Help me understand FX vs. DX

[Horoscope Fish]

True, but, what about wildlife in low light?

At some point I think it's a good idea to stop asking yourself, "What gear do I need to get the shot I want?" and start asking yourself, "How do I get the shot I want with the gear I have?"

I'm not a wildlife shooter, per se, but I have shot some wildlife. I'm not a BIF shooter, per se, but I've shot some BIF. I'm not one to shoot strictly in low light, but I find myself shooting in low light frequently. I don't consider myself a portrait photographer, but I shoot a certain amount of portraiture. All that being said, if I had to choose one format, I'd choose FX. I wouldn't like having just the one format but again, if it came down to one over the other, I'd choose FX.
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[mikew_RIP]

I only have the D7200 now,the 750 would not cut it in the birding situation ime in most of the time,24mp on DX is a good starting point and leaves room for cropping,the FX will be better in low light so its a swings and roundabout situation,to cover all bases you need both.
Posted this in another thread,ISO 6400 on the D7200 i would expect FX to walk all over it but it will do me,its the trade off i get for having good cropping on the small birds.

 

[J-see]

I'm

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.

If you get a D810 you have one cam that will do about everything. It has most megapixels which allows more cropping and thus is very close to the D7200 and has about the same signal quality and thus low light ability as the D750. It also has no AA filter (unlike the D750) which combined with the 64 ISO makes for a very sharp shot.

But it comes at a price.

Also keep in mind that if you want to get the most out of an FX, you'll also have to seriously invest in its lenses.

 

[Stoshowicz]

To make two matching photos, where a face fills the frame, the sum of all magnifications is basically the same for those final products.
Im told Larger sensors have a bit less noise due to spreading out of the photo sites, (but they cost 'exponentially' more.)
The smaller sensors are actually saturating at a lower ISO than the full frame , ( a big 'bucket' takes more photons to fill ) so one shouldnt compare
what they call 'equivalent exposures' to compare ISO related noise levels. One needs to compare based on the amount of incident light captured.
The efficiency ( how much incident light actually translates to digital signal ) of sensors is more complicated than just pixel size or spread.
Summary
I dont have the D810, its outside my budget, for indoor work it may perform better for 2500 bucks, with slightly less noise, than the 7200 costing about 1000 bucks ,it also may have better buffer capacity . Outside , with better light levels the 7200 is lighter to carry and I can replace it, if I drop it in a swamp or something.

 

[Horoscope Fish]

If you get a D810 you have one cam that will do about everything. It has most megapixels which allows more cropping and thus is very close to the D7200 and has about the same signal quality and thus low light ability as the D750. It also has no AA filter (unlike the D750) which combined with the 64 ISO makes for a very sharp shot.

But it comes at a price.

Also keep in mind that if you want to get the most out of an FX, you'll also have to seriously invest in its lenses.

Agreed: If there is "One Body to Rule Them All and in the Darkness Bind Them", it is the D810.

 

[480sparky]

True, but, what about wildlife in low light?

Again, no one simple answer.

You can either use an FX for it's low-light performance and pony up for a top-notch 800mm, or accept an affordable DX with a wallet-friendly 500mm.

 

[mikew_RIP]

12800 iso on the D750 but i would rather have MP and reach

 

[Chris@sabor]

I am dense so....

Light is diminished by focal length, correct?

So, at longer focal lengths it would seem like the fx would be able to shoot a faster shutter at lower ISO, no?

 

[480sparky]

I am dense so....

Light is diminished by focal length, correct?

So, at longer focal lengths it would seem like the fx would be able to shoot a faster shutter at lower ISO, no?

No. Focal length has nothing to do with how much light is passed through the lens.

A 100mm f/2.8 lens will pass the exact same amount of light as a 200mm f/2.8, or a 300mm f/2.8 or a 400mm f/2.8.

However, designing longer focal length lenses and maintaining larger aperture creates larger, heavier and consequentially more expensive results. So to reduce size, weight and price tags, the maximum apertures start to get smaller as you get into the longer focal lengths. This is a function of economy and marketing.

 

[hark]

I am dense so....

Light is diminished by focal length, correct?

So, at longer focal lengths it would seem like the fx would be able to shoot a faster shutter at lower ISO, no?

No. How much light reaches the sensor depends on how fast your lens is. A lens such as an f/4.5-5.6 won't be as fast as an f/2.8 lens.

 

[Chris@sabor]

No. Focal length has nothing to do with how much light is passed through the lens.

A 100mm f/2.8 lens will pass the exact same amount of light as a 200mm f/2.8, or a 300mm f/2.8 or a 400mm f/2.8.

Yes, thank you.
But when you compare dx to fx doesn't the crop factor have to applied to the aperture as well?

 

[RocketCowboy]

Yes, thank you.
But when you compare dx to fx doesn't the crop factor have to applied to the aperture as well?

Nope.

 

[RocketCowboy]

Remember, sensor size isn't changing anything about the actual geometry of the lightbox inside the camera body. The focal length is the same, aperture (f/4, for instance) is still aperture, and shutter speed is shutter speed. The only difference between DX and FX is the size of the sensor and consequently the size of the image taken from all the light that passes through the lens and camera.

Because DX takes a smaller image from the total available light, there is a smaller field of view which is what creates that "effective focal length" or crop effect, but the physics otherwise are identical.

 

[480sparky]

Yes, thank you.
But when you compare dx to fx doesn't the crop factor have to applied to the aperture as well?

No. The only difference between an FX sensor and a DX one is the DX sensor is physically smaller. This dimension has no effect on the lens. If you were able to magically swap sensors in a special-built camera, the lens will blindly pass the same amount of light regardless of which sensor sits behind it. If any sensor at all. Take the camera off the lens, and the lens will still pass the exact same amount of light through it.

 

[mikew_RIP]

There is a theory about wasted light if you use DX with a fx lens but when i tried looking at it i found it too boring and of absolutely no interest but the theory is out there somewhere

 

[Chris@sabor]

 

[J-see]

A sensor pixel will receive light depending upon the time it is exposed and the square surface of that pixel. Thus an FX pixel will receive more light than a DX pixel when shooting the same lens at the same aperture and shutter. But since the luminance value of a pixel is a certain percentage of its full well capacity, the exposure will end up being identical for both. The exposure, not the signal quality.

 

[480sparky]

There is a theory about wasted light if you use DX with a fx lens but when i tried looking at it i found it too boring and of absolutely no interest but the theory is out there somewhere

Well, ALL lenses have 'wasted' light as they project a circular image onto a square or rectangular sensor/film plane. The light is still passed through the lens, but is not recorded. It merely strikes the interior parts of the camera, which are painted black. Some is reflected back into the camera, and some is absorbed by the black coating.

 

[RocketCowboy]

Notice what they did there ... the upped the ISO 2-stops to ISO 800 when the closed down the aperture 2-stops to f/5.6. Or the reverse ... when they opened the aperture to f/2.8 (let in more light), the closed down ISO 2-stops to ISO 200 to accommodate. It's the same equivalent exposure.... they just traded aperture for ISO (or vice versa). Shutter speed remained the same, and focal length does not play a part in exposure.

 

[WayneF]

I am dense so....

Light is diminished by focal length, correct?

So, at longer focal lengths it would seem like the fx would be able to shoot a faster shutter at lower ISO, no?

Actually, no. If at the same ISO and shutter speed, then the same f/stop means the exposure is the same, regardless of all else.

Definition of f/stop = focal length / aperture diameter.
The purpose of f/stop is to keep the light the same.

So comparing say a 100mm and a 400mm lens,

The 400 magnifies size 4x, so the area seen is 1/16 the area (1/4 W x 1/4 H). You seem to be calling that diminished light, however the light per unit area stays the same. Because (to be the same f/stop number), the aperture diameter of the longer lens is 4x larger, allowing in 16x more light (at the same fstop number). The magnified subject area is 16x larger. The light per unit of area (the exposure) stays exactly the same... regardless of 100 vs 400 mm, regardless of FX vs DX, etc.

The entire concept of f/stop is that the exposure stays the same on all lenses set to same f/stop number, regardless. This lets cameras and exposure meters be useful.

Some troublemaker will pipe up and say that not all lenses exactly accomplish this (T-stops, etc). Modern coatings make that normally pretty minor though, and a whole different subject... The concept is that f/8 is f/f8, regardless.