Understanding lenses/focal length

[Tom Overton]

Greetings;

This is something that has been confusing me a bit... When I discuss lenses with my SLR friends, they always refer to the focal length. (for example using a 15-30mm lens for macro shots.) I do understand the basic principals here (I think)

Among digital P&S users, the same basic discussion involves magnification factors instead. (for example a 1.7x zoom lens, or a0.7 wide angle) As a beginner, I do find the latter to be a little easier to understand, but I also understand that these terms are far less precise in terms of the science of camera optics.

My camera, a Kodak dx6490 has a primary lens (is that the term?) that has a 35mm equivalent of 38-380mm. There is no way of knowing, outside of the vaguest of guesses, what my actual focal length is for any given zoom. If I add a zoom or wide angle to the equation... I'm lost... (my best guess X 1.7 = your best guess)

I'd like to undertand what my camera is doing, so I can understand what it is I need to do to move forward in my photgraphic education.

My question is firstly, why is there a difference in the terms of expression between my camera and a SLR/DSLR?

Secondly, Is there some form of "conversion chart" that would help to bridge the gap between the terminology?

Thirdly, though I'd like to understand lenses/focal lengths more, do I need this knowledge to become a better photographer?

Lastly, because I seem to be dwelling on a subject that digital P&S users apparently don't need to know, do I have the wrong camera? I love my 6490. It's my first good camera. Was it a bad choice for me?

Regards,

Tom, confused on Point Pelee, Canada

[Mikefellh]

With (most) SLRs you can remove the lens and pop on another one...you might have a 35-105mm that came with the camera, and buy a 100-300mm for more tele.

On non-SLRs the lens is not removable...you buy a camera with a 38-380mm you are stuck with it. For these cameras there are converter lenses...these lenses screw on the end of your existing lens (usually via a filter tube adapter as most digitals with retracting lenses don't have threads on them since they can't take any weight). These lenses are measured in an "X" factor because they convert the camera's lens by a multiplication factor, and they can be used on different cameras...

If I put a 2.2x on a 38-380mm it becomes a 83-830mm. I can put that same tele converter on a 35-105mm and it will become a 77-231mm.

If I put a .66x on the same camera it becomes a 25-250mm. If I put that wide-angle converter on a 35-105mm it will be a 23-69mm.

When it goes onto different cameras the combination becomes different ranges, but always by the same "X" or multiplication factor. I can also use these on SLR cameras, but why when the proper lens can be attached (although I did use my .66x on my mother's 50mm Canon prime lens to give it more wide angle, 33mm).

There are converters for SLRs too, but they go inbetween the body and the lens...you remove the lens, attach the converter, then reattach the lens...it extends the distance between the lens and the film plane giving you longer lenses...I've seen 2x and 3x.


As to your other questions...although some of this might be too basic for you, it might answer some of your questions:
http://209.196.177.41/

A lot of things come with experience...there are some rules about photography, but many can be broken. Many portrait photographers will use a 100mm lens...others may go more or less. The human eye is said to be 50mm (I have my camera programmed to start at this focal length).

Did you choose the wrong camera...all depends on you and what you do. I have a camera similar to the DX6490 in terms of the long lens, but I had always used manual cameras (adjusting shutter speed, aperture, etc. on my own) so miss the easy to dial manual controls (rather than pushing buttons)...I admit I have my eye on a professional dSLR but it costs more than my last car. Go into a camera store and play with the dSLRs...the Canon Rebel is a reasonably priced one (not that I'm recommending it).

If you are a point&shooter who wants more zoom, than the DX6490 isn't a bad choice (with the exception of shutter lag; time between pushing the button and the camera taking the picture).

[Tom Overton]

Hi Mike;

Thanks for your reply to my questions. (actually, I was hoping you'd read my post) Most of the first part of your answer I understood. (most) ...or at least that is what I assumed the answers to be. I'm ealing with new terminology here... DOF, focal length, aperture control, etc... I just thought perhaps I should know more.

Still, the question: Is it of value to know the length of my lens in a DX6490? Or is the speed of the lens a more important factor to be weighing? On that topic, I have two conversion lenses for my camera, an Oly tcon17, and the Schneider 0.7. As far as I know, neither of these lenses have any rating as to what effect they have on exposure.

I boughtthe Kodak because, for a beginner, it offered some control over aperture and other things I knew little about. It also had a very good automatic setting that helped compensate for my mistakes while I learned. Now that I have a little of that body of knowledge, I find that I am getting some very good results, so I'm not really sorry for my choice. I do wonder about the staying power of the camera as I develop. (no pun intended)

Thanks again for your response. There is so much more for me to learn. Next, I'm sure I'll be asking about manual vs automatil metering... but that's another day.

Tom, on Point Pelee, Canada

P.S. Thanks for the link. I've browsed those lessons before.

B.T.W. Though I approach photography mainly through its artistic appeal, I am trying to gain an understanding of the science of its aparatus.

[shene]

Tom Overton wrote:

My question is firstly, why is there a difference in the terms of expression between my camera and a SLR/DSLR?

With film cameras, the number of film sizes is very limited (e.g., 6x6, 645, 135, etc). Most camera can only accept one set of lenses that are designed for the film size of the camera. So, 135mm photographers use their own focal lengths such as 50mm, and 6x6 and 645 users use another set of focal lengths such as 80mm (the equivalent of 50mm in the 135mm size). However, digital cameras have so many different sensor sizes (more than the available film sizes) and we will see more in the future.A smaller sensor requires a shorter length to provide the given angle of view than a larger sensor does. For example, FZ-10's Leica lens has an actual focal length range from 6mm to 72mm, which is equivalent to 35mm to 420mm in 135mm format, with 9mm producing an angle of view similar to that of the 50mm lens. The Nikon 5700 has a lens from 8.9mm to 71.2 (35mm to 280mm in 135mm format), and produces an angle of view similar to the 50mm lens at 13.4mm. Therefore, the digicam makers prefer to quote the 135mm format equivalent rather than using the actual focal lengths to avoid confusion.

Secondly, Is there some form of "conversion chart" that would help to bridge the gap between the terminology?

Check your camera manual and you find it. However, there is no universal formula to do this job because there are so many different sensor sizes. The lens marking will show you the actual focal length like A-Bmm. Then, the specification of the camera will tell you the 135mm format equivalent, say C-Dmm. Now, compute (D-C)/(B-A) and let it be X. Then, given a 135mm focal length F, its actual focal length on your camera is F/X and conversely, given an actual focal length E of your camera, its 135mm format equivalent is X*E. More details can be found on the "Lens Overview" page in the "On-Camera Lens" section of my 5700 user guide.

Thirdly, though I'd like to understand lenses/focal lengths more, do I need this knowledge to become a better photographer?

No! Your artistic vision is more important. However, in many situations this lens and focal length knowledge CAN help you to realize your vision. You can find this type of information is any good photographic lenses book. An old and used one would be OK because the theory is all the same.

Lastly, because I seem to be dwelling on a subject that digital P&S users apparently don't need to know, do I have the wrong camera? I love my 6490. It's my first good camera. Was it a bad choice for me?

A camera is right one until it cannot fulfill your vision.

CK

http://www.cs.mtu.edu/~shene/DigiCam

Nikon Coolpix 950/990/995/2500/4500/5700 and Panasonic FZ-10 User Guides

[Tom Overton]

Greetings;

Thank you very much for your detailed reply. There is a great deal to digest between yours and Mike's posts. I am pleased that the information is available, though, as you say, not crucial to the art of photography.

The DX6490 lens is equivalent to 38-380mm, though it isactually 6.3-63.2mm. That will give you an idea of the relative sensor size. The numbers which may be more important are the lens elements which are rated f/2.8-f3.7. I still don't know what an add-on lens does to those numbers, or if the metered exposure is still accurate when you add lenses... but that's another question.

Thanks once again to you and Mike. I am a slow learner, but I do learn.

Regards

Tom, on Point Pelee, Canada (not that far from MTech)

[shene]

Tom Overton wrote:

The DX6490 lens is equivalent to 38-380mm, though it isactually 6.3-63.2mm. That will give you an idea of the relative sensor size. The numbers which may be more important are the lens elements which are rated f/2.8-f3.7.

The f2.8-f3.7 are the maximum aperture values, the f-numbers. It tells you how large the opening is. A smaller number means a larger opening. So, f2.8 is larger than f3.7 in terms of the size of the opening. f2.8-f3.5 means at the shortest focal length 38mm the largest opening is f2.8 and at the longest focal length 380mm the largest opening is f3.7. Therefore, when you zoom in, the maximum aperture of your lens gets smaller. A smaller aperture requires a faster shutter speed. To see the focal length change and its effect, please see the "On-Camera Lens" section of my Coolpix 5700 and/or FZ-10 user guides, where you will see a serious of shots taken with different focal length at the same position. You might also want to take a look at the "Exposure" section of my 4500 user guide. The focal length and exposure topics are camera independent.

I still don't know what an add-on lens does to those numbers, or if the metered exposure is still accurate when you add lenses... but that's another question.

Most good converter lenses do not change the aperture significantly. For example, if you use your cam with F5.6 and shutter speed 1/100 sec, adding a good converter lens may still cause your cam to use F5.6 and 1/100 sec or a very minor variation. However, a poorly designed converter lens would not guarantee that. Image quality of a converter lens is usually proportional to the price you pay. Better converter lenses are usually larger and heavier.

on Point Pelee, Canada (not that far from MTech)

Maybe cross Lake Superior. :-)

CK

http://www.cs.mtu.edu/~shene/DigiCam

Nikon Coolpix 950/990/995/2500/4500/5700 and Panasonic FZ-10 User Guides

[Tom Overton]

Thank you once again;

I have seen the results of poorly made converters first-hand. I tested some lenses in NYC in August, and ended up choosing an Olympus TCON-17. The lenses I did not purchase were, if not poor quality, certainly not matched to the existing optics on my camera. My other converter is a Schneider-Kreuznach 0.7 with a massive front element. It gathers quite a bit of light, and is very reliable. The TCON, while a good lens, is a little harder to recieve good results with. I suspect that it does stop down my exposure a couple of steps.

Am I correct in assuming, though, that the numbers that are displayed in my viewfinder are correct, regardless of the attachment on the lens.

I am beginning to get a handle on this. Considering how little I knew only a few months ago, I think I'm doing ok.

Thanks again,

Tom, on Point Pelee, Canada

[Mikefellh]

As you zoom in you do lose light, instead of sampling say 500 square feet of light, you might only be sampling 50 sqaure feet, and same goes with adding a teleconverter (some might cost you two stops of light (meaning two clicks of adjustment on the old cameras). And as you zoom in you need a faster shutter speed to compensate for the longer zoom, which is the opposite of what's happening as you zoom in (using automatic settings).

With photography there's always something new to learn...I'm planning to start experimenting with 3-d photography.

[shene]

Tom Overton wrote:

The TCON, while a good lens, is a little harder to recieve good results with. I suspect that it does stop down my exposure a couple of steps.

I guess when you were testing the TCON-17 you hand-held the camera/lens. In other word, you did not use a tripod. Right? If it is the case, the bad results is likely due to camera shake rather than a lens problem. Wiith a TCON-17 on your camera, the focal length is 380*1.7 = 640mm (in 35mm format equivalent). This is 640/50 = 12.8 times of the standard focal length 50mm. Consequently, a camera shake is magnified 12.8 times (with the TCON-17) of that of the 50mm. Even though a small camera shake that is invisible to human eyes would become significant after blowing it up 12.8 times! When would camera shake occur? As mentioned in my previous post, a 640mm focal length requires 1/640 sec for an untrained people to handle the camera/lens with camera shake. So, did you use that high shutter speed? If you did not, that was the problem. Next time, use a tripod!

Am I correct in assuming, though, that the numbers that are displayed in my viewfinder are correct, regardless of the attachment on the lens.

Yes, you are right. The numbers (i.e., aperture and shutter speed) displayed on the EVF or LCD are the ones to be used in the next shot.

Tom Overton wrote:

I suspect that it does stop down my exposure a couple of steps.

In general, the TCON-17 will not cause much difference in aperture and shutter speed. There will be some, but not much. The major loss is perhaps due to zooming the lens in. In terms of the maximum aperture available, there are two types of zoom lenses: variable aperture and constant aperture. A constant aperture zoom lens will not change itsaperture while zooming. In other word, if you choose F4 before zooming, then the aperture will fix at F4 no matter how you zoom the lens. This type of zoom lenses are usually expensive and only available as SLR/DSLR professional lenses. On the other hand, a variable aperture zoom lens does not guarantee a constant aperture while zooming. For example, if you choose F4 before zooming, then once you start to zoom in, the aperture will become smaller (e.g., F4.5). If you start to zoom out, the aperture will become larger (e.g., F3.5). Virtually every consumer level digicam and consumer level SLR/DSLR lenses are variable aperture lens. Why? Save cost! Maintaining constant aperture requires sophisticated optical formula and extra glass elements, which mean money. General consumers may not want to pay that mount of money, and live with changing aperture. In this way, design sacrifice constant aperture to save cost and still can come up with reasonably good design.

Variable aperture has nothing to do with metering. It is all about the length of the lens. When light enters the front glass element of the lens, ittravels in the lens barrel, go through the last glass element, and finally hits the sensor. As you may know, the longer a light ray travels, the weaker the intensity it is. This is the basic Newton law of light. More precisely, if the distance between your eyes and a light source is d, the intensity is proportional to 1/(d squared). Now, examine your lens. If you zoom it in, the lens barrel becomes longer, and if you zoom it out, the lens barrel becomes shorter. This means light travels a longer distance at the long focal length end than that of the short focal length end. This implies that the amount of light (oe intensity) that can reach the sensor at the long focal length end is weaker than that of the shorter focal length end. Converting to aperture, the maximum aperture you get at the longer focal length is smaller (larger f-number) than the aperture (smaller f-number) at the shorter focal length end. Hope this explains things well.

CK

[Tom Overton]

Gentlemen;

Thank you so much for your sharing of knowledge. Though much of what has been said I have read before in a more or less general way, it is good to see the same information in regard to specific questions.

I do understand that much of this information I do not need for the purposes of my camera, which for all intents and purposes is a glorified point and shoot... Still, I find an understanding of the basic principles of photographic sciences can only help in the long run.

Many years ago, when I was an undergrad studying music, renowned pedagogue William Ade spoke to our class. He said that it was essential for all musicians to dilligently practice their technique, regardless of how artistically gifted they may be. He continued to say that in even the most talented musicians' life, there may come a day that the muse fails them. That is the day that their technique studies will reward them, and they may come through the challenge relatively unscathed. (badly paraphrased after nearly 30 years)

...All this to say that thoughI shoot from the heart, even as an amateur photographer, I respect the science and technology that makes this art possible, and study it so that the day I am faced with a challenging opportunity, I may be able to do that subject justice. (sounds like I'm full of hot air)

Once again, thank you gentlemen.

Regards,

Tom, on Point Pelee, Canada