How to capture sharp images of multiple objects on a wall?

Question

I need to photograph a wall with multiple paintings with every painting under sharp focus. I believe depth of field doesn't play here as all the objects are at same distance. I tried with higher shutter speed and higher f-numbers but it only gets crisp image where focus points were floating.

Edit1: ●I am using kit lens 18-55 STM of Canon EOS 200d/SL2(alias in other regions). ●The wall is about 12×15 feet. To summarize, it seems like the camera is only capturing few objects in focus and thus sharp leaving the other objects slightly blur. This behavior can be observed in general photography too where only few parts of the Product(say a bag) would be crisp and other parts slightly blur.

Answer 1

You will get the best results using a lens designed specifically for imaging two-dimensional flat objects that are parallel to the camera's imaging plane. Such lenses are highly corrected for field curvature to give them good "flat field" performance. Most good prime (single focal length) macro lenses have fairly good flat field performance.

Many other lenses are less corrected for field curvature because correcting for field curvature also impacts other properties of the lens in ways that might be undesirable for other types of photography. For example, in order to render pleasing out of focus areas when using very wide aperture settings, many "portraits lenses", such as the Canon EF 85mm f/1.2L, leave field curvature mostly uncorrected. Even though the 85/1.2 costs about $1,900, the more modestly priced EF 100mm f/2.8 USM Macro (priced at about $600) would be a better performer for photographing flat objects.

I believe depth of field doesn't play here as all the objects are at same distance.

That's a bit of a false assumption. Since the field of focus is never perfectly flat (even in theory with a perfectly manufactured lens), stopping down to increase the depth of field will result in better acuity at the mid-frame and edges/corners of the frame with most lenses.

Answer 2

Your assumption that depth of field does not play a part is only partially valid. This is because objects near the center of your field of view are actually closer than objects near the edge of your field of view. However likely the distance difference is negotiable. So what’s the answer? Mount the camera on a tripod because even slight camera movement can be devastating. Choose an aperture setting that is about two f-stops reduced from the largest, likely f/8. This will be the sharpest lens setting, a comprise from the effect of diffraction that comes with tiny lens apertures and lowered acuity that comes due to the lens being set to its most wide-open aperture.

Answer 3

Using the gear you currently have.

1. Step the furthest away you can. Normally on the other side of the room.

2. Then choose your focal length for that distance. Depending on the room, and the area you need to cover of course. Stepping away will force you to use the longest focal length you can. Avoid wide angle lenses (a).

3. If you really have plenty of room, for example on a gallery you probably can define first your focal length (if you have for example a prime lens of 85 mm, then step back until you have the frame you want.

4. Use a firm tripod (a shaky tripod is not useful), point the camera exactly perpendicular to the center of the wall, and at the middle point of the area you want to frame. Do not tilt, do not pan.

5. Use the sweet spot of the aperture of your lens (as Alan Marcus pointed) probably f8.

6. Define your aperture and ISO depending on your camera. An ISO of 200 is fine.

7. Turn off any "Anti-shake" of your lens.

8. Use a remote shutter trigger, or if you do not have it, use a timer on your camera, of at least 3 seconds so you are not touching the camera when it fires.

9. If the room you are in has too little light you probably need to use some flashes, bounced around (depending on a lot of variables) so you reduce your shutter speed and avoid noise.

(a) If the target wall is too big, and you do not have much room to step back, probably you need to compose several shots.

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There is a chance you still need to correct some stuff on the image, eliminating some distortion of the lens.

Some of this can be corrected using your brand's software, or you can use DxO that has signatures of lens-camera combinations to correct some of the intrinsic problems of them.

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All this without thinking about lighting, white balance or noise.

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If you still do not get good enough results, you probably need to rent some better gear.