DX Vs FX Lens Sharpeness

OP
ntz

ntz

No longer a newbie, moving up!
Joined
Oct 29, 2020
Messages
658
Reaction score
319
Location
Central Bohemian, Czech Republic
Can others edit my Photos
Photos OK to edit
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.
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 ..

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
 

Braineack

Been spending a lot of time on here!
Joined
Jun 17, 2013
Messages
13,184
Reaction score
5,596
Location
NoVA
Can others edit my Photos
Photos OK to edit
did glass in past 15 years changed its characteristics so now the sensors can read through it more lines/mm ?

yes. Different materials, different shapes, different cuts, molded, aspherical, low-dispersion, extra refractive index just a few things you could google.

Or even just glance at the MTF charts the lens manufacturers provide.
 

Attachments

  • 1635255554277.png
    1635255554277.png
    41.5 KB · Views: 11
Last edited:

ac12

Been spending a lot of time on here!
Joined
Dec 5, 2017
Messages
2,350
Reaction score
763
Location
SF Bay Area, California, USA
Can others edit my Photos
Photos NOT OK to edit
You have to research, to find the resolution of the lens and camera.
Some companies make that kind of detailed info VERY hard/impossible to find on their site.

Some of the reviewers have the ability to do that kind of technical testing.

For cameras, the resolution number for a particular MP camera/sensor will be similar for similar MP cameras.
The sensor has X pixels in the H axis and Y pixels in the V axis.
From that, the number of pixels per mm is easy to calculate.

Example: Using round number for ease of calculations.
Sensor pixels: 2000 x 3000
Sensor size: 24 x 36mm
Sensor resolution = 3000 / 36 = 83 pixels / mm

CAUTION: There are different measurements of resolution, don't get them mixed up with each other. It is confusing.
- Lines Per MM, Line PAIRS per mm, MTF, Line PAIRS for Image Height, Line WIDTHS per picture Height, etc. etc.
Some of these may be different labels for the same measurement.

The "center of the image circle" is a subjective statement.
While generally the center of the image circle does have better resolution than the sides, how much better is variable, and says nothing about the resolution of the lens.
Unless you have the resolution numbers to back up that statement, it holds little water.

Like I said before, if the center resolution of the FX lens is the same as the DX lens, you gain nothing. And if you use a consumer grade FX lens, the center resolution could be worse than the DX lens.
FX vs. DX refers to the sensor size/image circle, NOT the optical quality of the lens.

DX DOES place a high requirement on lens optics than FX, for the same MP sensor.
That is simple math. You have the same number of pixels in a smaller space.

You CAN use older lenses on modern high rez camers. But, in general, you just won't get the best image resolution, because the lens has a lower resolution than the sensor. That is a generalized statement, because you have older lenses that range from poor to excellent. And the VERY excellent older lenses may still perform well on a high rez digital camera.


As @Braineack said, optical manufacturing has improved tremendously.
The kinds of lens elements in some of today's non-pro/consumer lenses, were in the past, ONLY available in EXPENSIVE pro lenses.

Super zooms like the Nikon 18-200 and Tamron's 18-400, was "Science Fiction" back in my early film days.
 
Last edited:
OP
ntz

ntz

No longer a newbie, moving up!
Joined
Oct 29, 2020
Messages
658
Reaction score
319
Location
Central Bohemian, Czech Republic
Can others edit my Photos
Photos OK to edit
@ac12 thank you very much !!!! I didn't know this page ... what and awesome resource


I am still a little bit lost with numbers ... Can somebody please tell how much lpmm requires for example

FX 45MP camera (like D850) ~ 8250x5500px
FX 24MP camera (like D750) ~ 6000x4000px
DX 24MP camera (like D7200) ~ 6000x4000px

I've learned, that lpmm values are the best among lenses usually with AP 5.6-8 whilst for example my DX 11-20mm Tokina has sensational results over 45 lpmm .. I am more interested about my FX lenses as far as I am planning to go towards the (used) D850 ..

I am now a little bit confused because the article says that decency is 30-32 lpmm but that statement was made back in 2010-12 so I would like to know, what is decency for ~45MP modern FX camera

thank you very much

ps. Z reaches absolutely megastar-like ~70 lpmm results o_O where the best and the most expensive nikon-F lenses have below 50

 
Last edited:

ac12

Been spending a lot of time on here!
Joined
Dec 5, 2017
Messages
2,350
Reaction score
763
Location
SF Bay Area, California, USA
Can others edit my Photos
Photos NOT OK to edit
"Lines per mm" target has a line and a space then a line and another space.
The number is based on number of Lines AND Space (20+20=40).

A pixel can display either the line or the space, so in line with lines and space.

FX sensor = 36 x 24 mm
DX sensor = 24 x 16 mm

Using the long dimension:
D850: 8250 / 36 = 229 pixels / mm
D750: 6000 / 36 = 166
D7200: 6000 / 24 = 250

So from this, and your numbers, it seems that the resolution of the Z lenses are way behind resolution of the sensor.

But because a pixel is only ONE color, you need THREE pixels to make a 3-color set (so you use a 2x2 set of 4 pixels) that will display ANY color. So does the calculation change to divide by two. 8250 / 36 / 2 = 114
I don't know.

This is not my area of specialty, so I don't know how they reconcile.
 
  • Like
Reactions: ntz

wfooshee

No longer a newbie, moving up!
Joined
Oct 28, 2014
Messages
689
Reaction score
197
Can others edit my Photos
Photos OK to edit
Summary of ac12's post: Better glass is always better, doesn't matter what size sensor it was made for.
 

Timppa

No longer a newbie, moving up!
Joined
Dec 15, 2016
Messages
375
Reaction score
186
Location
Finland
Can others edit my Photos
Photos NOT OK to edit
It does not matter.
If you use a FX or DX lens and you are happy with the quality, what's the problem?
I use both as well on my dx camera and I am very pleased.
 

Most reactions

New Topics

Top