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How to compare sharpness on two lens with different focal length?


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#1 iouzzr

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Posted 20 March 2017 - 10:39 AM

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Hi there,

I own a new loxia 85 F2.4 and want to test just the sharpness on this lens. However, I have only 35 F1.8 and 16-70 F4(both for a6300). It it reasonable to compare sharpness on two lens with different focal length? I have always see sharpness comparison on the same focal length, but not different...

Thank you.



#2 Guest_all8_*

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Posted 20 March 2017 - 07:59 PM

Why don't you try with the F4 and 70 and the Loxia (at 85 :-) ). Compare at f4 and f8, then report back with the results!

 

The one review I read of the Loxia 85 was that is was very good around f4.



#3 iouzzr

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Posted 21 March 2017 - 04:46 AM

Why don't you try with the F4 and 70 and the Loxia (at 85 :-) ). Compare at f4 and f8, then report back with the results!

 

The one review I read of the Loxia 85 was that is was very good around f4.

I will give a try when I have a tripod... maybe a stack of book is also ok? Should I compare the two lens at the same positon? Or try to make the object(maybe a dollar?) nearly the same size on photo?

Thank you.



#4 Username

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Posted 21 March 2017 - 04:48 AM

Check lenses independently of each other. 

Comparisons are not meaningful. You can 

mistakenly get the impression that lens "A"

is better than lens "B" by comparison. 



#5 Username

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Posted 21 March 2017 - 05:02 AM

I will give a try when I have a tripod... maybe a stack of book

is also ok? Should I compare the two lens at the same positon?

Or try to make the object(maybe a dollar?) nearly the same size

on photo?

Thank you.

 

Your approach to testing is so informal that it hardly 

matters exactly how you approach it. Just do your best 

and see if you like the images. 

 

You seem to be trying to approximate a controlled "lab 

test" environment. Unfortunately a lab test has to be a 

very precise approach. An approximated version of a 

precision operation is no longer precise. You cannot 

meaningfully approximate precision. It's contradiction !   

 

Just use the lenses in the most challenging conditions  

that you tend to encounter, and see how they support 

your efforts. And if you really need to know the lab test 

performance of your lenses, simply check the existing 

reports. If there are no such reports on a lens that you 

hope to enjoy, and you can't enjoy it without knowing 

how it performs under precision testing, then you can't   

realistically consider acquiring that lens. 



#6 iouzzr

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Posted 21 March 2017 - 05:28 AM

Thank you, ah, Username...

You are right, make approximate lab test is boring to me. However, the beginner nature of me forced me to convince myself the lens I have bought has outstanding quality is not a defect one(in my case is the sharpness).

 

So maybe I can restate my question: how to decide if my copy of lens is a defect one? 

 

This remind me that lens defection means that corner sharpness isn't constant among the four edges(center sharpness is always very good regardless the lens defection). So maybe I can compare on the same lens, on four edge corners to see if the corner sharpness is consistent... Is this ok?



#7 KMG

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Posted 21 March 2017 - 01:36 PM

iouzzr,

 

You may find this B&H post useful: https://www.bhphotov...-test-your-lens



#8 Guest_all8_*

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Posted 21 March 2017 - 04:07 PM

Folks over at Fred and Miranda go to some extreme lengths to check the centering of their Loxia lenses, I never got around to it myself.

 

This link has a test which seems easy to construct yourself ... about midway down : https://www.lensrent...ets-a-makeover/



#9 Username

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Posted 22 March 2017 - 12:38 AM

...............................  

  

So maybe I can compare on the same lens, on four edge corners

to see if the corner sharpness is consistent... Is this ok?

 

Can be done. Gotta be reeeeeally careful aligning lens 

axis perpendicular to the flat test target/subject. Most 

lenses focus to a slightly different distance at the outer 

regions than the focus at the center. This not a defect 

but a typical shortcoming of nearly all general purpose 

lenses. Some lenses are very specifically "flat field" 

lenses, and will focus center and outer regions to the 

same distance. Most lenses are not flat field and have 

a degree of curvature of field. Usually the outer regions 

are focused to a shorter distance than the center. What 

this means is that if you carefully align the flat target to 

be perpendicular to the lens axis, you expect to find the 

corner regions need refocusing. You should accept that,  

but upon refocusing to sharpen up the corner regions 

a lens that is properly centered will show all four corners 

to be at the same focus as each other [acoarst allowing

that the center is now imperfectly focused].   

 

I think that explains it :-) 



#10 iouzzr

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Posted 25 March 2017 - 04:58 AM

Folks over at Fred and Miranda go to some extreme lengths to check the centering of their Loxia lenses, I never got around to it myself.

 

This link has a test which seems easy to construct yourself ... about midway down : https://www.lensrent...ets-a-makeover/

DSC01924
DSC01923
DSC01922

 

the centering seems ok, so I don't test further, just go out shooting, thank you!




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