Help me understand FX vs. DX

Chris@sabor

Senior Member
Doing lots of research on my next camera...

Most of what I read says that DX is better for wildlife do to extra reach (crop factor). Here is what is puzzling me...

If a full frame camera receives more light at the same f-stop because you have to apply the crop factor to that as well, wouldn't a full frame image taken at the same f-stop and cropped to DX size actually be able to be shot faster and at lower ISO?

or...

If you shoot a DX sensor camera at 600mm f-6.3 (effectively 900mm @ approx. F8) or shoot a FX sensor camera at 600mm f-6.3 and crop to 900mm still at f-6.3 yield better IQ?

I think I have stated the question correctly, please help me understand if I haven't.

Thank You!
 

Horoscope Fish

Senior Member
Doing lots of research on my next camera...

Most of what I read says that DX is better for wildlife do to extra reach (crop factor). Here is what is puzzling me...

If a full frame camera receives more light at the same f-stop because you have to apply the crop factor to that as well, wouldn't a full frame image taken at the same f-stop and cropped to DX size actually be able to be shot faster and at lower ISO?

or...

If you shoot a DX sensor camera at 600mm f-6.3 (effectively 900mm @ approx. F8) or shoot a FX sensor camera at 600mm f-6.3 and crop to 900mm still at f-6.3 yield better IQ?

I think I have stated the question correctly, please help me understand if I haven't.

Thank You!
Would it be accurate to rephrase your question as, "All other things being equal, will zooming and cropping an FX image to DX dimensions yield the same image quality as shooting with a DX sensor?"
 

RocketCowboy

Senior Member
Light is light, so f-stops and ISO aren't specifically related to "receiving more light" in the larger FX camera body. The hole in the front of the lightbox that is either the DX camera or FX camera is the same, the same amount of light enters the lightbox. It comes down to sensor size that determines how well that light can be captured/processed.

In the DX body, the sensor size is smaller. In FX, the sensor is the same size as a traditional 35mm film negative. That size different determines the rest. The DX body with 24 megapixels has all those pixels compacted into a small area that makes the 600mm focal length lens effectively a 900mm focal length. On the FX body, the 24 megapixel sensor has the pixels spread out across a larger area ... so if you crop to effectively 900mm field of view, you're throwing away pixels and consequently IQ. At that point, you're comparing a 24 megapixel DX image to a ~12-14 megapixel FX image.
 

J-see

Senior Member
Taking megapixels out of the equation; the general rule is that for the same quality of signal as an FX you need about twice the amount of light with your DX.

That matters most when there is less light.
 

aroy

Senior Member
Doing lots of research on my next camera...

1)
Most of what I read says that DX is better for wildlife do to extra reach (crop factor). Here is what is puzzling me...

If a full frame camera receives more light at the same f-stop because you have to apply the crop factor to that as well, wouldn't a full frame image taken at the same f-stop and cropped to DX size actually be able to be shot faster and at lower ISO?

or...

If you shoot a DX sensor camera at 600mm f-6.3 (effectively 900mm @ approx. F8) or shoot a FX sensor camera at 600mm f-6.3 and crop to 900mm still at f-6.3 yield better IQ?

I think I have stated the question correctly, please help me understand if I haven't.

Thank You!

There are a few thinks that will clarify the issues

. In general DX sensors exhibit more noise that FX sensors of the same generation, so FX bodies will have better (lower noise) high ISO performance.
. Currently DX sensors have more pixel density compared to the FX sensors.
. The FOV of a DX sensor is about 1.5 times narrower than that of an FX sensor (24mm width V/S 36mm width). That is same as saying that from FOV point of view the same lens is behaving as a 1.5 time longer lens, which is what has given rise to the "effective Focal Length". For every thing else - DOF perspective etc, the lens is still behaving as its focal states.

If an image of an object is 20mm wide in a sensor (here it does not matter what the size of sensor is, the same lens will project image of the same size on either sensor) then for 24MP bodies the sensor has 6000 x 4000 pixels
- For FX this means 6000/36 = 166.67 pixels per mm
- For DX this means 6000/24 = 250.00 pixels per mm

So a 20mm image will have
- FX : 20*166.67 = 3333.33 pixels
- DX : 20 *250.00 = 5000 pixels

In effect you have increased the resolution of the image by just using a denser sensor. If you use 36MP sensor then there will be more pixels ~ 4100, but to achieve the same number as DX the sensor needs to be about 56MP.

2)
IQ depends on a number of factors, but sharpness (both center and corner) does increase with the first few stops, but then diffraction effect sets in and images become fuzzier. Most of the Supertelephotos are designed to be used wide open or at the most ones stop down. You may not get better IQ at higher stops, but you will nsurely get better DOF which may matter more in certain cases.

3)
Cropping will not have any bearing on IQ, only on number of pixels. Nor does cropping have any bearing on noise. So in your case if you use an FX sensor, you will get lower noise compared to a DX sensor, but what you gain in noise you will loose in resolution (which normally means a longer lens = more expenses). That is a trade off you have to decide on - shorter lens with more noise or longer lens with lower noise.
 

WayneF

Senior Member
Doing lots of research on my next camera...

Most of what I read says that DX is better for wildlife do to extra reach (crop factor). Here is what is puzzling me...

If a full frame camera receives more light at the same f-stop because you have to apply the crop factor to that as well, wouldn't a full frame image taken at the same f-stop and cropped to DX size actually be able to be shot faster and at lower ISO?

False premise. The FX light is NOT greater. Exposure is the same on FX or DX. Because the concept of fstops is that f/4 is f/4, same exposure regardless of focal length or sensor size, be it a tiny compact, or an 8x10 inch view camera. There can be other differences, but NOT exposure (not due to FX or DX).

When we use a handheld light meter, we are NOT concerned with sensor size or focal length (other than of course, we do desire to meter the actual captured area). If using an incident meter, we are totally unconcerned about the camera.



or...

If you shoot a DX sensor camera at 600mm f-6.3 (effectively 900mm @ approx. F8) or shoot a FX sensor camera at 600mm f-6.3 and crop to 900mm still at f-6.3 yield better IQ?

I think I have stated the question correctly, please help me understand if I haven't.

Thank You!

It is not an easy question, and there are various different variables, but the way to bet in general is on FX (because it is already larger). However, FX cost will be substantially greater, and be heavier and more bulky, etc.

Greater enlargement is a detrimental factor, and the smaller DX has to be enlarged half again more just to view at same size as FX.
DX is simply cropped smaller.

However, cropping FX to simulate the same DX crop reduces the FX frame to 41% of the pixels it originally had.
Where the DX would retain all the pixels it was designed with.
The best and fair comparison would be the 600mm on DX and 900mm on FX, to retain all pixels on both.

Greater sensor size could also imply larger pixel areas and lessor ISO noise, but of course this varies with megapixel counts. Not necessarily always true, for example a 36 megapixel D800 and a 12 megapixel DX. But if same megapixels, FX wins on pixel size and low noise.

The 600mm DX reach would only be considered a plus if we are too cheap to buy the 900mm for FX. :) But DX does have to be enlarged more, and might have smaller pixels and more noise.

And of course, the uncropped FX (with same lens on both) provides more field width, half again wider, which is a HUGE plus if you want wide angle.
A 14mm on DX acts like 21mm on FX, but 14mm on FX acts like 14 mm. Seems fair to say there is NO WAY to get that same view on DX. :)

DX has "more reach" only because it is cropped smaller and then enlarged more to same size. It is the additional enlargement that provides any "reach". Any telephoto effect is only because of the additional enlargement needed for DX. But images can suffer from excessive enlargement.

You can see that this way: Zoom any existing image in your editor, which crops the view you see to be larger size, and you see the exact same "reach" increase. It is the crop and enlargement that does it (simply because DX necessarily has to be enlarged half again more to view at same size). But as with cropping the FX, this zoom and crop also reduces the pixel count, which the DX crop could be designed to retain.
 
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Chris@sabor

Senior Member
Thanks Guys!

Just to make it crystal clear...

all things being equal, a D7200 will have better IQ than a D750 at the same focal length?

Lets forget about cost for this exercise.
 

J-see

Senior Member
Thanks Guys!

Just to make it crystal clear...

all things being equal, a D7200 will have better IQ than a D750 at the same focal length?

Lets forget about cost for this exercise.

Not necessarily. For wildlife using a long lens, it will outshoot the D750 because it has the same amount of megapixels but it will never outdo the D750 in terms of signal quality.
 

480sparky

Senior Member
Thanks Guys!

Just to make it crystal clear...

all things being equal, a D7200 will have better IQ than a D750 at the same focal length?

Lets forget about cost for this exercise.


It's still not that simple. While both are 24.3mp cameras, a pixel is not a pixel when it comes to sensors.

Given that you have more real estate in the 750's sensor than the 7200, this means the pixels are physically larger on the 750. This results in better low-light performance. So your question can only be answered in the context of 'how much light?'. In daylight situations, you won't see much difference. But in low light, the 750 will win out.

This is assuming you're using equivalent focal lengths for both cameras. Say, a 500mm on the 7200 and a 750mm on the 750. "All things being equal", and use one lens for both, there's no way to tell whether a cropped image from the 750 will fare against an uncropped shot from the 7200 in the same light.
 

RocketCowboy

Senior Member
Like J-see said ... it really depends on the final product. The D7200 has the same number of pixels as the D750, but compressed into a smaller form factor. The trade off on that is usually more noise, so the D7200 will not have the same high ISO performance that the D750 has. Whether that's an issue in this comparison depends on the conditions of the two shots that are being compared.

Likewise, if the D750's image needs to be cropped in order to match the "reach" of the D7200 (assuming using the same lens on both bodies), that cropping could impact the final result. Using different lenses on both bodies so that cropping is not required, I would expect that D750 to have better IQ ... but at a higher cost.
 

RocketCowboy

Senior Member
I just want to know which will make the best images at the same focal length, taking crop factor into account.

And unfortunately, there is no one single answer to that. It depends on what the subject is, the lighting conditions, etc. I know it sounds flippant to answer that way, but if there was a solid answer that one camera was hands down better than all others, guys like J-see wouldn't need to own three different bodies. :)
 

J-see

Senior Member
I just want to know which will make the best images at the same focal length, taking crop factor into account.

The D810. ;)

I have all three and when it comes to birding or wildlife the D810 and D7200 deliver about the same quality, taking crop factor into the equation. The D750 can't really compete there. But that is only when shooting the long lenses at maximum reach.

But when light gets low my D7200 goes on the shelf long before the other two do.

The question is; what do you intend to shoot and how much light have you available during the year? If you live somewhere sunny with loads of light and shoot long lenses, go for the D7200. If you have long winters or long periods of low light throughout the year, or don't shoot anything requiring long lenses, go for an FX.
 

hark

Administrator
Staff member
Super Mod
Contributor
Thanks Guys!

Just to make it crystal clear...

all things being equal, a D7200 will have better IQ than a D750 at the same focal length?

Lets forget about cost for this exercise.

Just to clarify the light loss thought...there was a comment posted here recently saying a DX is a teleconverter on its own. It isn't. There isn't any light loss.

The sensors between FX and DX are different. If you don't crop the FX, then FX is superior. Period.

BUT...if you take a photo with a 600mm camera on DX then switch the lens to FX without moving closer to your subject, you will need to crop the FX image to give you a comparable view of the image that the DX took.

That's where the image from the FX suffers. Cropping away those megapixels lowers the overall megapixels. If you were to blow up both images to 18" wide, the FX image will need to be enlarged more because it was cropped. You've cut away of the file size by cropping so it is smaller. And because the cropped FX image has to be enlarged to make the view the same as what you see in a DX image, any noise now becomes more noticeable (even though the FX sensors are MUCH better with handling noise).

If you choose to go with FX, there are 2 ways around it. You can get a teleconverter. However, there will be light loss, and because there is additional glass between the lens and the sensor, the image will be degraded slightly. The other option is to buy a more expensive longer telephoto lens for the FX so you won't need to crop.
 

RocketCowboy

Senior Member
I think Don has both cameras, maybe he would be willing to do a test at the longer focal lengths?

Don has a D7100 and D750. Would be similar to testing against the D7200 but not exactly ... the D7200 should have slightly less noise than the D7100 as I understand.

So to take a stab at this from a different angle, what would be the comparable shots to see? Again, I'm trying to be helpful, I hope it doesn't come across as being argumentative or confrontational.

What longer focal length is needed to test? 300mm on both cameras (using something like the 70-300 zoomed out to 300)?
What shutter speed?
What aperture?
What ISO on the camera?

Doesn't @mikew have both the D7200 and D750 and long glass that he could do a quick comparative sample?
 

480sparky

Senior Member
A simple, elegant solution is to shoot both. Use an FX body for when it works best for the subject, and pull out a DX with a long lens for wildlife, sports, etc.
 

Chris@sabor

Senior Member
I'm
The D810. ;)

I have all three and when it comes to birding or wildlife the D810 and D7200 deliver about the same quality, taking crop factor into the equation. The D750 can't really compete there. But that is only when shooting the long lenses at maximum reach.

But when light gets low my D7200 goes on the shelf long before the other two do.

The question is; what do you intend to shoot and how much light have you available during the year? If you live somewhere sunny with loads of light and shoot long lenses, go for the D7200. If you have long winters or long periods of low light throughout the year, or don't shoot anything requiring long lenses, go for an FX.

We were typing at the same time.

Thanks for the real world info!

I know there are so many variables it makes a definite answer difficult or impossible.

Just like telling you what I shoot...low light, hard light, landscapes, birds, BIF, and want to explore more.

I guess asking one or two cameras to do everything is nieve. It makes sense, you don't build a car or table with just one tool.
 

Chris@sabor

Senior Member
Don has a D7100 and D750. Would be similar to testing against the D7200 but not exactly ... the D7200 should have slightly less noise than the D7100 as I understand.

So to take a stab at this from a different angle, what would be the comparable shots to see? Again, I'm trying to be helpful, I hope it doesn't come across as being argumentative or confrontational.

What longer focal length is needed to test? 300mm on both cameras (using something like the 70-300 zoomed out to 300)?
What shutter speed?
What aperture?
What ISO on the camera?

Doesn't @mikew have both the D7200 and D750 and long glass that he could do a quick comparative sample?

Not at all, I appreciate your help!

I like most others am finding that I could always use more reach but also could use better low light capabilities. I am also demanding the highest IQ possible. So if the d750 could give me equal IQ at 900 mm by cropping then I will spend my money on it.
 
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