Testing lenses - the 'brick' test. Is it valid?

So I have this Sigma 56mm lens and I am not sure if it's 'soft' at the edges at low F-stops. So I did the standard 'brick' test. Stand in front of a brick wall (about 5 feet away) and tak a picture. But the Sigma goes down to 1.4 - and at 1.4 it has a VERY narrow depth of field. So, if the centre is in focus and sharp, at the edges, will it still be within that depth of field?

How do I tell? Here's an example of what I mean. Is it just me or is this soft at the edges?





Any thoughts?

Alan Sh said:

So I have this Sigma 56mm lens and I am not sure if it's 'soft' at the edges at low F-stops. So I did the standard 'brick' test. Stand in front of a brick wall (about 5 feet away) and tak a picture. But the Sigma goes down to 1.4 - and at 1.4 it has a VERY narrow depth of field. So, if the centre is in focus and sharp, at the edges, will it still be within that depth of field?

How do I tell? Here's an example of what I mean. Is it just me or is this soft at the edges?



Any thoughts?

Click to expand...

Hard to tell on my phone, but if all four corners look the same, it might be normal field curvature (the plane of focus isn’t flat). Try the same test, but focus in a corner - are all four equally sharp, and more so than the center now? My old Fuji 56 has very little field curvature, but many lenses do, especially noticeable at close range (not typically an issue with a portrait lens).

Also, make sure your lens is absolutely perpendicular to the wall (it doesn’t look like it is here). With very little DOF wide open and at close range, it really matters.

The flatness of a plane of focus at some arbitrary distance from subject is only relevant to a tiny percentage of scenes captured. My wife and kids' faces ain't flat. Landscapes ain't flat. Birds, wildlife, flowers, the world - all not flat... Lens reviewers shoot photos of flat static scene brick walls because they're easy to shoot in a reproducible consistent manner, but all of the things we actually use lenses to shoot are difficult to shoot in a reproducible consistent manner.

I have a pair of classic FF Vivitar "Series 1" f2.8 zooms covering 28-210mm that have some bizarre field curvatures at certain focal lengths, but that doesn't prevent 'em from taking some great photos out in the real world.

That looks great for being wide open. That is one of my go-to lenses for candid / editorial work.

How do you feel about the lens when you shoot an actual subject with it? The brick wall "test" is really only good to see if you have bad decentering which would jump out at you. If you don't see that, move on. Don't agonize over the edges and corners. Shoot subjects you would normally shoot with that lens and see if you like the look. If you make a compelling photo, no one is going to care about the extreme edges and corners.

Testing lenses as a hobbyist is very difficult. I went through the story of shooting test charts.

Meanwhile I am sure you will notice in your daily images if a lens has a flaw. I stay away from all brick wall tests and from shooting test charts.

The only test I do with new purchases is the quick decentering test as I call it. Is is - yes - quick, it is simple, fail proof, and significant.

And: please don't expect perfection from lenses.

Regards,

Martin

The "brick wall" test is incredibly difficult to do correctly. You camera needs to be absolutely perfectly perpendicular to the brick wall both horizontally and vertically. Vertical alignment can be checked with a bubble level, but side to side alignment requires a string or tape measure and careful measurements. Any error in alignment will give the impression that the lens is decentered when it actually is perfectly fine.

Looking at the brick wall sample you posted, it was not properly aligned horizontally or vertically.

Even if done correctly, the results can still be misleading. Many lenses do not have a flat field of focus. Instead of a flat plan, the area of focus will be more of a bowl shape which means the corners are in focus at a different distance than the center of the frame. With these curved focus planes, the brick wall test makes it look like the corners are really soft. The corners are not soft, they just are not in focus.

There is also the issue of subject distance. Some lenses will be sharper at one end of the focus range than the other. Testing a 400mm telephoto at 10 feet is relatively meaningless if you will always be using the lens for birds at 150 feet. How often will you be using the Sigma 56mm at only 5 feet from your subject?

Finally, lighting and contrast needs to be considered, and not just for the brick wall test. A monochromatic brick wall photographed in diffuse shade will make any lens look flat and dull. Is the photo dull because the lens has very low contrast, or was it because it was a dull scene? Ideally, you want to be testing in broad daylight on a bright, sunny day. Artifacts like color fringing will be much more apparent in bright light than they will be in dull shade.

If you are concerned about decentering, I did a post over in the M forum with a foolproof test that can be conducted almost anywhere and requires no complicated alignment - https://www.dpreview.com/forums/post/66151734

Beyond a basic decentering test, the best thing to do with a new lens is to go out and use it the way you intend to use it.

Thank you all.



The brick test is just something I do. It has found 2 decentred lenses for me. I was thinking more of the depth of field as a possible 'suspect' for soft looking edges.

I have been out tand taken pictures with this lens - results have been fabulous and not so good.

I don't know why, but I really like this one - maybe because the tree ilooks like it will topple any day now (but it won't).