What Lens Specification To Look For Sharpness At Distant Subjects When Cropped??

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rpszone Regular Member • Posts: 245
What Lens Specification To Look For Sharpness At Distant Subjects When Cropped??

Take two, different, 200mm lenses, same body and subject at 100 feet. Which one can crop better with sharpness and clarity?

Do I look at "perceived megapixel" rating, "resolving power", what makes one lens better than the other for this task?

ZilverHaylide Senior Member • Posts: 1,154
Re: What Lens Specification To Look For Sharpness At Distant Subjects When Cropped??
1

rpszone wrote:

Take two, different, 200mm lenses, same body and subject at 100 feet. Which one can crop better with sharpness and clarity?

Do I look at "perceived megapixel" rating, "resolving power", what makes one lens better than the other for this task?

If everything else were equal, then almost any sharpness or resolution metric could be used, and would probably indicate better cropability. But it's not equal. Sharpness varies over the field, virtually always better in the center than elsewhere, but falling off at different rates in different lenses, and additionaly, differently-dependent on f-stop and even subject distance. No single number can express that variability. Two lenses might have the same total "perceived megapixel" rating, but lens "A" might be fairly even in performance across the field, while lens "B" might have higher sharpness than lens "A" in the center, but lower sharpness towards the edges/corners. So take your crop from the center of that lens "B" and the result will be sharper than from the same location of the lens "A" image, but take your crop from the outer field of both lenses and the result from lens "B" will be worse than from lens "A".

John Crowe
John Crowe Senior Member • Posts: 1,197
Re: What Lens Specification To Look For Sharpness At Distant Subjects When Cropped??

I have never heard of "perceived megapixel" and I would not trust any reference to this.

You can compare MTF charts if you want, but in 40 years I have never bothered.

Independent lens testers will often give resolution in lines per mm or line pairs per mm at centre/edges/corners.

I don't mean to be rude but generally the $5000 200mm lens is better than the $1000 200mm lens.  There are also 200mm lenses for a couple hundred that are pretty close to the $1000 one.

Price will actually tell you a lot.  Other than that searching the internet for specific lenses should give you an idea from users how they compare.  If it is Canon or Nikon the lens comparison site is a good reference too.

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Chris R-UK Forum Pro • Posts: 21,311
Re: What Lens Specification To Look For Sharpness At Distant Subjects When Cropped??
1

John Crowe wrote:

I have never heard of "perceived megapixel" and I would not trust any reference to this.

I googled "perceived megapixel rating" and got no hits.  He might be getting mixed up with DXO's measurement of "perceptual megapixels", but that is for a camera/body combination and DXO has never explained how the measurement is obtained.

He might also be getting mixed up with the concept used by birders of "pixels on the target".  There comes a point when shooting at extreme range and with extreme cropping when the camera with the highest pixel density sensor will produce the "least worst" image.  That is why bridge cameras with tiny sensors like the Nikon P1000 still exist.

I am inclined to say to the OP that the best image quality will come from the lens being used by the photographer who shoots from the closest distance and waits for the best light and the best atmospheric conditions. 

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Chris R

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lehill
lehill Veteran Member • Posts: 6,135
Re: What Lens Specification To Look For Sharpness At Distant Subjects When Cropped??

rpszone wrote:

Take two, different, 200mm lenses, same body and subject at 100 feet. Which one can crop better with sharpness and clarity?

Do I look at "perceived megapixel" rating, "resolving power", what makes one lens better than the other for this task?

Each reviewer that quantitatively tests lenses has developed their own test rigs, methods, and means to express their results (eg, perceptual megapixels, blur units, lp/mm, etc). The camera+sensor used also has a big impact on results. This means you can't directly compare results between reviewers (eg Optical Limits vs Imaging Resource).

It's great when you can find one reviewer that has run their tests using the same sensor on the lenses you're interested in (example).

The one thing we have that's kinda universal are the theoretical MTF curves that the lens manufacturers publish. Even those are tricky to use for comparison purposes. This article is a primer about reading MTF curves.

Example MTF curve for Sony SEL90M28G

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Lance H

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OP rpszone Regular Member • Posts: 245
Re: What Lens Specification To Look For Sharpness At Distant Subjects When Cropped??
John Crowe
John Crowe Senior Member • Posts: 1,197
Re: What Lens Specification To Look For Sharpness At Distant Subjects When Cropped??
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Yeah, I don't give any credence to DXO.  I saw a guy on here quoting all kinds of DXO crap and he did not seem to realize that the differences he was basing decisions on were essentially within experimental error tolerances.

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rich_cx139 Senior Member • Posts: 1,779
Re: What Lens Specification To Look For Sharpness At Distant Subjects When Cropped??

rpszone wrote:

Take two, different, 200mm lenses, same body and subject at 100 feet. Which one can crop better with sharpness and clarity?

Do I look at "perceived megapixel" rating, "resolving power", what makes one lens better than the other for this task?

I think ZilverHaylide has given a pretty good explanation of the problem.

In addition, a few more thoughts:

If you are talking about a centred DX or other crop from a FF sensor output then life gets marginally easier in that it becomes easier to get some idea of performance from MTF graphs if you spend time understanding what they can and cannot tell you.  Since there aren't many 200mm lenses around then you may be looking at zooms and unless its a 70-200, or a 200-x, you probably will be left trying to guesstimate performance from nearby FL MTF data.

If you are cropping hard then you may need more data than the 10lp/mm and 30 lp/mm that Canon and Nikon typically give you but if its a modern lens produced with high mpx sensors in mind then you should be better off than with an older lens designed over 10 years ago for lower mpxs. There are no hard and fast rules however and some, even, film era 200 ish primes are still very very good.

A problem doing anything other than trying two lenses side by side yourself is that the resolution can vary significantly with the target distance - I have several where this the case. No one publishes data for a lens at different target distances and apertures.

The best guide is to see what people who use the lenses you are interested in actually say is their experience.

Price is usually a good pointer but not necessarily so since there are examples of really cheap lenses outperforming most people's expectation. Not sure about the converse: when prices get up and beyond 2K it would be surprising to find many 'lemons'.

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