the cropped sensor and image quality

[GeorgeTheGreek]

i often see here and other places people saying that a full frame lens is good on a crop sensor body because that body utilises the lenses' sweet spot.

am i right in thinking that what is meant by that is that a smaller section of the elements are used in order to project the image on the sensor? is this the same thing that happens when you stop a lens down? ( i know the f stop doesn't change, as the lens length doesn't- i mean would the lens have the image quality of itself being slightly stopped down)

i'm wondering, because I am very interested in the cheap and cheerful Nikon 28mm 2.8D for my D90. On my cropped sensor it is a full frame equivalent of as near as makes no difference to me 42mm- which is apparently the "normal" focal length for DX. Not only that, i do seem to use the lengths between 24 and 35 most of all when i am out and about general shooting. So i figure the little 28mm will be a very nice lightweight alternative to my zoom.

Now I ask all this about the sweet spot,because reviews all over have suggested this isn't the best of optical designs, and i can live with that, but it would make me feel better if i knew i was getting just that little bit more from the lens by using it on a DX camera

 

[chuxter]

i often see here and other places people saying that a full frame lens is good on a crop sensor body because that body utilises the lenses' sweet spot.

There are two "sweet spots"...

When an FX lens is used on a DX body, only the center part of the image is used. Since most aberrations occur in the corners and edges, the center can be termed a "sweet spot" because those aberrations are outside the frame.

am i right in thinking that what is meant by that is that a smaller section of the elements are used in order to project the image on the sensor? is this the same thing that happens when you stop a lens down? ( i know the f stop doesn't change, as the lens length doesn't- i mean would the lens have the image quality of itself being slightly stopped down)

Most lenses also have resolution and aberration differences with aperture...IQ tends to peak in the middle of a lens'es aperture range. Thus, the apertures around f/5.6 to f/11 are also "sweet spots".

i'm wondering, because I am very interested in the cheap and cheerful Nikon 28mm 2.8D for my D90. On my cropped sensor it is a full frame equivalent of as near as makes no difference to me 42mm- which is apparently the "normal" focal length for DX. Not only that, i do seem to use the lengths between 24 and 35 most of all when i am out and about general shooting. So i figure the little 28mm will be a very nice lightweight alternative to my zoom.

Now I ask all this about the sweet spot,because reviews all over have suggested this isn't the best of optical designs, and i can live with that, but it would make me feel better if i knew i was getting just that little bit more from the lens by using it on a DX camera

DX bodies require higher resolution lenses than FX bodies, so this scheme is probably not a good one.

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[Mako2011]

DX bodies require higher resolution lenses than FX bodies, so this scheme is probably not a good one.

No disrespect intended but could you elaborate? What would be a "high" resolution Lens vs. a "low" resolution lens? Why would a "high" resolution lens not be wanted on a FX body? I think of a 70-200 f2.8 as "Higher" resolution lens and a 70-200 f3.5-5.6 as "lower" resolution. Are you thinking better glass mitigates the higher digital noise common to DX? Thank You.

 

[phonebook]

well i was going to ask that, the OP seems to want to know if in image quality terms the lens will perform better on a DX body than an FX

 

[NetMage]

Resolution is unrelated to aperture it is dependent on lens design, glass quality and manufacturing tolerances. It is measured directly as MTF through the lens.

A crop sensor image must be magnified more for a given viewing size, and thus puts more stress on the lens image than the larger sensor.
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[Mako2011]

A crop sensor image must be magnified more for a given viewing size, and thus puts more stress on the lens image than the larger sensor.

I follow that...I think....so then, a DX sensor would be well served to have the more expensive FX lens (better glass) in front of it rather than the equivalent DX Lens (usally lower quality glass)....does that follow?

 

[TechOutsider]

DX glass is designed with the crop sensor in mind, thus it uses less glass versus a comparable FX lens. The DX lens may also feature more resolving power versus the FX lens, but then, it might not .

Personally, I feel this serves no purpose in practical photography. If you are concerned about lens performance, read up on reviews and look at sample pictures. I don't think any universal maxim will come out of this discussion such as all DX glass is inferior to FX glass on a DX body or anything like that.

And a crop sensor simply puts more stress on the lens because its photosites are smaller. Think about a D90 with 12 megapixels on a smaller sensor (DX) versus a D3 with 12 megapixels on a bigger FX sensor. Naturally, the D90 will have smaller photosites, making it harder on the lens to cleanly project the light into a single photosite. If it projects it on multiple photosites, you get a blur. The blur varies in intensity depending on how many photosites are involved.

A crop sensor image must be magnified more for a given viewing size, and thus puts more stress on the lens image than the larger sensor.

I follow that...I think....so then, a DX sensor would be well served to have the more expensive FX lens (better glass) in front of it rather than the equivalent DX Lens (usally lower quality glass)....does that follow?

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[Mako2011]

Personally, I feel this serves no purpose in practical photography.

What if you need really fast lens for low light/low ISO...DX options would be limited so it does seem to have some purpose.

Naturally, the D90 will have smaller photosites, making it harder on the lens to cleanly project the light into a single photosite.

Which seems to suggest that if one wants the best possible light path (cleanest) to his DX sensor...use high quality FX lens and take advantage of the central "Sweet Spot".

 

[TechOutsider]

Personally, I feel this serves no purpose in practical photography.

What if you need really fast lens for low light/low ISO...DX options would be limited so it does seem to have some purpose.

True ... thanks for pointing that out .

Nonetheless, reading individual lens reviews will be much more beneficial than making generalizations about lens .

Naturally, the D90 will have smaller photosites, making it harder on the lens to cleanly project the light into a single photosite.

Which seems to suggest that if one wants the best possible light path (cleanest) to his DX sensor...use high quality FX lens and take advantage of the central "Sweet Spot".

As explained, it's sort of a double edged sword. Note how full frame lens tend to perform worse on DX bodies in DPReview's lens tests in terms of sharpness.

http://www.dpreview.com/lensreviews/widget/Fullscreen.ashx?reviews=37,38&fullscreen=true&av=1,1&fl=50,50&vis=VisualiserSharpnessMTF,VisualiserSharpnessMTF&stack=horizontal&lock=&config=/lensreviews/widget/LensReviewConfiguration.xml%3F4

However, distortion is less on the crop body. Fall off is also non-existent on the crop body.

So using a full frame lens, you may sacrifice some absolute sharpness, but you reap other benefits. That's simply because a full frame sensor is less demanding than the smaller crop sensor.
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[Mako2011]

As explained, it's sort of a double edged sword. Note how full frame lens tend to perform worse on DX bodies in DPReview's lens tests in terms of sharpness.

http://www.dpreview.com/lensreviews/widget/Fullscreen.ashx?reviews=37,38&fullscreen=true&av=1,1&fl=50,50&vis=VisualiserSharpnessMTF,VisualiserSharpnessMTF&stack=horizontal&lock=&config=/lensreviews/widget/LensReviewConfiguration.xml%3F4

Amazing!!! I did not know that widget existed. Thank You. It may take a while for me to totally understand it.... But the real dif in sharpness doesn't seem to be from DX vs FX lens .... but, if I'm reading this right (say 70-200 2.8 vs 18-200 f3.5-5.6 at say 105mm).... it really does matter high quality vs good quality lens.

 

[TechOutsider]

Your welcome .

Sharpness is dependent on lens quality too, true .

Amazing!!! I did not know that widget existed. Thank You. It may take a while for me to totally understand it.... But the real dif in sharpness doesn't seem to be from DX vs FX lens .... but, if I'm reading this right (say 70-200 2.8 vs 18-200 f3.5-5.6 at say 105mm).... it really does matter high quality vs good quality lens.

--
http://www.flickr.com/photos/techoutsider

 

[GeorgeTheGreek]

thanks for all the replies to this

i have been having a rudimentary play arround with that widget now, and it seems based on a 50mm 1.8, that there is some advantage to using it on a crop sensor.

but yes, i shall have to go to the store with my camera and try out the lens