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03-21-2016, 12:15 PM   #1
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f-stop/aperture differences APS-C vs. Fullframe

Hey friends,

to avoid any more boredom during the wait of the arrival of our latest toy e.g. K-1, I thought about finally asking this question here.
I once saw a video from
, showing that the crop factor of 1.5 has to be applied also to the f-stop number to achieve the same effect in bokeh.

But now I was wondering for quite some time now... the situation I want to shoot is offering an amount of light no matter if I shoot APS-C or full frame. the world gets not brighter but the sensor gets bigger and can capture more light, right? Ok, now I try to articulate my question. will there be a difference in exposure time (can it be shorter) when using a full frame camera while using the same aperture on an APS-C camera shooting the same thing. Can one understand my train of thought here? Will it have a difference since the sensor has a greater area and captures more light?

03-21-2016, 12:20 PM   #2
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You'll have to buy a K-1 and let us know!
03-21-2016, 12:25 PM   #3
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haha, I won't sell my K5II before I could check that... thought maybe someone has more insight and could provide some interesting arguments to discuss.
03-21-2016, 12:26 PM   #4
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Any lens with any sensor (or film) will let in the same amount of light for a given f/stop. That means that the exposure time (shutter speed) will be the same.

03-21-2016, 12:31 PM - 3 Likes   #5
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I think there are multiple threads discussing this already...

:facepaw:
03-21-2016, 12:35 PM - 1 Like   #6
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Shooting the Sony Zeiss FE 35/1.4 ZA wide open on full frame (A7r) for me still yields ghastly results, especially portraits on the fly.

Eyes in focus, nose out of focus. Even mouth out of focus or one eye in focus and the other out of focus even if it looks like both eyes should be on the same focal plane.

The depth of field is so narrow I honestly think such fast lenses are a nightmare on full frame. The temptation to shoot wide open for maxed out bokeh ruins the subject that needs to be in focus.

I'm not very good mind you. F1.2 would be a disaster of yet another magnitude.

I'm hoping my FA31 will be easier to use on the K1, but then it's not really the camera and lens but the clown behind the thing driving it.
03-21-2016, 12:37 PM   #7
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QuoteOriginally posted by i5_david Quote
But now I was wondering for quite some time now... the situation I want to shoot is offering an amount of light no matter if I shoot APS-C or full frame. the world gets not brighter but the sensor gets bigger and can capture more light, right? Ok, now I try to articulate my question. will there be a difference in exposure time (can it be shorter) when using a full frame camera while using the same aperture on an APS-C camera shooting the same thing.
You need the same settings no matter what sensor size to achieve the same exposure. The crop factor on the aperture (or was it the crop factor squared?) is only for the bokeh of interest but not for a correct exposure. Total amount of light increases but so does the surface therefore the light per mm or in stays the same.
03-21-2016, 12:41 PM   #8
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QuoteOriginally posted by i5_david Quote
Ok, now I try to articulate my question. will there be a difference in exposure time (can it be shorter) when using a full frame camera while using the same aperture on an APS-C camera shooting the same thing. Can one understand my train of thought here? Will it have a difference since the sensor has a greater area and captures more light?
No. The shutter speed will be the same at the same aperture (and ISO) regardless of of format. F:2.8 is always equivalent to F:2.8. However, as you will use a longer lens on FF than on APS to get the same angle of view, you must use one stop smaller aperture on the former in order to get the same DOF (if thats what you want). To achieve the same exposure then you either have to shoot at slower shutterspeed on FF or higher ISO (or both) compared to APS.

03-21-2016, 01:04 PM   #9
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Short and simple version:

The F stop never changes at all. Neither does the depth of field, if you compare the same lens.

However, since a longer focal length is needed on FF to deliver the same angle of view as on APS-C, you'll generally have an easier time getting background blur on FF. I.e. a 24mm has less depth of field than a 16mm, yet they both deliver the same field of view when mounted on FF and APS-C, respectively.

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03-21-2016, 04:11 PM   #10
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QuoteOriginally posted by i5_david Quote
Ok, now I try to articulate my question. will there be a difference in exposure time (can it be shorter) when using a full frame camera while using the same aperture on an APS-C camera shooting the same thing. Can one understand my train of thought here? Will it have a difference since the sensor has a greater area and captures more light?
No. Modern sensors don't actually have different levels of sensitivity.
For simplicity lets say you are shooting JPEG (sRGB). Regardless of format the camera settings will be the same. All of you cameras meter for 18% gray or middle gray...... Well actually not all some meter for 12%, but lets stick to 18% which is the traditional number and translates into sRGB brightness of 50% (128,128,128 RGB). All cameras increase digital gain of the captured image to match this metering. This is why off camera meters work. Since the camera is always applying gain to reach a set scene brightness the camera has problems metering for scenes that are overly dark or light (snowy landscape). Its why a snowy landscape looks gray if you don't use exposure compensation.
03-21-2016, 04:47 PM   #11
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QuoteOriginally posted by i5_david Quote

But now I was wondering for quite some time now... the situation I want to shoot is offering an amount of light no matter if I shoot APS-C or full frame. the world gets not brighter but the sensor gets bigger and can capture more light, right?
Right- at the same exposure (f-stop and shutter speed) and assuming you're after the same FOV, the larger sensor will gather more light (and have less DOF.)

QuoteQuote:
Ok, now I try to articulate my question. will there be a difference in exposure time (can it be shorter) when using a full frame camera while using the same aperture on an APS-C camera shooting the same thing. ... Will it have a difference since the sensor has a greater area and captures more light?
Well... sort of. It can be shorter at the same f-stop and FOV, but you lose any noise advantage then because you'd have to boost ISO to maintain brightness, and that would show you the additional noise you introduced by using less exposure.

Also, a quibble on terminology - a wider aperture doesn't give you more 'bokeh', it changes the DOF. 'bokeh' has a distinct meaning from DOF. A scene with exactly the same DOF can have vastly different bokeh quality, it's lens-dependent and sensor size-independent.

Last edited by jsherman999; 03-21-2016 at 05:08 PM.
03-21-2016, 11:58 PM   #12
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QuoteOriginally posted by i5_david Quote
Hey friends,

to avoid any more boredom during the wait of the arrival of our latest toy e.g. K-1, I thought about finally asking this question here.
I once saw a video from Tony on youtube, showing that the crop factor of 1.5 has to be applied also to the f-stop number to achieve the same effect in bokeh.

But now I was wondering for quite some time now... the situation I want to shoot is offering an amount of light no matter if I shoot APS-C or full frame. the world gets not brighter but the sensor gets bigger and can capture more light, right?
For FOV and DOF to be the same between APS-C and FF, you need an FF focal length of f'=f*1.5 and an f-stop of a'=a*1.5. The f-stop is defined as the ratio of the lens's focal length to the diameter of the entrance pupil, i.e. a = f/d, hence for the diameter of the entrance pupil d = f/a = f'/a' = const, i.e. the same total amount of light per time enters the lens and falls onto the sensor. Since the APS-C sensor has a smaller surface, the light intensity, i.e. light energy per time and area, scales with a factor of 1.5^2, so for matching exposure, the FF either needs a slower shutter speed or a higher ISO setting (or a combination of the two) compensating for that. For equivalence of not only DOF and FOV but also motion blur, one would use the same shutter speed as for APS-C and ISO' = ISO * 1.5^2.
03-22-2016, 12:33 AM   #13
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QuoteOriginally posted by Ikarus Quote
... one would use the same shutter speed as for APS-C and ISO' = ISO * 1.5^2.
... which in turn means that, by virtue of the smaller light intensity being offset by a proportionally larger sensor area, that the total amount of light on the sensor will be the same between both formats for a picture with the same shutter speed, same perspective and same resulting depth of field. Thus in turn you will have roughly the same noise level, referred to the entire picture (for assumed equal efficiency of the sensors).

What you gain from a bigger sensor are options. With a bigger sensor you can work at shallower DoF at the same F-stop if your (longer) lens offers it, thus capture greater total amounts of light and this way improve noise level, or be further away from being diffraction-limited when stacking macros etc.
03-22-2016, 01:45 AM   #14
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QuoteOriginally posted by JensE Quote
What you gain from a bigger sensor are options. With a bigger sensor you can work at shallower DoF at the same F-stop if your (longer) lens offers it, thus capture greater total amounts of light and this way improve noise level, or be further away from being diffraction-limited when stacking macros etc.
That's a little twisted. Fact is:
  1. Whatever sensor size you use, you get the same DoF control, that is what equivalency rules of thumb tells us:
    1. on a larger sensor you can gain a tiny bit of less DoF on one extreme end if you want, but at the same time lose the option to get that tiny bit of more DoF.
    2. on a smaller sensor you can gain a tiny bit of more DoF on one extreme end if you want, but at the same time lose the option to get that tiny bit of less DoF.
  2. Diffraction is the same for same DoF, regardless of sensor size, so you gain nothing by using any sensor size. Again just simple equivalency rules of thumb at play.
As for the opening post just think of the following metaphor:


Person APSC has a small garden. Person FF has a large garden. There is a rain downpour for one hour. Certainly the total amount of rain in Person FF's garden is larger. But will it be more wet and muddy? No.

Two different aspects have to be kept separately:
  • total whatever and
  • per area whatever
Both have some effects. Total --> noise, per area --> brightness
03-22-2016, 02:02 AM   #15
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QuoteOriginally posted by i5_david Quote
some interesting arguments to discuss
LOL. This forum is already full of dozens of similar threads. I thought the subject was exhausted, with heavy casualties on all sides ...

But sure, why not have more. Feed the beast.
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