Dx mode vs real DX?

Kellynikon

I don't use the function often, but I am just curious about this. If you use the DX mode on a fullframe camera it crops a little bit in the center, the effective focal length does not change. However if you do this, do you need to multiply the shutter speed by 1.2 or 1.5 too, or doesn't it matter because it is just a cut-out and not a longer focal length?

spraynpray

When you go to DX mode you use a smaller area of the image circle so the focal length of the lens is effectively multiplied by 1.5. The shutter speed is not affected. Note I said effectively, because the focal length is always really the same, it is just that you are blowing a smaller cropped area of the image circle up to the same size as you would have viewed your FX image.

sevencrossing

As far as exposure is concerned you do not need to change the shutter speed
But as the magnification is effectively being multiplied by 1.5. you may wish to increase the shutter speed to minimise camera shake

heartyfisher

to be exact you don't multiply the shutter speed by 1.2 or 1.5 you go to the power of..
(that is only to reduce camera shake minimal shutter speed is usually 1 / focal length in dx mode the shutter speed value may need to be increased ..)

haroldp

DX mode in an FX camera is real DX.
There is no technical difference whatsoever between a DX sensor and using a DX size crop from the center of an FX sensor. The only difference is that the camera likely cost more, might be heavier, and can be switched back to FX mode when needed.

... H

funtagraph

@haroldp carefull with generalizing: Depending on the sensor there is of course a difference in resolution / pixel density. So, one can say DX mode of the latest FX sensor is like DX sensors one or two generations before they went from 16 to 24 MP

heartyfisher

High ISO 12MP DX ... thats what I like about DX mode on my D610.


> to be exact you don't multiply the shutter speed by 1.2 or 1.5 you go to the power of..
> (that is only to reduce camera shake minimal shutter speed is usually 1 / focal
> length in dx mode the shutter speed value may need to be increased ..)

PS : I was wrong .. I re read your first post .. and yes you multiply the Focal length by 1.2 or 1.5 and use that to get the minimum shutter speed .. sorry ...

Kellynikon

Thanks all, my question is answered

spraynpray

My question to you @Kellynikon is did you understand the answers? I only ask because in my experience is that when that question is asked there is usually a lot of confusing answers offered which only make sense if the person asking the question already knows the answer!

Put simply, the effect is that you have a lens on the camera that is 50% longer than is written on it. Comments about shutter speeds only distract from that simple fact. Chosen shutter speeds vary greatly on the ability of the user to hold the camera still, whether the lens has VR, what generation that VR is etc. etc. so the rough and ready rule of 1/ the (effective) focal length (no slower than 1/75th second for a 50mm on a DX body (75mm effective)) is *very* rough and ready.

Hope that helped.

haroldp

@funtagraph

Nothing in the question or other answers had anything to do with resolution, only with the geometry of the lens / sensor interaction.

So far, DX sensors have come from Nikon in, 2.7, 3, 5.4, 6, 10, 12, 16, or 24 mp. Other than that higher resolution makes all imperfections (motion, focus, aberations, diffraction) more visible, all of the answers apply equally.

Since the image of a DX frame is magnified 1.5 times over FX for any given display/print size, camera motion, focus errors and aberations will be magnified as well. Diffraction is more complicated, but at a given display size will e less visible with a 54mp FX sensor than with a 24mp DX.

In all of these respects, there is no technical difference whatsoever between a DX sensor and using a DX size crop from the center of an FX sensor.

.... H

funtagraph

@haroldp your answer is vaild in the context with the question. If you take the sentence isolated it will islead to wrong conclusions: "In all of these respects, there is no technical difference whatsoever between a DX sensor and using a DX size crop from the center of an FX sensor." And since people use all possible fragments of information, I tend to be careful with generalizing. But of course, you can write whatever you like.

WestEndFoto

@haroldp carefull with generalizing: Depending on the sensor there is of course a difference in resolution / pixel density. So, one can say DX mode of the latest FX sensor is like DX sensors one or two generations before they went from 16 to 24 MP

I don't quite agree with this. It sounds a little like the D4s sensor is behind the D810 sensor because it is 16mp, not 36mp. One is not better than the other, just more suitable than the other for certain scenarios.

For example, the DX portion of an FX sensor will beat a DX sensor for dynamic range.

funtagraph

Sure about that?

Those are different sensors and, referring to you one sensor's not better than the other: Why not? Define the scenario and you get the "better" sensor immediately

According to Senscore.org a D7100 Sensor got 955 points in dynamic range - not much of a diff to D810 with 1041, but a pretty huge difference to D4s with 1391. And they didn't compare DX mode of the FX sensors So if you want to see the diff between DX sensor and FX sensors in DX mode, I don't reckon much of a big difference.

WestEndFoto

The DX portion of an FX sensor will have the same dynamic range as its FX portion.

The larger sensor sites will perform better. I think the sensorscore numbers support that.

squoop

Given the same pixel density of the FX and DX cameras, an FX in DX mode vs a DX camera will have the same field of view. There is no actual increase in focal length for either.... just a crop resulting in a field of view perspective that is the same as it would be for a 1.5 greater focal length. But unlike an actual increase in focal length with an actual longer lens, with DX (or DX mode) you end up with proportionately less resolution, just like you would get if you cropped it yourself. As for shutter speed that is irrelevant to any of this, other than what @sevencrossing said. One difference though in comparing FX in DX crop mode vs DX body, is that even if they have the same pixel density, you will have about a 1-stop advantage with the FX due to greater light collecting ability.

sevencrossing

One difference though in comparing FX in DX crop mode vs DX body, is that even if they have the same pixel density, you will have about 1-stop advantage a with the FX due to greater light collecting ability.

spraynpray
My question to you @Kellynikon is did you understand the answers?


Well I certainly don't understand how you get a 1-stop advantage

Kellynikon

Yes I understand. My question was do I need a higher shutter speed (for camera shake) and the answer is yes

spraynpray

@KellyNikon: Great. Now I strongly suggest you stop reading this thread before others chip in and start the usual conversation about crop sensors not magnifying images etc. etc. You'll see.

squoop

One difference though in comparing FX in DX crop mode vs DX body, is that even if they have the same pixel density, you will have about 1-stop advantage a with the FX due to greater light collecting ability.

spraynpray
My question to you @Kellynikon is did you understand the answers?


Well I certainly don't understand how you get a 1-stop advantage

Quote from Thom Hogan:
"....all else equal, FX probably gives you another stop of high ISO capability over DX. By that I mean that--assuming everything has been managed perfectly equal, which isn't always the case--when we make a 24" print from a DX body at ISO 3200 we should probably get visibly indistinguishable results from an FX body at ISO 6400. ..... FX is a better choice for very low light work, while the DX system should hold its own against FX in almost any other amount of lighting."

And since the subject of this thread was "DX MODE VS REAL DX", despite the actual OP question I thought that fact was relevant to mention.

As for expounding on the crop explanation, that was more for the benefit of so many of those who continue to believe DX actually gives you greater magnification. It's a pet peeve of mine, sorry!

And as for shutter speed, unless you WANT it to be low (ie to slow down waterfalls for the smoothing affect) you normally want it to be as high as possible for the aperture you want to use, irregardless of focal length....the less motion blur from camera shake, the better. The best and easiest way to accomplish that, IMHO, is to set up auto-ISO for the range you want (maximum being what you can live with IQ-wise) and use aperture priority mode. That way you will always get the fastest shutter speed for the current light situation, for the aperture you desire, and within the ISO range you desire.

haroldp

Thom Hogans analysis is correct (it always is) , but presumes the full FX sensor being used.

An FX sensor in DX crop mode is a DX sensor in every respect, and identical in every respect to a DX sensor of it's density and construction including light gathering ability. There is no ability to tranfer photons from the 'masked' part of the sensor.

This is very simple concept and while I appreciate the motivation of wanting to cover all bases, all of the 'what ifs' do not add clarity, they reduce it.

An FX camera in DX crop mode is a DX camera period.

And if you like your DX (or FX) camera you can keep it.

.... H

WestEndFoto

Agreed Haroldp.

squoop

@WestEndFoto

If you agree with @Haroldp including his statement, "An FX camera in DX crop mode is a DX camera period." then please explain your earlier statement:

For example, the DX portion of an FX sensor will beat a DX sensor for dynamic range.

@Haroldp If my info on that one point was wrong then I stand corrected and thank you. I should have researched it further before posting, had been told it by another whose knowledge I respect. I had added it as an afterthought to my post which was otherwise not about that. I hope the rest of my post was helpful either to the OP or someone else reading, and did not reduce rather than add to clarity.

WestEndFoto

Hi Squoop, I can see how that might cause confusion. I am not comparing apples to apples. FX photosite sensors tend to be larger and more efficient than DX sensors because the individual sensor design is different. For example, pixel densities tend to be higher.

If you take an FX sized sensor with a DX portion cut out of it and installed in a DX camera, you will achieve exactly the same result as shoot the same FX sensor in an FX camera in DX mode.

Hopefully that clears things up?

squoop

Thank you WestEndFoto for your explanation. But wouldn't having larger and more efficient photosite sensors translate to greater light collecting ability, even in crop mode, than that of the same cropped area of a DX (assuming the same pixel density etc) ?

WestEndFoto

It would result in the same efficiency gain for DX or FX in crop mode, assuming the only difference in the sensor is the size (eg. The photosite are the same in both.)

sevencrossing

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Quote from Thom Hogan:
"....all else equal, FX probably gives you another stop of high ISO capability over DX. By that I mean that--assuming everything has been managed perfectly equal, which isn't always the case--when we make a 24" print from a DX body at ISO 3200 we should probably get visibly indistinguishable results from an FX body at ISO 6400. ..... FX is a better choice for very low light work, while the DX system should hold its own against FX in almost any other amount of lighting."

haroldp

Thom Hogans analysis is correct (it always is)

Thanks for explaining that
I had always assumed f stops related to lenses not cameras
eg a f2.8 lens has a one stop advantage over an f4 one