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.

Nikon 35mm F2D vs Nikon 35mm f1.8G vs Sigma 30mm F1.4 vs Nikon 50mm F1.8

The battle of the fast primes

Besides the obvious comparison, this article is also for anyone out who has a 50mm 1.8 on a DX sensor body & is considering an upgrade to a lens that delivers a true 50mm field of view perspective on a DX thats equivalent to a true 50mm on full frame FX.

What our contenders look like on the outside:



Sigma 30mm F1.4 EX DC HSM courtesy www.SigmaPhoto.com
 

Nikon 35mm F2/D courtesy www.nikon.co.nz

 

 

 

 

 

Main advantages & dis-advantages of each lens

A very important element around comparing these 4 lenses is to remember that the Sigma & the Nikon 1.8 G are DX only, & mainly designed for the smaller APSC crop sensor cameras. They thus have a built in AF motor so it will work with any DX camera including the entry level D3000/D5000 series nikon bodies.

But it will not be all that great on a FX camera, unless you use crop mode & are happy with the 50mm FX FOV picture angle on an FX camera.

Both the nikkor AF 50mm f/1.8 & the Nikon F 2/D are full frame sensor FX lenses, but will NOT Auto focus on the entry level Nikon DX bodies.

The Sigma is a tank, big & heavy. The nikons are light & easy to carry.

The sigma definitely has the best low light performance & its a noticeable advantage doing handheld low light photos which turn out quite well for subjects like pets & kids. Dont expect perfection out of any of these lenses in low light without some sacrifices.

The Nikon 1.8G is the sharpest wide open, so sharp that it beats the Sigma 1.4s at 1.8!!
I am serious. This Nikon lens is something Canon guys must miss as the performance of this $250 lens is better than some $700 lenses!

If you rely on short minimum focus distance, you should buy the Nikon 35mm F2D, the others are not macro, & definitely stay away from the Sigma. With a minimum focus of 45cm & a finicky focus, that is the worst thing at close distance, but yes, it works.
The 35 G is a good one, almost as close as the 35 F2.

Watch out for...!

If you are on a budget feel free to buy the Nikons used with relative peace of mind, but double check the 35mm F2d to ensure its not from a faulty batch back in 1999-2000 when lubricant leaked onto aperture blades & would randomly over expose your photos, it was nasty. Simple easy check, just slide the aperture actuater lever from the back of the lense & make sure the spring action is fast & crisp. The aperture should open & shut quickly.

Nikons usually last well so once they are checked ok you will have excellent life & re-sale value.

Be cautious buying the sigma used, make sure it doesnt have the usual sigma back/front focus errors or unreasonably low edge clarity / sharpness / contrast ok. Their focus becomes grindy & noisy very fast, ensure it still has plenty life left in it. Otherwise it will be fine.

If buying used, the Nikon 35mm f/1.8 G is the best value prime among the lot, but it has its downsides as explained later in the review.

If you are buying new & dont want to buy used & are on a budget, go with the Nikkor 50mm AF-D f/1.8 Its an absolute no-brainer when it comes to best value. Have a read of my review here for details on why I just love the Nikon 50mm f1.8 lens.

I wouldnt recommend upgrading to the 50 1.4 as its still the same 50mm on dx which is 75mm equivalent & often too narrow on dx, which is the point of considering the 35f2 etc

Sharpness / Contrast:

Nikon 35mm F1.8 G is the hands down winner, even beating the legendary 50 1.8 wide open on DX.
Important to remember this is a DX only lens, unless you want to use it in 'crop mode'.

Colour Rendition:

Both DX Lenses are opposites in this area.
If you want absolutely gorgeous skin tone & beautiful detail in the eyes, the 35 1.8G wins hands down.

If you like a warmer (arguably excessively saturated) image, for low light interiors or architecture, the Sigma is good.

But if you want something extremely plain to say shoot in RAW & then work the saturation in Lightroom, then get the Nikon 35 F2.
I must confess this is so neutral that it didnt impress me. I think Nikon got a bit too carried away in making it so neutral that it almost looks bland!
35mm F2 didnt give me the kind of skin tone image I like for portraits.

Close focus / macro performance:

The sigma is not a macro lens by any measure, & if you do get close, say hi to barrel distortion!
The Nikon 35 f2 is so good its almost a macro lens! It gives you 1.5 times the image you would get from the sigma.
But there there, these are not macro lenses, so I dont like to worry about it. Use the right tool for the job. If you want macro performance, either reverse a 50mm prime lens, or buy a macro lens :)
Saying that, if you dont do much macro, but want a versatile wide prime, the 35f2 is a great choice.

Distortion:

The sigma as expected has barrel distortion, & sadly, its really obvious! Dont attempt shooting straight lines, else this thing will disappoint, what a shame, as the rest of the image quality was quite acceptable for an otherwise cheap F 1.4 DX.
It also has some vignetting, but interestingly its not as noticible.

The Nikon 35 G is marvellous in this regard. It also has surprisingly low vignetting, which is great!

Some say the Nikon 35 f/2 is sharp even wide open at F2, but that was not the case for me.
It also has some vignetting, perhaps a tad more than the sigma when comparing both at F/2, but its ok.
And the vignetting stays even on smaller apertures, thats ok for some portrait photographers, but I personally do not like vignetting & the Nikon 50 f1.8 is a great lense in that matter.

Bokeh:

I am a big admirer of nice bokeh in a lens. Its because bokeh is one of those qualities in a lens that helps you choose between 2 similar products, & of course, its one of the few things that is open to personal opinion too.

I like creamy blurred bokeh with low contrasted edges, I don't like excessively overexposed sharp bokeh like those rendered by many Canon & Leica lenses.
I like this appealing arty bokeh in all my photos, especially portraits, where a distracting bokeh shifts the focus from the main subject.

For me, smooth silky bokeh with a non bright edge looks nice for portraits shots, where the subject isolation is more critical than to have bright highlights in your face. I dont like mild repetitive bokeh, I like very soft edges & shapes to be blown out quite a bit.

The lens I benchmark all my bokeh to is the Nikon 70-200mm VR, a lens that I absolutely adore!

In my opinion, the Nikon 1.8G is the sharpest but sadly the only 1 thing it doesn't do well is bokeh. It forms donut bokeh rather than proper circles, & it tends to be a little too bright to my taste.
This is still the lens I prefer for indoor portraits though as the sharpness in the eyes & skin tone rendition more than makes up for it.

But the Sigma 30mm f1.4 lens is great when you want to photograph people or kidswithout a flash in room light, & want good colour & good bokeh in situation with ambient light & varied backgrounds.

Though sigma bokeh didn't blow me away, I think it is quite well suited among the others.
Its bokeh is smooth, buttery/milky I would say, & its not bright, yet has decent edge clarity

The Sigma did tend to over-expose the bokeh sometimes, but thats also partly due to F1.4 & metering against a close subject with lighting levels being very a little different between the subject & the background. Still, I noticed the Sigma does tend to over-expose its bokeh quite frequently.

The Nikon 35 f/2 D is a very neutral lense in almost every respect. Its rather unimpressive bokeh is inline with its neutral performance. I think the bokeh is characteristic of any other lens thats versatile enough for every need & is designed to be sharp, above everything else.
The boken of the 35 f2 was really not to my likeing, & I think my Nikon 18-200mm VR does a better job even at f3.5

I would call Nikon's 35 f2d bokeh a tad bright, with low impact & lack of saturation. It had good edge clarity but wasnt contrasty enough to my taste, the sigma won out for me, but not by much.

If you dont care about bokeh, the 35 F1.8G will blow you away.

Final recommendations:

If on DX & don't intend to upgrade to FX soon, get the Nikon 35mm f1.8 G - hands down overall winner. Love it!
It is now my favourite lens, & the only lens I would consider carrying if I couldn't take my 18-200mm VR.

If on DX & bokeh is critical, then a used Sigma is the best features for price & has the best field of view, very versatile. But I wouldnt say that the extra stop is NOT the main point, other factors are important. Tread carefully, & if in doubt, get the Nikon G.

If going to upgrade to FX, then 35 F2D is a good choice. Great lens in almost every situation. But if on FX, strongly consider the 50 F1.8. A50mm on FX is pretty decent.

If on DX, & on a tight budget, buying new or used, the 50mm F1.8 is great.
If you were going to buy the 50 1.8 brand new though, I encourage you to instead buy a 35 1.8G used.
If you cant make up your mind & are going to be on DX for a while get the 35 1.8G.
But if youre already considering FX, get the 50 1.8 It will not disappoint. The only downside is the narrow 50mm while you will use it on DX ie 75mm picture angle, otherwise there is nothing wrong with it.