Perceived mp

Nostromo

Nostromo

Suspended / Banned
Messages
6,236
Name
Dominic
Edit My Images
Yes
I was watching a YouTube video about, is using full frame lens on crop sensor bodies good or bad (well not so good). And this "perceived mp" kept coming up. What does this actually mean? If I'm to believe what was being said, then it would seem that there is no improvement in IQ (in fact it may even be worse) when using FF lens on crop bodies.
I'm just interested to know.
 
Nostromo said:
... it would seem that there is no improvement in IQ (in fact it may even be worse) when using FF lens on crop bodies.
Click to expand...
No improvement compared to what?
 
Compared to using a lens of similar focal length designed for a crop body.
 
Nostromo said:
Compared to using a lens of similar focal length designed for a crop body.
Click to expand...

I suppose... the bigger the sensor the less the image has to be magnified and the less hard the lens has to work... So... A lens may perform very well on ff but weaknesses may show up if you use it on APS-C or MFT or any other smaller system because the picture has to be magnified more and the shortcomings of the lens will therefore stand more chance of being visible because of the greater magnification. I've seen this when using old film era lenses on MFT and ff, on the ff camera the pictures appear much sharper.

Lenses specifically designed for APS-C or any other smaller system may be designed to be sharp so that the image quality damaging effect of the additional magnification is reduced a little... and I think we do see this with some of the smaller systems for example some MFT lenses are very good and if they could be scaled up they'd be very very good FF lenses. Maybe :D
 
Unless you share a link to the video I suspect this is going to go around in widening circles of "wtf is he talking about?"
 
It's to do with resolving powers of lenses and how much the camera/sensor magnifies/enlarges the image and therefore put more demands of the lens and shows up the flaws more I believe, but @sk66 will be able to explain it better.

DXO gives a good guide as to how well a camera/lens combo resolves detail, although as with a lot of these things you need to take them with a pinch of salt.
 
Last edited:
Alastair said:
Unless you share a link to the video I suspect this is going to go around in widening circles of "wtf is he talking about?"
Click to expand...

I concentrated on the ff lenses on smaller format bodies bit... and I agree that this will probably drop the image quality as effectively you're cropping/enlarging the image and this can only lead to a drop in image quality, if viewing from the same distance etc.
 
Potentially you could have slightly better edges on a crop sensor with a ff lens as you're only using the centre of the lens.
 
Last edited:
stevelmx5 said:
Potentially you could have slightly better edges on a crop sensor with a ff lens as you're only using the centre of the lens.
Click to expand...

You'll avoid using the possibly less good part of the lens towards the edges of the frame and only use what's possibly the best bit in the middle but you're still stuck with the effects of greater magnification.
 
woof woof said:
I concentrated on the ff lenses on smaller format bodies bit... and I agree that this will probably drop the image quality as effectively you're cropping/enlarging the image and this can only lead to a drop in image quality, if viewing from the same distance etc.
Click to expand...
This assumes so many variables as constant though - principally pixel size and pitch, and assumes that you don't shoot to the capabilities of the equipment. Comparisons between sensor size generally assume varying degrees of "the photographer's an idiot/dumb user" and make little mention of how the different sensor sizes might be used to best effect.
 
Alastair said:
This assumes so many variables as constant though - principally pixel size and pitch, and assumes that you don't shoot to the capabilities of the equipment. Comparisons between sensor size generally assume varying degrees of "the photographer's an idiot/dumb user" and make little mention of how the different sensor sizes might be used to best effect.
Click to expand...

I'm talking about real world use. Fit an old lens to a ff camera and a smaller sensor camera and use them in the real world as you would and then view the images as you would and if you look for the differences you'll probably see that the smaller sensor pictures are softer.
 
My main question is what does perceived mp actually mean?
 
woof woof said:
I'm talking about real world use. Fit an old lens to a ff camera and a smaller sensor camera and use them in the real world as you would and then view the images as you would and if you look for the differences you'll probably see that the smaller sensor pictures are softer.
Click to expand...
In the real world I'm not idiot/dumb user enough to use the same lens on a FF and crop sensor camera and expect the results to be directly comparable for the same use.

Give an example of a situation where you could use, say, a 50mm on both a crop and FF sensor and expect the same result? These arguments are entirely theoretical and don't relate to practical usage.
 
I've got no idea what "perceived MP" is to be honest. Megapixel is a physical measure of the sensor resolution so has nothing to do with the lens. If you physically crop an image, you will reduce the megapixels (assuming you don't then resize it) but again, that's nothing to do with the lens.
 
As above, I've never really agreed with half of the stuff Tony Northrup posts. He never makes any sense but gets lots of hits.
 
Nostromo said:
This is the link (done on my phone so not sure if it been done right)
View: https://youtu.be/YDbUIfB5YUc
Click to expand...
Lost me at "Let's look at some real numbers from DXO Mark".. I then skipped through, and yup.. it's an entirely theoretical mansplaination that doesn't consider a single actual photo.

.. I think there are some GWC who aren't actually in it for taking photos..
 
Perceptual MP (PMpix) is DXO's lens sharpness measurement. They take test measurements to determine the lens' MTF (lines per mm reolved) and convert that to how many pixels are required to reproduce those lines (system MTF).
What is interesting/significantly different from other methods is that, instead of reporting MTF at a given contrast percentage (i.e. MTF50/MTF30/etc), they calculate what you can perceive based upon the human contrast sensitivity function (CSF for 20/20 vision).

What's even more interesting to me is that the MTF that most closely correlates to the human CSF for standard viewing distances is 90%, and a higher contrast requirement results in a lower resolution requirement/capability. That might seem contradictory, but it makes sense when you correlate it to the fact that the standard for sharpness (COC) requires no more than 2MP from a FF sensor. And the max resolution a human can perceive is somewhere around 12-14MP in an image (@ standard viewing distance).

Basically, what Tony says in that video is technically correct. But IMO it is making a big deal about things that don't really matter (much like the whole "equivalence" thing).
 
sk66 said:
Perceptual MP (PMpix) is DXO's lens sharpness measurement. They take test measurements to determine the lens' MTF (lines per mm reolved) and convert that to how many pixels are required to reproduce those lines (system MTF).
What is interesting/significantly different from other methods is that, instead of reporting MTF at a given contrast percentage (i.e. MTF50/MTF30/etc), they calculate what you can perceive based upon the human contrast sensitivity function (CSF for 20/20 vision).

What's even more interesting to me is that the MTF that most closely correlates to the human CSF for standard viewing distances is 90%, and a higher contrast requirement results in a lower resolution requirement/capability. That might seem contradictory, but it makes sense when you correlate it to the fact that the standard for sharpness (COC) requires no more than 2MP from a FF sensor. And the max resolution a human can perceive is somewhere around 12-14MP in an image (@ standard viewing distance).

Basically, what Tony says in that video is technically correct. But IMO it is making a big deal about things that don't really matter (much like the whole "equivalence" thing).
Click to expand...
That's for taking the time to write that. So in your opinion if i put a canon 70-200 f4 is, instead of say a canon 55-250mm stm, on a canon 50d (i know you use nikon so may not know any thing about my 50d) that i wouldn't be wasting my money.
 
Nostromo said:
That's for taking the time to write that. So in your opinion if i put a canon 70-200 f4 is, instead of say a canon 55-250mm stm, on a canon 50d (i know you use nikon so may not know any thing about my 50d) that i wouldn't be wasting my money.
Click to expand...

In your specific example the answer is that you probably wouldn't be wasting your money.

Have a look at the following comparison of the 2 lenses:

http://www.the-digital-picture.com/...meraComp=736&SampleComp=0&FLIComp=0&APIComp=1

However, this does not mean that all full frame lenses will always outperform all crop lenses. Comparison between the canon 55-250mm stm and the Canon 70-300 DO IS from the same website:

http://www.the-digital-picture.com/...3&Sample=0&CameraComp=736&FLIComp=0&APIComp=0

Crop lenses can be made smaller and lighter than full frame lenses for the same focal length and aperture. Depending on the quality of the optical design and the manufacturing quality, one may outperform the other or vice versa on a crop body.
 
Nostromo said:
That's for taking the time to write that. So in your opinion if i put a canon 70-200 f4 is, instead of say a canon 55-250mm stm, on a canon 50d (i know you use nikon so may not know any thing about my 50d) that i wouldn't be wasting my money.
Click to expand...

As well as being built differently optically, you're also paying for the weather sealing, more efficient af system, constant F4 aperture and the white lens effect ;0) However, I wouldn't discount the 55-250 entirely. It's an excellent optical performer as well as being 50mm longer.
 
Nostromo said:
That's for taking the time to write that. So in your opinion if i put a canon 70-200 f4 is, instead of say a canon 55-250mm stm, on a canon 50d (i know you use nikon so may not know any thing about my 50d) that i wouldn't be wasting my money.
Click to expand...
Based purely on DXO figures the 70-200mm would give you 2 more Mpix, but is still a relatively low 7 Mpix which isn't great value for money imo. Of course this doesn't take into consideration AF performance, build, constant aperture etc etc which of course increases the value of the 70-200mm. In terms of sharpness upgrading your body and keeping the 55-250mm would actually give you a better upgrade, for example the 750d with 55-250mm give 8Mpix, probably simply due to it being 24mp vs 15mp.
https://www.dxomark.com/Lenses/Comp...-on-Canon-EOS-50D__1212_1010_1212_272_255_272

Sticking the 70-400mm on the 750d gives 10 Mpix so more sharpness still. The 70-200mm f4 clearly is much better on FF giving a much better 18 Mpix on the 5D3 even though it gives up 2mp resolution compared to the 750d (22mp vs 24mp).
 
snerkler said:
The 70-200mm f4 clearly is much better on FF giving a much better 18 Mpix on the 5D3 even though it gives up 2mp resolution compared to the 750d (22mp vs 24mp).
Click to expand...
It's simply a matter of "size of dots," the lens can deliver dots of a size closer to the larger dots (pixels) on the FF sensor.
 
Nostromo said:
That's for taking the time to write that. So in your opinion if i put a canon 70-200 f4 is, instead of say a canon 55-250mm stm, on a canon 50d (i know you use nikon so may not know any thing about my 50d) that i wouldn't be wasting my money.
Click to expand...
I would be much more concerned about what a lens allows you to accomplish rather than pixel level detail... especially if you are not displaying the images large or viewing them particularly critically (A2+/short viewing distances where you can't see the whole image at once). Unless that one stop at the long end makes a difference for what you are shooting (probably not), or the AF speed is needed (USM should be a bit faster), I think it would probably be a waste of money.
 
stevelmx5 said:
Potentially you could have slightly better edges on a crop sensor with a ff lens as you're only using the centre of the lens.
Click to expand...

This is not true.
The whole of the area of a lens surface contributes to the formation of the image at every individual point across the plane of focus. So using a FF lens on a crop sensor - the edges of that lens as still contributing to the image on the sensor despite its area being smaller - try stopping down the FF lens on a crop sensor and you will notice the brightness of the image dims, the edges of the FF lens when fully open was therefore still contributing to the crop sensors image.
The main difference between a FF lens and a purpose designed crop sensor lens is the size of the image circle that the lens produces at the plane of focus - FF lenses have a much bigger image circle to cover the larger sensor area.
 
jbw said:
This is not true.
The whole of the area of a lens surface contributes to the formation of the image at every individual point across the plane of focus. So using a FF lens on a crop sensor - the edges of that lens as still contributing to the image on the sensor despite its area being smaller - try stopping down the FF lens on a crop sensor and you will notice the brightness of the image dims, the edges of the FF lens when fully open was therefore still contributing to the crop sensors image.
The main difference between a FF lens and a purpose designed crop sensor lens is the size of the image circle that the lens produces at the plane of focus - FF lenses have a much bigger image circle to cover the larger sensor area.
Click to expand...

I was about to reply to your post with regards to the image circle but you already stated it. So if the image circle produced by the FF lens is larger than the total area of the crop sensor, aren't you therefore recording the image from the centre of the lens (and cropping the edges)?

If I mount one of my medium format lenses (that cover 60x60mm) on my crop A6000, am I not also only recording the centre of the entire image circle, regardless of how that image circle is produced?
 
Last edited:
stevelmx5 said:
I was about to reply to your post with regards to the image circle but you already stated it. So if the image circle produced by the FF lens is larger than the total area of the crop sensor, aren't you therefore recording the image from the centre of the lens (and cropping the edges)?

If I mount one of my medium format lenses (that cover 60x60mm) on my crop A6000, am I not also only recording the centre of the entire image circle, regardless of how that image circle is produced?
Click to expand...

I think you may be mixing terminology up rather than the concept - what I think you mean is that a crop sensor only uses the centre part of a FF lenses image circle - this is true. The edges of an image circle tend to carry most of the image imperfections so just using the centre area should discard most of those imperfections.
However, the crop sensor is not using the centre part of the lens - even every small pixel on the sensor ( as long as it lies within the image circle) has light falling on it contributed from the whole area of the lens. Therefore whole of the lens contributes to the formation of every point on the image. If you stick a small plaster any where on the front of the lens no single part of the image will be seen to disappear - the image will now just become dimmer and will have less contrast.
 
jbw said:
I think you may be mixing terminology up rather than the concept - what I think you mean is that a crop sensor only uses the centre part of a FF lenses image circle - this is true. The edges of an image circle tend to carry most of the image imperfections so just using the centre area should discard most of those imperfections.
However, the crop sensor is not using the centre part of the lens - even every small pixel on the sensor ( as long as it lies within the image circle) has light falling on it contributed from the whole area of the lens. Therefore whole of the lens contributes to the formation of every point on the image. If you stick a small plaster any where on the front of the lens no single part of the image will be seen to disappear - the image will now just become dimmer and will have less contrast.
Click to expand...

I think maybe we're talking semantics here and just differing terminologies. You're correct in that the entire lens focuses an image to produce the specific image circle at the sensor or film plane. Whilst the entire lens element area is used, a crop sensor will only use the centre portion of a full frame lens image circle.
 
You must log in or register to reply here.
Back
Top