50mm vs 85mm lens: is there a visible difference in image quality?

Lets say i have a 50 and 85mm Sigma Art - from a theoretical photo-physics aspect, does the difference in focal length provide different/better quality?

I noticed that on DXOMark, the best prime lenses were often 85mm with a wide range of cameras that they have tested so i wondered if this is an odd coincidence?

With the materials in use currently, yes there is a difference in favor if 85mm at least for distortion. It's always a tradeoff in optical design, but you'll see that longer lenses, even the big primes, often have slight pincushion distortion and lenses less than 85mm tend towards barrel distortion. Is the difference visible... maybe not depending on the size of display, your vision, and the camera used. For instance, CA less than 1/2 pixel width doesn't really show, so on a sensor with large pixels and two really nice lenses the difference may actually not be discernable. Unfortunately I'm not so deep in lens design as to be able to explain the physics of these tendencies without a textbook.

Define IQ (and forget DXOmark). Possible factors are:

• resolution
• contrast
• flare resistance (or the opposite, depending on what you want)
• corner performance
• bokeh
• color rendition
• vignetting

85mm and 50mm provide very visible difference in the images; sharpness is one of the least visible ones.

tesla23 said:

Lets say i have a 50 and 85mm Sigma Art - from a theoretical photo-physics aspect, does the difference in focal length provide different/better quality?

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It's not the difference in focal length as much as the difference in the field of view.

Many lens aberrations increase as the angle of view increases. Hence a lens that covers only a narrow angle can be better corrected more easily than one that covers a wide angle. You see this in practice with almost all lenses: the resolution and image quality drop off as you move away from the centre of the image. Long focal length lenses (with same sensor size) are effectively using the centre portion of the image only.

I remember twenty years ago seeing very comprehensive MTF test results for a large number of 35mm film camera lenses and the lens that obtained the top score for resolution across the frame was a Canon 200mm lens.

the unspoken factor here is that the fifty mm lens used for thirty five mm photography has been around since the 1920s and has benefitted from a hundred years of development. the eighty five not so long but for all practical purposes and easier lens to design. So for the two lenses your asking about there will probably be little if any visible difference when the image size is the same, but then try say the summicron aspherical or the top or the line zeiss fifty and see what a fifty is really capable of

I suspect there is a "sweet spot", in terms of focal length. Shorter FLs need to bend light more acutely, and bear in mind the sensor plane is flat - surely more difficult to cope with than for longer FLs.

But also, the longer FLs have larger glass elements, and on top of that, lens aberrations will tend to be magnified more.

So, there's probably an optimum FL, and 85mm sounds quite plausible to me!

Futax said:

I suspect there is a "sweet spot", in terms of focal length. Shorter FLs need to bend light more acutely, and bear in mind the sensor plane is flat - surely more difficult to cope with than for longer FLs.

But also, the longer FLs have larger glass elements, and on top of that, lens aberrations will tend to be magnified more.

So, there's probably an optimum FL, and 85mm sounds quite plausible to me!

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You should try my 35LII!

tesla23 said:

Lets say i have a 50 and 85mm Sigma Art - from a theoretical photo-physics aspect, does the difference in focal length provide different/better quality?
I noticed that on DXOMark, the best prime lenses were often 85mm with a wide range of cameras that they have tested so i wondered if this is an odd coincidence?

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Frankly, I don't know.

I have several primes and several zooms. I use the lens appropriate for the image I want to capture. I am not interested in the "test charts" nor the test done by any independent test labs. I go by my perceived IQ to suite my purpose.

If you suspect the FL provide difference in Image Quality, may I suggest you compare the images taken with a zoom lens.

Since this is not a beginners forum, I assume you would know what and how to do the process.

There will be no difference in image quality with a change in focal length.

it is the actual lens that will make the difference.

It seems to me that 85 mm lenses typically have a different design goal compared to the typical 50 mm lens. 85 mm lenses seem to be optimized for portraiture, intended to be used opened up, giving a smoother background blur and decent center sharpness when opened, at least compared to traditional 50 mm designs at the same aperture setting. Standard consumer 50 mm lenses tend to be quite soft with a busy, distracting background blur when opened wide, although premium lenses will differ.

Tighter apertures tends to be the great equalizer amongst lenses, and so differences will be less noticable. Wide apertures tend to separate the men from the boys, with awesomely expensive ciné lenses usually being excellent performers when opened up: they simply don't have too many distracting defects.