# Crop factor and aperture



## Balinus (Dec 7, 2012)

Hello!

I have a question concerning the crop factor and the aperture of a given lens. I can't remember where I read that but in a review about the Panasonic 22mm f/1.7 lens, the reviewer said that the aperture was the 35mm equivalent of a 3.4. Is it true?

I'm wondering because my brother has a Sony A57 (crop factor 1.5) with a lens that has an apterture 2.8. So, I was looking to have something at least similar beacuse I like the performance in low light of his lens.

Any idea about that?

Thanks!


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## KmH (Dec 7, 2012)

The crop factor is about the size of the image sensor.

Many cameras that don't allow lens interchangeability use 35 mm equivalent focal length and aperture values.

In this illustration, crop factors are shown on the left. Note the smaller the image sensor, the larger the crop factor.






This file is licensed under the Creative CommonsAttribution-Share Alike 3.0 Unported license. - MarcusGR


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## snowbear (Dec 7, 2012)

I've never heard of this so I did a quick test:
With my 24mm f/2.8 on the N90: ISO 200, spot meter, and aimed at a floor lamp.  At f/2.8, the meter zeroed out at 1/640.
I moved the lens to my D40 with the same setup (ISO 200, spot meter) and get the same result. There was a little bit of fluctuation with the  D40, but it was pretty much centered at 1/640.  The D40, having a Sony sensor, is also 1.5x.  I'd have to say it isn't so, at least with this lens.


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## Ysarex (Dec 7, 2012)

When you see equivalent values for f/stops between crop versus full frame cameras they're offering a comparison relative to DOF performance not exposure/light transmission. In terms of low light ability f/2.8 is f/2.8 no matter what the sensor size. However smaller sensor cameras force a smaller magnification image which increases DOF -- the reviewer was comparing relative DOF performance between the two sensor size cameras.

Joe


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## Trever1t (Dec 7, 2012)

I think what is meant but not technically correctly stated is that with FX you will need to be coser to the subject to fill the frame equivalent to DX and therefore the OOF areas are more pronounced. Whereas larger aperture + closer distance = shallower DOF.


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## Balinus (Dec 7, 2012)

Oh I see! Thanks. 

I guess the reviewer was indeed refering to the DOF, as a lot of articles are refering to it.

So, I'll look for either the f/1.4 25mm of the f/1.7 20mm then


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## Balinus (Dec 7, 2012)

snowbear said:


> I've never heard of this so I did a quick test:
> With my 24mm f/2.8 on the N90: ISO 200, spot meter, and aimed at a floor lamp.  At f/2.8, the meter zeroed out at 1/640.
> I moved the lens to my D40 with the same setup (ISO 200, spot meter) and get the same result. There was a little bit of fluctuation with the  D40, but it was pretty much centered at 1/640.  The D40, having a Sony sensor, is also 1.5x.  I'd have to say it isn't so, at least with this lens.



Thanks for the field experiment! Just tried the Panasonic 25mm f/1.4 in-store. Will be perfect to take pictures of my (fast-moving) kids!


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## TCampbell (Dec 7, 2012)

Crop factor has nothing to do with exposure values.  f/2.8 is f/2.8 ... on every camera/lens combination that can do f/2.8.  A focal ratio is based strictly on the lens and not the sensor.

The crop factor has the obvious impact of (1) image size and (2) angle of view at a given focal length.  It has some indirect impact as well... since the angle of view changed, you might stand at a different distance to your subject to achieve the similar framing.  That distance change (given that you're using the same lens) will now effect the depth of field.  So you'll hear people say that a larger sensor can offer shallower depth of field and thus create more background blur.  While it's "true" that you'll see the results, the reason it's true is because it'll change the behavior of the photographer (they will change their shooting circumstances because the larger angle of view allows for it.)  So indirectly, a larger sensor impacts the "look" of the image.  But if the photographer agreed to stand in the exact same spot and not move when going from a smaller crop factor to a larger format camera, you would really just see the angle of view / field of view change but the depth of field wouldn't actually change as long as they use the same lens and focal ratio.

There is no difference in light.  There is some confusion (and I can understand why) that since not "all" the light lands on the sensor (given that it's a smaller sensor) then you must be "losing some light" and ending up with a dimmer image - thus a "larger" sensor should work in lower light.

It turns out physically larger sensor usually can work better in lower light but NOT because of focal ratio.  The f-stop (focal ratio) can be thought of as a "photon density" per given area.    f/2 is a full stop wider than f/2.8.  That means f/2 collects "twice" as much light.   Some people will consider that an APS-C (crop factor 1.5) camera has a total surface area about 1/3rd smaller than a full-frame sensor so the image must be dimmer.  BTW, I've even come across the opposite argument... that a "full frame" camera has to spread the same photons out across a larger area.  Both turn out to be wrong.   A better way to think of it as "photons per square centimeter".  At f/2.8 let's suppose our photon density is 50 photons per square centimeter per second and at f/2 our density is 100 photons per square centimeter per second (clearly I've made up these values).  The fact that the total image size is larger or smaller doesn't change the density of light.  Also the fact that on crop-frame sensor some of the photons are landing on an area above/below/left/right of the sensor and missing the sensor entirely does NOT have any impact on the photons that DO land on the sensor.  

I mentioned a larger sensor usually will do better in lower light, and here's how that works.  It turns out the the sensor is covered with light sensitive points called "photo sites".  The photos sites are behind a colored mask that looks a bit like a 3 colored checkerboard (called a "Bayer mask").  The combination of 4 of these photo sites (2 green, 1 red, & 1 blue) combined create a single "pixel".  But it's the "photo site" that we care about.  These photo sites have a given size usually measured in microns.  It turns out if each individual photo site is physically larger, then more "photons" will land on it.  It gets better saturation of light.  That improved light gathering power (simply because it is physically larger) causes the photo site to more accurately record light and be less likely to create random image "noise".  Physically larger sensors generally ALSO have larger photo sites.  That means they tend to naturally do better in lower light while exhibiting less image noise.  Don't take this entirely as gospel because photo site physical size is merely ONE factor among many that influences noise.  It's still possible to have a camera sensor with smaller photo sites which still has lower noise as compared to a larger photo-site sensor.

Hope that helped.


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## Helen B (Dec 7, 2012)

You can apply the crop factor to both the focal length (for equivalent field of view only) and the f-number (for equivalent depth of field only, at the same camera location and field of view). For example, if you have a crop factor of 2, a 25 mm lens set at an f-number of 1.4 will be equivalent to a 50 mm lens set at an f-number of 2.8 on a full frame camera, in terms of field of view and depth of field.


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## Helen B (Dec 7, 2012)

TCampbell said:


> I mentioned a larger sensor usually will do better in lower light, and here's how that works.  It turns out the the sensor is covered with light sensitive points called "photo sites".  The photos sites are behind a colored mask that looks a bit like a 3 colored checkerboard (called a "Bayer mask").  The combination of 4 of these photo sites (2 green, 1 red, & 1 blue) combined create a single "pixel".
> 
> Hope that helped.



That's not usually the case. One photosite becomes one pixel in most cases.


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