cropped Prime lens shots v zoom uncropped ?

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stevewestern

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I have often wondered just how they compare - a shot taken with a prime lens then cropped or a shot taken with a zoom lens that fills the frame. I am talking about Canon L quality equipment here, such as say a 70-200 f2.8 mk 2 v a 200mm f2.8 or 135 f2.

Now I know this is not really possible to answer as it depends on just how cropped etc, but any thoughts ?

I use zoom lenses as my subjects (kids in classrooms mainly) move fast and I often cannot move for fear of falling over a child or desk but see so many sports photographers using primes such as the 300 f2.8 and their cannot move during a game so have to assume there is still some advantage to a prime.

Educate me please !
 
Optically all lenses are better in the centre, on axis

Generally speaking a prime is optically optimised for its focal length, that is the form of all of the elements is optimal, thus reducing aberrations

Generally speaking prime lenses are simpler, which means the calculations needed to optimise the lens are easier. Only recently has computing power being so good that the very complex optics of zooms could be optimised to any great effect

DX lenses are like FX lenses, but only use the central part of the optics (see my first point)

Pro sport photographers have FX cameras, need fast sharp lenses and at the moment the zooms don't hack it. to compensate for lack of flexibility, they take a pile of cameras, each with a different lens on, however, in reality, a footballer in the lens choice they make covers more of the pitch then you would think (whilst still filling the frame with something useful)
 
Thanks Richard - that explains some of the technical side, but just how do the images compare if for example I were to use a 100mm prime and crop by 50% and a 70-200mm zoom at 200mm and not crop is more the kind of information I am hoping to find out.
I have owned a couple of short primes (a 50mm and 85mm) and found I rarely used them as changing lenses takes too long and I do need to be able to zoom in or out often, meaning my 24-70 got used a lot but the idea of something like a 135 or 200mm prime keeps coming back to me like an itch....
 
To take your example, if you crop to 50% you will only have a quarter of the original resolution (e.g. 16 megapixels become 4 megapixels), and I would say that the loss of quality due to this would outweigh the poorer original quality of a half decent zoom lens.
 
I have the 135mm prime and used it a lot.. nothing in a zoom compared at all.. they they brought out the 70-200 mkII and that does compare good enough for me to use that over the 135mm ..I cant go techie for you.. accept to say they compare well now and I ahve hardly used the 135mm..

At the other end I ahve a 35mm prime and a 17-40L lens.. the 35mm F2 (non L) wipes the floor with the 17-40 every time.. easy better by a country mile esp in low light situations (yes both at f4 compared)
 
roseway said:
To take your example, if you crop to 50% you will only have a quarter of the original resolution (e.g. 16 megapixels become 4 megapixels), and I would say that the loss of quality due to this would outweigh the poorer original quality of a half decent zoom lens.
Click to expand...

not in practice... a cropped picture from my 400mm f2.8 will beat the equivelant zoom any day of the week... IMHO :)
 
roseway - that is largely what was behind my thinking..

KIPAX - it was your replies on another thread that made me start thinking !
I am thinking of getting a 70-200 f2.8 mk2 and selling on my 70-200 f4 but did wonder about keeping my f4 and getting a 200mm f2.8 prime. I guess the new zoom is looking even more likely now...
 
stevewestern said:
Thanks Richard - that explains some of the technical side, but just how do the images compare if for example I were to use a 100mm prime and crop by 50% and a 70-200mm zoom at 200mm and not crop is more the kind of information I am hoping to find out.
I have owned a couple of short primes (a 50mm and 85mm) and found I rarely used them as changing lenses takes too long and I do need to be able to zoom in or out often, meaning my 24-70 got used a lot but the idea of something like a 135 or 200mm prime keeps coming back to me like an itch....
Click to expand...

The zoom would win hands down.

Cropping is absolutely the worst thing you can do if best image quality is the aim. You're asking the lens to work much harder to deliver much higher resolution and you're throwing away pixels and precious photons big style, promoting noise.

It's like expecting a crop format camera to beat full frame, which it won't, ever.
 
Hmmm.

This is the second time I've said this in the last couple of weeks, sorry Richard.

Cropping does not reduce image quality.

All you are doing when you crop an image is cutting bits off it and that can not reduce the image quality. The reduction in image quality comes when you then enlarge the cropped image beyond it's own size as the information within needs to be spread out or added to to make the image bigger and if viewed from the same distance it may not look as good. That's where the perceived degradation comes from, not from the cropping as such.

Lets imagine that we have a camera that will produce a 100% image of A3 size. If we then cut an A4 image out of it when viewed at 100% there is absolutely no difference in image quality from the bigger A3 image because all we are doing is cutting a slice out of it. However, if we increase the A4 image to A3 we'll have a problem... so I suppose that when cropping we should keep in mind the size of the final image we wish to end up with.

Other than that a zoomed image will use more pixels than are used in a cropped image and will therefore contain more detail and will make a more detailed big print.

PS. Assuming that the lenses are of equal quality. If your prime is a star and your zoom is a dog all this goes out the window.
 
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woof woof said:
Hmmm.

This is the second time I've said this in the last couple of weeks, sorry Richard.

Cropping does not reduce image quality.

All you are doing when you crop an image is cutting bits off it and that can not reduce the image quality. The reduction in image quality comes when you then enlarge the cropped image beyond it's own size as the information within needs to be spread out or added to to make the image bigger and if viewed from the same distance it may not look as good. That's where the perceived degradation comes from, not from the cropping as such.

Lets imagine that we have a camera that will produce a 100% image of A3 size. If we then cut an A4 image out of it when viewed at 100% there is absolutely no difference in image quality from the bigger A3 image because all we are doing is cutting a slice out of it. However, if we increase the A4 image to A3 we'll have a problem... so I suppose that when cropping we should keep in mind the size of the final image we wish to end up with.

Other than that a zoomed image will use more pixels than are used in a cropped image and will therefore contain more detail and will make a more detailed big print.

PS. Assuming that the lenses are of equal quality. If your prime is a star and your zoom is a dog all this goes out the window.
Click to expand...

Now why didn't I think of that :shrug:

On your PS, the two lenses can never be of equal quality, because even the same lens used in this way is being required to deliver a much higher level of resolution for the cropped image and contrast will have dropped accordingly (basic MTF principle).

Even a relatively modest quality zoom (uncropped) will beat the best prime (cropped) when heavily enlarged, regardless of how many pixels you throw at it. It simply destroys all aspects of image quality - first rule is to always use the right focal length for the job.
 
We're bordering on another CoC discussion here ;) :D
 
OP:
Before you outlay a load of dosh on the lens... zoom or prime... Can you arrange to borrow one from a (hopefully) friendly TP in a meet to take the same shot with the different lenses and minimal light change?

Alternatively if you are that much unsure, see about hiring the lenses you want to buy (if possbile) for a week and shoot in different conditions along with the zoom you have.

At least you'd be able to justify which lens you prefer then?

good luck choosing!
 
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