View Single Post
Old Mar 11, 2006, 2:33 PM   #112
DBB
Senior Member
 
Join Date: Jan 2004
Posts: 2,483
Default

VTphotog wrote:
Quote:
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) ()

Dave
DBB is offline   Reply With Quote