[vodanh1982]

Is the purpose of FF just for shallow DOF only? Is there any IQ different between a crop with fast lens (33.3mm f/2 ISO 3200) vs FF slow lens (50mm f/2.8 ISO 6400) assuming lens have the same quality, sensor has the same technology, DXO standard ISO?

[Adam]

Is the purpose of FF just for shallow DOF only? Is there any IQ different between a crop with fast lens (33.3mm f/2 ISO 3200) vs FF slow lens (50mm f/2.8 ISO 6400) assuming lens have the same quality, sensor has the same technology, DXO standard ISO?

Yes, the main difference is that the FF will generally exhibit much less noise, as the sensor is more than twice as large. Thus, the image will appear to be a bit clearer.

[jatrax]

Is the purpose of FF just for shallow DOF only? Is there any IQ different between a crop with fast lens (33.3mm f/2 ISO 3200) vs FF slow lens (50mm f/2.8 ISO 6400) assuming lens have the same quality, sensor has the same technology, DXO standard ISO?

I have no idea how anyone could answer that honestly.

The 'purpose' of FF is not anything, it is just another sensor size like m4/3 or Aps-C or APS-H or the MF sensor in the 645D. The larger the sensor the less depth of field at a given focal length. Whether that is good or bad is a different story. Larger sensors also tend to have less pixels per square inch (or mm) so they have less noise in some cases. What will happen when the pixel density climbs to the same as that in an APS-C is a good question.

[vodanh1982]

But the crop is at 1 stop lower ISO than the FF. From DxO Mark, the FF is about 1 stop better ISO noise performance.

[jatrax]

Which sensor has the same technology and is available in both Aps-C and FF?

[vodanh1982]

Which sensor has the same technology and is available in both Aps-C and FF?

We just assume it for easy comparison just in case we compare a today crop to a 2 years old FF sensor.

I would like to add "4/3 sensor 25mm f/1.4 ISO 1600" to the comparison also.

[jatrax]

We just assume it for easy comparison just in case we compare a today crop to a 2 years old FF sensor.

Then you are comparing apples to oranges and no valid conclusion can be reached. Only generalities.

[m42man]

...The larger the sensor the less depth of field at a given focal length...

We need to be careful here: for a given focal length, you get more depth of field the larger the sensor. However, for a given angle of view, you get less depth of field the larger the sensor.

Regarding APS-C versus FF in general, for a given technology, a FF sensor has bigger pixels and therefore lower noise, so can operate to higher ISOs. Also, the bigger pixels means that fewer demands are made on lens resolution (though the lens needs to project a larger image satisfactorily).

[Laurentiu Cristofor]

Is the purpose of FF just for shallow DOF only? Is there any IQ different between a crop with fast lens (33.3mm f/2 ISO 3200) vs FF slow lens (50mm f/2.8 ISO 6400) assuming lens have the same quality, sensor has the same technology, DXO standard ISO?

As usual, the answers can be found by checking this reference article - btw, they are "no" and "yes", respectively (the difference between APS-C and FF is not the 1 stop difference between your sample lenses, but is closer to 1.3 stops).

[snake]

A high end APS-c sensor like in the D7000 and K-5> low to mid FF sensor. Maybe equal to the latter. For proof, try bringing up shadows in the 5DII and III and see what happens.

Good cams, regardless.

[Laurentiu Cristofor]

But the crop is at 1 stop lower ISO than the FF. From DxO Mark, the FF is about 1 stop better ISO noise performance.

Keep in mind that if you use the FF camera at the lowest ISO available to the APS-C one, you can no longer get an equivalent setting on the latter, so you can no longer achieve equivalence. So larger sensor cameras always have an advantage at low ISO.

[vodanh1982]

As usual, the answers can be found by checking this reference article - btw, they are "no" and "yes", respectively (the difference between APS-C and FF is not the 1 stop difference between your sample lenses, but is closer to 1.3 stops).

Nice article. It is the answer that I am looking for.

[RonHendriks1966]

Well with the same image capture the bigger sensor catches more light and thus needing lower iso-setting making a better imgae quality.

[zapp]

Lenses designed for 35 mm mount SLR cameras have to give way for the mirror of a full frame camera. As a consequence short focal length lenses (35mm and shorter) have the rear lens element farther away than the focal length from the sensor. This retro focus design typically results in image degradation and larger distortion for wide angle lenses in SLR cameras. So, it makes huge difference especially for wide angle lenses if you use a wide angle for FF on an APS-C camera as normal angle lens. You also get a lot more glass and have to pay much more money to end up with simialr specs. This is extremely basic knowledge about image quality and THE major drawback of SLR cameras. This is the major reason why Leica M cameras are superior wide angle cameras and why there is a special glass of wide angle cameras for medium and large format film.
A bigger sensor does not save more light or provide better image quality. Only larger individual pixels can gather more light, if the density of pixels is the same, there is no difference. Image quality degrades to the edges and sensor design makes it quite tricky to get decent results in the image corners, so APS-C cameras use the "better" quality area of a lens if it is designed for full frame. Again, this is basic knowledge.
The whole concept of depth of field is also outdated. A cirlce of confusion of 0.030mm cover so many pixels these days that a drop in contrast can be obeserved much sooner - and 100% crops on the monitor do not coincide with the 0.03 criterion. So one could acrgue that a digital APS-C cameras has less DOF than a fullframe film camera at the same angle of view.

Most of the other hints are not inherent to FF sensors, but are specs of some FF sensors.

[vodanh1982]

Only larger individual pixels can gather more light, if the density of pixels is the same, there is no difference

Not true accoding to DxOMark Camera Sensor
Bigger pixel is better than smaller pixel is the myth. Bigger sensor is better than smaller sensor is the truth.

so APS-C cameras use the "better" quality area of a lens if it is designed for full frame.

A FF lens on APS-C sensor will use center area but a FF lens stop down 1 stop also use the same center area.

Because of lens limited resolution, the FF has an advantage when the sensor outresolve lens resolution. (More picture height).
From here
Q: How can a softer lens on a larger sensor resolve more than a sharper lens on a smaller sensor?
A: This point is directly addressed here. On the quick, the following example explains the logic: Consider the Zuiko 150 / 2 on 4/3 and the Canon 300 / 4L IS on 135, which are equivalent lenses on their respective formats -- that is, both have the same AOV and maximum aperture diameter. The 150 / 2 tested at 49 lp/mm wide open, whereas the 300 / 4L IS tested at 36 lp/mm wide open. Since the 4/3 sensor is 13mm tall, and the 135 sensor is 24mm tall, these figures translate to 49 lp/mm · 13mm/ih = 637 lp/ih for the 150/2 and 36 lp/mm · 24mm/ih = 864 lp/ih for the 300 / 4L IS. In other words, even though the 150 / 2 is the sharper lens, the 300 / 4L IS out resolves it on the larger sensor.
Equivalence