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01-03-2020, 05:42 PM   #1
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A question (or two) about crop factor and field of view.

This newb stuff, please bear with me, as I may be incorrect describing something.

I am going to try having fun with an old k-mount 50mm manual lens. I understand how to use it, regarding the steps involved for metering and all that. At least I hope I do. What I am curious about is how the field of view is affected due to crop factor with my old lens. If I understand correctly, the light that enters the lens is providing an image a fair bit larger than the aps-c sensor on my K-50. If that is correct, when I look through my viewfinder, do I just guess how much of what I see will be available to the sensor?


I should add that when I turn on the K-50 it prompts me for length, and I enter 50, which seems correct to me (the lens is not magically changing diameter).

Once again, this is a newb question, so I apologize if I am saying this all wrong.

My thanks in advance to any who might shed some light on my confusion.

01-03-2020, 05:46 PM - 2 Likes   #2
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Don't worry about it. In fact, don't even think about it. Everything you see in the viewfinder will available to the sensor.

It will be a tighter field of view than a 35mm lens, and a wider field of view than a 70mm lens. Just enjoy it.
01-03-2020, 05:46 PM   #3
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What you see in the viewfinder is what the sensor will see. And it's still a 50mm lens, which is a physical property of the lens, so just enter the focal length as 50mm as that's what it is. In fact, don't think about any of this equivalence stuff, it will make life simpler.

Though if you choose to think about it, the 50mm lens on an APS-C sensor will have the same field of view as a 75mm lens on a FF sensor. But it is still a 50mm lens, not a 75mm lens now. The physical properties of the lens do not change with whatever sensor you have.
01-03-2020, 05:46 PM - 1 Like   #4
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QuoteOriginally posted by Schick Quote
when I look through my viewfinder, do I just guess how much of what I see will be available to the sensor?
What you see through the viewfinder is exactly what the sensor sees. The only difference is that if you mounted the same lens on a full frame camera, you'd see more.

The animation on this page may help explain exactly how this works:
The Crop Factor Unmasked - Articles and Tips | PentaxForums.com


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01-03-2020, 06:04 PM   #5
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great advice so far

so to confuse matter -

the full size sensor is the same size as what is/was found on a SLR camera a certain size rectangle shape

the ASP-C sensor is smaller

so the same amount of light enters but a smaller amount is captured by the smaller sensor and the rest of the light falls outside of the sensor

think rectangles

the full size sensor is bigger, the smaller sensor fits inside the larger

you lose the edges compared to the full size sensor

Last edited by aslyfox; 01-03-2020 at 06:10 PM.
01-03-2020, 06:17 PM   #6
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QuoteOriginally posted by aslyfox Quote
great advice so far

so to confuse matter -

the full size sensor is the same size as what is/was found on a SLR camera a certain size rectangle shape

the ASP-C sensor is smaller

so the same amount of light enters but a smaller amount is captured by the smaller sensor and the rest of the light falls outside of the sensor

think rectangles

the full size sensor is bigger, the smaller sensor fits inside the larger

you lose the edges compared to the full size sensor
So to cap it all off: A crop sensor sees a crop of the image from a full frame sensor.

And to add to Allen's info:
FF sensor is 36x24mm
APS-C sensor is 24x16mm
01-03-2020, 06:46 PM   #7
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lens with a known focal length at x aperture lets in y amount of light

that light falls onto the sensor

the larger the sensor, the more of the light is captured on the sensor

so FF sensor, 36x24mm, captures more of the light

than the APS-C sensor 24x16mm

the remaining light falls outside of the sensor

01-03-2020, 06:55 PM   #8
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QuoteOriginally posted by Sandy Hancock Quote
Don't worry about it. In fact, don't even think about it.
Best advice on this subject I have ever read. Really, just forget you ever heard the term "crop sensor", put the lens on and enjoy. Trying to make any adjustments or corrections is just overthinking it.
01-03-2020, 06:55 PM   #9
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The great thing about digital is you can just take a picture and see the result on the screen right after to see what you got.
01-03-2020, 07:19 PM - 3 Likes   #10
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It's already been explained, but I made this diagram years ago for my local club. The lens produces an image. The sensor size only determines how much of it is captured.



Luz = light
Lente = lens
Largo focal = focal lenght

Ismael = me 😀

Thanks,
01-03-2020, 07:28 PM   #11
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QuoteOriginally posted by ismaelg Quote
It's already been explained, but I made this diagram years ago for my local club. The lens produces an image. The sensor size only determines how much of it is captured.



Luz = light
Lente = lens
Largo focal = focal lenght

Ismael = me 😀

Thanks,
I knew others had done better explanations
01-03-2020, 07:31 PM - 1 Like   #12
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I think we should stop discussing equivalence in this thread. It has been done to death already.

The original poster uses a crop camera. As far as he/she is concerned, there is literally no point discussing what a 50mm lens does on any other sensor size.

I have interchangeable lens cameras with sensor areas from 41.5mm2 to 1452mm2, so I think I have a working understanding of equivalence.

For anyone who shoots one format only it is totally irrelevant.

Last edited by Sandy Hancock; 01-04-2020 at 12:13 AM. Reason: Precision
01-03-2020, 07:41 PM   #13
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Back to the OP:

QuoteOriginally posted by Schick Quote
I should add that when I turn on the K-50 it prompts me for length, and I enter 50, which seems correct to me (the lens is not magically changing diameter).
You are absolutely correct here, although (strictly speaking) what you are entering is focal length, not diameter

Also be aware that entering the focal length only affects the camera's shake reduction function, as well as providing the focal length to the images' EXIF data.
01-03-2020, 08:17 PM   #14
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QuoteOriginally posted by Schick Quote
I just guess how much of what I see will be available to the sensor?
All of it. That is the short answer and is also the long answer.


All of the stuff above regarding crop factor and equivalence is irrelevant. It truly is and may safely be ignored. You purchased a single lens reflex camera with 100% coverage viewfinder. As a result, the borders of what you see within the frame will be the borders on the captured image. This is true regardless of what lens you mount.


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01-03-2020, 11:57 PM - 1 Like   #15
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QuoteOriginally posted by Sandy Hancock Quote
I think we should stop discussing equivalence in this thread.
For someone who only shoots APS-C it is totally irrelevant.
I wonder if whoever invented “crop factor” gets a penny for every time it is mentioned in a forum somewhere.
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