Digital Cameras, a lie of omission.

Discussion in 'Beyond the Basics' started by Bobby Ironsights, Dec 17, 2007.

  1. Bobby Ironsights

    Bobby Ironsights TPF Noob!

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    The following post was in error. I was mistaken about one thing* Apparently, the 85mm effective focal length given by salesman and oft-quoted, is not true. When using a digi cam with a less than full sized sensor, you are not changing the effective focal length at the time of exposure, you are just magnifying a smaller picture to a larger extent when final printing occurs. I'll leave the posts up, so anyone who wants to read it can.

    You know, I've seen a mistake repeated here, many times. It's the salesman that are to blame, I think.

    A lie of omission.

    I use film camera's, exclusively, except for my avatar, for which I used my webcam.

    You see, I often hear people cleverly say, "since I have a sub-frame digi-cam, that 50mm f/1.4 prime will be like an 85mm! And that nice USM 4.5-5.6 85-200 kit zoom, now is like a 150mm to 340mm zoom! The salesman told me so! Isn't that great???!!!"

    Now there is a flip side to that though, what he didn't tell you, is that the nice and fast f/1.4, will now become a turtle slow f2.4.

    That kit lens? You better glue it to a tripod, because it's now a glacially slow f/11, give or take, and unlike stopping a prime lens down to f/11, that's WIDE OPEN, where optical performance is worst.

    There's no such thing as a free ride. A 35mm maximum diameter exit pupil, has a limit to how much light it can let in, and a smaller sensor just wastes light.

    And if that wasn't bad enough, where does all that extra light that doesn't hit the sensor go anyway?

    It just bounces around, corrupting your image sharpness and contrast.

    So, if you've just noticed your bum hurts, there's a reason for that....and it's not because you sat on a tack.:greenpbl:
     
  2. Josh66

    Josh66 Been spending a lot of time on here!

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    Wouldn't the same thing happen with film? I mean, the lens is round - so there will always be extra light that isn't hitting the film/sensor.

    Isn't the inside of the camera coated with non-reflective materials to prevent this?
     
  3. Bobby Ironsights

    Bobby Ironsights TPF Noob!

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  4. Steph

    Steph No longer a newbie, moving up!

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    As I said in another thread, that is wrong. The max aperture only depends on the lens, not the camera you put behind it.
     
  5. Josh66

    Josh66 Been spending a lot of time on here!

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    OK, I see. This -> ([]) vs. this -> ( [] ), right (hope that makes sense)?
     
  6. Bobby Ironsights

    Bobby Ironsights TPF Noob!

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    absolutely, jeep grille lookalike handle dude.
     
  7. Bobby Ironsights

    Bobby Ironsights TPF Noob!

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    Absolutely, the NOMINAL focal length of the lens when focused to infinity is technically the same, but the EFFECTIVE focal length of the lens is increased by the change in angle of view.

    I can quote the basic physics here from Wikipedia.

    If two cameras of different format sizes and focal lengths have the same angle of view, and the same aperture area, they gather the same amount of light from the scene.

    However, while in the case of a 35mm slr lens, the aperture area remains the same.....the ANGLE OF VIEW decreases, which, while not exactly the same as changing the focal length, has the same effect on the equation as if the focal length had changed. What I mean to say, is the effective focal length has changed.

    So yes, if what you are saying is that I'm not quoting the EXACT pedantic def'n of the According to Hoyle of optics, I'll agree. But most people don't have university level physics.

    Soooo... giving the simple explanation helps most people. (at least I thought it would)

    The lens was designed, so the vertex of focus at infinity impacts on the film plane. In order to keep the same WORKING aperture, the smaller sensor would have to be significantly closer to the lens than the film, AND the lens would have to be designed to focus well PAST infinity to compensate for the closeness of the sensor to the rear element of the lens.
     
  8. Battou

    Battou No longer a newbie, moving up!

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    Sorry, I am a dumfck when it comes down to digital photography, but the omission of information is common place in any marketing campaign. People only want to sell stuff, not teach them how to use it or inform them there is better. They would rather wait until the sale is final and then sell you the better things. Two sales for the price of one.

    People should go in knowing they are not gonna get all the information, and be prepaired to ask for it.
     
  9. Steph

    Steph No longer a newbie, moving up!

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    Of course if you use a 50mm f/1.4 lens (for example), the full frame sensor will gather more light than a cropped sensor but only because it is bigger. This has nothing to do with the effective maximum aperture changing from full frame to APS-c sized sensors. The amount of light gathered per square millimetre is the same on both sensors.
     
  10. Bobby Ironsights

    Bobby Ironsights TPF Noob!

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    The intensity of light per square millimeter would be the same, if the effective focal length was the same. But it's not right????

    Well, I've spoken to someone else about this, and there may be a fly in my ointment.

    Apparently, the sensors DON'T give an effective focal length of 85mm. They just give a smaller window, on the same 50 mm focal length of 50mm.

    They just crop. So the end user, when they make the final print, is having the effect of enlarging a smaller negative to fit the size of the print. That's where the only Enlarging occurs. Extra light IS NEEDED, but it comes from the "Virtual" light source, of a printer.

    If you were printing film, you'd have to raise the lamphouse up higher, and leave the timer on longer.

    The 85mm thing, is just a Big Fracking Lie! GRRR!
     
  11. Steph

    Steph No longer a newbie, moving up!

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    On that we agree.

    I don't quite understand this and how it relates to the maximum aperture of a lens...:scratch:

    Not in terms of field of view. A 50mm on a APS-c camera has the same field of view as a 85mm on a full frame camera.
     
  12. patrickt

    patrickt TPF Noob!

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    Wow, a salesman told you something that wasn't correct. Amazing. And, it must have been a lie.

    A 50mm lens made for a 35mm film camera has a focal length of 50mm, a FOV equal to a 75mm, and the aperture is the same.
     

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