:
a sufficiently fast (apologize LOL) portrait lens for FT is the 2/50mm
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Would you argue that Samsung f/1.8 is actually faster?

http://www.dpreview.com/news/1002/10022005samsungex1.asp

Here is a portrait lens for 4x5 cameras,

http://www.bhphotovideo.com/c/product/36884-USA/Nikon_1312_135mm_f_5_6_Nikkor_W_Lens.html

Is it fast enough?

--
- sergey
 
Rriley said:
You ought to be banned from this place permanently, and prosecuted for the costs of keeping you out of the place, just as a measure of sanitation
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... banning you permanently would be a far better option as it would help make this board a much saner place as it would reduce the toxicity levels dramatically.

I am sure you don't realise it but you are one hell of a noxious and poisonous influence on the board and it would be best for everyone if you were prevented from accessing the place.

Just my opinion. No hard feelings.

--
http://dakanji.com

'Of what use is a philosopher who never hurts anyone's feelings?'
Diogenes the Cynic, 300 BC.

Ken Rockwell Supporter
 
still my sentence is valid:

"a fast lens is one you would use if your standard lens is not fast enough. "

and your 5.6/135mm lens is for another format. so i do not know what are normal apertures for 135mm lenses for large format cams. maybe it is a fast lens.

rgds gusti
 
mcabato said:
still my sentence is valid:

"a fast lens is one you would use if your standard lens is not fast enough. "
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A fast lens is one you would use if you don't have any better - I am not sure this defines anything.
mcabato said:
and your 5.6/135mm lens is for another format. so i do not know what are normal apertures for 135mm lenses for large format cams. maybe it is a fast lens.
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It is a fast lens for a particular format, just like an f/2 is fast for FT.

--
- sergey
 
Sergey_Green said:
mcabato said:
still my sentence is valid:

"a fast lens is one you would use if your standard lens is not fast enough. "
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A fast lens is one you would use if you don't have any better - I am not sure this defines anything.
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...
...
Sergey_Green said:
- sergey
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you are misinterpreting.
word picking and - you are not sure ...

(normally, you will be using, due to price, size etc. a standard lens.
e.g. 4/300 or 3.5-5-6/28-70 or something like this.
this is standard equipment. cheap and good enough for normal shots.
but if you need faster, you will take a heavier and more pricy piece of glass.

and then you are with the 2.8/300 fo 3-times price and 3-times weight and the 2.8/28-70 of 3 times price and 2-times weight.)

rgds gusti.

btw.: it is not about getting right. it is about getting understood and giving good advice.
 
mcabato said:
Sergey_Green said:
mcabato said:
still my sentence is valid:

"a fast lens is one you would use if your standard lens is not fast enough. "
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A fast lens is one you would use if you don't have any better - I am not sure this defines anything.
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...
...
mcabato said:
- sergey
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you are misinterpreting.
word picking and - you are not sure ...
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Am I ;) ?
mcabato said:
(normally, you will be using, due to price, size etc. a standard lens.
e.g. 4/300 or 3.5-5-6/28-70 or something like this.
this is standard equipment. cheap and good enough for normal shots.
but if you need faster, you will take a heavier and more pricy piece of glass.

and then you are with the 2.8/300 fo 3-times price and 3-times weight and the 2.8/28-70 of 3 times price and 2-times weight.)
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Yes, but the price and the weight has nothing to do with how fast the lens is, does it? As long as you are comparing within the same format the smaller numerical aperture value will always indicate that the lens is faster than the previous one. Only as long as you are not mixing the formats, then it becomes different.

--
- sergey
 
EJohnE said:
To bobn2 who is still rabbiting on about what he wrongly assumes to be my opinion that a 2x converter becomes another 300mm of light travel. Dear, oh dear. If I look at a postage stamp through a magnifier I see more detail, but have in no way changed its physical size As to why a lens plus converter is different from a telephoto lens: the lens can be used on its own, the lens/converter can be used together, the converter on its own will give nothing but unfocused illumination. If the (negative, is it?) elements are removed from the telephoto lens, what you have is an expensive repair job. To save embarrassing him I will refrain from commenting on his version of that parable.
Once the holiday season is over, he should find it easier to obtain Prozac.

As for my cluelessness re the inverse square law: that relationship was discovered, not devised by, say, Fred Flinstone. It applies to all sorts of things. An image falling on a lens does not go as we see it straight through to the film plane. The top of a building's image goes to the bottom of the sensor/film. It goes to a crossover point within the lens, and then the whole captured image expands to fill the format's frame. That nodal point may be considered to be a point source. For a 50mm lens, a light reading 25mm behind that point would be four times higher than at the 50mm distant sensor.
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Hi John. At last, some sort of answer to my question, though a bit hard to construe. Lets go back to your original quote:

'Where you go wrong is that you forget or don't know that the "focal length" means that the light has to travel twice as far (50 to 100 mm) to reach the sensor, and that the inverse square law applies.'

This statement seems quite clear to me. A 100mm lens has double the focal length of a 50mm lens, which means, apparently, that the light has to travel twice as far to reach the sensor. Here's an example of a 50mm lens and a 100mm lens.





The top one is a 50mm lens. The second is a 100mm lens formed by taking the original 50mm lens and placing a -60mm lens 20mm behind it. So, my question is this: If the reduction in image brightness is caused by the inverse square law due to the extra 50mm (double the distance) that the light travels, where is this extra 50mm? I cannot find any image forming ray here that will travel an extra 50mm.

--
Bob
 
Sergey_Green said:
As long as you are comparing within the same format the smaller numerical aperture value will always indicate that the lens is faster than the previous one. Only as long as you are not mixing the formats, then it becomes different.
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I prefer the terminology we use in Swedish where we talk about the "ljusstyrka" of a lens. I believe you use the same terminology in German as well ("Lichtstärke").

--
Rikke
 
Rikke Rask said:
Sergey_Green said:
As long as you are comparing within the same format the smaller numerical aperture value will always indicate that the lens is faster than the previous one. Only as long as you are not mixing the formats, then it becomes different.
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I prefer the terminology we use in Swedish where we talk about the "ljusstyrka" of a lens. I believe you use the same terminology in German as well ("Lichtstärke").
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[edit] disregard the post.

--
- sergey
 
mcabato said:
neither price, not dof, nor weight are connected with lens speed.

why should i further discuss, if you do not believe me and always introduce your own invented concept?
just read here. fusllstop.
http://www.lensrentals.com/news/2009.08.15/fast-lenses-a-primer
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I only glanced at it and I already see few gotchas that you did not pay attention to. Obviously the author meant one format for comparison, and so he states,

HERE’S WHERE FAST LENSES COME IN.

As numbers, the difference between f/2.8 and f/4 might not seem like much, but it’s enough to make all the difference in the world to your photographic outcome. f/2.8 lets in twice as much light as f/4, allowing a lower ISO setting (reducing the risk of noise) and providing you with the shutter speed you need (or desire) to keep things from blurring.

But between the FT f/2.8 and FF f/4 there is not twice as much light. In fact the f/4 on FF gets still more light on the sensor than the f/2.8 on FT.

WHEN F/5.6 IS CONSIDERED FAST.

.. But, there are always exceptions and, in certain photographic circles, f/5/6 and f/4 are plenty fast enough for the job that needs to be done …

Well of course, what gets the job done. What f/2 does on FT the f/2.8 does on APS-C and f/5.6 on the FF. Provided we use the same AoV, etc ...

So again, as long you are comparing it on the same format you are fine. Stray outside and things become very different and very fast.

I did not read the whole article.

--
- sergey
 
Rikke Rask said:
Sergey_Green said:
Well of course, what gets the job done. What f/2 does on FT the f/2.8 does on APS-C and f/5.6 on the FF. Provided we use the same AoV, etc ...
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You mean f/2, f/2.8, and f/4 respectively, of course.
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You know, this is almost stupid of me. I have seen many times others do it, and I kind of winked, and there we have it, I write it myself. Yes, I meant f/4.

--
- sergey
 
EJohnE said:
From that "Great Bustard" (? spelling of that avian noun?) in the previous filled thread of the same name. - "And the aperture diameter for 100mm f/4 is 100mm / 4 = 25mm, and the aperture diameter for 50mm f/2 is 50mm / 2 = 25mm. In other words, f/2 on 4/3 gives the same aperture diameter as f/4 on FF for the same AOV. "

In both cases as you state, the diameter is 25 mm. Where you go wrong is that you forget or don't know that the "focal length" means that the light has to travel twice as far (50 to 100 mm) to reach the sensor, and that the inverse square law applies. This is the same as a guide number for flash, in that double the distance from the light source, and the lens has to be opened up two stops to give correct exposure.

I have both a 35 mm lens and a 350 mm lens. At (for convenience) I set the 350 at f10, the aperture is 350/10 = 35mm aperture. On your faulty logic, it is therefore equivalent in light-gathering power at the sensor of my 35 mm lens at f1.0. The 35 mm is f2.8 maximum.

Please go back to arithmetic classes.

John.
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Amazing, John, that traveling 50mm (according to you) extra through glass and air will lose SO much light! One should switch lens manufacturers, if that is the case.

(Well, it is not... nor is Olympus glass that opaque! It simply is nonsense what you write, John.)

About flash..... So, a subject 5 meters from you, needs less light flashed in than a subject 10 meters from you. Makes sense, as the... light SPREADS OUT and therefore less will hit a subject. Nothing to do with loss through distance... unless it is REALLY foggy!
 
Sergey_Green said:
Rikke Rask said:
Sergey_Green said:
Well of course, what gets the job done. What f/2 does on FT the f/2.8 does on APS-C and f/5.6 on the FF. Provided we use the same AoV, etc ...
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You mean f/2, f/2.8, and f/4 respectively, of course.
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You know, this is almost stupid of me. I have seen many times others do it, and I kind of winked, and there we have it, I write it myself. Yes, I meant f/4.

--
- sergey
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Regardless of your failure even to get the holy "equivalence" right (I note that Joe has now "promoted" this to having a capital "E" ... rotflmao), it is still wrong anyway ...

ISO 100, f/4 at 1/250th will give identical exposure, no matter what the format. If you do not understand this, you have never used a light meter, and have no understanding of basic photographic principles ...

--

-
 
John King said:
ISO 100, f/4 at 1/250th will give identical exposure, no matter what the format. If you do not understand this, you have never used a light meter
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Nobody has ever denied this, so I'm unsure as to why you repeat it so often. The issue is what is the significance of having the same exposure.
John King said:
and have no understanding of basic photographic principles ...
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that boot would be on your foot.
--
Bob
 
bobn2 said:
John King said:
ISO 100, f/4 at 1/250th will give identical exposure, no matter what the format. If you do not understand this, you have never used a light meter
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Nobody has ever denied this, so I'm unsure as to why you repeat it so often.
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Just possibly because you bunch of stooges keep yelling from the rooftops about how f/2 on FT is "really" the same as f/4 on 135 format ... Maybe? This indicates that you have zero understanding of basic practical photography ...
bobn2 said:
The issue is what is the significance of having the same exposure.
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Getting the exposure correct for starters?

But then you have already indicated that you haven't got a clue about that here:
http://forums.dpreview.com/forums/readflat.asp?forum=1022&thread=37320027
AND in a multitude of other posts and threads ...
bobn2 said:
bobn2 said:
and have no understanding of basic photographic principles ...
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that boot would be on your foot.
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Why? I understand how to set my camera to the correct ISO, etc. OTOH, you patently struggle with the basic concepts involved, probably because you have decided that all that old fashioned stuff about ISO, aperture and shutter speed should be thrown away because you think it is meaningless. It isn't ...

Go and get a basic book on photography and educate yourself. You are patently incapable of, and uninterested in, accepting any education from people who actually do understand practical photography. Possibly because you consider the acceptance of knowledge from others to be both beneath your dignity, AND damaging to your fragile self-esteem and severely threatening to your obvious inferiority complex ...

END of 'discussion'.

--

-
 
John King said:
bobn2 said:
John King said:
ISO 100, f/4 at 1/250th will give identical exposure, no matter what the format. If you do not understand this, you have never used a light meter
Click to expand...
Nobody has ever denied this, so I'm unsure as to why you repeat it so often.
Click to expand...
Just possibly because you bunch of stooges keep yelling from the rooftops about how f/2 on FT is "really" the same as f/4 on 135 format ...
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Well, it 'really' does the same job. This is something competent photographers have known about for a very long time.
John King said:
Maybe? This indicates that you have zero understanding of basic practical photography ...
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Rather the reverse, I would think but never mind.
John King said:
John King said:
The issue is what is the significance of having the same exposure.
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Getting the exposure correct for starters?
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What do you mean by 'getting the exposure correct'? That is a genuine question. When 'correct' exposure can be altered simply by adjusting the ISO control, what is 'correct' exposure?
John King said:
But then you have already indicated that you haven't got a clue about that here:
http://forums.dpreview.com/forums/readflat.asp?forum=1022&thread=37320027
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It was DPR staff who set the exposure in those shots, so how does it illustrate that I don't have a clue about exposure?
John King said:
Why? I understand how to set my camera to the correct ISO, etc.
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What is 'the correct ISO'?
John King said:
OTOH, you patently struggle with the basic concepts involved, probably because you have decided that all that old fashioned stuff about ISO, aperture and shutter speed should be thrown away because you think it is meaningless. It isn't ...
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There seem to be quite a few people asking the same questions and coming to similar (if not identical) opinions. People like Iliah Borg, Joe Wisniewski, Emil Martinec. Do they not understand the basic concepts involved either?
John King said:
Go and get a basic book on photography and educate yourself. You are patently incapable of, and uninterested in, accepting any education from people who actually do understand practical photography.
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Strange, I thought I had gained rather a lot of knowledge on practical photography from people like Iliah Borg, Joe Wisniewski, Emil Martinec....
John King said:
Possibly because you consider the acceptance of knowledge from others to be both beneath your dignity, AND damaging to your fragile self-esteem and severely threatening to your obvious inferiority complex ...
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You could see it that way, I suppose, but the way I see it is that I'd rather accept knowledge from people who have it.
--
Bob
 
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