No longer a newbie, moving up!
- Oct 29, 2020
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- Central Bohemian, Czech Republic
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now logical Q: - how do I know how many lines/mm does specific lens resolve ? How do I know how many lines/mm do I need from lenses to resolve on specific camera body ..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.
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.
let me return back to the beginning - I never cared for question like that and I am using FX lenses on DX bodies just like from the beginning - also I was assuming that it brings benefits instead (eg I am using the center of circle, I can stack circular magnetic filters on them freely without vignetting, and so on) .. I know that the concern about old lenses on new high-MPs camera bodies is probably legit - I've read in lot of reviews (on photographylife) that this or that 15+ years old lens is not supporting properly high-MP chips - but I didn't know how is that possible and what makes the difference .. so my question is: what makes the difference ? did glass in past 15 years changed its characteristics so now the sensors can read through it more lines/mm ? Moreover lines/mm are not stated anywhere in the lenses SPECs .. What exactly makes 15+ years old lenses to unfit for high MPs sensor cameras ? Then if density matters, then the DX should be more demanding than FX - DX D7200 has same amount of lines as for example FX D750