Full Frame FX vs Crop sensor DX - DOF

Full frame vs DX for DOF

A lot of online posts talk about how the key point behind FF is low light ISO performance, followed by Depth of Field, but no one has properly shown it side by side with examples.

It was this talk about subject isolation & DOF that really got me interested in FX in the first place.
So I asked myself the key question:

If I take the same exact photo with the 'equivalent focal length lenses' (eg 50mm on FX & 35mm on DX) , then how much more shallower DOF will I actually be getting with the Full frame camera?

After doing a fair bit of research into considering whether Full Frame was my next big step, I realised that the only way to know the answer would be to buy one & then publish my results side by side.

The article is still in progress with photos etc but Ill publish it as the outcome is quite clear.

Short Answer:

If you don't want to read the whole thing but want my confirmed opinion based on my photos, here is my verdict:

If you are an experienced amateur or semi-pro, FX has a significant advantage for subject isolation & DOF to create a smooth bokeh. We're not talking small difference, we're talking game changer difference. FX wins big. I illustrate the benefits by placing multiple side by side photos in the article below. But if you don't care about the shallow depth of field, then there is not enough benefit in moving to FX.

If you are a new comer to DSLR world I actually recommend DX because of this feature.

 The limited depth of field in FX needs control. If you buy a f1.8 prime lense & just leave the camera sitting on Program mode & are taking family pics, the camera will use f1.8 for every shot to compensate for the low light, so it can cause a lot of your subjects to be out of focus, or one eye in focus & the other blurred etc, even if they are slightly tilted.

If you are new to photography, get a DX camera, there is more than enough subject isolation possible for a beginner even on DX until you improve your aperture selection.

Other Observations:

ISO performance is better in my D600 than any previous camera, but having used a D7000 before, I don't consider ISO performance to be the key point of a Full frame camera. ISO performance & low light benefits in newer DX cameras are very comparable to equivalent class FX cameras & don't justify spending double to move to FX.

If you want super sharp beautiful portraits & exceptional detail in your images at equivalent resolutions to the DX camera, then FX helps. Ie, a 24MP FX will beat a 24MP dx in matters of detail & dof.
If you don't need insane subject isolation & own good lenses on DX, then from the cost stand point there is no other benefit moving to FX. Image Quality on both systems is equal when you compare equally well made bodies with good sensors. IQ doesn't include DOF. IQ refers to mainly things like low light performance & noise. Yes the FX is marginally better, but not significant enough to make a move. If you don't need dof control, don't bother moving to FX.

Objectives of Experiment:

This article focuses on the DOF aspect of the 2 formats, & not necessarily on ever single aspect of the image difference, because there is a significant different between my D300s & D600 in terms of what generation their sensors are etc.

We know for a fact that within reason, not manufacturer has made 2 cameras that are equal with only the limitation of exactly the sensor. Nikon came close to it by making the D7000 & D600 with same senor & AF system, but different number of pixels. Then when they upped the pixels in D7100, they changed the sensor & AF system.

So, lets be very clear about the fact that we will never be able to truly compare apples with apples in regards to image quality, & that no manufacturer will ever make 2 cameras where we can easily prove one format better over the other, because that would hurt their own marketing.

But what we can compare with great ease is the DOF, which is my main area of interest behind putting up this post.

About the Comparison Images

We did the test with primes & zooms.
First with prime lenses set to aperture at 1.8 on FX as the baseline, & then check what the aperture did on FX with the equivalent focal length on DX. Ie, comparing a 50mm on FX to 35mm on DX
And then with zoom lenses set to 2.8 on DX. Comparing a 17-50mm f2.8 to a 28-70mm f2.8.

We start dropping the aperture down on the DX until the DOF looks similar to that set on the FX.

I used the best possible lenses I could afford, but even if they weren't the sharpest, that wouldn't matter so much as all we are focusing on is the DOF differences between the 2 systems.

All images taken at iso 400 aperture priority.
Focal point is always dead centre, which is the black plastic piece in centre of frame.

Prime lenses & large apertures:

I used a Nikon 50mm F1.4D on D600 FX & Nikon 35mm F1.8G on D300s DX.
These are the best lenses Nikon makes for DX & FX respectively.

D600 50mm at f1.8 vs D300s 35mm set to different apertures in every shot

Test 1 50mm f1.8 vs 35mm

The D600 at f1.8 is the clear winner, & you will see this pattern in every single photo. The FX will have a shallower DOF.

 

Test 2 50mm FX at f1.8 vs 35mm DX set to f2.8

 

 

Test 2 50mm FX at f1.8 vs 35mm DX set to f3.2

Common question - Should I buy the best DX or the cheapest FX eg Nikon D7100 vs Nikon D610. Please click on this to read my earlier blog post.

Ramblings & Criticism of some current opinions on the FX vs DX saga

I like to genuinely be neutral & unbiaised when reporting things, I don't have my ego attached to my opinions. And I always change my opinion when a better one is presented.
I also feel that what suits you best is for you as at that point, & could change again.
I was on DX for 10 years before I moved to FX.
I don't care whether something is called DX or FX, what I care about is the findings.

Despite me suggesting that DX is the better option 99% of the time if you don't care about this excessive shallow DOF, I want to address some of the other stuff I have read, which doesn't do justice to either party.

1) The Politically Correct 'Its a choice' Opinion

Many new marketing driven blog posts are now saying its a 'choice'. Ok, you can put it like that, but if its just a 'choice' then there should come a day where FX will cost the same as DX & you'll just 'choose' - but wait, that will never happen, & not because it costs a lot more to make, but because premium cameras will only ever continue to be made in FX to optimise the profits.

If you have all the money in the world & want to buy the best camera AND the best lenses then you have but naturally got the best camera, then you want FX. A big 'AND' above because without the equivalent best FX lenses you are wasting your money, glass before body, always remember that no matter what you buy.

If you cant afford the FX, go for a DX, but don't disillusion yourself into thinking that the DX is somehow 'better' to FX. No its better for you to go with DX as it will save you the dollars as at that point in time, like it did for me, for a whole 10 years I stuck to DX!! But that doesn't mean that FX will not be better.

2 ) DX crop ratio of 1.5x gets me more 'zoom'

Well, to get the said 'zoom', you could take the hires 24MP image you get from your FX camera & crop it as much as you please.
You'll argue that means you're loosing resolution. No, youre loosing 'Megapixels', & you might be thinking that all those extra pixels lost are going to make a worse image, but infact, as we know well, MPs have little to do with IQ. So, the zoom argument in my opinion is moot in todays age of insanely high MPs available in every DX & FX camera out there.

People debate that by putting a 70-200mm F2.8 on DX they are getting the equal of 300mm f2.8
No, they are not. They are getting 300mm at f4.5 because the DOF on DX is less than FX.

By using 70-200mm f2.8 at 200mm f2.8 on a 24MP FX camera, I can then crop it into a 10MP image which will be 300mm f2.8.

Now what would I rather have a 10MP 300mm F2.8 or a 24MP 300mm F4.5??
If you want to follow the megapixel hokum, you're welcome to pick the 300mm f4.5, but any real photographer knows that MP don't mean anything in the real world, where 10MP is more than enough.

3) FX may give me the subject isolation but I loose light so need to push ISO which makes my images noisier.