DX Vs FX Lens Sharpeness

ntz

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Hello,

why is talked so much that using FX lens on DX body will bring issues, for example with sharpeness - eg that DX lenses are sharper (and made to be sharper) than FX lenses ? I am living all my life in the impression, that this topic is absolute nonsense. There are prominent speakers that repeatedly mention (for example controversial Northrup photo-family on youtube) that using FX lenses on DX bodies actually brings more disadvantages in the IQ than using DX lenses on DX bodies ..

Could somebody please share his/her thoughts over this ?

thank you and regarads, ~dan

ps. I've been asked by my other nephew, I've initially told him that it's nonsense that DX lenses are specifically build for DX bodies in the meaning of sharpness, he shown me later several videos from youtube where some people discuss that matter claiming that DX lenses perform better than FX lenses on DX bodies
 
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ntz

ntz

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You've got it backwards. FX lenses are (in general) sharper than a DX lens on a DX body as you're only utilizing the 'best' center part of the image circle.
yeah, I know this ... this is what I've initially told to my nephew ...

problem here is, that obviously there is a lot of confusion about this matter and surprisingly this matter is often discussed over and over .. I don't want to put a links here to youtube, but there are a lot of videos with vlogs, where there are people seriously discussing that ..

some of them claim, that DX lenses are built optimized for to be used on DX sensors with high density of smaller pixels .. My technical knowledge in the area of optics and coating and glasses is limited, but my nephew asked me - what then means, that some glasses are not optimized on high megapixels count cameras and *if* it has something do with this ... personally I do not understand what specifically must have the glass to employ the support of high megapixels chips better than other glass, but he's implying, that DX glasses are made with this "feature" implicitly (because of smaller chips with higher density of smaller pixels) which sounds weird to me ...
 
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480sparky

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DX lenses are designed to sufficiently cover the smaller sensor size. I'd like to know what is done to 'optimize' it for sharpness for sensor pixels with a higher density. If there really is something, then manufacturers would incorporate that same technology into FX lenses and really make them astounding.
 

Ysarex

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I use a lot of FX lenses on DX bodies and have not noticed any such problems. Here's an example where the lens is an FX 90mm macro used on my Fuji XT-2 (DX).

tomatoes

The tomato top (your left) has a visible speck on it just a tad left of center towards the bottom. That's a spider. It has legs and you can see them if you pixel peep. I think that's sharp enough.
 

Trever1t

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It's wrong and not surprising so called experts have no idea what they are talking about to make revenue on YouTube. As stated, simple physics, the FX lens casts a larger ~35mm circle than the DX lens so that the FX lens on the DX body, the DX sensor is only really using the center portion of that circle. This is why you can actually run older lenses such as the 70-200VR1 on a DX body and get super crisp images.

DX lenses are generally made cheaper, from cheaper materials and less lens elements/coatings. A high resolution body will out resolve a DX lens.
 
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ntz

ntz

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thanks much for the answers ... can somebody please try to shortly explain in some simplistic form what is that some lenses are "not-ready-a-high-MPs-cameras" ? What is the difference there Vs the lenses, that support "better" (or properly) chips with high amount of MPs (eg old Vs new, in past was standard 12-16MPs and now the standard is say 24-36MPs)
 

RAZKY

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Hello,

why is talked so much that using FX lens on DX body will bring issues, for example with sharpeness - eg that DX lenses are sharper (and made to be sharper) than FX lenses ? I am living all my life in the impression, that this topic is absolute nonsense. There are prominent speakers that repeatedly mention (for example controversial Northrup photo-family on youtube) that using FX lenses on DX bodies actually brings more disadvantages in the IQ than using DX lenses on DX bodies ..

Could somebody please share his/her thoughts over this ?

thank you and regarads, ~dan

ps. I've been asked by my other nephew, I've initially told him that it's nonsense that DX lenses are specifically build for DX bodies in the meaning of sharpness, he shown me later several videos from youtube where some people discuss that matter claiming that DX lenses perform better than FX lenses on DX bodies
Your question makes no sense. How would anyone here know why the Northrups make such stupid statements??
 

ronald_michael

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No comment! Seems to me like a moot point ! I've used a tamron 10-24 FF on Nikon d3200 with no problems. AF worked fine.
Just my comment.....not looking for discussion.
 

Strodav

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Both situations happen so you need to do your research for a given lens on a specific camera body. I start at DxOMark.com where you can see the ratings of various lenses on specific bodies. Most of the time FX glass, which is generally more expensive, has a higher rating than DX glass, which is generally less expensive, but you will find some instances of DX glass out performs FX glass.
 

ac12

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Which instance(s) do you have in mind, Strodav?

Easy, you stack the deck. Apples to Oranges.

The same way you can put a FX pro grade lens on a DX camera and get better IQ than the DX kit lens.

You can take a DX pro-grade 16-80, and compare it to an older FX consumer grade lens.

It is about optical quality of the specific lens.
 

ac12

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The old saying applies: "a chain is only as strong as its weakest link."

It is down to the optical design.
What the designers are designing for.
The devil is in the details.
DX vs. FX is just one of the variables in the equation.

Example:
I take a pic of a car with a FX camera with a 2x magnification (100mm) lens, and the image on the sensor is 10mm long.
I take the same pic on a DX camera with a 2x magnification (70mm) lens, the image on the sensor is 7mm long.
To get the SAME IQ of the car, that smaller 7mm DX image has to show the same details as the larger 10mm FX image.
To do that, the DX sensor AND lens have to be able to resolve at 1.5x the resolution of the FX sensor and lens.
So if the FX sensor+lens resolved at 40 lines/mm, the DX sensor+lens would have to resolve at 60 lines/mm, to capture the same level of detail.
Sensor: The same 24MP sensor sizes on the D7200 and D750 makes that the same.
Lens: Because the image on the DX sensor is smaller (7mm vs. 10mm), the lens has to be able to resolve more, to get the same detail into the smaller sensor image. So the DX lens has to have 1.5x more optical resolution than the FX lens, to get the SAME IQ.

I shoot a DX D7200, and at one point I was considering upgrading to FX.
Then I got a 70-200/4, and the IQ was clearly better than what I was getting with my 18-140.
That told me that a few things.
1) The resolution of the 70-200 was better than my 18-140, irrespective of the FX vs. DX image circle of the lenses.
2) The IQ of my D7200 was being limited by the quality of the lens, not the size of the sensor.
3) I did not have to upgrade to FX to get better IQ, I just needed to use better quality lenses to get the most out of the DX camera.

Note: The 70-200 is an (optically) pro-grade FX lens.
If I used a non-pro grade FX lens on my DX D7200, I would have gotten a very different result.

With high MP sensors, you need better quality optics to make use of that higher sensor resolution.
If your old low MP sensor can only resolve 30 lines/mm, then a lens that resolves 30 lines/mm is just fine.
But if the new high MP sensor can resolve to 50 lines/mm, but the lens can only resolve to 30 lines/mm, then the lens is limiting the IQ of the camera. The high rez sensor will only show an image at 30 lines/mm, even though it is capable of more. To get the max IQ out of the high MP sensor, you need to use a lens that can match the resolution of the sensor.
In the case of my D7200, the resolution of the 18-140 lens could not match the resolution of the sensor.

Think of it as a modern lens that is designed to 50 lines/mm resolution, or a old lens that is designed to 30 lines/mm.
Or a Pro lens designed to 50 lines/mm resolution, vs. a non-pro lens designed to 30 lines/mm resolution.
Although it is not that simple.

You have to be careful with generalizations and terms.
From an optical POV: you have PRO-grade (high resolution) and non-pro grade (lower resolution) lenses.
Then you have construction/marketing PoV: PRO vs consumer/non-pro.
The new non-pro lenses have better resolution than the old non-pro lenses. Designs improved over time.
Then you have new non-pro lenses that have better resolution than the old pro lenses. Designs improved over time.


Back to your topic.
If your DX lens will resolve at 30 lines/mm, and the FX lens will resolve at 50 lines/mm. Then irrespective of the sensor size, the FX lens will give you a better image on the DX sensor.
But if you put a FX lens that resolves 30 lines/mm on a DX camera, you will not get any IQ improvement, as the lens resolutions are the same.
And if you put a FX lens that resolves 20 lines/mm on a DX camera, you will get worse IQ than the DX lens that resolves at 30 lines/mm.

That is the flaw in the DX vs. FX argument. The use generalities or specific lenses in their comparison to support their argument.
The devil is in the details.
 
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