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Occasional Contributor
Posts: 5
Registered: ‎05-09-2019

What makes a good lens "good"?

I recently upgraded from my old XTi to a 77i.  I bought the body only and I am currently using the stock lens from my XTi while I save up for a better lens.


The EF-S 17-55mm f/2.8 IS USM is what I'm looking at but I have been trying to figure out what it truly offers over the EF-S 18-55mm f/4-5.6 IS STM lens.


In reading up, I don't think for a novice like me the STM vs USM will make a difference one way or the other in my use/perfromance.  Please let me know if I'm wong about this.


I see there is a noticeable difference in the lens construction elements.  Is this the reason for the price difference?  Is the difference in performance in line with the step up in price?


What would I gain (or lose) if I got the EF-S 18-135mm f/3.5-5.6 IS STM instead of the EF-S 17-55mm f/2.8 IS USM?

Esteemed Contributor
Posts: 3,279
Registered: ‎02-17-2016

Re: What makes a good lens "good"?

What makes a lens "good" is what you value in a lens.


For the 17-55 vs 18-135:

The 17 lets in more light and is constant aperture across the 17-55 range, so it works better in low light situations.


The 18-135 has a focal range from moderately wide angle to telephoto, and so is a good walking around lens.


Construction is probably better for the 17-55.

Posts: 10,996
Registered: ‎12-07-2012

Re: What makes a good lens "good"?

The decision you need to make is, do you want to stay in the beginner class or do you want to move up to a more professional type lens?  The Rebel T7i is a fantastic camera and capable of fantastic pictures.  It will do even better if given better glass.

The Canon EF-S 17-55mm f/2.8 IS USM Lens is that lens and was designed specially for the Rebel and xxD line of cameras.


It is not just better optics and the vastly better f2.8 constant aperture.  It is also built better.  IMHO, USM is better than STM and most full on pro lenses have USM motors.


"...would I gain (or lose) if I got the EF-S 18-135mm f/3.5-5.6 IS STM instead of the EF-S 17-55mm f/2.8 IS USM?"


I would choose the EF-S 17-55mm f/2.8 IS USM Lens 100% of the time given that option. The f2.8 constant aperture is a huge improvement. If you need longer FL buy another lens.  One of the 150-600mm super zooms would be a nice addition.


So which is it? Stick with entry level or move up? Any Canon lens and the T7i will make great pictures just some are even better.

EOS 1DX and 1D Mk IV, even less and less other stuff.
Reputable Contributor
Posts: 501
Registered: ‎01-25-2018

Re: What makes a good lens "good"?

I agree with Ernie about going with the 17-55 F2.8 over the slow wider range zoom.  Don't handicap a capable camera with just so so glass.  


Making an extremely wide range "zoom" like the 18-135 involves compromises, especially when trying to hit a lower price point, and one of those compromises is a narrower aperture which not only impacts what you can do in terms of depth of field and shutter speed/iso choices for lower light situations but also compromises autofocus performance since AF is done with the lens wide open.  A faster lens also provides a brighter viewfinder image since the lens is kept wide open until the image is taken (or the DoF preview button is used).


My go to set of lenses are Canon 17-40 F4,  24-70 2.8 and 70-200 2.8 lenses; they are somewhat expensive however the 24-70 f4 is a reasonably priced alternative especially used and I think something in the 24-70 range should be the first quality zoom lens for most photographers.  These three lenses provide an extremely versatile range although if I was buying now I would go with the 16-35 f2.8 at the wide range but it is a huge step up in price for a lens where I don't really need the extra stop.


Good glass will be something you will use on this and future camera bodies so it makes sense to buy the best glass you can and go with quality over quantity adding more lenses as needs and finances allow.  



EOS 1DX M2, 1DX, 1D M2, EOS 650 (film), many lenses, XF400 video
Honored Contributor
Posts: 7,972
Registered: ‎08-13-2015

Re: What makes a good lens "good"?

First and foremost, know and fully understand your gear.  Understand how auto focusing works.

With a T7i you are creeping into the photo-enthusiast range of camera bodies.  When a camera focuses, it does so with the lens aperture wide open.  This allows the most light to reach the AF and exposure sensors.

Many of the newest camera bodies have AF points whose performance changes with wide aperture lenses, f/2.8 or better.  When given a wide aperture lens, the cross-type AF sensing systems kick in.  The suggested EF-S 17-55mm lens can take advantage of the most acccurate AF points in the T7i camera body.


You can get sharper focusing with a “fast” lens on most camera bodies.  But, there is a catch.  On many camera bodies, only the center AF point is capable of this type of behavior.  In other words, manually select the center AF point, instead of letting the camer pick whichever AF point it wants.


Another good investment that anyone who wants to capture more than just snapshots is a fast prime.  A fast prime can be far lest costly than a fast, constant aperture zoom.  I feel the latest Canon “nifty fifty”, the EF 50mm f/1.8 STM, is a must have lens.


"I don't rent software. I use Photoshop CS6, ACR 9.8 and Lightroom 6.8 ."
Frequent Contributor
Posts: 70
Registered: ‎11-14-2018

Re: What makes a good lens "good"?

[ Edited ]

What makes a good lens good? IMHO a really "good" lens produces sharp images that can stand some cropping. It also produces an image with color that best represents the subject. It also focuses quickly (a real difference between STM and USM focusing systems) and focuses accurately. It should be as durable as is necessary for the task at hand. If you are asking what is a good lens for me, only you can answer that after research and some experience and as others have said it should do what you want it to do. All of the options affecting a lens are tradeoffs. Weight, Price, Image Quality, Speed of focus, zoom range, max F stop, Prime, … You must decide the tradeoffs you are willing to accept.





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