Aperture on crop

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Peter
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Where do you all stand on the affect on aperture when a FF lens is used on a crop body. I totally get the part that the DOF is affected when a FF lens is attached to a crop body, but often hear arguments for and against the aperture changing. Obviously using the FF lens on both a FF body then a crop body will yield two different looking images, but say you cropped both pictures down to a size where they looked like identical images, assuming the same focal point is selected on both. Does this now give you say an F2 aperture on both final images or does the original crop body image now display a F3 or 3.2 (depending on crop factor). OK I can already sense people thinking why the hell does it matter or thinking just get the correct equipment for what you want to achieve, but I have been party to an ongoing dispute between a couple of photography friends and I was just curious on what others think. o_O
 
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Jonathan
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<Sigh> It makes no difference at all whether the lens is FF or 'crop', the aperture is what is marked on the lens. Neither does the depth of field change. All the crop camera is doing is recording a smaller section of the image circle. The physical characteristics remain the same.
Do you really think there's going to be any difference in the image produced by, say, Sony's 50mm f1.8 E lens compared with the FF 50mm f1.8 FE when used on the same APS-C body?
 
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Peter10d
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Peter
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It makes no difference at all whether the lens is FF or 'crop', the aperture is what is marked on the lens. Neither does the depth of field change. All the crop camera is doing is recording a smaller section of the image circle. The physical characteristics remain the same.
Do you really think there's going to be any difference in the image produced by, say, Sony's 50mm f1.8 E lens compared with the FF 50mm f1.8 FE when used on the same APS-C body?
Thanks for your reply and I agree with you that based on your example there will be no notable difference in the image produced. I am also aware that my original question is similar to pixel peeping and has no real outcome of the final image, however I am just curious on the physical aspects of what is happening.
By the way if you were to search YouTube, there are many so called experts saying that the aperture is affected when using a FF lens on a crop body.
 
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Jonathan
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So much for YouTube......
The thing is that there are no physical aspects as nothing is happening. As we've had to say countless times here, a lens's physical properties don't magically change depending on the format on which it's used.
 
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Jason
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Indeed the glass is the same whatever size thing you project it on to. The focal length is physically and demonstrably the same absolute focal length, the width of the aperture is again demonstrably and measurably the same therefore the ratio between them (f number) is the same.

so called expert hits the nail firmly on the head.
 
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sirch

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Chris
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I wonder if some of the confusion here is related to terminology? f-stop is related to depth-of-field so perhaps when people are saying "aperture change" they are thinking about depth-of-field change. Of course the DoF doesn't change either for a given focal length at a given aperture but on a crop sensor for the same field of view a shorter focal length is needed which then does change the depth-of-field.
 
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Phil
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Do you really think there's going to be any difference in the image produced by, say, Sony's 50mm f1.8 E lens compared with the FF 50mm f1.8 FE when used on the same APS-C body?
Mathematically correct.
Photographically incorrect.
DoF is indeed fixed to focal length - but if we use a different size sensor - we need a different focal length to take 'the same shot'. So the Sony APSC camera will need a longer focal length lens to take the same shot as the 50mm on the FF camera.

You can check this with any DoF calculator, increase the focal length to keep the same subject frame with the smaller sensor and you'll see a greater DoF.

In simple terms a 35mm focal length lens on a 5x4 camera is an extreme wide angle and commonly needs f64 or thereabouts to produce reasonable DoF, a 16mm lens on FF would need F16 and a phone camera would give similar at about f4

edit to add - the iPhone 11 pro has 3 focal lengths 2.87mm (ultra wide) 4.25mm (wide) and a 6mm 'telephoto' that's actually closer to what we'd call a standard lens.

Put a 6mm lens on a FF camera and focus 5ft away and you'll have front to back DoF just like you have on the iPhone - but you won't have the same image.
 
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Steven
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The easiest way I can explain it is to think about using a FF lens on a FF sensor to photograph a white wall. This creates a 35x24mm image (digital negative) that is uniformly illuminated.

You then crop this result to some smaller size. It does not matter if it is done using a crop sensor, or in post editing; you are discarding image area that contains light that was transmitted by the lens aperture. This is equivalent to having used a different lens, that would not require cropping, at a smaller aperture. 1 stop smaller for DX, 2 stops smaller for 4/3, etc (i.e. 4/3 is 1/4 the area remaining from the FF original; 1/4 light is 2 stops less).

Simultaneously, instead of having a 35x24mm image area you are left with 24x16mm for a DX size crop, or 17x13mm for a 4/3 size crop. Again, it does not matter how the crop was done. For any equivalent image output size the smaller image area has to be enlarged/magnified more... this is exactly the same as zooming into the FF image to 150% (DX) or 200% (4/3). And this greater enlargement/magnification makes any lack of sharpness more apparent, which is exactly what having less DOF means.

So if you change nothing but increase the crop/crop factor you wind up with less light and less DOF... but different images (image area remaining).

If instead you want the same image remaining you have to back up; 2x the distance if it's a 4/3 sensor, and the Inverse Square Law (ISL) dictates that due to light spread over distance 2x the distance equals the same 2 stops less light intensity.
However, greater distance (and shorter FL) has 2x the effect on DOF that aperture does; because it makes the recorded details smaller, which makes them appear sharper. This is exactly the opposite of increased magnification and results in a greater recorded DOF.
Alternatively you could use a shorter FL on the crop body, which again reduces the size at which details are recorded and increases the relative DOF. And if the same aperture # is used the total light is gain reduced due to a smaller aperture opening; i.e. f/4 = ~12mm diameter for a 50mm lens, and ~8mm diameter for a 25mm (4/3 crop). 12mm diameter is 113mm in area, and 8mm diameter is 50mm in area... 2x less area = 1/4 light = 2 stops.
 
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Phil
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I'm not a big landscape shooter - I'm a people photographer.

On my 6d my favourite lens is the Canon 135 f2L. On my crop Canon's the 'equivalent' lens is my 85mm 1.8, taking the same shot wide open on both cameras, the crop camera gives marginally more DoF despite the fact it's actually a half stop wider aperture. Here's the data from a DoF calculator for both cameras focussed at 6 feet

7d 85mm at F1.8
Near limit 5.95 ft
Far limit 6.05 ft
Total 0.1 ft

6d 135mm at F2
Depth of field
Near limit 5.97 ft
Far limit 6.03 ft
Total 0.07 ft

To achieve the same DoF on the 7d I'd need the 85mm f1.2 lens
 
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Steven
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I should say that you asked about the physics of what is happening, and I tried to explain it simply/clearly.

But all of this "equivalence" stuff is largely pointless IMO... there are simply too many factors that make generalized comparisons nearly pointless. And do you really know what the final composition/viewing size/viewing distance is going to be? Do you have any control over that? If not, then you don't know what DOF is required nor what it will be (relative magnification to the viewer).
And F-stop is not exactly the same as actual light transmission (T-stop) which varies between lenses. Different cameras may/probably use different exposure basis (ISO standard/ISO offset). And different cameras/sensors may/probably have different efficiencies, especially between generations/technologies.
 
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