# Effective iq for same dof on ff vs. crop @ same shutter speed?



## Marsu42 (Sep 19, 2013)

This is a thought experiment I've been wondering about, I hope I understand the basics correctly  ...

Let's say I want to have a certain depth of field and use a full frame and a crop camera with the same shutter speed. When going from crop to ff, I'd need to close the aperture to get the same dof for the same field of view. At the same time, I could raise the iso to counter this light loss with the ff and to remain at the same shutter speed.

Question: Which iq (noise, ...) is better: crop with lower iso or ff with higher iso? When shooting handheld with a macro lens at macro distance, maybe shooting with a crop is even better than ff if a large dof is wanted?


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## ajfotofilmagem (Sep 19, 2013)

I had this dilemma when I use film SLR (even full-frame DOF) and change for digital APS-C. At the time of film, I was forced to close the aperture to F5.6 in groups of people. With APS-C I use F3.5 and I will have the same DOF. So today I do not think of "return" to full frame DOF. Actually, now I use ISO1600 without problems. At the time of color positive film, I was only available until ISO400. I do not need ISO 6400, and I'm fine with APS-C.


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## ajfotofilmagem (Sep 19, 2013)

The gain of quality of the noise is approximately 1.5 stops between APS-C and full frame at the same generation. Diaphragm opening to match the DOF is also 1.5 stops. Therefore, what is gained on the one hand is lost by the other. It is true that at ISO6400 or higher, the advantage of full frame is close to 2 stops, but I do not really need to go that far. Another issue is finding a standard zoom lens for good quality full frame. Canon 24-70 F2.8 II is a dream, but the price is a nightmare. Also the total weight of camera + lens + flash full frame is disheartening to me.


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## Marsu42 (Sep 19, 2013)

ajfotofilmagem said:


> Diaphragm opening to match the DOF is also 1.5 stops.



This is the same for all apertures & distances? Is it different for macro with an even thinner dof?



ajfotofilmagem said:


> Another issue is finding a standard zoom lens for good quality full frame. Canon 24-70 F2.8 II is a dream, but the price is a nightmare.



The Tamron looks fine to me, the Canon crop f2.8 zoom in that range is also rather expensive and not even sealed - so that's not my problem with ff.


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## ajfotofilmagem (Sep 20, 2013)

Marsu42 said:


> ajfotofilmagem said:
> 
> 
> > Diaphragm opening to match the DOF is also 1.5 stops.
> ...


I have not done scientific testing to confirm, but it seems that the difference in DOF versus diaphragm opening, remains valid for any distance, considering a framework evenly. Yes, Tamrom 24-70 seems a good option on the Canon. However, in the APS-C is available a zoom equivalent to 27-88mm F2.8 (which activates focal points of dual cross tipe) and also has image stabilizer. In full frame does not available such a lens. Some will say: Canon 24-105 is longer zoom and captures more photons than 17-55mm. It may be true, but not change the fact 24-105 is not really a F2.8 lens.


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## ajfotofilmagem (Sep 20, 2013)

I'm not campaigning against full frame.  But my personal experience has shown that APS-C is more advantageous for those who need more DOF off most of the time. On the other hand, full frame is more advantageous for those who need to use ISO 6400 for most of the time.


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## rs (Sep 20, 2013)

To calculate the equivalent (in FF terms) focal length of a lens on crop, multiply by the crop factor (1.6). To calculate the equivalent aperture (in FF terms) of a lens on crop, again multiply by the crop factor (1.6). This holds true for both DoF and the total quantity of light delivered to the sensor (the measured aperture eg f2.8 is a unit of brightness, total light is a combination of brightness and area). 

Conversely, due to the larger surface area of a larger sensor (FF is 1.6x horizontally and 1.6x vertically bigger than crop), it can be used at 2.56x as high an ISO as crop to deliver the same S/N ratio. This is of course based on both sized sensors using the same tech - which rarely is the case. 

So shooting a scene at 50mm, f2.8 and ISO 2,500 on crop is the equivalent of 80mm, f4.5 and ISO 6,400 on FF. Shutter speed would of course remain the same.

In other words, if you switch to a larger sensor, feel free to stop down the lens by 1.3 stops and boost the ISO by 1.3 stops should you want to retain the larger DoF you had with crop.


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## ajfotofilmagem (Sep 20, 2013)

rs said:


> To calculate the equivalent (in FF terms) focal length of a lens on crop, multiply by the crop factor (1.6). To calculate the equivalent aperture (in FF terms) of a lens on crop, again multiply by the crop factor (1.6). This holds true for both DoF and the total quantity of light delivered to the sensor (the measured aperture eg f2.8 is a unit of brightness, total light is a combination of brightness and area).
> 
> Conversely, due to the larger surface area of a larger sensor (FF is 1.6x horizontally and 1.6x vertically bigger than crop), it can be used at 2.56x as high an ISO as crop to deliver the same S/N ratio. This is of course based on both sized sensors using the same tech - which rarely is the case.
> 
> ...


Great explanation. In this scenario, I could choose between two options:

Canon 70D + 50mm F1.4 = U.S. $ 1550.
Canon 6D + 85mm F1.8 = U.S. $ 2400.

I seem to have some advantages 70D over 6D except high ISO.


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## Marsu42 (Sep 20, 2013)

rs said:


> In other words, if you switch to a larger sensor, feel free to stop down the lens by 1.3 stops and boost the ISO by 1.3 stops should you want to retain the larger DoF you had with crop.



... and since the advantage of ff vs. crop is said to be about 2 ev (max) you're agreeing that if you need a deep dof using a ff is not even on par with crop, but a disadvantage?


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## ajfotofilmagem (Sep 20, 2013)

Marsu42 said:


> rs said:
> 
> 
> > In other words, if you switch to a larger sensor, feel free to stop down the lens by 1.3 stops and boost the ISO by 1.3 stops should you want to retain the larger DoF you had with crop.
> ...


If one also considers the price and weight of cameras and lenses equivalent. Seems that APS-C is even more advantageous for those who want to keep more objects within the depth of field.


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## Marsu42 (Sep 20, 2013)

ajfotofilmagem said:


> Seems that APS-C is even more advantageous for those who want to keep more objects within the depth of field.



Until someone comes up with a better explanation than the above, I admit I'd have at least expected ff to be equivalent to crop @same dof ... but as also mentioned crop really falls apart @iso3200+ and then it's no match, so you might be safer pushing the dof limit with ff than with crop in dim light.


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## neuroanatomist (Sep 20, 2013)

If you mean 1:1 macro, the 1.6x effect on DoF doesn't apply. The DoF is deeper on APS-C results from being further away to match framing (or using a different focal length). At 1:1, you're at the MFD of the lens, so no change in distance. In that case, the DoF is actually (very slightly) _shallower_ with APS-C. 

Take the example of a 100mm Macro lens shooting a coin at the minimum focus distance for the lens (i.e. 1:1 image magnification). 

The FF will give you a larger FoV at 1:1 magnification. On APS-C, an Indian head gold dollar (a coin smaller than a dime) at 1:1 will just fill the frame. On FF, that gold coin will not come close to filling the frame, but you could put a Canadian quarter next to that little gold coin and still fit both in the frame at 1:1. 

OTOH, with current sensors, the APS-C will put more pixels on target, meaning the same object photographed at 1:1 on both would look bigger at 100% on the monitor in the APS-C image. For the same pixel-level magnification, you'll be further away with APS-C, and have deeper DoF (but that wouldn't be the case, for example, comparing the 5DII to the 20D, since they have the same pixel pitch).


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## Marsu42 (Sep 20, 2013)

neuroanatomist said:


> If you mean 1:1 macro, the 1.6x effect on DoF doesn't apply.



I meant macro as in "very close up" like in the "macro" focus limiter range of the 100L - I very seldom really shoot at max. mag 1:1, too short working distance and too long exposure times for handheld or anything that moves. But for "just" close up macro the shallow dof is also a problem, so I was wondering if the same effect as in other shooting distances apply (i.e. higher iso capability gets nullified by the need for smaller apertures).


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## ajfotofilmagem (Sep 20, 2013)

Marsu42 said:


> neuroanatomist said:
> 
> 
> > If you mean 1:1 macro, the 1.6x effect on DoF doesn't apply.
> ...


As I understand the explanation of our friend Neuro, to fill the frame with a pair of rings (maybe 0.5X magnification), full frame oblige you get closer to the object, and use more closed diaphragm to match the depth of field of APS-C.


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