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Old Mar 8, 2006, 7:05 PM   #11
DBB
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tmoreau wrote:
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Don't worry about magnification, that's an internal mechanical technicality that has no meaning in real-world use. If you crop a 35mm negative and print an 8x10, or print a full-frame 8x10 from a digital file, they are the same (assuming same camera location and setup). If your concerned about sensor pixels per inch, or theoretical film resolutions, and resolving power of the lens, then the 100mm and 150mm lenses may have some slight physical differences, but the argument is pedantic. I don't work for nasa and I don't have a 82mp sensor, so the horribly minute optical differences between using 1 square inch of glass per megapixel or 2 square inches of glass per megapixel are unimportant. Here in the real-world, they do the same thing. You can use any amount of magnification, and any size image capture media, but when you print to final media the magnification is UNDONE. The only differences are resolution and field of view.
I follow your reasoning, but this "cropped paragraph" I have to disagree with. My shooting depends on magnification. And the cropping factor cannot compare with REAL magnification. All well and good when the difference is small, 50mm as opposed to 75mm. But some people actually think their 500mm telephoto will produce the same detail as a real 750mm. And such is not the case.

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Old Mar 8, 2006, 8:48 PM   #12
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DBB wrote:
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... My shooting depends on magnification. ...
I don't understand that at all. Shooting should depend on filling the sensor/film with the subject anddesired surrounds. If several cameras, say 8x10" view camera, dSLR with an APS sized sensor, film SLR, ... are set in the same possition and have lenses that cover the same subject, there will be no difference in the photos (resolution, lens quality, ... aside). So it make sense to say that a 12" lens on the view camera is equivalent to a 43mm lens on the SLR and equivalent to a 28mm lens on the dSLR. So it would be very reasonable to say that the 28mm lens is equivalent to a 12 inch lens - except that very few people would have any idea what you are talking about.

That is the whole point of quoting 35mm equiv focal lengths - lots of people have a feel for what that means.

tmoreau gave asgood an explanation as I have seen.
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Old Mar 9, 2006, 6:03 AM   #13
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The problem is that it creates a false perception. If you have a 200mm lens on a camera and are looking at some writing but you cant read the writing because it is too far away but put on a 300mm lens and now you can, you wont get the same effect by switching camera bodies from a 35mm to a crop. Although the equivelant focal length may change from 200mm to 300mm by switching bodies, the writing will still be the same size, only less of it, so you still wont be able to read it, you will still need to put a 300mm lens on to be able to read it no matter which camera you use.
So in practice if you had a 35mm camera for bird watching and your 200mm lens didnt allow you to get close enough to see the bird effectively, some people may think "great, this crop camera will give me a 300mm lens so I will now be close enough to see detail" but it wont, you still have the optical magnification of a 200mm lens, so if you want to get close enough to see detail you will still need a 300mm lens. The optical magnification is what gives you the detail, not the digital magnification. So if something is out of range to be clear with a 200mm lens, such as writing but is close enough with a 300mm lens, then changing bodies isnt going to make the writing clearer as it would if you changed lenses because you are beyond the optical range of the lens. Its like if you try reading something too far away, the writing will be blurred, if you blow up that image it is only a bigger version of a blurred image, it doesnt become clearer as you blow it up. That is all crop cameras do, they take a smaller image and blow it up.
This is where the marketing has taken a wrong turn, because they are trying to deceive people by making out that their 200mm lenses will become 300mm lenses if put on a crop camera.
If you take a photograph of Mars, it doesnt matter how many trillion pixels you have and how much you blow it up, you will never be able to blow it up to look like anything more than a dot of light unless you use optical magnification to be able to see the detail.
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Old Mar 9, 2006, 6:20 AM   #14
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That would depend on why you can't read the text or see the bird well enough with the 200mm lens. Is the problem the lens resolution or the sensor resolution? In other words, is the lens not producing a sharp image or are the sensor photosites too big or too far apart to resolve the detail that the lens is providing? If the lens isn't sharp enough, then your 300mm would have to be sharper than the 200mm to do any good; if the sensor is the problem, then switching to a smaller sensor (smaller photosites closer together) may well allow you to see what the lens sees.

There is no "digital magnification" going on. It's like changing eyepieces on a telescope. You have a fixed focal-length objective lens, and when you switch to a shorter focal-length eyepiece (which just lets you see a smaller bit of the image circle made by the objective lens), you "see" higher magnification. If you switch to a longer focal-length eyepiece (shows more of the image circle), you see less magnification. You're still the same distance from the subject and the focal length of the objective lens hasn't changed, so the magnification must come from "seeing" a smaller portion of the image circle--which is exactly what a smaller sensor on a camera does.

As for your Mars example, if you have enough pixels and a sharp enough lens, yes, you could get detail by "blowing up" the photo (the limit would be atmospheric distortion/attenuation).
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Old Mar 9, 2006, 7:07 AM   #15
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Mark47 wrote:
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The problem is that it creates a false perception. If you have a 200mm lens on a camera and are looking at some writing but you cant read the writing because it is too far away but put on a 300mm lens and now you can, you wont get the same effect by switching camera bodies from a 35mm to a crop. Although the equivelant focal length may change from 200mm to 300mm by switching bodies, the writing will still be the same size, only less of it, so you still wont be able to read it, you will still need to put a 300mm lens on to be able to read it no matter which camera you use.
Sure you will, if the sensor is outresolving the lens. The sensor isoften the limiting factor versus the lens itself, and most lenses are sharper in their center, too (and if you use a smaller sensor on a lens designed for 35mm cameras, you're using the "sweet spot" of the lens).

I'm as guilty as anyone in how I explain this, since the "popular" thing to do is call it a crop factor.

But, when you think about it, there is no "crop factor" involved here. Remember, you've got lenses designed specifically for a DSLR with an APS-C size sensor, with smaller image circles.

Are you still looking at a cropped view then? Nope.

Do you still need to use a multiplier to see how the angle of view would compare to the same focal length lens on a 35mm camera? Yes.

Look at a camera like the Olympus E-1, or E-300. It's sensor is even smaller than the one in most DSLR models. Do they have a cropped view, just because you need to multiply the actual focal length of the lenses by 2x to see how the angle of view compares to a 35mm camera? Nope.

How about a model with an even smaller sensor. Let's look at something like my little Konica KD-510z pocket camera. It's lens is marked as 8-24mm. Is it being cropped in some manner to give me the same angle of view I'd have on a 35mm model using a 39-117mm lens? Nope.

The lenses are correctly marked with theactual focal length of the lens (and in the case of fixed lens consumer models, the "35mm equivalent focal range" is also given in their specifications).

What changes is the angle of view, which will depend on the sensor (or film) size, for any given actual focal length.

The mainreason that you see the 35mm equivalent focal lengths of the lenses mentioned, and crop factors/focal length multiplierspublished, is so that users of 35mm cameras have a better idea of how the angle of view compares to the same focal length lens on a 35mm model.

This is only because 35mm cameras are so popular!

Let me repeat that.

This is only because 35mm cameras are so popular!

What compounds the confusion, isthat DSLR bodies are often designed so that a lens from the same manufacturer's 35mm bodies can be used on it. In this case, the image circle is larger than the sensor. That's notwhat makes the angle of view different.

You see the same types of differences when you go to film that's larger than 35mm, too. The actual focal lengths of the lenses are still marked on them. Yet, you'll have a widerangle of view for any given actual focal length compared to a 35mm model (versus a narrower angle of view like you'd have using the same focal length lens on a model with a smaller sensor orfilm size).

For example, a 45mm (actual focal length) lens on a 645 format camera would have a 35mm equivalent focal length of approximiately 29mm. What I mean by that is the angle of view would appear to be the same as a 29mm lens would have on a 35mm camera.

So, if 645 format models were more popular than 35mm cameras, we may be seeing focal length multipliers/crop factors for 35mm cameras (so that users would understand how the angle of viewcompares, when using the same focal length lens on a 35mm camera, that they were accustomed to using on a camera using a larger film size.

If 35mm models weren't so popular, there would be no reason to even mention a "35mm equivalent focal length" or "crop factor".It's done for comparison purposes, since the actual focal lengths of the lenses is are how they are marked.

Does it make any difference that the sensor happens to be smaller than 35mm film for detail purposes? Not if the sensor is capable of capturing what you need, just as if you were comparing 35mm film witha larger film format.

Ditto for things like angle of view or depth of field. Sure there is a difference. That's because of the sensor or film size, and how yourangle of view changes with the film or sensor size,for any given actual focal length lens.

Should we abandon 35mm because 645 is larger, or abandon 645 because I can get an 8x10" camera? LOL

You use works best for you.

Are focal length multipliers/crop factors useful (even if these terms are relatively inaccurate)?

Sure they are.

They help users that are familiar with 35mm cameras determine how a lens at a given focal length compares when using a DSLR that has a sensor size that is smaller, from an angle of view perspective.

Now, just to make things even *more* confusing, the angle of view is often published with lens specifications.

But, the specifications will *assume* that the lens is being used with a given film or sensor size. If you look at a lens with a 45mm focal length designed for a 645 format camera, it's going to show you a wider angle of view than you'll find in the specifications for a 45mm lens used on a 35mm camera.

The lens manufacturers compute the angle of view for the format the lens is designed for.

Interesingly, I'm seeing some lenses designed specifically for DSLR models with sensors smaller than 35mm film, that show an accurate angle of view in their specifications. Nikkor DX lenses are a good example of this.

For example, the widest angle of view for the Nikkor 18-70mm f/3.5-4.5G ID-IF AF-S DX lens is shown as 76 degrees at a focal length of 18mm (as you would expect when a lens with a focal length of 18mm is used on a DSLR model with a smaller sensor).

If you look at a non-DX lens, the angle of view shown in the specifications for a given focal length assumes it will be used on a 35mm model.For example, the Nikkor 18-35mm f/3.5-4.5DED-IF AF lens (non DX lens) shows an angle of view in it's specifications of 100 degrees at a focal lenth of 18mm (as you would expect when an 18mm lens is used on a 35mm camera).

The focal length at the widest setting for both lenses is identical. What changes is the angle of view, depending on the size of the sensor/film the lens is being used with. In the case of Nikkor DX lenses, the manfacturer is just taking the format the lens isbeing used on into consideration when the specifications are published.

You'll see the same thing in specifications for larger format cameras (lenses designed for them show the correct angle of view for the filmformat being used.

I haven't looked at the specs on other "made for digital only" lenses, but you may see the same thing in them, too.

But, most users don't understand angle of view, they understand how a given focal length lens looks from an angle of view perspective when used on a 35mm camera.

Here is a handy online application on Canon's web site that actually shows the angle of view clearly in the graphics:

http://consumer.usa.canon.com/app/html/EFLenses101/focal_length.html

But, the angle of view shown is for a given focal length lens when used on a 35mm camera (that's what this application was designed for).

If you were using the samefocal length lens on a model with a larger sensor or film size, you'd have a wider angle of view.

If you were using the same focal length lens on a model with a smaller sensor or film size, you'd have a narrower angle of view.

That's what the so called "crop factors" and "focal length" multipliers are good for -- so that users familiar with using a 35mm camera have a better understanding of how the angle of view compares, for agivenfocal length lens,when it's used on a DSLR with a smaller sensor.

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Old Mar 9, 2006, 7:13 AM   #16
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Mark47 wrote:
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The problem is that it creates a false perception. If you have a 200mm lens on a camera and are looking at some writing but you cant read the writing because it is too far away but put on a 300mm lens and now you can, you wont get the same effect by switching camera bodies from a 35mm to a crop. Although the equivelant focal length may change from 200mm to 300mm by switching bodies, the writing will still be the same size, only less of it, so you still wont be able to read it, you will still need to put a 300mm lens on to be able to read it no matter which camera you use.

If you have 2 8 Mp digital cameras 1 full Frame and 1 with a smaller sensor and you take a photograph of some writing on a distant wall with a 300mm Lens on the FF and a 200mm Lens on a crop camera the image will be the same and will have the same number of pixels used for each letter. Whether you can read the writing on either image will depend on the quality of the lens. If the 200mm lens is not of high enough quality to resolve the detail and the 300mm lens is then you are correct. However if the 300mm lens is a cheap plastic lens and the 200mm lens is a top of the line pro lens then you'll see more detail in the shot taken with the 200mm lens. When using a crop camera you are in fact using only the centre of the lens, which is the best part of the lens so even with similar quality lenses it's not a clear cut case that the 300mm lens will out perform the 200mm lens.
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Old Mar 9, 2006, 8:39 AM   #17
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JimC wrote:
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Mark47 wrote:
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The problem is that it creates a false perception. If you have a 200mm lens on a camera and are looking at some writing but you cant read the writing because it is too far away but put on a 300mm lens and now you can, you wont get the same effect by switching camera bodies from a 35mm to a crop. Although the equivelant focal length may change from 200mm to 300mm by switching bodies, the writing will still be the same size, only less of it, so you still wont be able to read it, you will still need to put a 300mm lens on to be able to read it no matter which camera you use.
Sure you will, if the sensor is outresolving the lens. The sensor isoften the limiting factor versus the lens itself, and most lenses are sharper in their center, too (and if you use a smaller sensor on a lens designed for 35mm cameras, you're using the "sweet spot" of the lens).
No you wont as in the example of taking a photograph of Mars. If you took a photograph of mars with a 300mm lens and a trillion megapixel camera, it wouldnt matter how much you blew it up you would never get any detail. the further something is away from you, the more the light will disapate and distort before it gets to you and the only way to correct that is through optical magnification, you cant do it digitally. This is why digital magnification on video is inferior.
Imaging shining a torch into space, what happens to the light? It doesnt stay straight does it? So if you were orbiting the earth, by the time it gets to you there is hardly any left to make a picture with. A longer focal lenth lens allows you to capture more light, so there is more to work with and therefore less distortion. A 300mm lens will take in more light from a smaller area at a greater distance than a 200mm lens. Its not taking in more light but more light from a smaller area.
So imaging you are orbiting the earth and someoe is shining a torch at you. With a telescope you can focus enough light to enable you to see but with a small 300mm, it just doesnt take in enough of the torch light, so no matter how much you crop or blow up that image you wont get any detail.
Light travels in waves and expands outwards, to correct that you need a more powerful lens.
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Old Mar 9, 2006, 8:55 AM   #18
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Your analogies may zero sense at all to me.

The brightness of a lens is not determined by focal length, and often you'll have shorter focal length lenses that are brighter.

Don't confuse the diameter of a lens, and how much glass it's got with brightness. If you have a lens with larger available apertures for a given focal length, sure it's going to be larger. If you've got a lens with smaller available apertures for a given focal length, it's going to be smaller. The aperture of a lens (as expressed in f/stop) is a ratio between the focal length of the lens and the area of the diameter of the aperture iris.

As far as how much light is being gathered, a larger format lens needs to spread the light over a larger surface area (the film or sensor). A smaller format lens designed for a smaller sensor or film size doesn't need to be as large to give the sensor just as much light, because the surface area of the sensor is smaller.

The confusion comes in because many lenses that can be used on DSLR models were originally designed for 35mm cameras. So, people assume that the angle of view they see is because of a crop. It's not, it's because of the focal length combined with the size of the sensor or film.

An 8mm lens on a camera with a tiny 1/1.8" sensor that has f/2.8 available gathers just as much light for the sensor, with the same angle of view that you'd have using a 39mm lens with f/2.8 available on a 35mm camera. The lens for the 35mm camera is larger. That doesn't necessarily mean it's better or brighter, as far as how much light is going to the sensor.
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Old Mar 9, 2006, 8:56 AM   #19
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Nagasaki wrote:
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Mark47 wrote:
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The problem is that it creates a false perception. If you have a 200mm lens on a camera and are looking at some writing but you cant read the writing because it is too far away but put on a 300mm lens and now you can, you wont get the same effect by switching camera bodies from a 35mm to a crop. Although the equivelant focal length may change from 200mm to 300mm by switching bodies, the writing will still be the same size, only less of it, so you still wont be able to read it, you will still need to put a 300mm lens on to be able to read it no matter which camera you use.

If you have 2 8 Mp digital cameras 1 full Frame and 1 with a smaller sensor and you take a photograph of some writing on a distant wall with a 300mm Lens on the FF and a 200mm Lens on a crop camera the image will be the same and will have the same number of pixels used for each letter. Whether you can read the writing on either image will depend on the quality of the lens. If the 200mm lens is not of high enough quality to resolve the detail and the 300mm lens is then you are correct. However if the 300mm lens is a cheap plastic lens and the 200mm lens is a top of the line pro lens then you'll see more detail in the shot taken with the 200mm lens. When using a crop camera you are in fact using only the centre of the lens, which is the best part of the lens so even with similar quality lenses it's not a clear cut case that the 300mm lens will out perform the 200mm lens.
We were not talking about quality of lenses, that goes without saying, of course in this argument you would be referring to lenses of the same quality. Although you have the same number of pixels the variable is the quality of light coming to you to. If you were to photograph a laser coming towards you, the further you are away, the more the laser will have distorted. A 300mm lens will correct that distortion more than a 200mm lens therefore you will get a clearer image.

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Old Mar 9, 2006, 8:57 AM   #20
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DBB wrote
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My shooting depends on magnification. And the cropping factor cannot compare with REAL magnification.
I understand that a narrow field of view is required for some types of photography, such as birds or sports. With a narrow angle of view you can fill the frame with something small and distant. What I dont get is why you claim magnification has anything to do with it. Whether you capture a distant bird at 1:1 life size on an 8x10 sheet of film, or reduce it in size to smaller than a dime on a tiny digicam CCD, when you print both at 8x10 they will be the same. One was not magnified or reduced, the other reduced dramaticly. This effect is reversed, at any rate, when you print. If you assume that the digicam has enough resolution to create a nice 8x10, what is the difference, exactly? Rather, lets try this. Suppose you capture something full frame on a 1 megapixel DSLR, and the same shot on a 6 megapixel digicam. The DSLR image, inside the camera, is magnified much more. Which will give a better print? The techical thins that happen from the front of your lens until the print, arent particularly important.

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you will still need to put a 300mm lens on to be able to read it no matter which camera you use.
How can you claim that? Again, if your using the SAME camera, SAME sensor, you get more detail with a longer lens. When you have a shorter lens and a higher pixel density sensor, you'll get the same result as a longer lens and lower pixel density sensor. Your not comparing apples to apples, this is why equivilency factors are so neccasary.

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So in practice if you had a 35mm camera for bird watching and your 200mm lens didnt allow you to get close enough to see the bird effectively, some people may think "great, this crop camera will give me a 300mm lens so I will now be close enough to see detail" but it wont, you still have the optical magnification of a 200mm lens, so if you want to get close enough to see detail you will still need a 300mm lens. The optical magnification is what gives you the detail, not the digital magnification.
Thats some leap of faith you take after running out of logical explainations. It would only make sense if the lens can not resolve enough detail. We know that a well constructed lens resolves plenty of detail for most any digital camera on the market. The film or sensor are the only items limiting detail, at this point. At some point you run into real physical limitations of light difraction and all that, which is again irrelevant to this discussion.

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If you take a photograph of Mars, it doesnt matter how many trillion pixels you have and how much you blow it up, you will never be able to blow it up to look like anything more than a dot of light unless you use optical magnification to be able to see the detail.
Ah, thanks for summing it up. WRONG. As long as were not trying to do things with light that are not physically possible (which were not, with whats on the market) you CAN get a detailed picture of mars using a 40 terrapixel sensor and you favorite 50mm lens. Why wouldnt you? Are you saying that the lens is unable to resolve more than X megapixels? True, but it seems that X is more than 10mp with the most common SLR lenses, and more yet with quality lenses.

Bottom line: I've only used 35mm film cameras a few times many years ago, yet I know that in the 35mm format I need a 28mm lens to get a good wide view inside a house. That required an actual 7.2mm lens on my Minolta A1, a 18mm lens on my 5d and a who knows what mm lens on my old P&S. In all cases, its a 75° angle of view that I'm after (though, I had to look that up. Its not a commonly published figure). Now watch out, is that horizontal, vertical, or diagonal? What if the aspect ration changes from 3:2 to 4:3? Well crap, that wont work either! Guess your SOL, there is NO WAY to compare different cameras and different lenses. Unless you want to use equivilancy factors, and aproximate for different aspect ratios.

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you would never get any detail. the further something is away from you, the more the light will disapate and distort before it gets to you and the only way to correct that is through optical magnification, you cant do it digitally.
Thats some great theory, but it DOES NOT APPLY to the cameras on the market today. We are well within these limits, sorry. The difference from 200mm to 300mm is indescribably small.
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