" It could of been that i bought a bad lens "

VersusNYC

I been reading reviews of various lenses and people will sometimes say, "the one i bought was bad" or i have to send my "bad" lens back... Maybe i am asking an idiot question.. but what do they mean by it and how do you know you have one..

Moogle Pepper

VersusNYC wrote:

I been reading reviews of various lenses and people will sometimes say, "the one i bought was bad" or i have to send my "bad" lens back... Maybe i am asking an idiot question.. but what do they mean by it and how do you know you have one..

It could be either-

a.) I bought a lens that I didn't want afterall. It didn't meet my needs or requirements.

or most likely,

b.) I bought the lens I really wanted, but got a bad copy. So I need to either return and exchange or send it back to manufacturer to adjust it or fix it.

How you find out if option b needs to be sent back is if it takes too long to focus if it is supposed to be a non hunting lens, whacky sounds coming from the lens, or it is incorrectly focusing.

If you have a worry, you can post test images here and members here can help you out.

ziggy53

Moogle Pepper wrote:

...

If you have a worry, you can post test images here and members here can help you out.

Great advice!

VersusNYC

Sorry for the vague post. I am in the market for a new lens (probably the 17-40 F/4L) Before plunking down the cash i have been reading many a review, and once in a while ill come across a review stating that they received a bad copy. I figured people have way too high expectations from the glass they are buying.

But some cite issues like back/front focusing problems( i have no idea what this is) or just general blurriness. I just want more education so that i know when i have an actual bad lens as opposed to what Moogle Pepper said

jrmyrnsm

A lot of times you can check front/back focus by setting your camera to the center focus point and then taking a picture of something like a ruler at 45 degrees from the camera. Then you can aim the camera at one of the numbers on the ruler and take your picture. Then check your picture and zoom to see where on the ruler is actually in focus. If the actual focus point of the picture is in front of or behind where you aimed it your lens could have a front/back focus problem. Someone please correct me if that doesn't sound right but thats how I usually test my lenses to see if they focus right.

ziggy53

jrmyrnsm wrote:

A lot of times you can check front/back focus by setting your camera to the center focus point and then taking a picture of something like a ruler at 45 degrees from the camera. Then you can aim the camera at one of the numbers on the ruler and take your picture. Then check your picture and zoom to see where on the ruler is actually in focus. If the actual focus point of the picture is in front of or behind where you aimed it your lens could have a front/back focus problem. Someone please correct me if that doesn't sound right but thats how I usually test my lenses to see if they focus right.

Unfortunately, using a ruler to try to detect front or back focus issues may induce those issues.

Even selecting a single focus point is not as finite as the display might indicate. The actual region affected is quite a bit larger. Because of this a lens/camera may choose a mark other than what you might think or intend.

I suggest using a single target with a sharp "cross" mark oriented at roughly perpendicular to the camera and lens. The background around and in front and behind the target should be as non-distinct as possible.

The idea is to give a single distinct target so that the AF sensor does not get confused by marks before or after the target. If you want to supply a scale, make sure that it is well to the side of the target and away from the marked regions of AF in the viewfinder.

I know this goes against a number of tools and sites that exist to supposed measure front and back focus, but frankly, they haven't researched the impact of what they do.

Assuming that you do properly construct a single target like I suggest, front focus is the tendancy of a lens and camera to place the prime focus at the focal plane in front of the target.

Back focus likewise tends to consistently focus behind the target.

Be sure to take into account the minimum focus distance (MFD) for the lens involved, which can change in many zoom lenses depending on the focal length chosen.

I prefer to use twice MFD for testing front and back focus. Most lenses do not do their best at MFD, the exception being true macro lenses.

eyusuf

VersusNYC wrote:

people will sometimes say, "the one i bought was bad"

if the lens stinks then you got a bad lens.

cmason

If you take a photo with the lens, and the image is not the quality you were expecting, then you may have a problem. If you are happy/delighted with the images, its a keeper

eyusuf

cmason wrote:

If you take a photo with the lens, and the image is not the quality you were expecting, then you may have a problem.

the problem does not necessarily come from the lens though. could be poor technique or body needs calibration?

cmason

eyusuf wrote:

the problem does not necessarily come from the lens though. could be poor technique or body needs calibration?

Sure, but you know when you have bad technique, you can diagnose it and explain a problem you see due to it. Lens issues are unexplainable. The calibration thing is a possible resolution to a lens issue, not necessarily the problem.

jrmyrnsm

Hey I just found this focus test you can do with your camera. It includes a page you actually print out and you can see if your lens has a front/back focus problem. Anyways heres the page: Focus Test. Good Luck!

ziggy53

jrmyrnsm wrote:

Hey I just found this focus test you can do with your camera. It includes a page you actually print out and you can see if your lens has a front/back focus problem. Anyways heres the page: Focus Test. Good Luck!

Unfortunately, depending on the lens tested, this is one of the tests which I am saying can and I believe does yield false indications of front or back focus.

The mark you are supposed to focus upon has enought additional markings around it that the autofocus can become confused and choose the wrong focus point, giving you the impression that the lens and camera are mis-focussed.

Feel free to do your own testing but do not accept the results of this singular test as accurate. It has proven in my tests not to be accurate enough of the time that I have discarded using it for any purpose.