What affect has smaller sensor on a lens? Let's understand it

Discussion in 'Photography Equipment & Products' started by nossie, Oct 24, 2007.

  1. nossie

    nossie TPF Noob!

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    An 85mm lens on a 350D camera.
    It seems to me that many people think that you simply multiply the 85mm by the crop factor and you have a 136mm lens.

    Here's what I think - You don't! Instead of more you get less. Go out and take a photo on a full frame camera with the 85mm lens, print it say 16" x 10", cut 1.6 from the sides leaving the middle 10" X 6.25" of the original size and that's what you get, a cropped photo.
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    | |-----| |
    | |1.6x | |
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    [no expense spared graphic]
    Now if I had used a 136mm lens and would have been 1.6 times further back I would have got the same shot.
    At no time did the 85mm lens turn into a 136mm lens we just simply cut off the sides to give the same viewing angle.
     
  2. newrmdmike

    newrmdmike TPF Noob!

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    m m m m m . . . .


    the effect is that you don't use all of the lens. smaller sensor on same lens will result in narrower field of view, like the cropped photo.

    more importantly to me, you cannot obtain as shallow depth of field as with a larger sensor with the same framing.

    ex.

    5d shooting a headshot at 85mm, standing 5 ft away
    d200 same, but to frame it the same you need to stand further away, but you can't achieve the same depth of field with that framing

    because depth of field is affected by distance to subject.


    anyone correct me if i mis-spoke.
     
  3. Helen B

    Helen B TPF Noob!

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    It is also affected by the degree of magnification of the recorded image, so that offsets some of the increase in depth of field made by moving further away. A rough guide, should you want such a thing, is that if the focal length and the aperture are kept constant the depth of field varies with the square of the distance divided by the crop factor, ie u^2/CF. As the two are related, the depth of field ends up being roughly proportional to the crop factor, for the same size of final image.

    Best,
    Helen
     
  4. nossie

    nossie TPF Noob!

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    Yes this point I left out too... the smaller sensor works the middle of the lens which has a few secondary effects. If you can imagine f1.8 on the 85mm being stopped down to f2.8 then that's hardly going to be noticed on a smaller sensor, not exactly right but generally.

    I'm not wishing to get super scientific here but rather to get rid of the general zooming misconception in favour of people understanding the cropping effect.
     
  5. Helen B

    Helen B TPF Noob!

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    The flip side of that is that the image from the cropped sensor has to be magnified more than the full-frame image, which may mean that the lens' performance appears to decrease. The other, similar, effect is that diffraction becomes predominant at lower f-numbers with a cropped sensor than with full frame.

    Best,
    Helen
     
  6. Big Mike

    Big Mike I am Big, I am Mike Staff Member Supporting Member

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    This has been discussed, to death, in the last few years....and yes, it's not fully understood by most.

    Here's my take on it: It's not really important.

    The 'crop factor' is listed and talked about all the time, and this confuses people who are trying to learn about cameras and lenses...and makes buying them more complicated that it needs to be.
    The crop factor is a comparison to the old standard, 35mm film. However, if the person in question has no preconceived notion of what a lens feels like on a 35mm camera...then why in the world do they need to know that comparison? All they need to know is how a lens looks on the camera that they have or want...which in most cases is a 1.5 or 1.6 'crop factor'.

    Now on the other hand, if someone is familiar with 35mm film cameras and lenses...then the crop factor is important, if they are looking at buying a camera with a sensor smaller than 35mm film.

    DSLR sensors are not a standard size, we have 1.5, 1.6, 2.0 crop factors and we have full frame sensors. So I guess there is a need for a way to compare them, and 'full frame' 35mm film is probably the best thing to use a comparison. But, for the majority of DSLR users...the crop factor is something that they should just forget about.
     
  7. Helen B

    Helen B TPF Noob!

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    I agree with Mike. Having different size 'sensors' is as old as photography and those of us who are used to multiple formats just think in the particular format we are using. I don't need to refer to the equivalent 35 mm lens when I think about 4x5 lenses, for example. I guess that it is like language - when you are learning a new language you think in your language, then translate. Later you think in the language you are speaking. Or you stick to your first language.

    The use of 35 mm equivalent doesn't apply only to digital cameras - many people getting into, say, medium format from 35 mm like to have comparisons with the 35 mm lenses they are used to.

    Crop factor is a useful and simple way of translating many things between formats - angle of view, degree of magnification, depth of field, diffraction. It's a shame that it gets misunderstood.

    Best,
    Helen
     
  8. nossie

    nossie TPF Noob!

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    and then there's... on a full frame sensor I can get a wider angle from my lens and I can crop down if I want to.
     
  9. Garbz

    Garbz No longer a newbie, moving up!

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    No one said you get a 136mm lens. Only that you get a 136mm field of view which is 100% right. If you had used a 136mm lens on a film camera you would be standing in the same spot to get the same photo as the 80 on the APS sensor. No one ever claimed that the lens changed, just that the perceived field of view is the same as 136mm lens.

    As a side note who cares? Work with what you have. If you need a 75mm, FF equivalent, mount a 50mm lens on your camera just like any other camera user who doesn't use 35mm format does. Unless your job and reputation relies on extremely shallow field of view the APS sensors put you at no real disadvantage. Yes diffraction sets in lower? So? You don't notice it till you zoom in at 100% anyway. I've wooed people before with a photo taken at f/16 printed 12x8, no one ever complained about the sharpness because it is still sharp! Diffraction is a much smaller issue compared to buying a crap lens.

    I for one think too many people fret over these issues rather than just going out and taking damn photos.
     
  10. Helen B

    Helen B TPF Noob!

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    You seem to be the only one fretting this issue.

    Best,
    Helen
     
  11. Garbz

    Garbz No longer a newbie, moving up!

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    Errr what? How did you arrive at that conclusion? I'm right with Mike and uou on this topic. Crop factor is a number that describes stuff. There's no use debating about how good / bad it is. It's a different format, nothing more, nothing else. An APS sensor doesn't make a lens any less useful than a 35mm. Especially DX or other reduced circle lenses which wouldn't work at all ;)
     
  12. nossie

    nossie TPF Noob!

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    Well people do say it and that's the point of the thread.
     

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