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Old Mar 11, 2006, 2:31 PM   #111
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I came in to this a bit late, and will admit to having just skimmed some of the posts, so I am sure I missed a good bit.

I am sensing that the root of the matter may be in the difference between focal length and lens diameter.

A300mm diameter lens will certainly gather more light than one of 200mm diameter, whatever focal lengths each has. How much light they provide to the sensor, depends on the quality of the glass, number of elements, etc.

In general, a 300mm focal length lens will have a larger objective diameter than one of 200mm, in order to maintain a reasonable minimum f/D ratio. However, it is possible to have two lenses of the same focal length with very different objective diameters, and this happens a lot (and can cost a lot, if you want a fast, long focal length lens).

A telescope with 900mm focal length with a 200mm diameter objective will (other things being equal) provide a sharper and brighter image than one with the same focal length, but 100mm diameter.

It is the "All other things being equal" which has been missing from a lot of this discussion, which may be the source of some of the confusion, because, of course, they never are.

If I am misreading what is going on, I am sure someone will let me know.:G

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Old Mar 11, 2006, 3:33 PM   #112
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VTphotog wrote:
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I came in to this a bit late, and will admit to having just skimmed some of the posts, so I am sure I missed a good bit.

I am sensing that the root of the matter may be in the difference between focal length and lens diameter.

A 300mm diameter lens will certainly gather more light than one of 200mm diameter, whatever focal lengths each has. How much light they provide to the sensor, depends on the quality of the glass, number of elements, etc.

In general, a 300mm focal length lens will have a larger objective diameter than one of 200mm, in order to maintain a reasonable minimum f/D ratio. However, it is possible to have two lenses of the same focal length with very different objective diameters, and this happens a lot (and can cost a lot, if you want a fast, long focal length lens).

A telescope with 900mm focal length with a 200mm diameter objective will (other things being equal) provide a sharper and brighter image than one with the same focal length, but 100mm diameter.

It is the "All other things being equal" which has been missing from a lot of this discussion, which may be the source of some of the confusion, because, of course, they never are.

If I am misreading what is going on, I am sure someone will let me know.:G

brian
Personally, I think at this point that any reasonable post, such as yours deserves an immediate flamming, reducing you to toast...:lol:

Here I am on this thread, almost from the beginning, and I believe there are a number of separate disputes - you have touched on ONE of them.

What is the name of this thread? "Equivalent Focal lenght."

So let me rehash, as best I can. The focal lenght multiplier, "The Cropping Factor" is described as the equivalent of magnification - meaning that a 200mm lens on a camera WITH a 1.5 cropping factor gives the same amount of magnification as a 300mm lens on a camera with NO cropping factor.

The entire conversation about light gathering ability is actually a separate dispute - True it is related, but it really is a side issue. Those beating each other over the heads about this are completely unaware of this...:?

(OR, maybe I'm completely unaware that MY point is a side issue)

From the point of view of science, the cropping factor DOES magnify the image. The reason I am busy flamming the opposition is that there is another aspect of this - "Resolving power."

In other words, I am saying to others that you can indeed magnify an image, but does it increase the resolution of the image? I say "not quite."

With digital photography, you do indeed increase the clarity, by getting more magnification, but you cannot resolve a detail that is not there. Only with a larger focal lenght lens can you get increased resolution.

If we created a test situation where a sensor had an enourmous amount of MP's but a low power lens, could it match the resolving power of a very high powered lens, attached to a camera with relatively few MP's?

To be specific, a 39 Meg Mamiya Medium format, with a 100mm lens, compared to a Nikon D1 with 1.75 MP's and an 1100mm lens. At 100 yards, the Nikon will be 13 feet from the target, the Mamaya, 150 feet. The target is a one inch diameter flower. A nice violet if you will.

I will say, that no matter how many MP's you have, that target will either be invisible or a tiny (but clean) blur. While it will be easily seen as a flower with the Nikon.

To back up this rather irrational theory of mine, I have posted from various sites that use microscopes and are unaware of the heated disputes in photographic forums.

Now coming back to reality for a second, Jim has pointed out, (pardon the quotes - I'm putting words into Jims mouth) "Ok, and so what? For all practical purposes yours is not a real world problem. The camera's we use cannot even match the resolving ability of present lenses - and the cropping factor DOES magnify an image and DOES clarify the result. So to describe it as a real benefit to those who use telephoto's is a meaningful use of the words."

And he's RIGHT. Except for people like me, who invest big bucks in huge glass, and want to know where we stand and what we can expect to resolve. And of course, I like a scientific use of words to be scientific.

The Cropping factor DOES magnify the image, but it does NOT increase the resolution. (And I swore I wouldn't post in this discussion - so you can call me a liar) ()

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Old Mar 11, 2006, 3:39 PM   #113
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Err, umm...

That's not what I'd call a quote. :-)

I don't even like to use the term crop (since it's not a crop if you have lenses designed specficially for a given sensor or film size, yet you'll still have angle of view differences).

Here's an exact quote (one of my posts in this thread):

Quote:
Ok guys...

If you talk actual versus theoritical limits, as already pointed out numerous times in this thread, you get the same detail with a 100mm lens on a DSLR with an APS-C size sensor compared to a DSLR with a 35mm size sensor wearing a 150mm lens, for the same subject framing, provided the sensors are of equivalent resolution and image processing is equivalent.

You want a controlled conditions sample? I don't like to post links to other sites, but I'll make an exception this time.

If you look at the resolution charts for Phil Askey's review of the Canon EOS-5D, you'll see an 85mm f/1.8 lens used, stopped down to f/9. This is a 12 Megapixel sensor.

You'll also see resolution charts for a Nikon D2x, wearing a 50mm f/1.4, stopped down to f/9, setup for the identical framing with a much shorter focal length lens.

Guess what? The resolution charts are almost identical because both cameras have a sensor that's the same resolution. Any differences between them are probably due to image processing versus lens limitations, and if you shot raw with both and processed the images the same way, they probably would be the same.

The D2x actually outresolved the 5D shooting in jpeg.

http://www.dpreview.com/reviews/canoneos5d/page31.asp

There are other examples, too.. where even a prosumer model with a *very* short focal length lens can outresolve some DSLR models with about the same resolution.

In real world use, we haven't reached lens limitations that would show a difference yet, unless you buy poor quality lenses.

Also, it's not a crop if you have lenses designed specifically for a given sensor size. But, you'll still have angle of view differences, based on the sensor size, for a given focal length.

You have the same thing with different film sizes. You'll need a longer focal length for the same angle of view if you use film that's larger than 35mm. You'll need a shorter focal length for the same angle of view if you use film that's smaller than 35mm.

You see more benefit when going to larger film sizes, because you're increasing resolution of the film with surface area (given equivalent film characterstics, grain size, etc.).

But, with sensors that have the same number of photosites, regardless of whether it's an APS-C size sensor, or a sensor the size of 35mm film, it really doesn't make any difference whether you use a 100mm lens on the APS-C size sensor, or a 150mm lens on the 35mm size sensor for the same framing.

The detail captured will be the same with good quality lenses.
P.S.

Here's another (where I did ask "what difference does it make")/

Quote:
What difference does it make in real world use with these cameras?

Who cares how they resolve the detail if the results you can get are the same using a shorter focal length on a smaller sensor, versus a longer focal length lens on a model with a larger sensor?

As long as the lens is capable of resolving the detail a sensor is capable of capturing, it doesn't make any difference which solution you use for detail purposes.

Now, if you look at other differences (for example, depth of field), then one solution may be preferrable versus another, for what you are trying to accomplish.
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Old Mar 11, 2006, 4:05 PM   #114
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Hey... I'm an open minded guy....

But, I just haven't seen limitations yet. We may get there at some point, though.

I've seen too many review samples and resolution charts from prosumer models with *very* short focal length lenses that can resolve about the same amount of detail as some of the DSLR models wearing much longer lenses (provided ISO speeds are kept low).

From time to time, someone even starts a thread on a subcompact model wearing a very tiny lens, with links to review samples elsewhere shot at the same time with a DSLR, noting that the pocket camera's pics were better with more detail captured. It happens. ;-)

Now, I'll concede that I have read comments from photographers using full frame models complaining that lenses are far more important with them.

But, I'm not so sure that's because the cameras are outresolving the entire lens either. I think it's more likely due to edge softness.

Given the surface area of some of the tiny sensors used, with extremely short focal length lenses, you could do a LOT of scaling to MUCH higher pixel counts and be OK from a lens resolving power perspective with an APS-C size sensor, if the same quality you find in some of these lenses was also scaled larger.

That's not to say that all current lenses are that good (we all know about soft lenses). But, I think the technology is there to improve lens quality so that we don't outresolve higher resolution APS-C size sensors for a long time to come. Other problems are more likely to be limiting factors (for example, noise) versus lens resolution from my perspective.

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Old Mar 11, 2006, 5:06 PM   #115
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JimC wrote:
Quote:
Hey... I'm an open minded guy....

But, I just haven't seen limitations yet. We may get there at some point, though.

I've seen too many review samples and resolution charts from prosumer models with *very* short focal length lenses that can resolve about the same amount of detail as some of the DSLR models wearing much longer lenses (provided ISO speeds are kept low).

From time to time, someone even starts a thread on a subcompact model wearing a very tiny lens, with links to review samples elsewhere shot at the same time with a DSLR, noting that the pocket camera's pics were better with more detail captured. It happens. ;-)

Now, I'll concede that I have read comments from photographers using full frame models complaining that lenses are far more important with them.

But, I'm not so sure that's because the cameras are outresolving the entire lens either. I think it's more likely due to edge softness.

Given the surface area of some of the tiny sensors used, with extremely short focal length lenses, you could do a LOT of scaling to MUCH higher pixel counts and be OK from a lens resolving power perspective with an APS-C size sensor, if the same quality you find in some of these lenses was also scaled larger.

That's not to say that all current lenses are that good (we all know about soft lenses). But, I think the technology is there to improve lens quality so that we don't outresolve higher resolution APS-C size sensors for a long time to come. Other problems are more likely to be limiting factors (for example, noise) versus lens resolution from my perspective.
Now, now - Here what I wrote about my "quotes" of you:

"(pardon the quotes - I'm putting words into Jims mouth)"

I "quoted" YOU, in the manner that I did, because your response to one of my threads did give me pause.

After all, even though I am technically right about resolving power, in the real world of digital photography it doesn't make much, if any difference.

So I took the liberty, with apologies, to "quote" the voice of reason...:lol:

Dave () (man do I love this little bouncing emoticon)

PS. I am now the founder of the new one issue political party called the "Resolvers." We intend a Constitutional amendment banning the use of the term "Focal lenght multiplier."

Internal Party disputes will leave the term "cropping factor in peace.
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Old Mar 11, 2006, 5:21 PM   #116
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Well, perhaps some optical engineer that's smarter than all of us will chime in at some point, with figures on line pairs/mm for sharper lenses, and how that relates to photosite density and compute where we'll end up seeing limitations with sensors.

That way, we'll all have something to go by (if the figures are accurate).

It's good to know where we may run into limitations, and I can imagine that the camera manufacturers are studying it carefully, too.

P.S.

Of course, there are higher resolution medium format digital backs available if you need larger optics (from an image circle size versus focal length perspective). That way, your lenses could have less resolution in lp/mm and still get good quality, since you're projecting the image on to a larger, less dense medium.

But, with meidum format, you need longer focal length lenses for any given angle of view compared to models with smaller sensors. Hence, we'd go around in circles with the original debate on this issue.

That's one reason I don't like the term crop (since you still have the same issues with angle of view differences based on sensor or film size, regardless of whether or not the lenses are designed specifically for a given media size.


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Old Mar 11, 2006, 6:02 PM   #117
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JimC wrote:
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That's one reason I don't like the term crop (since you still have the same issues with angle of view differences based on sensor or film size, regardless of whether or not the lenses are designed specifically for a given media size.
Ah, that's the beauty of the post I made from the Microscope site. They literally spelled it out, how you can magnify by cropping, but that doesn't result in an increase in resolving power.

With digital photography, with the present limitations, cropping is all you need. You are simply magnifying what you are already capable of resolving!

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Old Mar 11, 2006, 6:42 PM   #118
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Indeed, I was addressing only one point, which seemed to me to be a major one. Sorry if it wasn't the main one.

As to the original point of the thread, I don't think the camera makers are trying to 'scam' anyone by using the '35mm equivalent focal length', just trying to give people a reasonable idea of what sort of image they are likely to get with a given lens, using the most popular format as a basis.

I don't worrry too much about resolving power, as long as I get acceptable results. I spent over a decade shooting nothing but Kodak Tri-x. The grain in the film is larger than the resolving power of most lenses. So in that case, the resolution is sensor limited. Currently, my 5MP camera gives me results pretty closely equivalent to what I got shooting 35mm, so I'm happy. In order to double the resolution I have now, I would have to quadruple the MP count to 20. That might require better quality lens than what I have. Don't see that in my future, though.

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Old Mar 11, 2006, 9:59 PM   #119
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Aumma45,
I agree that many things being said are factual. But Mark47 does have an error is his logic that has been pointed out to him at least twice now. I'm going to try again. but there is clearly some miscommunication going on.

tmoreau,
Thanks for that link. I trust your numbers & I've actually wanted a page like what that one lists for awhile. Every now and then I see quotes of that info (size of photosites on different sensors) but I've never seen it gathered somewhere before.

I fully agree that sensor noise limits resolution. There are many, many differences between cameras that make a difference in their performance. noise being a huge one. In my example I wanted to remove that factor (noise) because this discussion started out purely about lenses.

I would generally agree with you that cell phones might, some day, remove the bottom end of the market for cameras. There are many forces working against that, but it is certainly possible. phones keep getting smaller and its a ruthless market with minimal margins. Adding in a "better" camera (at least right now) wouldn't seem to garner lots of extra sales. So they have little reason to do it, except for charging you a per-picture cost to do something with them (like download them.)

But if the market and social dynamic around phones changes, it could really hurt the camera makers.

Mark47
Could you please define the term Aperture? I am very interested in your definition because I believe it is different than mine.

Once you have done this please answer me this question:
If I have a 400mm f4 lens and I take a proper exposed setting at 1/1000 f5.6, what are the settings I would use to take the same properly exposed image with a 500mm f4 lens? I accept that I couldn't be standing in the same physical location, that doesn't change the exposure settings used.

I ask this question because you have stated on several occasions that the longer focal length lens (the 500mm in my example above) gathers more light than the shorter lens. To quote you:
Quote:
While all the factors are the same, there is one difference between the two set ups. One is using a 300mm lens and the other a 200mm lens. The difference is in the glass. To see this difference it is easiest with my example of two magnifying glasses. The more powerful magnifying glass will bring more light through it, therefore being able to burn you. When something is far away from you, the light is more disperssed, just like the light from a torch therefore you need a more powerful lens to capture more of it.
What you describe above has a problem. It violates my premise. You say my example is wrong by violating an assumption in the situation I lay out. I am saying that the 300mm & 200mm lenses are *exactly the same* except for the focal length. If they have the same aperture, then they gather the same amount of light. Maybe I should have stated that assumption better. What I meant was they had the same aperture, same quality optics, same number of atmospheric elements, the are the same optically.

What you describe with your example of the two magnifying glasses of different sizes is equivalent to two lenses with different maximum apertures. Note I didn't say the lenses in my example have the same diameter front elements (which would be the same as what you describe.) When a photographer says "these two lenses have the same aperture" he/she is referring to Aperture (with a capital A) or f-stop. A 400mm f2.8 and a 20mm f2.8 both gather exactly the same amount of light. They have the same Aperture. If you measured them they would be radically different in size. In fact, they are:
20mm f2.8 has a max diameter of 3.1"
400mm f2.8 has a max diameter of 6.4"
(both numbers taken from www.canoneos.com)

They use the same exposure settings (shutter speed, ISO, f-stop) in the same lighting conditions. They gather the same amount of light.

You keep saying over and over that some how inherently a 300mm lens gathers more light than a 200mm lens. That is absolutely and totally incorrect because that isn't enough information. A 300mm f4 lens gathers *exactly* the same amount of light as a 200mm f4 lens. Period. That is what an f-stop of 4 means, and is independent from focal length. In fact a 400 f2.8 lens gathers *more* light than a 500mm f4 lens. Yes, even though the 400mm lens has less focal length. That is the definition of what f2.8 and f4 mean. An f-stop of 2.8 gathers twice as much light as an f-stop of 4. The 400 f2.8 will weight more than the 500mm f4 because it has a much bigger front element (11.8 lb vs. 8.5lb.) Because a bigger front element (a bigger magnifying glass, like in your example) will gather more light. But that is independent of the focal length of the lens.

If f-stop/Aperture didn't mean this, then many fundamental concepts in photography since it was invented would have been wrong. (I'm ignoring photography where no lenses are used - pin-hole cameras for example.) The sunny-16 rule is a prime example.

Mark47, I'm really trying to help here. It has been pointed out before by others that you are misunderstanding the concepts of Focal Length and Aperture in photography. But you seem to have not noticed this. If you just make a lens physically bigger (larger diameter) you can always make it gather more light without changing its focal length. Because of this, in photography the ability to gather light is effectively independent of focal length.

Eric
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Old Mar 11, 2006, 10:21 PM   #120
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eric s wrote:
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I would generally agree with you that cell phones might, some day, remove the bottom end of the market for cameras. There are many forces working against that, but it is certainly possible. phones keep getting smaller and its a ruthless market with minimal margins. Adding in a "better" camera (at least right now) wouldn't seem to garner lots of extra sales. So they have little reason to do it, except for charging you a per-picture cost to do something with them (like download them.)

But if the market and social dynamic around phones changes, it could really hurt the camera makers.
10 Megapixels in a camera phone is coming this year from Samsung:

http://news.yahoo.com/s/pcworld/2006...pcworld/125020


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