Need technical advice on testing a lens: I'm interested in a used Canon 24-70 lens that's available from a local owner. He says it has a scarcely perceptible blemish on the rear...

gym Loc: Athens, Georgia

I'm interested in a used Canon 24-70 lens that's available from a local owner. He says it has a scarcely perceptible blemish on the rear element not evident in either stills or video. I'd like to take some photos with this lens and enlarge them to see if I can see anything that might cause greater concern.

My question: What's the best subject to photograph in order to make such an evaluation? Should I simply use a solid color background?

BigBear Loc: Northern CT

gym wrote:

I'm interested in a used Canon 24-70 lens that's available from a local owner. He says it has a scarcely perceptible blemish on the rear element not evident in either stills or video. I'd like to take some photos with this lens and enlarge them to see if I can see anything that might cause greater concern.

My question: What's the best subject to photograph in order to make such an evaluation? Should I simply use a solid color background?

I would find a subject that has as much depth and detail and close your aperture down and see what it looks like.

snowbear

A newspaper?

Wabbit Loc: Arizona Desert

gym wrote:

I'm interested in a used Canon 24-70 lens that's available from a local owner. He says it has a scarcely perceptible blemish on the rear element not evident in either stills or video. I'd like to take some photos with this lens and enlarge them to see if I can see anything that might cause greater concern.

My question: What's the best subject to photograph in order to make such an evaluation? Should I simply use a solid color background?

Hey Doc ..... blemish's on the objective lens normally affects nothing, so look closely because in this case it may. If the blemish on the lens is lets say six o'clock ..... on your picture it will be at the 12 o'clock position (always opposite)

Nice Blue sky should be a good test.

RaydancePhoto

Sky is good, white paper as well.

gym Loc: Athens, Georgia

RaydancePhoto wrote:

Sky is good, white paper as well.

Thanks.
Would I be looking for something subtle - like a slight off color blur?

RaydancePhoto

gym wrote:

RaydancePhoto wrote:

Sky is good, white paper as well.

Thanks.
Would I be looking for something subtle - like a slight off color blur?

Something consistent in each shot that is not supposed to be there. Shooting a solid color or white will usually show best.

stevelew47 Loc: Edmonds, WA

A fast way to test a lens is to glue a newspaper to poster board. Set your camera up on a tripod. I would use available light if possible. Window light works good or outdoors in open shade. Set the lens at the widest aperture f2.8. Take a few shots and check the focus. Be sure the camera is exactly perpendicular to the newspaper. You will notice if the lens is good or not. Look at the image on your computer screen and zoom in. The blemish may show up. Good luck!

St3v3M Loc: 35,000 feet

http://www.uglyhedgehog.com/t-44829-1.html

PhotoArtsLA Loc: Boynton Beach

When lenses have any form of physical damage or blemishes on the glass elements, shooting for sun flare will reveal image degradation generated by the lens issue, if any. Short of such extreme shooting, a small amount of issues on a lens generally will not show, unless severe enough.

jeep_daddy Loc: Prescott AZ

Shoot an overcast sky or a solid sky that is even.

Rufus Loc: Puget Sound area, WA

I've worked in labs that used carefully taken shots of newspaper pages as targets for subjective determinations of image quality. If this approach sounds useful to you, use the precautions suggested by Stevelew in his item above.

BHC Loc: Strawberry Valley, JF, USA

snowbear wrote:

A newspaper?

Absolutely! Take pictures flat and at 45 degrees to test detail and depth of field. And test short, middle and long zoom settings. Use a tripod and timer or remote; lock up mirror if you can.

gym Loc: Athens, Georgia

Thanks to everyone who offered advice. I'm doing some evals now. The lens was not what I was expceting. It is a 28-70 and not a 24-70. I know this lens also has a good reputation, but it's a LOT older, has a narrower view, and if I ever need repairs, I'm wondering if parts would be easily available.

Suddenly it's gone from what looked like a possible 'good deal' to one that's questionable.
Thanks again.

stevelew47 Loc: Edmonds, WA

That lens was popular back in the film days. If it's in good condition, it may be a great buy. If your camera is full frame, then this lens would be great. I use a 24-70 on my 5D and a 17-40 with my 50D. Pretty close to the same spread and both lens are very sharp.